Through September 2007 CNC Blog Archive
I'm taking a day off from my other activities to do some
work on this blog and hopefully get down to the shop.
If you want to run flood coolant, you'll need an enclosure
around your mill or the coolant will go everywhere. Here are a few example
enclosures that might spark some ideas:
The splash guard on this Tree CNC mill would
be a lot easier to build than a full enclosure. It will limit the size
of work that can be done, however. The Widgitmaster
has built a very similar enclosure for his Bridgeport clone:
The Widgitmaster Enclosure being
bolted down to mill table. Note the shiny plate is some work he will dial
in. It is located against two pins at the rear.
Locating pins and clamp...
Pins on the bottom locate the enclosure
against he mill table...
Here's the enclosure in use...
It seems like it would be straightforward
to make one of these and to leave room for a vise as well. It's an ideal
setup to key in your vice so it will be properly indicated in when you
drop it on. It's basically a fixture plate with plexiglass walls.
Kap Pullen uses a very simimlar enclosure
on a mill at his work...
Here's a more traditional enclosure. I
love the way the whole front hinges downward for access. Notice how the
coolant is channeled into the drip pan at the bottom...
I've always liked the idea of making
an enclosure from 80/20 aluminum extrusions:
Here is the Flood Coolant in action...
It drains from the bottom into a tub. Electronics
on top are high and dry!
You can see from the pictures why I like
the 80/20 route: it goes together with a minimum of fabrication and comes
out looking extremely professional. The extrusions aren't cheap, but I
really think they're a very attractive way to go.
Speaking of attractive, here is a totally
awesome fabricated enclosure for an IH Mill:
Shiny diamond plate sure looks good!
Electronics are right underneath the mill...
Not Machine Tool Oriented, But I Want One!
If you're like me, there is a gaggle of boxes and cables
under your desk that are associated with your computer. There are routers,
DSL modems, power strips, and every manner of wall wart power supply.
It's scary to even go under there and try to plug a new one in or fix
the old. Enter the pegboard under-desk organizer:
Such a cool idea!
Slow Times at the CNC Cookbook!
You've no doubt noticed a bit of a hiatus in the postings
here. That's because I'm in the process of changing jobs. I'll get back
into this thing once the new job is up and running smoothly, but in the
meanwhile, things will be a bit sporadic. Bear with me!
Great Idea for a Vibratory Polisher
I couldn't pass this one up because it seems so cheap
and easy to try:
Take four pieces of 1in pipe weld them to a flat piece
of steel. Make it so a five gallon plastic bucket will slide up and
down loosely between the pipes. Then pick up a cheap jitterbug sander
from harbor Freight. I got one for $9.99 on sale. put a long piece of
threaded rod up through the center of bucket to bolt the top down then
epoxy a block of plywood to the bottom of the bucket drill out hole
to fit over rod and nut on bottom of bucket. bolt the sander to the
block of plywood. You put your parts in the bucket with some crushed
walnut hulls You can get 13lbs of them on ebay. for 8 bucks. add a little
polishing compound to the media. You attach a air hose to the Jitterbug
sander. when you set the whole thing bucket and sander down between
the rails of pipe it hits the trigger lever which is on top of the sander.
it points down when bolted to the bucket. as long as air goes to it
the whole bucket will dance It took me 15mins to build this and guess
what it works.
From the Anodizing
Yahoo Group. Sounds like a clever solution to me!
Mach 3 Controller Limitations
I recently came across an interesting thread on the Mach
3 boards talking about the limitations of the various controllers. As
you may know, there is a relatively new crop of controllers that Mach
3 supports that do the step pulsing in the controller, relieving Mach
3 of the task. Supported controllers include the Gecko GRex, Galil, and
the NCPod. In theory, these boards allow much higher performance because
they can deliver a smoother pulse train to the step or servo drivers and
many more pulses per second. But here is the rub--each outboard controller
has a limitation on the number of moves per second it can accept, and
all of them accept a lot fewer moves than a parallel port:
Parallel Port: circa 10,000 moves/sec
NCPod: 1,000 moves/sec
Galil: 250 moves/sec
GRex: 50-100 moves/sec
Those are some big differences! Does this mean the outboard
controllers are all doomed to poor performance relative to the parallel
port? Not necessarily, things are more complex than that. Think of the
moves as being a rough estimate of the maximum rate g-code moves can be
evaluated. If the g-code is making 0.0001" moves to simulate some
sort of smooth 3D curve, you get the following maximum rates:
Parallel Port: circa 10,000 moves/sec = 1"/sec of
0.0001" moves = 60 IPM or 1" of 0.0001" moves
NCPod: 1,000 moves/sec = 0.1"/sec of 0.0001"
moves = 6 IPM or 0.1" of 0.0001" moves
Galil: 250 moves/sec = 0.025"/sec of 0.0001"
moves = 1.5 IPM or 0.025" of 0.0001" moves
GRex: 50-100 moves/sec = 0.005 - 0.010"/sec = 0.3
- 0.6 IPM or 0.005 to 0.010" of 0.0001" moves
Wow! Those get to be pretty darned slowed speeds at some
point. In fact, these are theoreticals, and there are mitigating circumstances
of various kinds. For example, the GRex is far more likely to hit its
theoretical maximums than the other boards because it has an internal
move planner that looks ahead and smooths acceleration. The others have
to deal with start/stop jerking, and so won't hit their theoretical maximum.
The parallel port's maximum is mitigated by the ability to generate the
necessary pulses to move at those speeds, and as Art will tell you, 60
IPM is in the grey zone. Some stepper motors can be tuned up to perform
that fast, but many cannot. I know the motors on my CNC
MiniRouter cannot--30 IPM is about the best I can do there.
So what's to be done about this? It rears its ugly head
most often for 3D profiling operations on a mill. What are the chances
you need to profile to 0.0001"? What are the chances your mill is
accurate enough that its even possible? You probably don't and can't,
but the bad news is your CAM program may be throwing out such moves regardless.
I took a look at OneCNC XR2 Mill Advantage, the program I use, and there
is a parameter for the tolerance on the 3D profiling. I tried creating
a program to profile the rifle stock I designed,
which is a demanding 3D task. I set the parameter to 0.0005" tolerance,
and posted. The result is a 17,000 line g-code program that OneCNC says
would take 3 1/2 hours to run. Incidentally, for wood it wants a feedrate
of 4 IPM, which is faster than really small g-code segments can go according
to the calculations above, although GRex should be able to hit about 1.5
to 3 IPM with 0.0005" segments. I loaded the g-code into Excel and
wrote a quick sheet to determine how many moves were less than 0.0005".
Out of the 17,000 odd lines, only 75 made moves less than 0.0005".
The total distance moved in this way was 0.023".
