Fly Cutter Problems

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Glenn Brooks
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Fly Cutter Problems

Post by Glenn Brooks »

Guess I need some advise on fly cutters.

I’ve been trying to level and finish surface a 6” x 10” bit of 1” plate to use as a base for a shop made tapping fixture.

Decided to try a 2.5” fly cutter that’s been laying around in my tooling cabinet for a couple of years.

Several problems showed up:

1) each finish pass (.003” doc) leaves a little bit of edge overlap on the sides -I can feel the lip with my thumb.

2) one fairly deep finish pass- maybe .005 to .008” DOC actually left a noticeable raised area over one half the path of the cut. See photo. It’s almost as if the work pice is controlling the DOC, rather than the mill. You can sort of see the raised area on the left side of the photos- middle of the board. The edge appears as a line running right to left.

3) finally, I tramed the quill but still end up with a rough surface on one axis. The cutter bit cuts a smooth finish on the front side of the cut. But the back side of the cut started leaving a rough surface, half way through the job. It’s as if the cutter scratches the back side .0005” or so and leaves it like it was hit with 60 sand paper. I assume this a tram problem, but why would it suddenly materialize. (OK. Just noticed Mr. Ron’s tramping thread from a few days ago- going to go recheck my alignment on the machine with the comments people offered. Also will take a photo of the tool tip and post)

The mill is A NOS Bridgeport clone built in 1980, but in still in excellent shape, having been used lightly by a hobbyist since new. Still has scrapping on the ways, all around. Power feeds on the table, all around..

BTW, I ground and polished the tooling edges just prior to using the fly cutter. Speed around 250-300 RPM, very slow feed rate. 1” per minute or less.

Here’s a pic of the raised surface area, with a finish recut over the top.

Can anybody advise the most likely avenues to examine for finish and DOC problems with a fly cutter?

Thanks,
Glenn
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armscor 1
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Re: Fly Cutter Problems

Post by armscor 1 »

I set my head 1 thou over and zero nod.
You will always get deflection and small steps are unavoidable.
Abom has seen the light with his Shapers, does beautiful work, wish I had the space for one!
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BadDog
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Re: Fly Cutter Problems

Post by BadDog »

Assuming alignment or flex issues have been eliminated, your bit may be ground wrong for a fly cutter. A fly cutter bit is quite different from a typical left hand lathe bit in that it needs more clearance on the end, and the feed is toward the end, not the side. So the edge on the end is where you want any rake, and a fine sharpened edge.

I can't tell from the pictures, but it looks like you may be pushing a displacement bur indicating an improper (rubbing or dull) bit, which causes pressures to rise, and finish to suffer. It's never going to provide a ground finish, and there is always going to be a visible transition, but once you get it sorted, it should produce a nice looking flat finish surface that won't hang a fingernail.
Russ
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pete
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Re: Fly Cutter Problems

Post by pete »

Maybe a few combined issues from what I see Glenn. Not one of the better grades of steel that are known for producing easy to get good surface finishes since it looks like the surface is showing some tearing. 1018? And what you already know an obvious tram issue if your getting a step at each pass. Imo despite how common they are BP's and the clones are a really flexible combination of parts even in brand new condition with a spindle suspended a long ways out in space as far as having something really rigid to cut with. And fly cutting just high lights there built in rigidity issues. I spent a great deal of time once trying to get as close to perfect as possible tramming job on my mill and finally managed to get it to under a couple of 10ths over 12" in X and the same in Y for the full distance of my Y axis travel. A really frustrating job that I spent a couple of hrs doing. I still got minor tool marks from my fly cutters tool tip dragging on the back side of the cut. With those larger diameter cuts and the leverage they exert on the tool itself, plus the spindle bearing clearances and all those parts of the head, tilt/nod assembly, ram etc a dead perfect spindle tram to the table is still only a static condition. It's all that minor flex throughout all the parts that requires a minor bias on the tram so the tools not recutting or dragging on the back side of the cut. And I'd agree about .001" off tram is probably enough.

Something else that might be worth a try even on a D.O.C that light. Years ago I bought what was then a new type of flycutter that was supposed to have been developed for machining oxy/act and plasma cut edges. It uses the round carbide inserts. Mine oddly doesn't have the manufacturers name stamped on it, New Dimension??? New ???? something anyway was there name and I think there now out of business. Anyway it really is the smoothest cutting flycutter type I own. Conventional thinking is the depth of cut is supposed to be as much or more as the radius on the tool tip. I think the flycutter I have gives the surface finishes it does most times because it's large radius is acting like a wiper on the previous cut surface during each revolution. You might try a much larger radius on the tool? Those round inserts mine uses have a positive rake built into them and also a bit of rake ground into the tool shank where the insert sits. You also didn't say if that was a HSS, Braised Carbide or insert your using. If it's HSS at that diameter your about 1.6 times too fast at 250 rom and double what you should be at 300 rpm so the tool edge would be breaking down, if it's carbide you could be half as fast or more as you should be. So I think your cutting rpm might be part of it as well. Russ has a very good point, it looks like a dull cutting edge due to the size of that burr. And if your ram isn't already as far back as possible I'd do that and also re-check the machines tram after you do. If you already aren't doing so, lock every machine axis except the one your using. Those spacers under the plate are probably because your planning on drilling some holes after surfacing it. Those don't help deaden vibration but it's hard to tell if that might in any way be part of the problem or not.
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Harold_V
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Re: Fly Cutter Problems

Post by Harold_V »

Hmmm. When I dial in the head (I don't like the term "tram". That's related to the timing of a steam engine, not the condition of a mill spindle) of my mill, I want it to be dead perpendicular (keeping in mind that a new mill is generally sent out with the front of the table slightly higher than the rear of the table). When it is, there is no step, although you'll never eliminate the pattern line, as that's related to the huge overlap of cuts as compared to the marks made @ a 90° angle from the edge. It will be visible, but shouldn't be distinguished by a ridge that can be felt.

