View Full Version : Fly Cutting Mill Damage

10-29-2012, 10:22 PM
Over the years I've heard that fly cutting is hard on a mill. I have not needed or used a fly cutter and have avoided using them because of the extra wear on the mill. I'm now in a position to take advantage of using a fly cutter.

It looks to me as though the main damage would be to the quill bearings. I can see that replacing the quill bearings is not trivial but it shouldn't be that difficult.

Are there other areas of potential damage? Am I underestimating changing the quill bearings?

10-29-2012, 10:46 PM
What mill? If a BP knee mill, the main issue is typicially to the spindle splines. No fix there expect for a new spindle. Bearings can handle it.
Same happens with a face mill if not cutting on several inserts. OK, so they get noisy... but you've cut a lot of metal to make them that way!

Deus Machina
10-29-2012, 10:52 PM
Probably not much more, for the wear:material ratio.

I do know it played hell on my Sieg mini, though. Plastic gears instead of steel or a drive belt did not get along with interrupted cuts.

Bill Pace
10-29-2012, 11:01 PM
I suppose theres a grain of truth that a fly cutter is harder on a mill (the splines like lakeside said, not bearings) but geez its nothing like accelerated wear like 50 uses of a fly cutter and you gotta overhaul it. In the home shop after a 2-3 thousand times you might pick up a slight rattle, if then...

A fly cutter is one of my favorite tools, getting that almost mirror finish on a piece of crap dug out of the scrap yard is a satisfying thing, I use em a lot!

10-29-2012, 11:09 PM
Lets put it this way: It will likey cause less wear to your mill then removing the equivilent amount of material with 10+ passes of an endmill and ending up with a multi tool path finish, compared to a (nicer) flycut finish.

If you are worryed about the wear, then don't use a flycutter for bulk material removal, use a multi insert facemill or a roughing endmill.

michigan doug
10-29-2012, 11:09 PM
If you don't use it, it won't wear out.

Go wear it out and have some fun. Once you wear it out, you deserve the next bigger/better mill.

Any questions?


10-30-2012, 12:28 AM
If it's hammering and rattling more than you feel comfortable with, then make some changes. I've done a fair amount of flycutting without getting any unusual noises, so it's not like it's something to avoid. The usual rules apply- keep the cutter sharp, keep the material solidly clamped, keep the material as low to the table as practical, keep the depth of cut within reason, keep the SFM in the correct range, try to keep the cutting forces off the gibs-

Also look at how the cutting edge enters the material. You don't really want it slamming straight on to an edge.

10-30-2012, 01:02 AM
Frank Ford designed a clever fly cutter that has two opposed cutters, one a roughing cutter, the other a finishing cutter. That seems to me a good idea.


Paul Alciatore
10-30-2012, 04:37 AM
Here is my version of a two cutter, balanced fly cutter:


It uses standard 1/4" HSS lathe tools. I cut one in half for two short cutters.

Another neat trick is to set the diameter to 4". This makes the linear cutting speed (FPS) approximately equal to the rotary speed in RPS. No tables or calculator needed to set the cutting speed you want.

This one has a threaded hole to mount on my SB lathe spindle, but other types of mount can be used.

Forrest Addy
10-30-2012, 04:52 AM
The shock of a single tooth flycutter certainly has an effect on a milling machine and it's exacerbated if the dynamics result in back lash being present when the tool contacts the work. But that's minor.

The big concern are those mills whose spindle does not feature a robust keyed drive. I refer to the keyed flange of 30, 40 and 50 milling tapers.

The R8 spindle typical of turret mills and many mill-drills is at a particular disadvantage. It has no key other than a tiny collet indexing dog point screw which has no effective torque capacity on the scale of cutter loads. If the dog point of the index setscrew shears the broken end stuck in the tool shank's keyseat will damage the straight portion of the R8. Extraction of the tool may be difficult.

Damage from shearing the R8 indexing feature is no doubt one origin of how flycutting is "bad" for a mill. I strongly suggest those of you who use R8 equipped machinery remove the indexing setscrew as a precaution. The occasional slip of an R8 tool in its taper is several orders of magnitude less damaging than shearing the indeing setscrew.

I've run flycutters on mills small to very large and encountered no damage to tooling or machine as a result. The disadvantage to flycutting is its slow and uneconomical. OTH some jobs seem to be made for it.