Another way to look at this is to look at how much travel
can be had with these short segments before the controller starts to get
behind. The parallel port cna go 1", NCPod 0.1", Galil 0.025",
and GRex 0.005 to 0.010". With the worst case being the GRex, we
assume that moves of less than 0.005" can eventually lead to falling
behing. Since the buffer is 50 to 100 moves/sec, let's assume 50 moves
for GRex, 250 moves for Galil, and 1000 moves for NCPod. On my rifle stock
sample, the maximum number of moves that are 0.005" or less is 29.
It seems to me that the GRex could tolerate up to 50 such moves before
it fell behind, so I should be okay.
What's my conclusion? I'm not going to worry about this
particular buggaboo. The rifle stock model I tested is very curvy, yet
OneCNC generated very few g-code moves short enough to cause problems
with the GRex. Your mileage may vary!
Lathe Touch Off w/ Edgefinder
I knew there was a reason I'd need one of these cool electronic
edgefinders after I saw this picture of using one to touch off a CNC
converted Lathemaster 9x30:
BTW, this conversion apparently will do 125
ipm! This is the $175 model from J&L Industrial, made by XYZ. Apparently
this fellow tried some cheaper ones with poor results.
CNC'ing the "Hula Hula" Steam Engine
I've been working through the drawings
for the Hula Hula engine (a Philip Duclos design) and converting them
over so I can build the engine using CNC techniques. So far I am just
at the Rhino drawing stage, but I'm hoping this can be one of my first
projects after completing the conversions on my lathe and mill. Here is
a sample drawing:
As you can see, I have slightly redesigned
the parts. Some things that were more easy on the manual mill Duclos used
can be better done a different way with CNC. These cylinder backplates
and associated holes are a prime example. Duclos originally had these
pieces with nothing but square cuts and pointed tops. The point on the
top would locate the backplate on the Engine Body, and all holes were
to be drilled using the backplates as a guide for greater accuracy and
match up. With CNC, it's easier to profile the round shapes shown, and
the holes can be drilled with enough precision under CNC control that
it isn't necessary to match up the pieces.
Tubing Bender and "Unbender"
How about this beautifully made tubing bender for model
And the matching "unbender" for
These gorgeous tools were made by McGyver,
who recently displayed
them on the HSM boards. They were so neat I added them to my projects
wish list page to do somewhere down the line.
Making Square Holes With Round Pegs
I've mentioned in the past that I like to allocate a certain
amount of shop time to experimentation. I find that if I do all my learning
on projects, I get too fixated on finishing the project, and may shortcut
the learning process. When experimenting, everything is scrap, and the
only outcome is knowledge. A little of both practical projects and experimentation
goes a long way towards making the whole greater than the sum of its parts.
Today, I decided to try to make a square hole in a piece
of steel. A friend was asking if I knew where to get sockets with square
holes for some unusual bolts he had. I suggested a couple of possibilities,
but then stepped up and said, "We can make one too out of a donor
socket and some scrap metal." I decided I'd better get going with
an experiment before he shows up locking for his socket.
My plan was to do the old trick where you cut a square
slot, and then "glue" two of those together to make a square
hole. In this case, I took a 3/8" end mill, slotted the end of a
piece I had squared earlier, sawed then end off, cut that part in two,
and then Tig welded the result together. After grinding down the welds,
I found the technique to be eminently workable:
Square Hole: Mission Accomplished!
A couple of things I learned or that should
- The left and right sides are not exactly
square--they seem to curve to the left. This is because I cut the slot
in a single pass the width of the cutter. You want to use a cutter narrower
than the slot so that each side gets a nice pass all to itself without
cutting forces deflecting things and curving the walls. This is no biggie,
I knew that even as I was making the cuts. This was just a test, but I
want you to be sure you understand.
- Z depth control is critical to squareness.
I have some techniques I've written about to get me within 0.001"
in the past. Make sure you can do the same if you want your hole to come
- I started with a block I had squared. Good
- I needed to make a few practice passes
with the Tig, but instead I just dove in, so the bead is pretty nasty.
However, I ground off the bubbles and it is okay. I wouldn't want to run
a 5HP motor shaft with it, but it will work to turn square headed bolts!
Next time I'll lay a couple beads on some scrap before diving in.
- I tried a slitting saw to cut the slotted
piece in two. This was my first time. They work great! I looked up some
feeds and speeds and found they want to go real slow. In fact, I found
the feed speed was too slow for power feeding, so I just fed manually.
As long as you take it easy, I found they cut extremely well and leave
a very clean accurate slot behind. I'll be trying one of these again sometime
So all in all, it was a pretty happy 1 hour
segment in the shop to learn a couple of things. I'll hand my pal the
test piece and see whether he wants to go ahead making up a custom "square
Schaublin Rear Cutoff Tool
A number of commercial lathes I've run across have an
option for a rear cutoff tool, usually pneumatic or servo operated. I
really liked the manual lever operated tool on this Schaublin lathe:
Something like that would not be too hard
to build and seems like it would be very handy!
Another Gantry Crane
this nice one must be nature's way of reminding me I've made no progress
on mine nor on my Tejas Smoker which
is the reason I need such a beastie!
That color would work in my
shop, eh? This fellow recommends 2 dollies on the crane as coming
in handy a lot...
An Insanely Nice Southbend Lathe Restoration
I'm not too sure I could bring myself to cut any chips
on this newly
restored Southbend lathe if it were mine:
It belongs to a lady named Paula, who says
she uses a removeable metal pan to catch chips so the wooden top is protected.
But you ain't seen nothing yet! See the Rivett below.
Museum Quality Rivett Lathe
The Southbend above was beautiful, but the
Rivett is even more so. They're a more precision machine than the
Southbend design. Just look at all the hand scraped surfaces:
Super Titanium Penlight
Came across this really
neat AA flashlight on the PM boards:
Nicely made design for an AA flashlight...
Obsessed With Precision? (Moore Angle Plate)
I guess you could say I'm obsessed with precision. At
least I'm obsessed with having the ability to measure and align precisely.
A lot of the shortcomings of cheap machine tools can be at least partially
overcome if you can measure precisely. For example, adding a precision
DRO to a tool can greatly increase the precision of the work you can do
on the machine.
The science of measurement is called Metrology, and its
a complicated business. I keep my eye open for items on eBay that give
me a chance to substantially improve my chances for precision. One such
that came along recently was this Moore Angle Plate:
The picture doesn't do it justice, because
this thing is 36" long and its two surfaces are hand scraped:
Hand scraped precision surface...
The name "Moore" is synonymous
with ultraprecision in the machine tool world, so I reckon getting my
hands on this piece for $140 shipped was a real bargain! I now have a
very precise standard that I can use a a straightedge or for checking
90 degree angles. If I wanted to make a set of box ways, I could use this
angle plate as a standard for scraping them in. The reality is it will
probably sit in its box and see only occassional use, but that's okay.
I feel like I've significantly extended my shop's capacity for precision.
I've made a couple of other recent eBay purchases
in the precision department. First up is a pair of Browne & Sharp
ground parallels. Doesn't sound to impressive? Well, these parallels are
1" x 2" x 12", so they're really BIG! These are ground
to 0.0002". I figure they may be handy for a precision setup of some
kind some day. For example, to spread the clamping force if I am milling
a big plate I could put these on the edges atop the plate and clamp on
I've also got a cylindrical square on the
way that looks pretty nice. They're just the thing for checking spindles
on mills and that sort of thing.
