Parting & Cutoff
or cutoff is the operation of cutting a piece off by slicing a groove
all the way through it with a special parting or cutoff tool. These tools
are generally thin blades of HSS, but there are also carbide insert tools
available for the task or you can grind a cutoff tool out of HSS. I haven't
tried the HSS blades, although my QCTP came with a holder for them.
A cutoff tool ground from HSS...
I ground one
tool from HSS-Cobalt, which is shown above. It is about 1/8" wide,
which is pretty wide. A thinner tool would have been better. If the truth
be told, I just plain got tired of grinding. You have to remove more material
to make a parting tool than most any of the other blade types. This was
on a 1/4" blank and it probably took me a good 15 to 20 minutes of
grinding to get it right. Notice there is a lot of positive back rake
on the tool as well as plenty of side relief.
Keep in mind
that parting off is a fairly demanding operation for your little Asian
lathe. Rigidity is extremely important. I find that locking my carriage
is an essential step to take in order to maximize rigidity. Another important
step is to make sure the cutting tip of your tool is right on the center
line of the piece you are parting. Make sure your tool is exactly perpendicular
to the axis of rotation as well. Another thought on parting is to try
to do your parting as close to the chuck jaws as possible. This is another
way to maximize the rigidity and make things go smoother. I have heard
some authorities suggest you can't part more than 1/2" from the chuck
jaws, but I've had no trouble parting 1" or 2" out at all.
like to cut with the parting tool upside down and behind the work piece.
This is a more rigid configuration plus the tool will tend to pull away
instead of digging in with this arrangement. I haven't tried anything
like that yet. I really haven't had that much trouble parting. It does
generate more heat and smoke than the other operations, but it isn't too
bad. Do be sure to use a lubricant of some kind. I use either oil or Tap
Magic depending on what's closer to hand. I like to apply the lubricant
to a chip brush and then use the brush on the rotating workpiece.
I have also
heard the recommendation that you have to do parting operations at extremely
slow spindle speeds. This has not been my experience, particularly not
when cutting with carbide. I'm not saying to get carried away, but the
speeds I use are not that different from turning speeds, only slightly
slower. Of course you can look up all this data in Machinery's Handbook.
Aloris No. 71 Carbide Insert Cutoff and Parting Tool
cutoff tool is made by Aloris. Being an avid devotee
of carbide tools, you shouldn't be surprised to see that it is similar
to the HSS blades except that it is uses carbide inserts. This whole thing
then fits into an AXA-series QCTP toolholder. I got mine at Enco, and
it wasn't cheap. Boy does it work good though!
An Aloris Carbide Insert Cutoff Tool: My Favorite!
stainless like butter:
Parting off a piece of stainless steel with the
Leaves a Good Surface Finish...
Parting off always leaves a little nub...
The GTN-3 Carbide Inserts are odd looking little
works great, achieving rapid cutting speeds and good surface finish. As
usual, when I first started out, my biggest problem was that I wasn't
cutting at quite the right speed. I've developed a habit of really boosting
the feed speed when I don't like the way the swarf is forming to see if
carbide cuts better. It often does, which is an indication I need to increase
speed. In the parting off example in the pictures, the cutting got progressively
smoother as the cut deepened. This was an indication I was spinning too
fast. As the cut got deeper, the diameter was less and hence the surface
speed was also less. Someday I will have a true variable speed DC motor
set up and it will be easy to try faster or slower speeds to really dial
things in. Meanwhile, be experimental and just try some different speeds
until things settle in.
When the Aloris
is running at its optimal speeds and feeds, the swarf peels off like nice
fine ribbons (sorry, didn't get a good photo there) and it is a real thing
of beauty to see. This is a pretty expensive gadget (cost more than the
QCTP), but it sure is nice not to have to grind any more parting tools!
If you want one of your own, here are the pieces I picked up:
- Aloris No.
71 Holder. This was the most expensive part, and some enterprising amateur
could surely make a holder that would take the blade.
- Aloris No.
71 Self-Locking Throwaway Insert Blade. Others make similar blades, but
this one is nice and small for the 9x30 lathe I own. Most of the others
are pretty large.
- GTN-3 Carbide
Inserts. I ordered 10 so I would have spares on hand.
Handy Pre-ground Parting Off Tools from eBay
I found these great pre-ground parting
off tools one day on eBay:
They're available from eBay seller samsws,
and are usually listed as "Cut-Off Parting & Grooving Mini Lathe
Tool". The price for 3 was $16, and I've found they work great. You
can just pop them into a regular toolholder. Not as heavy duty as the
Aloris above, but they do make a much finer cut, so I use them for smaller
Up Your Parting Blade
While I love the little parting blades
mentioned above, I find they benefit from a little "tuning up."
Here's how I do the tune up:
First, use the radius on the edge of your
grinding wheel to put a little positive rake in the blade. This greatly
reduces chatter on a lot of materials. Easy does it, don't take too much
Next, take on of those inexpensive pocket
diamond hones and use it to make the tool really sharp. I QCTP holder
with the tool on its side on a flat surface, and stand the hone up on
it's side. A couple of swipes as shown are all it takes to make the blade
This last tip is not really necessary for
parting, but I somethings use my parting blade as a turning tool, for
example on my Verburg Steam Engine Team Build
connecting rods. A radius like this is essential for such cuts. Put the
radius on the side you'll be moving into the cut. The radius shown gives
you a tool that can take shallow turning cuts moving from tailstock to
Turning with a parting blade makes it easy
to get nice square shoulders if you need your smallest OD between two
larger OD's. Here, we are about to plunge the blade and we'll be turning
to the left shoulder that's visible. Don't try too much depth of cut.
A sure sign of trouble is a build up of material on the part at the cutting
point. Eventually something will break if that's happening--take a shallower
cut! For this little brass part 0.010 to 0.015 on the dial (0.005 to 0.0075
actual DOC) worked well and gave a decent surface finish...
Here we are starting a pass. You can see
the positive rake radius in these two pictures...
Nice square shoulders thanks to the parting