# A fly cutter, and some questions



## PaulL (Jul 31, 2022)

I spent a few hours in the shop today turning a failed shaft extender part into a fly cutter.  It reaches out to a 4" radius, more comfortably set here at about 3".  I don't yet have some big shell mill to surface with.






It "works".  After thinking about the geometry and spin direction, I managed to cut it exactly backwards.  So I run it with the mill in reverse ;-)  And the screw holes aren't in a very good line as I have to basically free-hand them on my wobbly drill press as I don't yet have a chuck mandrel that will fit any of my collets.  That's in the post, but a week out.

So now, quesions:
My surface finish is spotty.  Here's some cuts in some no-name scrap steel tubing I had lying about:





The feed direction has the work travelling from left to right.  In sections it's good and smooth, and then I get all the gouges.  I played with my cutter speed, from ~100fpm to ~300fpm, and carriage speed from near-dead-stop to pretty zippy.  Slower was smoother (maybe I should round my cutter more?), except for the gouges.  But most difficult to control are the extra gouges that come in every once in a while, and sometimes very frequently.  Are those chunks of the material stuck to the cutter?  I threw some oil down, but I don't think it does a great job keeping chips away from the cutter.  Time to test the flood cooling?
Thoughts or advice?


----------



## YYCHM (Jul 31, 2022)

Not following...... Using a fly cutter on steel tubing?


----------



## PaulL (Jul 31, 2022)

Just skimming it.  It's easy to hold and pretty flat to start.
Which is to say I'm just using it as a test surface.


----------



## Janger (Jul 31, 2022)

Post more pictures of your setup. Is the quill locked? Is the column locked? Rpm? Feed rate? Depth of cut? Some of this you already answered…


----------



## jcdammeyer (Jul 31, 2022)

I finally bought a 4 cutter facing mill.   My surfacing of aluminum castings went from looking like your example to silky smooth.  I did manage to get nice finishes with the flycutter but it was always a bit hit and miss.
I think the imbalance and load/unload of the cutter creates a vibration all the way into the spindle bearings.  Enough I suspect to cause that sort of variation in cutting depth or speed.


----------



## PaulL (Jul 31, 2022)

Janger said:


> Post more pictures of your setup. Is the quill locked? Is the column locked? Rpm? Feed rate? Depth of cut? Some of this you already answered…


Quill is locked.  Depth of cut 5 thou.  Knee is not locked, though seems unlikely? Rpm between 125 and 300.  
I'm suspecting my cutter grind is pulling swarf around.
More relief?

I need to spend an hour with a good cutter geometry text - the whys and wherefores rather than the "cut these angles".  I hate not understanding why.


----------



## jcdammeyer (Jul 31, 2022)

Here's a couple of pictures of my mill flycutting my castings.  No idea if the geometry on the bit was really correct.






Surface finish wasn't bad but you can see (just barely that the knee isn't completely perpendicular to the spindle so on each pass resulting in a slight, almost imperceptible edge between passes.  Clamping the knee helps but we're talking 0.0001" to 0.0003" tilt.


----------



## trlvn (Aug 1, 2022)

PaulL said:


> I need to spend an hour with a good cutter geometry text - the whys and wherefores rather than the "cut these angles". I hate not understanding why.



It is not a textbook but Mikey on HobbyMachinist is kind of a guru of HSS sharpening.  I think the following post is a pretty good summary:



> ... if I were to grind a flycutter bit:
> 
> Almost every cut will be an interrupted cut so I would only use M2 HSS; cheaper, more impact-resistant, fast to grind.
> For a shape, I would grind a roughing tool shape or possibly a general purpose shape like our square tool. Depending on depth of cut, cutting loads may be very high.
> ...











						Models for grinding HSS Lathe Tools
					

I don't know what the other guys think but I can give you my take on a flycutter tool. I used to grind a left hand roughing shape with a fairly generous nose radius - slightly larger than 1/32". I wasn't too deep into tool modifying back then but I wasn't too thrilled about tool life, although...




					www.hobby-machinist.com
				




Craig


----------



## mickeyf (Aug 1, 2022)

You have not mentioned whether you have power feed or are feeding manually. Always lock everything that does not need to move for the job at hand! Nose radius will definitely make a difference (should be relatively generous), but we don't know what you have now. How (with what) are you holding the cutter in the mill? It's not clear from the photo, but it looks like your tool bit is parallel to the surface being cut, not at an angle as is the fly cutter in jcdammeyer's photos. This is fine as long as the bit projects below the holder and the geometry is such that only the cutting edge can contact the work, with sufficient clearance. 