Lastly, I've been on the lookout for Dial
Indicators. I mean the plunger style, not swinging indicators, which are
called Dial Test Indicators. I've made do for quite some time with a really
cheezy Central Tool indicator, and I wanted something nicer with decent
travel. I scored a brand new Starrett with a snazzy 2" dial for about
$20 on eBay. The difference in the feel of the action between the Starrett
and my cheap indicator are like night and day. The Starret is silky smooth!
Having some extra indicators will let me
keep them set up in their fixtures for faster use.
A Shopmade Follow Rest and Box Cutters
Turning a small diameter piece that's long and skinny
is difficult. The piece is going to want to bend and flex and generally
make it difficult. Imagine trying to work this piece, for example:
That long skinny piece would give
Steady rests and follower rests were made to pick up where
the support a tailstock can lend is not enough. Most lathes come with
these gadgets, and most folks don't use them often enough--I know I certainly
don't. Part of my problem is I have the usual cheap Asian brass tipped
units, and I just can't feature that brass riding very happily along the
workpiece. It has been in the back of my mind to add some ball bearings.
I've got a ton of cheap skate bearings floating around the shop that should
be perfect for the job.
fellow that did the dollar bill piece above came up with a solution
when turning that piece:
Frank Ford, whom I've mentioned here before (be sure to
visit his awesome site!) has a mini-follower
built right into a QCTP toolholder:
Here's another version, the one I think Frank
said he modeled on, that you may be able to purchase
from King if you don't have time to make one:
The only thing I don't neccessarily like
about the QCTP mounted versions is your tool has to be square onto the
workpiece. For some operations, that isn't the case, so I think mounting
one to the carriage as in the first example might be a little more versatile.
Sine Jaws for the Kurt Vise
After cogitating on the idea of putting dowel pins in
vise jaws (of which more below on how a fellow did that for another purpose),
I did a Rhino drawing to figure out the placement of pins needed to provide
a set of angle stops in 5 degree increments from 5 to 45 degrees:
This would be a dead simple thing to do with
CNC, but I went ahead and listed DRO coordinates for those who want to
play with it on a manual mill. A rotary table would let you do the work
very easily as well. The idea is to drill and ream holes for 1/8"
precision dowel pins. As you can see these holes are divided between a
2" and a 2.5" radius so they don't land too close together.
One could either make a little bar with two pins to use as a stop to align
the workpiece at the desired angle, or just use two dowel pins in the
appropriate holes. I would bore the opposing jaw with holes slightly oversized
so the dowel pin ends go in when tightening the vise on thinner workpieces.
I got the idea to use dowel pins from Jim
Sehr whom I mention below for this set of jaws:
Headstock Adjustment Bolt
Birmingham lathes have a bolt to adjust the headstock
so it cuts true to the ways with no taper:
Nifty Soft Jaw Tricks
Soft jaws for your Kurt vise are fantastically useful
gizmos and easy to make. But there are a few clever things I saw recently
while prowling the web:
Use slots instead of holes and
the soft jaws are quick release...
A series of holes for precision dowel pins
and you get:
An easy way to hold round parts for drilling.
Normally I would make do with a V-block in the vise...
And here is one of the most interesting
tricks. This is a mini sine bar. Depending on which holes the two pivot
pins use, you get angles from 0 to 90 degrees. Exotic, but useful! Available
Posted Some New Workshop Pix
There've been some updates since I first posted about
my workshop! Check the workshop page for
the newest photos:
QCTP Indicator Holders
Someday in the not too distant future I plan to give my
lathe a tune up. I want to check spindle runout, adjust the preload on
the spindle bearings, check the headstock and tailstock alignments, and
generally give it a little TLC. I want it running as accurately as possible
before I tackle making the end blocks to hold the ballscrews when I convert
the lathe over to ballscrews. Angular contact bearings need some pretty
close tolerances for installation. I'll also be turning the ends of the
ballscrews, so it pays to have it all shipshape.
One of the things that I've been seeing for a long time
and thinking I need to build is a QCTP holder with an indicator in it.
saw another one and thought I'd do a little roundup article here so
I've got the details all in one place.
Needs no Dovetail Cutter
If you don't have a dovetail cutter for making QCTP holders
(I made one, it isn't hard!), you might
consider this fellow's approach of just doing it by milling the dovetails
as separate parts:
The QCTP Indicator Holder...
Tilting Vise Fixture to Mill the Dovetails...
The Components. Note How the Dovetails
Are Bolted to the Holder...
"Flapper" For Irregular Shapes
Klotz gave us the "Flapper" design for dialing in irregular
shapes or square stock in the 4-jaw:
This one just uses a magnet to attach itself...
5Bears Indicator Holder
5Bears (the Swede) modified an unused QCTP
(looks like a knurler) for this purpose:
CNCCookbook "Instant" Indicator
As I was writing this, I was staring at an indicator holder
that fits onto a height gage I got off eBay. These replace the carbide
scriber and can be used to increase sensitivity and accuracy of the height
gage. eBay seller discount_machine
(I think that's Shars) has them for $8.95:
If you want one (I ordered a second after
seeing how useful they can be), do an eBay search for "HEIGHT GAGE
INDICATOR". They only have them on "Buy it now" in their
store, so you may have to look carefully.
I took this little gadget together with the
QCTP knurler holder (everyone has one and they aren't that hot if you
get a scissors knurler, so its great to reuse it) and put them together
to get this:
It wouldn't take much to rework the mounting
bar so it was just like 5-Bears holder.
Frank Ford's Holder
has not one, but two versions, although the second is just an improvement
he made to the first:
The Mark I...
Mark II: Now with a blade so you don't
care if it's on center!
BTW, Frank Ford's "Frets"
site is filled with wonderful tips and projects. Do check it out if you
haven't found it already!
Someone is Finally Putting Linear Rails on an Asian
I got the idea to try this when I came across a Tormach
mill for sale on eBay that had bad column ways. I bid on, but did not
get the mill. This
fellow has actually started to make the mod and its looking quite
The router is being used to mill a nice flat
spot on the side of the column using the original dovetail ways and saddle
as a guide. This looks pretty cool!
The linear rail looks very beefy as well.
Can't wait to see how this turns out. This fellow is also doing a belt
drive conversion on the mill head.
Need More Lathe Precision? Add Some Indicators to the
It's tough to be more precise than you can measure (although
some lathes will be less precise than they can measure!), so maybe you
should improve your lathe's capabilities in that department. This
fellow builds his own super performance model engines for R/C boat racing
and added tenths indicators to both axes of his Emco Maier Compact 11
Tenths indicators on both lathe
You could achieve the same result with a
very sensitive DRO on each axis too.