Those without positive keyed drive take care you don't spin the tooling in the spindle taper. Those with positive keyed drive procede with due caution.

A.K. Boomer
10-30-2012, 08:45 AM
Don't hog with them - keep an ear for major rattles - even if not hogging you might hit a hardened spot in your material or something, LOCK your quill lock tight or you will put wear in the quill bore... use the knee to adjust depth.

10-30-2012, 12:03 PM
If your cutter cuts a square shoulder then it will hammer like hell. If you use a 45 deg leading angle on the cutter it will cut fairly smooth. Also, use about 1000+ rpm flycutting and use the feed that feels the best. I have seen and used BP mills that have been used with flycutter for years with no issues.

10-30-2012, 12:29 PM
The big concern are those mills whose spindle does not feature a robust keyed drive. I refer to the keyed flange of 30, 40 and 50 milling tapers.

To be fair, it does take a LOT of torque to slip a properly fitting 40 taper toolholder even when not using the key feature. I have a 4" flycutter I use in my iso40 equipped mill (it mounts into a clarkeson autolock) to surface heads etc and I've never had slippage when using it. I imagine the 50's are even more difficult to get to slip.
I bought a iso50 collet chuck once by accident and when it arrived I had to use two hands to lift the box it was such a monster. One day, I plan to try and machine it down into something useable on my current machine sizes, I'm just glad I didn't buy the machine it was out of by accident too and turn up with my usual lifting gear of a pallet jack and a panel van and some steel ramps :)

10-30-2012, 01:07 PM
I've said it before ..depends how you position the work to be cut and the angle of attack.

rather scruffy drawing ..but hope you get it


just experiment with a piece of metal ..cutting it different ways ..you'll hear the difference .

all the best....markj

10-30-2012, 03:53 PM
Mark is absolutely right - the difference is very large. To further smooth the cutting action, I favour a heavy fly cutter - a big solid disk, not the wimpy little cutters that are often sold as ready made fly cutter bodies. My mill is a typical German style machine with loads of gears transmitting the power. The amount of banging that can propagate through maybe ten spur gears needs to be heard to be believed if you get the cut setup wrong. Since some of those gears cannot economically be replaced I worry about this type of thing.

10-30-2012, 05:23 PM
Don't use negative rake tools/inserts.

If using HSS keep them sharp.

All things in moderation with the usual run of HSM-er type machines.

Sometimes if the set-up allows it - use climb milling - but use light cuts - as climb-milling engages the work with a positive amount of cut when starting.


http://en.wikipedia.org/wiki/Climb_milling#Conventional_milling_versus_climb_mi lling

I usually use "high rake" insert utters in lieu of a fly-cutter as theyare "smoother" witjh more teeth engaged and less "noise/rattling".

I use these on a Sieg X3 square column mill:


I use these on a HF-45 square column mill:


I will use a fly-cutter when necessary for cutting into "corners" and for "light skim" cuts - but there are no hard and fast rules - just use what is needed and what is available - and common sense.

10-30-2012, 07:44 PM
Take a look at what MarkJ writes, he is right on.
The analogy is just like a hand held Skil saw. (circular saw for wood).
This is the reason you don't set the blade to stick out too deep.
(doesn't chip out the wood). For flycutting it makes a smoother cut as well.

My Gorton mill has a spline backlash eliminator. Tree mills have a ball spline
to eliminate lash. This helps much when flycutting. Some horizontal mills
incorporated a flywheel into the spindle to add mass, which helps keep the
spline or gear torque loaded. Tip- Make your flycutter as heavy and large
a diameter as possible. You could even "turn" it true in the mill.
Side note (this is for sir John)- Any clatter from the spline on Bridgeport
mills is amplified by the aluminum belt housing (think loudspeaker).
The J head is bad for this, the M head is even worse.


A.K. Boomer
10-30-2012, 11:55 PM
Positioning your cutter to the work is great when you can - and then there's things like cylinder heads with coolant jackets and head bolt holes and combustion chamber cut outs...

light cuts and a proper cutter and lock the quill down solid, also run your quill adjustment up and thread it against it,
My mills smoooth as silk cutting multiple interupted cuts as in cylinder heads - but it's also a belt drive with no gears in between - really makes for a chatter free milling experience.... (in general) I can fly cut steel with excellent results even in multiple interupted cuts... near mirror finish...