I made a fly cutter, but 1) the tool bit was not radially adjustable in the head and so was less able/likely to move in the slot and, 2) I put a counterbalance piece 180 degrees opposite the cutter. I never tried it without the counterbalance piece so I don't really know if this made a difference. 3) The holder fit directly into the mill spindle (MT2, with a draw bar) - it did not have to be held in a chuck or collet.

In my case the tool bit was held perpendicular to the work surface, and all the clearance angles were in the tool bit itself, not the holder. It should not matter which approach you take as long as the geometry is "correct" with regard to the cutting edge vs the work.


----------



## RobinHood (Aug 1, 2022)

YYCHM said:


> Using a fly cutter on steel tubing?


HSS (hollow structural steel) tubing also comes in square and rectangular (not just round) cross sections.








			http://www.cim.mcgill.ca/~paul/HollowStruct.pdf
		


He is fly cutting a rectangular/square piece of HSS.

I have done that to a large rectangular piece when I needed a spacer for my dividing head because the gears for differential dividing (127 division in this case) were “hanging“ below the base of the DH when installed on the mill table. The spacer solved that problem. Both DH and TS have a HSS spacer under them.

Here is the thread:









						SM1340 Change Gears
					

My Standard Modern 13X40 lathe is all metric. I want to be able to cut inch threads as well. It did not come with the change gears. Called SM to see if they are available: YES. Great, how much? $$$$$$$$s! OK thanks, I'll pass for that price. Off I went to make my own. This time I had my mill and...




					canadianhobbymetalworkers.com


----------



## PaulL (Aug 1, 2022)

Thanks Mikey - I'm power feeding, which makes this at least easy to experiment with!  The head is held in a 1/2" collet, and doesn't seem to be slipping.  I need a proper end mill holder so I can get a set screw on it.
My cutter is at a 10 degree angle to the surface; the photo doesn't see that well.
This morning I reground my cutter, adding some nose radius (maybe 1/32"?), and that helped.
I've started locking down the Y and knee, to no appreciable difference.  But that did lead to me thinking in terms of vibration/chatter and a lightbulb went on.  My cuts are appreciably smoother on the leading edge than the trailing edge.  The leading edge is taking a bigger bite, providing more resitance to chatter, but the trailing edge is effectively taking a spring pass, but starting from an interrupted cut and bouncing over the ridges left from the leading edge cuts.  And those are chattery as heck when I look at them closely.
So probably my little 1/4" HSS bit is just too small to be rigid enough.  And I can replicate the tail-end chatter on the front end by taking a very light cut.
So now to find a bigger piece of HSS for the tool.  My slot will fit 5/16.  I'll likely enlarge it to fit the 3/8 cutter I have in my drawer.


----------



## Chip Maker (Aug 1, 2022)

What type of mill is it? If it's a Bridgeport style and the leading edge is giving a smoother cut there's a strong possibility the head needs to be squared ( trammed ) and the cutter is dragging. I would also suggest a cemented ( brazed ) carbide lathe tool and increase the rpm.


----------



## Susquatch (Aug 1, 2022)

PaulL said:


> Quill is locked.  Depth of cut 5 thou.  Knee is not locked, though seems unlikely? Rpm between 125 and 300.
> I'm suspecting my cutter grind is pulling swarf around.
> More relief?
> 
> I need to spend an hour with a good cutter geometry text - the whys and wherefores rather than the "cut these angles".  I hate not understanding why.



I feel exactly the same way. I don't mind formulas and charts, but I am never happy till I understand why.

My thoughts on your surface finish are that the tool isn't sharp where it needs to be which is catching swarf instead of cutting it. In this case, because the tool is rotating counter clockwise when viewed from above, the cutting edge is the bottom edge of the face that is sweeping toward you. So the edges that need to be sharp are that edge and its left most corner - which should have a small radius on it, as well as any amount above the corner that is included in your depth of cut. 