Door Hinge Thingey
I dunno what it is, but the thought was that it would
look nice as a door hinge on a hot rod maybe:
Stunning Paintball Pistol
Came across this completely awesome custom paintball marker
made by DocsMachine:
Electronic solenoid-actuated paintball gun
(heavily-modded WGP Autococker) with infrared break-beam ball detection,
programmable firing modes, and operated off 4,500 psi compressed air.
(Regulated, of course, down to around 180 psi operating pressure.)
The workmanship and technology here is just
Things People Like to Make
It's cute, and I might need something
for the kids...
Quarter Scale Tractor Pulling!?!!
My Internet Service Provider had a serious hardware failure
so the site was down and out for two days. They never did get everything
properly restored as far as I can see, but I think I was able to get everything
back. Drop me a note if you encounter any odd behavior. Thanks for your
Super Precision Machine Accuracy Checking: Ballbars
and Circle Diamond Tests
Tree machine tools used to ship a circle-diamond test
part with every CNC mill to prove that the machine performed to spec.
The part looked something like this:
It's an interesting looking test, and one
I had never seen much written about. I recently did find a
brief article describing the test that I found interesting. Low and
behold the circle-diamond test is an official government test that machines
used to have to pass for aerospace work. The test is called "NAS
979", where NAS is an acronym for "National Aerospace Standard".
The NAS 979 test is designed to measure:
- 5 Deg ramp and .005" taper cuts:
Uniformity of servo response and slide way stiction by visual inspection
of the surface finish.
- Outside Square surface for:
Dimensional accuracy, flatness, squareness, parallelism, and Surface
- 5 Deg Ramp for: Angular deviation
- The circle:
Dimensional accuracy, roundness, diameter variation, and finish
- The center 45Deg Canted square:
Dimensional accuracy, squareness, parallelism and surface finish
It's supposed to be a pretty complete test of a milling
machine, so sometime I may try to get together some g-code for it. As
originally concieved, it is supposed to be cut from a 14" x 14"
x 2" block of aluminum, so I may try something a little smaller!
Now let's fast forward to the invention that has replaced
the circle-diamond test, something called a "ballbar":
Ball Bar: Precision linear measurement
held between 2 spheres...
The ballbar was invented in the mid-80's
at Lawrence Livermore Labs and consists of a precision linear transducer
held between two spheres--one in the spindle and one on the table. The
transducer tells the ballbar computer software whether the two spheres
are moving closer or futher apart as the machine moves through a series
of circles around the fixed sphere on the table. A ballbar can determine
Control Loop Errors
- Servo Mismatch
Servo mismatch occurs when the servo loop gains of the axes are mismatched,
resulting in one axis leading the other causing an oval shaped plot.
The leading axis is the axis with the higher loop gain.
- Reversal Spikes
When an axis is being driven in one direction and then has to reverse
and move in the opposite direction, instead of reversing instantaneously,
it may pause momentarily at the turnaround point, causing a ‘Spike'
to appear in the plot, and a flat on the work piece.
- Back Lash or lost motion
This is usually associated with excess clearance within the drive system,
or guide way mechanism.
- Cyclic errors
Often associated with badly worn/manufactured, drive system elements
like Ball Screws and nut, rack and pinions and their encoder devices.
- Scaling Error
Indicates the linear accuracy relationship between two axes within the
test area. The Ballbar software provides linear accuracy values to help
determine whether each of two axes are unequal and/or correct, due to
either servo positioning or slides way mechanical errors.
Axis Slide Way Errors
When one axis of motion is not at 90 degrees to the other.
Measures any deviation in the axis of motion from a nominal straight
line, within the test length.
- Lateral Play (slop)
This comes from excess clearance in the axis guide way system, allowing
sideways motion of the element or table as it changes direction. A common
cause of lateral play may be excessive gibb clearance.
- Stick – Slip and Vibration
These errors result from poor isolation and/or damping from either internally
generated or externally induced disturbances, which cause an axis to
move erratically. Potential surface finish problems are identified by
the Ballbar through the vibration and/or stick-slip characteristics,
but these represent only a small portion of the likely sources of poor
surface finish on machined components.
The spindle and it's bearings, along with the cutter/work piece interaction
are two primary sources of vibration, in addition to work piece material,
configuration, clamping, and process details such as cutter feeds and
speeds. While Renishaw Ballbar plots clearly illustrate many of the
potential problems affecting part surface finish, the software does
not actually diagnose or provide a calculated value for vibration.
- Positional Tolerance
This is a calculated estimate of the likely, bi-directional, positioning
capability of the two axes within the test area. This true position
calculation makes use of the diagnosed values for backlash; scaling
error; cyclic error; straightness; squareness and lateral play.
Pretty nifty device, eh? Shops use the ballbar
as a way of tracking their machine performance, anticipating when maintenance
may be required, diagnosing machine problems, and providing documentation
to customers that their machines are in working order.
How About a Really Nice Monarch 10EE?
There's something about the look of these lathes that
just can't be beat. Some other lathes, like the Hardinges are also nice,
but the Monarch 10EE is machine poetry at its finest. Here is a particularly
nice example, recently restored for a total cost of $12,000 (phew!):
How about those monster leveling
I don't know that I'll ever own one of these
beauties as I am pretty firmly committed to CNC. It would be an awful
shame to convert one to CNC too as far as I'm concerned.
On the Matter of Cheap vs Expensive Angular Contact
Bearings for Ballscrews and Spindles
Without ball bearings of various types, machine tools
would be impossible. Their most critical applications involve ballscrew
mounting and spindles. Unfortunately, these very same critical applications
often call out for very expensive bearings that are out of reach for hobby
class machine work. I have a confession to make: I harbor a deep
resentment for those expensive "machine tool quality" angular
It may be an unreasonable resentment from some perspectives.
NCCams over on CNCZone will tell you all day long that you get what you
pay for and you have to buy the most expensive bearings you can't afford,
but my resentment leads me to wonder whether it is all really necessary.
Yes, if I'm building a vertical machining center with micron accuracy
that's capable of 600 ipm rapids, I'm sure they're necessary. NCCams has
built a precision machine used to make camshafts for NASCAR winning seems,
surely a very exacting application, and one that needed great bearings.
But do I really need those costly bearings to do an 6K
rpm spindle for a hobby mill? Do I need them for a ballscrew bearing
block on a machine I hope will be repeatable to a thousandth? I feel the
resentment is reasonable for the hobby machines. Someone needs to speak
There are tantalizing clues about this conundrum that
I run across from time to time, and it always perks up my interest. Some
- I am told by various sources that garden variety bearings
of today are every bit as accurate as the machine tool quality bearings
of the 40's and 50's.
- Many Asian-built machines such as the Tormach mills
do not use ABEC7 bearings, they get by on lesser grades.
- See my blog post below "When the wrong bearings"
wherein I explore the use of multiple deep groove bearings to achieve
levels of stiffness comparable to angular contact bearings. I can't see
why 4 of these dirt cheap bearings couldn't be made to perform like $400-600
worth of expensive AC bearings. This article is also copied on the belt
- See my notes on the belt
drive page about how to go about hand fitting unmatched bearing pairs
to be preloaded duplex pairs.