10-31-2012, 04:33 AM
Here is a (drum) brake disk machines on a HF-45 square column mill using a high-rake insert-mounted cutter:

(It is not "rust" but a photo artifact);



The cut - from memory - was about 0.030" deep.

I had the mill on high speed and could not manually rotate the rotaty table fast enough to have any effect.

I could have used a HSS fly-cutter but I'm not surehow it would have coped with skin.

No complaints from the mill at all.

10-31-2012, 04:40 AM
My Gorton mill has a spline backlash eliminator.

[wakes from half-doze] Eh, wot? How does that work? My 8-1/2D has a lot of backlash.

10-31-2012, 07:16 AM
Having thoughts here ..

about the statement made earlier on about the flycutter being weighty to give plenty of bulk and momentum .

I'm thinking, if you could make a large lump of metal flywheel at least 10 lbs or more .boss sticking out of it with a hex in it ...plonk it on the top of the drawbar ..
Would only work when the quill was fully retracted ..but you're using the mill that way anyway in these circumstances ...
it then puts all the weight out of the way ..enables you to use minimum of flycutter down below.

it would also counteract the backlash...you could even build into it a motorcycle cush drive .
which. if you don't search on a specific motorcycle and just search cheapest BIN, will be cheap.


Good or bad idea ?

All the best.markj

10-31-2012, 08:37 AM
Mark -

The cush drive idea is an interesting one You could build a flywheel with an integral cush drive hub that would drop onto the spindle splines, if your mill has that arrangement. I don't use a flycutter enough to invest the time to make one of those, but still a good idea. Incidentally, flywheels were a feature of some old milling machines. I had a quick look for a photo, but couldn't find one. I have shown this photo of my fly cutter body before - mounted on a 40 taper tool holder, it is quite an effective flywheel.


10-31-2012, 10:39 AM
I have a small bench top column mill and have found that a heavy fly cutter does a much better job than the lighter fly cutter sets sold here and there. Here's a picture of a couple I've made. The larger one works great, the smaller one works in tight places. Both use reclaimed drills for cutters. The center drill shown isn't "reclaimed" yet :)


10-31-2012, 01:02 PM
Place I once worked had two lathe insert holders welded on a shank to present the inserts for a 45 degree cut. T series inserts in the 4xx or 5xx size, IIRC. We used that to cut to height abrasive resistant steel which was first heated red and bent into a 'J' for welding as a wear plate on some bucket machine. Also for deep weld grooves. This would sound terrible and was hard on the spindle bearings. But that cutter would eat metal. It was amazing how deep a cut it would take without straining the mill motor. Feed rates were high, too.
The $'s made on that job paid for new bearings a couple of times over, I'm sure. I would not use that thing on my mills, unless I was making a LOT of money on the job.
I do use a fly cutter on my mills, but doc is not very heavy. I only use one for finish or width coverage.

10-31-2012, 01:35 PM
I made this one, 10-3/4"cut, after profiling the bottom still weighs almost 4-1/2lbs. , runs smooth and always leaves a beautiful finish.


10-31-2012, 02:40 PM
You are definitely in the lead for the biggest and best looking fly cutter contest. That is a monster. Is that R8? If so what sort of mill do you run it on?

10-31-2012, 02:55 PM
Yep R8,Grizzly9902 BP clone.

10-31-2012, 09:48 PM
Thanks for all the feedback. Once again, the HSM group puts its wealth and depth of knowledge to work. This set of information is more than I had hoped for. I have an R8 mill and this completely answers my question. I'll take the set screw precaution and proceed with fly cutting using improved techniques and knowledge. THANKS AGAIN

A.K. Boomer
11-01-2012, 08:44 AM
You are definitely in the lead for the biggest and best looking fly cutter contest. That is a monster. Is that R8? If so what sort of mill do you run it on?

I second that! too perty to use - I think id have that one hanging on the mantle above the fireplace...

nice job JR

Aboard I like your idea of the cush drive, but like Wilmac stated earlier I think it should be integral with the spline drives, and then put them on gear head mills,
My experience (which is not allot) is that the belt drive mills are much more chatter resistant that the gear head ones.

as far as flycutters go - it might help to put a tight fitting piece or rubber around the cutters base if design allows - kinda like a harmonic balancer on a crank pulley - worth a try - who knows anything from an old vacuum cleaner belt or even a bunch of thick rubber bands could take the edge off just enough to reduce the frequency...

