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Fly Cutter: Ultimate Surface Finish Rewards

Learn what a fly cutter is and how to use one to get the best possible surface finish on a milling machine.

A Fly Cutter is any single point cutter that is used on a mill…

What’s a Fly Cutter?

A Fly Cutter is any single point cutter that is used on a mill. The most common fly cutters are used as surfacing tools. But, you also see single point form tools referred to as fly cutters on mills, for example a form tool being used to create an involute on a gear tooth.

I get asked about Fly Cutters a lot, there is a lot of traffic to CNCCookbook on those keywords, and Fly Cutters are very commonly used by manual machinists. While the CNC crowd will more often prefer facemills, even many CNC machinists realize that a very fine surface finish may be better done by fly cutting. Remove all but one insert from your facemill, and finish improves.

The exception are those most expensive facemills where you can individually adjust the cutting height of each insert to 0.0001″, because that’s what it takes, and that’s why fly cutters can leave a better finish. Many say their secret weapon for fine aluminum finishing is a fly cutter with a PCD (diamond) insert.

So, for CNC’ers one reason to use a fly cutter is to achieve a fine finish. In most cases, the best finish will be one that can be done in a single pass, which implies a large diameter tool and therefore probably not a face mill.

Speaking of CNC’ers, here is a video of David DeCaussin’s scratch built mini VMC. You’ll see a fly cutter in the tool changer that comes into use at about 11:05 on the video. He’s using it to make a fine finish on a part in one pass:

David DeCaussin’s Mini VMC uses a Fly Cutter at about 11:05 on the video…

David has probably forgotten more about CNC than most people will ever know. If you don’t recognize the name, he is the “D” in FADAL and was one of the original founders of the company.

Here’s one more reason to use a fly cutter–it’s a great tool for surfacing your spoilboards on a CNC Router. Typically, you need to take just a little bit off a very large surface area. Running a cutter with a larger diameter helps speed that operation considerably.

Flycutters Can Give Better Finish than Face Mills,
But Why?

You often hear fly cutters can
give the best possible finishes (albeit at slower speeds), but why? If you remove all but one cutter
from a face mill this converts
the facemill to a flycutter which has no runout. The alternative is a facemill whose individual inserts can each be adjusted for height to eliminate the runout. With facemills that can’t be adjusted for height so they all match, each insert cuts a different chip load and this is what results in imperfections in the surface finish.

Fly Cutter Feeds and Speeds

For Fly Cutter Feeds and Speeds see the Fly Cutter page in our Feeds and Speeds Cookbook.

Fly Cutter Geometry

The trick with fly cutter geometry is converting the geometry of an appropriate lathe tool to behave given the geometry of how the fly cutter will engage the workpiece. There’s no use talking about it, as it’s a very visual thing. Speaking of which, here are a bunch of fly cutter geometry pictures:

Fly Cutter Geometry

Not bad, but the point needs a big radius and more positive rake…

Big Radius Fly Cutter

This fly cutter has a honkin’ big radius…

Fly Cutters

These fly cutters are intended to use a geometry created by just grinding an angle on a piece of HSS round, for example, from another cutter that was broken. There’s no positive rake, but there is clearance from the angle and a big radius so the geometry is not bad. Note: The center drill is just a placeholder and is not being used as a cutter!

fly cutter geometry

This one is very close, but the face is vertical and should tilt back away from us to create positive rake. The radius is about the minimum you’d want, but fine. Most people make the radius way too small…

HSS Fly Cutter Grind

Ideal fly cutter geometry for aluminum: big radius and tons of positive rake. This is Widgitmaster’s design…

Widgitmaster’s Homemade “Really Big” Fly Cutter

Why such a big fly cutter? Widgitmaster likes to finish all surfaces in a single pass to improve the finish. You’ll also see him using really large end mills for the same reason in many cases. He remarks that even though they may chatter, it still gives a better finish than multiple passes.

Big Fly Cutter

The head was trammed to an angle, special vise jaws go on the outside of the vise, and look at Widgitmaster’s homemade fly cutter!

Plans for Big Fly Cutter

Here are Widgitmaster’s plans to build his fly cutter. I’ve got to try one of these and see how I like the finish. He takes 0.010″ cuts with it. To make the square hole for the tool in the arm he cut it open, slot milled it, Tig welded it back together, and then turned the arm so you couldn’t see the weld. Slick!

 

Flycutting German Style With a Boring Head

Came across this picture recently on a German site:

I’ll bet the tool was ground from an old dull endmill judging from the finish, but I could be wrong about that. I would think a small radius would lead to a nicer finish…

A Tale of Two Flycutters

You get the best finish when you can surface with a single pass of the cutter. This becomes difficult on wide plates. It’s one reason why flycutters are popular, aside from being cheap: they cut a wide swath. But not all flycutters are equal.
It’s nice to have one that cuts the widest possible swatch, and it’s nice to have smaller ones for more rigidity when cutting smaller work pieces. 

Here are two flycutters:

Note the flycutter
on the left can be extended to a wide circle…

It takes two
passes with the smaller flycutter, which leaves an ugly line between the
passes…

Note how you
can see both sides of the fly cutter cutting. If they’re even, your mill’s
head is properly trammed. This one is close, but no cigar!