- I constantly see examples where machinists are able
to get superior performance from worn out or inferior machinery because
they know the right tricks. Why can't that be true here too?
All of this will sound like a lot of sour grapes belly
aching on costs and snake oil selling to those professionals who think
nothing of just specing the expensive bearings designed to do the job.
They may be right, but I have a sneaking suspicion there is more to it
than this. Why might the professionals use expensive bearings if cheaper
ones would do? I can think of several reasons.
First, look at it from the standpoint of manufacturing
repeatability. Machine tools have to be warranted to certain performance
levels despite variations in their construction and component parts. Tightening
up the specs on the components makes it less likely the tolerances will
stack up poorly and a machine will leave the line that isn't up to specs.
The science of Six Sigmas and quality control will tell you it is cheaper
to set up the manufacturing process to avoid these mistakes in the first
place rather than find them after the machines are built and have to rework
the out of spec machines. So the overall cost to mass produce a machine
may be less while the individual cost of a single machine may be more.
Second, consider the warranty aspects and especially durability
and wear considerations. An expensive machine tool sold for production
business use will be expected to run tirelessly around the clock day after
day in order to justify its cost. A hobby or light use machine need not
be so durable in order to fulfill its purpose. Also, for many manufacturers,
the warranty cost of a failure is very expensive to cure. The cost to
an individual to fix their hobby machine themselves may be so much lower
they're much more willing to risk it. I read somewhere on CNCZone of a
fellow running a CNC router shop that uses hardware store routers and
buys crates of bearings for them. He says it costs about $2.50 to replace
each bearing and he gets 100 hours of continuous routing from a bearing.
To him it is worth it to keep on replacing cheap bearings. To Haas, who
sold you a very much more expensive gantry router and a warranty that
causes them to have to send someone out to fix your bearings if they break,
it isn't worth it.
Third, commercial machines have a radically different
performance envelope than hobby machines. We kid ourselves we can do what
they do, but we can't. We may be able to get a part made that is very
close, but it will take us much longer to do it. We don't run nearly the
rapids, we often run steppers instead of servos, cutting loads are probably
much less, we are babying our cutters and our machines, while the pros
are cranking out 110% on the spindle load meters and creating so many
chips our little home shops would be buried in no time if we tried it
(not to mention all the other problems!). What we really care about most
in these hobby class machines is accuracy and repeatability, and not all
that much of that. Maybe someday the cutting speeds and efficiencies will
matter to us, but for now, we'd just like to get the parts made reliably
to a thousandth or so. I submit that this is a far simpler requirement
than what most of these high end expensive bearings are being designed
to deliver, and that we can therefore get by on less. The level of accuracy
and performance needed to cut precision cams to be used in zillion dollar
NASCAR racing may be a touch more than what we really need to build tabletop
Lastly, there is a labor intensiveness factor that matters
more to the manufacturer and less to the hobbyist (or to the Asian manufacturer
for that matter!). If I am a hobbyist, I can take the time needed to take
two relatively unmatched angular contact bearings and grind them for a
desired preload. I can then hand fit them to the shafts and bores they'll
live in, lapping, honing, or using whatever means is necessary to achieve
a good result. If an experiment of this kind is marginal or fails, I can
always try again and perhaps do better the second time. Most of the investment
I have in is just time. OTOH, if I am a manufacturer, unless my labor
is extremely cheap, I want no part of that process. I will spend quite
a bit of premium to buy a matched pair of preloaded AC bearings off the
shelf so I don't have to mess with grinding them and trial and error.
A really fancy set of these bearings is maybe $800. It takes surprisingly
little shop time for someone to run up $800 of labor, and they may screw
it up! If I just buy the bearings, they're guaranteed and someone else
takes the risk for the screw ups. Hence I just buy the bearings if I'm
a manufacturer. Things were not always this way. Bridgeport used to hand
fit spindle bearings, for example. Not because it made a better mill,
but because it was cheaper than buying more expensive bearings at the
labor rates in those days.
This piece has been more of a rant than anything else,
so I understand if you arent' convinced, but let me leave you with the
anecdote I read this morning that set me off. A
fellow on PM was asking about ballscrew bearings for his Bridgeport Series
I CNC. He got on the phone to Bridgeport at the suggestion of one
member, and was told that each axis would cost $650 to $875 just for the
bearings. OUCH!!! He probably paid less than the cost of the 3 sets of
bearings for the whole machine!
Along comes another member with the same machine who bought
a cheap pair of unmatched AC bearings off eBay for $35. He got after them
with a surface grinder to grind the inner races to create a preload condition,
installed them on his machine, and measured 0.0008" play while still
being able to turn the ballscrew relatively freely (too much preload will
make the screw stiff and there will be a tradeoff between less play and
too much stiffness). Backlash of 0.0008" would be fine for hobbyists
wanting to machine with 0.001" accuracy. The price is right and a
small amount of labor delivered this happy ending. Would a manufacturer
do it this way? They probably would in China, but not from one of the
"big name" machine builders they wouldn't!
Computer Upgrade Time
I dread upgrading because everything stops working for
a little while. However, this is a good time for me to find the hours
needed, it's been 3 years so machines have gotten a lot faster, and AMD
recently put some big price cuts on their CPU's. That being said, I just
ordered up a new set of "guts" (I like to keep reusing my custom
Borg Cube case)
from my favorite dealer, NewEgg.com.
Here's what the new machine will have going for it:
AMD Athlon64 X2 6000+ 3.0GHz
This is a dual core CPU running at a much higher clock speed than
the single core chip I have today, so it should be about twice as
AMD and Intel periodically trade places for the speed champ, with
Intel usually offering a slightly faster chip for a lot more money.
At the moment they seem neck and neck with AMD still a tad cheaper.
ASUS M2N32-SLI DLX WIFI AM2
I've been partial to Asus boards for a while now. They're extremely
well made and stable. This one isn't cheap, but it is crammed to
the gills with features and will make a nice platform I can upgrade
at least once with better video and CPU.
ASUS EAX1950PRO/HTDP/256M R
Not the absolute fastest board available, but it is the fastest
board under $200 at the moment. It will be dramatically faster than
the old GeForce card I have today!
1Gx2 Corsair TWIN2X2048-6400C4D R
There's faster memory for a premium, but this is 2 gigs of memory
that is plenty fast enough as I don't intend to overclock this system.
Western Digital 10K rpm 150GB 16M SATA WD1500ADFD
I love these Western Digital 10K rpm drives--awesome speed! My
current machine runs 2 of them in a RAID configuration for even
better speed. The RAID formats are not necessarily compatible between
motherboards, so I bought this disk to use to back up the other
two so I can then bring them up on the new mobo. It will take some
backing and forthing, but I gotta have that RAID speed. When I'm
done, this disk will remain to use for nightly backups.