Lew Hartswick
11-01-2012, 08:46 AM
I made this one, 10-3/4"cut, after profiling the bottom still weighs almost 4-1/2lbs. , runs smooth and always leaves a beautiful finish.

Nice looking tool. The big question I have about fly-cutters with two bits is
how do you ever get the two to cut evenly?????

A.K. Boomer
11-01-2012, 09:47 AM
I don't think you need to be as critical with it as most would think,

here's my take on that, just lower the flycutter with the mill off almost to the workpiece,
then loosen the bits individually and drop them down to the workpiece and tighten,

your good to go - don't worry about being a little off as one will just turn into the finishing cutter,
Your end result is a perfectly balanced cutter and one that is effectively reducing your feed rate by half due to it taking off half as much chip load at a time... I bet it's very smooth. (but just a guess)

11-01-2012, 11:49 AM
Nice looking tool. The big question I have about fly-cutters with two bits is
how do you ever get the two to cut evenly?????

There are two concerns. Both cutters need to be close enough that DOC on the Z axis allows both to cut. The second is they both need to swing on or very close to the same radius so they both do work on the X or Y axis. There is always rebound in interrupted and even uninterrupted cutting and this is seen in a finishing pass on a lathe where no infeed is done between the final two passes. The difference with having two cutters is that this second pass can happen in the current arc. There is no relationship between the feed rate and the tool position, so either cutter will randomly initiate a cut or end a cut, or both, depending on feed rate and RPM.

So you work to manage this. In Frank Ford's two-cutter example he purposely places a shim under one of the cutters to produce a vertical offset of known dimension. The two cutters are completely asymmetric in design and intended to do different things. The deeper cutter will control the final finish, and the overall finish will be dependent upon cutter quality, SFM, and feed rate.

10-07-2013, 12:14 PM
As a manufacture of large diameter fly cutters for the past 8 years we commonly will get questions regarding wear on the machines spindles. Our fly cutters are very free cutting single point cutters and we run them daily in our own shop on all of our mills from our R-8 Bridgeports to our 40 taper CNC mills and on our 40 taper Devlieg boring mills. We have not had a single spindle wear repair issue and have very tight tolerance accurate machines. Milling machines are tough and made to cut. As long as a the operator follows some basic common sense rules of machining and does not attempt excessive cut depths a fly cutter is = or less harmful to the machine than other cutters such as face mills that require more horsepower and can stall the machine in deep cuts or with worn inserts. Here is a link to our YouTube video of 1 of our fly cutters, cutting the scale off a HRS steel plate at 650rpms and 6 IPM at a depth of .018" the plate is 6" x 8.25" the cutter should be slightly offset to the conventional milling side (as opposed to climb) to load the spindle bearings in one direction as shown in the video, check out our other fly cutter videos while you are there: http://www.youtube.com/watch?v=JcoQq4Y1ks4

10-10-2013, 09:20 PM
So I need a really flat surface, to the limits of my machine's accuracy, and I'd it done quickly. I think I am going to pick a cutter less than or equal to the spindle diameter with the right combination of multiple edges to use 100% of the motor hp as shown by my clamp on ammeter. A tiny bit of tram error and tool deflection due to the spindle bending won't be giving a slope entering or leaving the plate either. The finish won't be as impressive as a flycutter but it will be whole lot flatter as will be shown on a surface plate using a dial indicator.

J Tiers
10-10-2013, 09:49 PM
Ancient thread..........., but before everyone wants flycutters, it's worth mentioning that the humble shell end mill can do as well or better than flycutters, and have no banging.... At shop tag sales, they go cheap (around here at least)

The banging is typical of a single cutting edge.... With multiple cutting edges, some are always in the work, and the alternate acceleration and then sudden deceleration when the cutter hits the work, do not occur. The same applies to a horizontal mill, a hand saw, or nearly anything where it is possible to have fewer than 3 teeth "in" the work.

The front surface shown here was cut with a shell mill, held on a shop-made arbor.
http://img.photobucket.com/albums/0803/jstanley/block01.jpg (http://smg.photobucket.com/user/jstanley/media/block01.jpg.html)

http://img.photobucket.com/albums/0803/jstanley/shellmillarb4.jpg (http://smg.photobucket.com/user/jstanley/media/shellmillarb4.jpg.html)