I would avoid a long bottom cutting edge as any imperfection in the angle would almost unavoidably trap swarf and thereby score the surface. Similarly, the tool needs some relief on the bottom surface and left radial face so that swarf trapping and imperfect cuts cannot happen there either.


----------



## Ian Moss (Aug 1, 2022)

Whether cutting in normal rotation or reverse rotation, will your holder keep the cutting edge of the tool on the radius line of the spindle rotation? I don't think the same grind can be used for rotation in both directions. The cutting tip needs to be on the radial line and all the tool angles approximately right.


----------



## Susquatch (Aug 1, 2022)

Ian Moss said:


> Whether cutting in normal rotation or reverse rotation, will your holder keep the cutting edge of the tool on the radius line of the spindle rotation? I don't think the same grind can be used for rotation in both directions. The cutting tip needs to be on the radial line and all the tool angles approximately right.



Unless I misread the OP's original post, that's exactly why he has to run the mill in reverse. The centerline was on the wrong side of the tool.


----------



## PaulL (Aug 2, 2022)

Yes, I have the tool running backwards, but also adapted properly to put the cutting edge at the outside radius.  I see a relief angle on some drawings that I haven't applied though, so I'll try that.

I'm back to tramming in - I thought I had it right, but the back edge dragging has me wondering, so I want to make sure.  I turned the shaft on the body directly to guarantee concentricity, but I'm wondering if I messed something up in the cutter that could introduce a wobble. I'll put a DTI to it and see.


----------



## Tom Kitta (Aug 2, 2022)

Pictures of the work make me think its not cutting but rubbing or sheering metal off. This means the HSS is not properly ground for the task. 

Tramming the milling head is of course nice to have but it would not have an effect on finish unless it was WAY off - where cutter was cutting say 15 thou vs. 2 thou. 

You should end up with "gray" finish on your square tube. Its relatively soft steel so it would be hard to get a mirror. 

I would fly cut with something small first, not say 3". 

I mostly use a face mill to do the job and get great finishes, on aluminum I can use the piece as a mirror & its only slightly worse then the one in the bathroom.


----------



## PaulL (Aug 2, 2022)

Tom - you had the right of it.  My grind was off, with the attack too square to the work.  Bevelled that in a bit (10 degrees or so) and now I get a much better finish.  First picture, halfway down the stock, stopping to see leading vs trailing.  Quite fast feed rate, 10 thou cut:





Second picture, slower feed rate, 3 thou cut, very high macro - this piece of 1018 is 1" wide.





Feels smooth, though not polished.  I'll play with nose radius to improve that.
Thanks!


----------



## PaulL (Aug 2, 2022)

More slightly modified geometry: rounder nose, higher back relief.  The surface now feels silky smooth.





Next is to sort out the step between the leading edge and the trailing edge.  Both are equally smooth, but I can feel a step between the two.  I put a DTI on it, and there's just about 1 thou difference in height.


----------



## Susquatch (Aug 3, 2022)

Perfect!

The step is just tram. Your head is rotated slightly.


----------



## PaulL (Aug 3, 2022)

Ok, checked my tram, carefully.  Table was good.  Vise was out by (get this) 2 thou in 6".  Or just over 1 thou between the leading and trailing edge of the fly cutter - dead on the error.
Turns out I had somehow not adequately tightened one of the screws holding the fixed jaw when I trammed it in.  I expect some of my "learning" moments nudged it out of true.
So now it's true again and I get this finish, running the feed pretty fast:






Note the small trailing edge scuffs, except for that outlier where it looks like I grabbed a good piece of swarf.
I can do better still, I'm certain, but this meets my current needs.


----------



## Susquatch (Aug 3, 2022)

PaulL said:


> Ok, checked my tram, carefully.  Table was good.  Vise was out by (get this) 2 thou in 6".  Or just over 1 thou between the leading and trailing edge of the fly cutter - dead on the error.
> Turns out I had somehow not adequately tightened one of the screws holding the fixed jaw when I trammed it in.  I expect some of my "learning" moments nudged it out of true.
> So now it's true again and I get this finish, running the feed pretty fast:
> 
> ...