These goodies, plus the power supply, DVD-R/W, and other
bits and pieces in my current machine should give me quite an upgrade
in performance. That will free up my current machine's guts to trickle
down and improve my kid's machines. Isn't it funny how it always costs
right around $1000 to do the PC upgrade you'd like to do?
I plan to buy another machine before too long to use for
my CNC mill conversion. I think a Shuttle PC will fit inside the NEMA
enclosure quite nicely along with the rest of the electronics. Stay tuned,
I won't tackle that until the main machine has been upgraded!
Nifty Shop Photos: Truing a Lathe Chuck, Steady Rest,
4th Axis Gear Cutting...
Here's some nifty photo postings I recently came across
Truing a Lathe Chuck. Note the washer to
keep the jaws loaded while grinding them true, as well as the sparks flying!
Heck of a Steady Rest! Big ball bearing
Parts for the Steady Rest...
I need to make a tailstock for my Phase
II table just like this one...
A Fellow GRex Rack Mounter!
I came across this fellow's thread on CNCZone and felt
a kindred spirit. His rack system is pretty similar to my CNC
lathe's rack chassis:
There are several features of his case that
I really like. First, It was definitely simpler to just mount the Geckos
to the wall rather than use the heatsink I did. Second, I like bringing
out the fuses with pilots to the front panel for all axes. I think adding
a disable switch to each axis would be nice as well, though it wouldn't
get used an awful lot.
The GRex is so powerful and compact, you
can fit a lot into one of these small cases!
Network Connectivity to the Shop
Networks and printers usually make for a frustrating day
working on computers. Getting network connectivity down to my shop was
no exception. I decided to try to do it wirelessly, thinking this would
be easier than dealing with cables. So, I bought a wireless router at
Staples and a USB wireless connection for the shop computer and I was
off to the races. I had only a modicum of the usually nuisances that accompany
this sort of job. For example, the "Quick Start" software that
came with the router to make it easier completely failed to work. I don't
think I've ever seen a network "Quick Start" utility that did
work, which seems pretty ridiculous in this day and age. In any event,
I had wireless down to the shop and I could log into my network from my
shop computer. The chief advantage to this was not having to schlepp g-code
files from the CAM program in my office down to the machines.
Having crossed this hurdle without singing too many tail
feathers, I thought, "Why not keep going and get the lathe's GRex
to run via the shop computer?" Capital idea! I had, unfortunately,
screwed up royally in my approach. You see, the wireless modem on the
shop computer plugs into its USB port and the GRex wants a CAT5 connector.
Back to staples looking for a wireless guzinta (because I need something
that guzinta that kind of jack). No such animal. How about Radio Shack?
Never heard of it. Back home, I went to look at NewEgg. By now it seemed
that nobody had thought it would ever be a good idea to take a wireless
connection back into a CAT5. How silly is that?!??
I don't give up too easily on these network jobs. If you
show fear or weakness, you're done for on this kind of thing. So, I started
reading up on the Belkin web site, that being the brand of wireless router
I had purchased, and discovered it has a "Wireless Access Point"
mode. Great! This must mean I can extend my wireless reach with a second
router and it would have plenty of nice CAT5 plugs on the back. Back to
Staples and a new box was had. More configuration conundrums. As soon
as I configured the second router as an Access Point, the PC immediately
quit being able to communicate with it in any way shape or form. Drat!!!!!
I went through the hard reset process with it about 4
times before deciding that was for the birds. Out came a big long cable
with CAT5 connectors on either end. I plugged that into a distant connection,
ran it all the way to the shop, and plugged it into a cheap 8 port switch.
I plugged computer and GRex into that switch, booted the computer and
low and behold I was connected! I did the usual little back and forth
between Mach 3 and the GRex before they were willing to communicate (just
one retry, no biggie), and all was well.
Why didn't I do things the simple way to start with? Sigh...
Debugging the Lathe Electronics
Having gotten things connected down in the shop via LAN,
nothing would do but that I needed to fire up the stepper motors for the
lathe and verify I could spin the shafts under Mach control. Why? Well
why not??? I'd seen this work before, but under much more experimental
circumstances in my office and not down in the shop. If I could make it
work in the shop, I could bolt the cover on, stick it in the rack next
to the computer, and get going finishing off the lathe.
Alas, this was just not an easy day. It's funny, but I
must have carried over some bad vibes from that last Friday the 13th or
something. I hit the jog keys in Mach 3 and nothing. I could at least
see the proper axes were lighting their LEDs on the GRex, so I know I
had everything good to that point. What gives???
Out came the multimeter and I started tracing down how
far the AC was getting to the DC power supply. The answer was, "Not
very far." Doh! By this time it was 9 o'clock in the evening, so
I shut down the shop and came upstairs to write this little missive. I
suppose progress was made, but it certainly seemed minute. Still, it can't
be all that hard to track that AC down and figure out what's gone wrong.
Dropping Out Parts in CNC
Let's say you're making a part that is going to be machined
all the way around. How do you make sure it drops out nicely from the
stock without hanging up on the cutter?
For example, let's take my EZ-Clamp design:
I want to machine the clamp so it is as thick
as the stock. How do I make it drop out?
- Machine it to partial depth and then flip
it over and clamp it in soft jaws for the vise that are machined with
a "negative image" of the part. This method does require production
of the softjaws, and so is perhaps better suited to production of more
than one part.
- If there is a hole, bolt the part down
to a fixture and machine away everything but the part and the fixture.
- Leave some tabs 0.010" thick that
hold the part but that are easy to file through and then sand off. Apparently
Mastercam has a feature to put the tabs in automatically. I believe some
of the VCarve programs will too.
- Machine the part on the end of a piece
of round stock and then part it off on the lathe. It will be helpful if
the depth you cut the part is a little greater than required.
- Clamp half the part with 2 clamps. Machine
the part that is free of clamps. Shift the clamps to the other side one
clamp at a time. Finish machining.
- Consider super glue and double sided tape.
These can work if the cutting forces and vibration are not too great.
Proper Installation of Precision Dowel Pins
I recently bought some precision dowels on sale somewhere
to use for locating parts. You shouldn't be using bolts to locate parts,
they're simply there to act as fasteners. Use dowel pins to precisely
locate parts. The trick in having them work precisely is in how you make
the holes to receive the pins. So, I went out and tracked down a
thread that talks about it. There's always a thread to be found on
I will spare you the reading of the thread by cutting
to the chase. There are 2 preferred methods. The first will be a little
faster, and slightly less precise:
- Spot drill: And folks were at pains to point out that
you should not be using a center drill for this purpose!
- Drill 1/32" under.
- Plunge an undersized endmill.
- Finish by reaming.
The second method, which is more accurate but also slower,
is to use a boring head.
Consider metric sizes for the undersized endmill. For
example, 6mm is 0.2362", which is a reasonable undersize for 0.250".
If the fit is really crucial and you can drill the pin
bores all the way through, consider clamping the parts in their proper
alignment and machining the dowel bores in a single operation. You can
go back and oversize ream one of the two holes for clearance.