Using a flycutter
that is wide enough for a single pass is much nicer. Note that it makes
the tram issue worse when the cutter arm is longer (visualize the geometry
to see why). Now we only see marks from one side cutting…

Neat Fly Cutter Tip for Milling in Lathe

You can set up a mill to act like a lathe or a lathe to act like a mill. Neccesity is the mother of invention. When you have to get by using their lathe as a mill, it is amazing what can actually be done in this fashion. How about this nifty way to face the end of a big block with a fly cutter in the 4-jaw:

Lathe Fly Cutter

Block bolted to cross slide, fly cutter in the 4-jaw…

More Fly Cutter Hijinx

Some good posts on a flycutter that Evan Williams made over on HSM:

He’s got a hand ground HSS cutter on one end and an insert on the other. I don’t think he means to cut with both at the same time, they’re just options. I like the flat to reduce the chance the bar will slip. I’d also like to try an insert, but I would likely use TCMT’s based on my experience making a dovetail cutter. Someone on the thread also suggested a set screw or other feature so that if the bar slips it is captured in the hub and won’t go flying across the room.

A close up of the geometry on Evan’s hand ground HSS fly
cutter bit. It sits vertically, so acts more like a facing cut on a lathe…

Monster Big Fly Cutter

Saw this recently on Facebook:

Weighs about 15lbs and cuts a 10.5″ circle. Currently in use on a 50 Taper mill…

Precision Fly Cutter

Here is a slightly different kind of flycutter that came up on eBay the other day as a “Precision Fly Cutter”:

I don’t know how “precision” they are, but they’re interesting. Note how they’re designed to fit a shell mill arbor. It’s a good idea to give them as much support as possible as they have a lot of leverage on the spindle. I like the idea of using a shell mill arbor as opposed to putting a 3/4″ shank on one and trying to stick it into a collet. Not a big fan of multi-cutter fly cutters. If you have more than one cutter in play, you should just use a Face Mill.

Tangential Tool Flycutter

Tangential tools are highly thought of for small manual lathes. They’re easy to grind and the geometry makes them very rigid. Here is an example of the same principles applied to a fly cutter by the Mike’s Workshop Blog:

Tangential Fly Cutter

I’d still want way more radius on the tips than is shown in Mike’s article for the best finish.

Clicksprings Large Diameter Fly Cutter

Great videos on clockmaking from this site, but he also has a nice write up on a large diameter fly cutter he made:

large diameter fly cutter

A couple of thoughts. First, I like the full circle design on these large fly cutters. Keeping the cutter balanced is a good thing, especially if you’re after the ultimate surface finish. I would even go so far as to add a little weight to the side opposite the cutter. It should be easy to determine exactly how much is needed by simple balancing the cutter on a knife edge.

Second, he uses a hand ground HSS cutter. I prefer carbide, and it would be pretty easy to adapt a round insert to a design like this to get a big radius for a nice finish.

Here’s a nice shot of the finish he gets from this fly cutter:

A Fly Cutter May Fit the Toolchanger

All toolchangers have a maximum diameter tool they can accomodate. But here’s a trick–if your machine orients the spindle to the same position every toolchange, you may be able to fit a flycutter in a toolchanger that won’t allow a similar diameter shell (multi-insert) mill.

The fly cutter goes in with the arm oriented so it points to the center axis of the toolchanger carousel. Depending on your machine, that lets it slide in without interfering with adjacent tools.

Use a Fly Cutter to Increase Your Travels

I got a nice note from Steve, a toolmaker in Indiana, who wanted to pass along some valuable fly cutter lore.  You’d think the modern facemill had eliminated the need for these simple cutters, but nothing could be further from the truth.  That link back there goes to the CNCCookbook page of fly cutter lore, and it’s worth checking out because there are situations where nothing but a fly cutter will help.

Take Steve’s case, for example.  Despite having a big old Haas VF-5 vertical machining center, when it came time to resurface a table saw table, he’d run out of travel.  The answer was to use a fly cutter that could reach into the area where the machine just couldn’t travel:

FlyCutterTravels

That’s a beefy looking shop made fly cutter he’s got there.  He goes on to describe the special tip he ground to reduce chatter:

For the wood saw table top, I got chatter with a tip ground to the geometry you are suggesting. I had to switch to sharpening it a different way, a hook on the top that was a cylindrical groove parallel to the face of the cutter (which in this case was perpendicular to the surface being machined).

Since I was taking about .050 off in the first pass, the cut was stabilized by the amount of material I was taking off–all force was directed into that .050 thickness of metal coming off (almost all radial force and almost no axial force.

The cutter you are describing, due to the radius on the bottom, had forces pushing up against the tool(axial) as well as partially against the rotation of the tool (radial).

It takes a skilled machinist and toolmaker to understand hand ground cutters well enough to come up with that approach.  Steve says there’s a lot of good content on CNCCookbook (thanks Steve!) but that we’re missing around 20% in his estimation.  Every time I talk to someone like Steve I think there’s a lot more than 20% yet to be written about.  We do appreciate all the kind folks like Steve who write in with helpful ideas or to share their projects.

Fly Cutter: Ultimate Surface Finish Rewards [Complete Guide]
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