A fly cutter, and some questions

[PaulL]

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]

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

 

[PaulL]

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]

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]

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]

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]

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]

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.
• Flycutters run at high speed, which lowers cutting forces so we can afford to keep relief angles near baseline levels for strength.
• Since the relief angles are stronger, we could afford to increase side rake to boost chip clearance and lower forces more so I would go maybe 4 degrees above baseline.
• Almost all cutting is at the tip so I would boost back rake by 5 degrees and this will put all the cutting action at the tip.
• Then I would grind a nose radius just a tad bigger than 1/32".

I'm pretty convinced that a LH Square tool would work pretty good if you gave it a slightly larger nose radius. The only way to know is to try it. Of course, you would change the side cutting edge angle of the tool to be sure the nose was the contact point but that's pretty simple to do.

Dunno' ... what do you think? Why not make a LH square tool and try it, then report back and we can brainstorm on it.

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

Perfect!

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