Unusual Edge Finding Accessories: Toolmaker's Chairs
Edge finders are important gadgets to have so you can
make sure your mill is lined up on a particular feature. Normally, they
are just little gizmos you stick in the spindle that work either electronically
or mechanically. The classic design spins and when it just touches the
edge it "kicks out" to tell you that you've found that edge.
There are more elaborate schemes available, however. SPI
makes a couple of edge finder accessories that are called "Toolmaker's
Chairs". They look like this:
As you can tell, they have embedded magnets
to hold them in place, and they are precision ground to 0.0001"!
You use them to precisely locate using a dial test indicator. To find
a corner, the chair on the left is places on the corner and then you indicate
on the circular hole that is precisely centered over the corner. To indicate
on an edge, use the slotted tool and your DTI.
Recently, I came upon a brief
writeup that showed a shopmade variant of these that Marv
This interesting tool also serves for center
Here is a similar Japanese-made commercial
A Few New Goodies Arrive on My Doorstep
Kabel Schlepp cable carriers (used to keep cables from
tangling on CNC machines):
A precision ballscrew for my gang
A planer gage, sorry no pix! And an ER40
collet chuck in 30 taper which may someday be used on my mill
belt drive conversion:
Interesting Lathe Modification: Headstock Tramming
I came across one of the more unusual lathe modifications
I've seen in a long time on CNCZone. This fellow has added setscrews that
bear against the 4 mounting bolts for the headstock of his Asian minilathe
so that he can tram the headstock for more accurate results:
Allen wrench is used to adjust
It's an intriguing idea, but I wonder how
well it works in practice? It seems like he is just cocking the head on
the V-shaped ways, which would have to introduce some odd side effects,
reducing rigidity at the least. OTOH, someone else on the thread opined
as how this is a standard feature for some 9x20 lathes from the factory.
Perhaps the 9x20's don't sit on the V-shaped ways? Another remarked that
its important to torque the main bolts carefully, constantly rechecking
alignments lest they be knocked out again. I will also add that I read
some posts on CNCZone by Widgitmaster that shows his Birmingham 14x40
lathe has bolts similar to what this fellow is using for headstock alignment.
I had heard a better way to approach this
problem is to first
level the lathe and then apply controlled twist the bed to counteract
whatever the remaining errors are. One fellow in that thread claims this
was the "official" approach advocated by Warner Swasey for their
lathes, and certainly a number of the fellows I've come to respect are
very supportive of this approach.
At some point I mean to try the bed adjustment.
I want to go through a full "accurizing" process with my lathe
after I get the CNC conversion completed to see what the maximum level
of performance is that its capable of. FWIW, folks on the aforementioned
bed twisting (sorry, trying to humorous) thread feel that taper of less
than a thousandth over 6" on a piece with reasonable diameter so
it won't flex is fine. The
factory specification for Monarch 10EE's was 30 millionths of taper
in 2" of length. That's 0.000030"!
Lathe Progress Continues
I had to make a little faceplate to go over the
breakout board I'm using to connect the control panel. This was a
perfect job for the Mini-Router. Took
hardly any time at all and the end result in 1/8" aluminum was perfect:
I also did some serious design work on how
the spindle control circuitry will work. Here is the schematic:
For a further exploration, please see the
driver electronics page. I'm still
not done on this design, but I am much closer!
Cutting T-Slots in Cast Iron for Machine Tables
My Gang Tool Slide
for the lathe is going to require me to cut T-slots, so I went poking
around and came up with some tips on cutting T-slots and cast iron.
First thing is you'll need a T-slot cutter. These are
special milling cutters made for the purpose of cutting the bottom extra
wide area and they're not cheap!
Here is what one looks like:
Note the unusual staggered tooth pattern.
It's designed to help pass the chips through the confined spaces of the
Okay, given the requisite cutters, the first
task will be to square the cast iron piece if you haven't done so already.
The Fidgiting Widgitmaster makes these T-slot tables for a living, and
has recommended that when roughing cast iron, one should use low rpm and
high feed rates. Within the limits of my machine's HP ratings, my 3"
indexable face mill could be used at 764 rpm, 20 ipm, and 0.050"
depth of cut. The Widgitmaster goes on to suggest that for a fine finish,
try a 0.005" depth of cut and slow feed. I'm thinking I'll use my
newly purchased big fly cutter to do this job.
Next step is to rough cut the table slot
with a regular square end mill. You need to make the slot wide enough
to allow clearance for the T-slot cutter's shank, but it need not be much
wider than that. Some folks like to cut the table slot so it will be a
little deeper than the T-slot's bottom. The allows extra clearance for
the T-bolts so they don't jam up as easily. It also reduces the likelihood
the T-cutter will dig in. All in all, it seems a good idea to me.
Having roughed in the table slot, the next
step is to run the T-slot cutter. These cutters have a hard life--you
can't sneak up on the cut at all, they have to cut full width and they
have to do the job down in a hole. Be sure to make arrangements to clear
the chips down in that hole! I'm told most people prefer to cut cast iron
without coolant, so one could use compressed air, but a shop vac rigged
up to suck out the chips seems an even better idea and will keep from
spreading nasty cast iron dust all over the shop. I'm also thinking I
want to run that T-slot cutter without touching the Y-axis at all after
having milled the table slot. Feed and speed for the T-slot cutter are
interesting. I've heard a recommendation of 1/2 the spindle speed of an
equivalent sized end mill but much faster feedrate because of all the
The last step is to do the finish cuts on
the table slot. You want these to be well finished and true, so now is
the chance to cut only on one side instead of full width. Take care the
slot stays well centered on the T-slot and use optimal finishing speeds
and feeds for the cast iron as well as a nice sharp 4 flute end mill.
Lifting Heavy Fixtures and Tooling
Lifting heavy tooling and fixtures such as vises, chucks,
or rotary tables can be difficult. I've come across several aids in my
travels that I thought I would share. First is this nifty vise caddy that
I have added to my project to do list:
Isn't that cool? It's a copy and somewhat nicer looking
version of a gizmo that SPI sells for almost $400. Attaches to the base
of your mill and then you can move your vise on and off the table easily.
I could see keeping a shelf back behind the mill that is accessible via
Next I have a tip for owners of larger lathes wanting
to change chucks. It doesn't come with pictures, unfortunately. The suggestion
is to chuck up a suitable section of pipe, place a block of wood on the
ways to act as a fulcrum, and then use the pipe to lever the chuck off
the lathe and over to where it's going. If you keep your chucks standing
on edge you can simply chuck the next one onto the pipe and use the same
strategy to manuever it back onto the lathe. You should probably use aluminum
pipe or conduit to avoid gouging your spindle with the end.
How is this for a portable lifting gadget for moving fixtures
and tooling on and off the mill tables? It was created by a machinist
on CNCZone named Geof:
That seems like a pretty handy doodad. It
is set up to straddle the mill base on his Haas VMC's. Geof is not done
with tricks yet, however. Consider this tidy installation on his Mini-Mill:
A similar lifting arrangement with hydraulic
cylinder and arm is bolted to the floor. He's got those 2 Kurt Vises on
a tooling plate...