Looks great to me. 

The trailing edge scuffs are to be expected. I think the only way to get rid of them is to deliberately tilt the head a bit which has its own set of issues. 

Damn those lonely swarf devils! 

Are you putting your work on parallels? 

If so, I'm not sure I understand how the Jaws create this issue. The parallels should locate the part parallel to the vise base independent of the Jaws. Unless you are certain of what happened, you might want to recheck everything again. 

If not, then I assume you have ledge type jaws. Which would explain what you are getting.


----------



## PaulL (Aug 3, 2022)

Susquatch said:


> Looks great to me.


Thanks


Susquatch said:


> Are you putting your work on parallels?


Oh oh.  Yes, I am, and see immediately that tramming in that edge does nothing.
So *by luck* this worked.  I removed the vise, cleaned and stoned it , and re-mounted it carefully (and for this purpose uselessly!) checking the top edge of the vise.  I should have been checking the bottom and thinking of shims, if needed.
So how in heaven's name did I "fix" it? I have to assume I had a spec of something under the vice, and I never knew because I made the same error the first time.  I'm guessing the vise is better machined than I've given it credit for!


----------



## Susquatch (Aug 3, 2022)

PaulL said:


> Thanks
> 
> Oh oh.  Yes, I am, and see immediately that tramming in that edge does nothing.
> So *by luck* this worked.  I removed the vise, cleaned and stoned it , and re-mounted it carefully (and for this purpose uselessly!) checking the top edge of the vise.  I should have been checking the bottom and thinking of shims, if needed.
> So how in heaven's name did I "fix" it? I have to assume I had a spec of something under the vice, and I never knew because I made the same error the first time.  I'm guessing the vise is better machined than I've given it credit for!



Yes, I'm glad I asked about it. Everything makes more sense now. 

Ya, dust matters. Even a human hair can throw things off a thou or three depending largely on colour.....


----------



## PeterT (Aug 3, 2022)

Your 1018 is I suspect cold rolled? Even though flycutting is small DOC & there is likely no heat in cutting, the material is probably stress relieving as you are progressively removing material. Very common for some distortion once of out of the vise. Actually it can distort even clamped inside vise under certain conditions. So its a good practice to flip flop the part to mate the parallels better & help equalize bending. The raggy edge you get on one side due to tool rotation direction on single pass flycutter. Some prefer to traverse the cutter the same so like up the left & down the right. This is actually more significant for end milling (higher DOC & more chance for chip tearing the edge off vs cutting into it). Remember to dress the edges with file & cleaning before resetting in vise.

The finish looks nice. 1018 finish can vary with the same tool just as function of the stock for some reason, at least that's been my experience.


----------



## jcdammeyer (Aug 18, 2022)

When I was over at Paul's for a shop tour he showed me the fly cutter he built.  We were discussing the supporting edge and seems what I remembered wasn't accurate as to what edge was supporting what.

Contrary to what I remembered the cheap BusyBee cutters do not press the leading edge of the cutter along the slot centerline.  In fact the two anchor screws push it away from the center line. 




I have a fairly round tip on my cutter with back and front relief.




You can see it this last photo the cheap setscrew still in there is also cracked which is why the other one was replaced with a hex cap screw.


----------



## PaulL (Aug 18, 2022)

It's clearly the side with the most "meat" left over to hold the screws.  But yeah, thinking about it it's odd to have the screws pushing away from the center line.


----------



## Susquatch (Aug 19, 2022)

PaulL said:


> It's clearly the side with the most "meat" left over to hold the screws.  But yeah, thinking about it it's odd to have the screws pushing away from the center line.



I think it looks and sounds worse than it is.

Besides the fact that it obviously works, the very slight offset from center is probably more or less meaningless at that kind of radius.

It might become more important if you used a smaller piece of hss tool - say 3/16 instead of 3/8 or whatever that is and thereby ended up much further off center.

Even so, one could simply adjust the relief angles at the tool tip such that the attack and relief angles are all based off of the vector from the center of rotation to the cutting tip.


----------