So the whole thing comes right out and
can be dropped onto a rolling cart...
But what's lurking in the corner? Rotary
4th axis hangs from a turnbuckle. Once the vises are out of the way, it
can be dropped down onto the table...
Of course there are those shops with a jib crane standing
next to every machine, and my all time favorite for wretched excess, a
home shop with overhead travelling crane:
Handy Digital Bevel and Angle Gage
I have coveted the expensive digital levels for quite
a while, but didn't know how well they would work. Along came the opportunity
to buy this cheap and cheerful little brother to those tools and I decided
to go for it. I forget which supplier I got it from, but the cost was
minimal--$39 or something similar. I stuck it in a corner and figured
I would forget about it until I needed to use it. Along
came a thead on HSM which motivated me to haul it out and run a couple
of tests to see how well it works. Here's a little photo review essay.
Suffice it to say that the device seems pretty accurate and is a keeper!
They're cute little goobers, aren't they:
Digital "Bevel Box" shows
my granite surface plate is level...
My 1-2-3 block is level...
Now there is some question. I think this
arrangement may be level but the Bevel Box is cocked a little bit. You
sometimes need to jiggle it to get it to sit flat...
My 30 degree angle plate is surprisingly,
ahem, 30 degrees...
Lathe is level according to Starrett, what
will the Bevel Box show?
You can see the awful truth on the rhs
1-2-3 block. The gage block was hidden under the Starret level and I need
to get my lathe shimmed up!
When the Wrong Bearings May Work for a Spindle or Ballscrew
or, How You Can Make A Mill from a Drillpress
Any casual reader of CNCZone will eventually run across
one of the famous bearing rants for either spindles or ballscrew mounting.
Closely allied are the drill press mill rants. Some
noob will inquire with much enthusiasm how to go about converting the
Asian drill press they just got for $39.95 into a CNC mill capable
of slicing through solid green kyrptonite at 300 ipm with an accuracy
of 10 microns and the old hands will just come unglued at the absurdity
of it all. While this can be entirely entertaining to watch, one does
feel a bit like the beginners are receiving an initiation flogging they
don't really deserve and are ill-equipped to understand.
The bearing question is similar. Someone wants to mount
a ballscrew or spindle in "ABEC7" skate bearings that were purchased
cheaply on eBay and the ranting from the old hands starts in again. Pretty
soon the fur is flying and we're talking about the need for $800 20TAC47
bearings on an Asian mill that didn't cost that much more than that
and everyone wonders how we got there.
What's funny is that every now and again, someone actually
manages to do what the experts have said is impossible. For example, there
is a Mech E professor that has built a pretty
nice little milling machine from a drill press:
You too can build a milling machine
from a drill press...
Someone commented that this design was a
nightmare and the guy obviously didn't know what he was doing because
he had installed 5 deep groove ball bearings and a single tapered bearing
that was in backwards of all things. You just can't do that--it ain't
"Hmmm," says I. When I hear that
you can't do something, I kind of want to know why the guy did it anyway
and how well it worked. It occured to me that perhaps this guy was clever
like a fox. I sniffed around his site a bit more and learned he was a
Professor of Mechanical Engineering with full Piled Higher and Deeper
credentials. Now I am nto one who is intimidated by credentials having
attended graduate school and met many of these sort of fellows. At the
same time I do not immediately assume any PhD is an idiot either. This
guy piled on 5 deep groove and 1 upside down tapered roller bearing for
a reason, and it became my mission to figure it out.
It didn't take me too long to decide that
maybe he was just stacking the bearings to make up for their inherent
weaknesses. One often hears about stacking 3 or even 4 angular contact
bearings to increase rigidity. So I dragged out my bearing catalogs and
had a look at what this might mean.
From the NSK bearing catalog I found a nice
comparison of the strengths of various kinds of bearings. We can see that
deep groove bearings are primarily limited in that their load capacity
is not as good as angular contact bearings:
From the NSK Bearing Catalog: e1102c.pdf
Deep Groove Ball Bearings
Angular Contact Ball Bearings
Tapered Roller Bearings
Radial Load Capacity
Axial Load Capacity
Fair in Both Directions
Good in One Direction; Takes 2 bearings for 2 directions
Good in One Direction; Takes 2 bearings for 2 directions
Excellent: All tolerance classes available
Excellent: All tolerance classes available
So I decided to try to set up a comparison
of the radial and axial load capacities for similar sized bearings of
different types. There are formulas in the NSK catalog that may be used
to compute the load capacity of up to 4 stacked bearings:
- Double: 1.62x Radial, 2xAxial
- Triple: 2.15xRadial, 3xAxial
- Quadruple: 2.64xRadial, 4xAxial
You can see that axial loads are additive
but radial loads don't get 4x the value when you stack 4 bearings. In
fact they aren't even 3x as strong radially when 4 bearings are stacked.
That's going to be the weakpoint I suspect. The results are interesting.
Multiple 6204’s Back to Back
6204's are standard deep groove ball bearings, typically
considered wholly unsuitable for spindle and ballscrew use. You can buy
plain vanilla 6204's for $7.69 apiece while ABEC7 quality 6204's are $77.
Here's what you can get by stacking them:
Consider that for the ballscrew application
the load is going to be largely axial as we are trying to prevent the
screw moving along its axis and introducing backlash.
So how do they compare to equivalent angular
The plain vanilla angular contact equivalent of a 6204
is simply a 7204. Your basic 7204 costs $23.88 so already we could have
bought 3 6204's for the price of a single 7204 and for two 7204's we can
surely stack up our 4 6204's. A matched duplex pair of ABEC7 7204's are
a cool $200.
Here are the specs on stacked 7204's:
Guess what? The double AC bearing configuration
is bested by a triple 6204 bearing arrangement! Now if we want ABEC7's,
the duplex AC bearings are still a bit cheaper, but if we're fooling with
more "stock" bearings, it seems like we can get a more rigid
arrangement for less money using the deep groove bearings.
I'm sure it is probably not quite so easy,
but it is certainly intriguing. It wouldn't cost much to build a test
rig and see how well the deep grooves perform when stacked. Now I'm sure
the bearing gurus are spinning up to full whirling dervish speed to jump
all over this concept, but I remain unrepentant until I see someone hook
them up and make them play.
What about the really expensive bearings?
A duplex pair of the much vaunted 20TAC47B
purpose-built for ballscrews angular contact bearings turns in an axial
load value of 26,600N. That's better than 3 stacked 7204's! However, note
that the quadruple 6204's begins to approach this value at 26,400N. Also
note that 20TAC47B's cost $800 the pair.
Can 4 of these cheap bearings do ballscrew
duty as well as the $800 TAC's? That's a scary thing to spring on the
It remains to be seen, but I would sure love
to try the experiment someday!
No, but it would have been an appropriate
post to keep people guessing.
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