When the head of a mill is properly dialed in, the cut results in a cross hatch pattern. However, that pattern is likely to be come and go, as you're dealing with a mill, not a grinder of high precision, so there's movement in the machine that causes variations.

When one dials in the head with error, the resulting cut isn't flat, but a huge radius.

Yeah, I get it---it's exceedingly large, and generally of no consequence, but it's still an error that need not be.

Do keep in mind, any error that is intentionally dialed in will yield an edge, all depending on which axis is used for the cut.

H
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Glenn Brooks
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Re: Fly Cutter Problems

Post by Glenn Brooks »

Pete, Harold and all,

Thanks for the comments. Good discussion. I am using HSS tooling... the cutting edge did show some signs of wear and rubbing on the bottom, so reduced speed from 312 to 80 rpm, as Pete suggested. Made a world of difference in the finish - even with the degraded edge. Still getting a bit of roughing with the back side of the fly cutter, so once I lift the work off the table I intend to try Harold’s method dialing in the quill and see how it lays on the 4 corners of the axis (12: 6: 3: and 9 o’clock positions)

When I changed my hold down clamps to mill the ends (under the clamps) the surface of the work changed vertical position a thou or two. Which ruined the flat surface with the next cut.

So changed out the fly cutter out for a big shell mill with 6 carbide cutters, and just finished the job. It worked a lot better, and did not leave the trailing scuffing as did the fly cutter. So thinking the tool wear is part of the roughness problem. Anyway, finally I decided I had learned enuf fly cutter stuff for the day. As in - don’t use em if the shell mill will fit the job. :shock:

Next time I think I will try a surface finish with the Logan shaper I bought at an estate sale last fall.

Glenn
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Harold_V
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Re: Fly Cutter Problems

Post by Harold_V »

Do keep in mind that in the process of machining a side of a piece of material that stresses are relieved, so flatness will be sacrificed. In order for you to achieve flatness, you must remove like amounts of material from each face, alternately taking cuts. When you have achieved a full cut on both sides, you may enjoy success, but getting something truly flat with a mill borders on impossible. You'd do well if it was a thou out. I'd expect more. You'll notice that the material moves when you remove the clamps. That's caused by the stress.

When running a fly cutter in steel, you can't ignore surface speed. I'll assume you were swinging a 3" circle. That's about 245 sfpm, far too fast for HSS. You likely would have enjoyed success with Stellite, however. Carbide, too, would have performed better, assuming you chose the correct grade.

H
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armscor 1
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Re: Fly Cutter Problems

Post by armscor 1 »

Watched Adam Booth on youtube using the G&E shaper shear cutting plates, beautiful finish and I guess pretty darn flat.
The bygone shapers still have their uses even though snubbed by many including me but I have a completely different view point now.
whateg0
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Re: Fly Cutter Problems

Post by whateg0 »

I'm not sure you understand what to expect from a fly cutter, or for that matter, really any rotating cutter. The finish shown in the last pic above is what you should be expecting from the tool. As Harold said you should expect that crosshatched pattern. As the tool comes around the front of the cut, the leading edge, it takes a cut equal to your DOC. The tool will leave tiny ridges each time it comes around. On the backside, it has already cut, but essentially drags across those ridges, leaving the crosshatched pattern. If your tool is only cutting on the front side, then the back is too high. If the tool cuts on the front and makes a chip on the back, the front is too high. You will see the result of each of those as lacking the crosshatched pattern, only seeing semi-circles going in one direction. The same happens with a facemill, or even an endmill. Because of the large diameter of the cut, the fly cutter exagerates any misalignment of the head. When you say it leaves a ridge, where is the ridge felt? between passes? Or are you talking about the surface finish after the tool has made it's full rotation through the already cut surface?

Dave
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Re: Fly Cutter Problems

Post by John Hasler »

I agree that a faint crosshatch is normal but I usually get a better finish than that.
Glenn Brooks
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Re: Fly Cutter Problems

Post by Glenn Brooks »

Yes, I expected the cross hatch pattern, and viewed that part as normal. The roughness is left by the back side of tool- passing around the rear path of the spindle. And by roughness I mean rough enuf to catch at a shop rag and pull small, fine lint off the rag when cleaning up the work. If this were a piece of hardwood, You could say the front end of the cut was finish sanded with 150 grit sandpaper, and the rear of the cut was scuffed with 50 grit. Anyway, when I reduced the cutting speed the tooling left a noticeably better finish. Not perfect, but a lot smoother.

Also on several passes one tool path was several thou higher than the others - but only in the middle of the plate. This was unacceptable to me as this is to be a taping fixture, and I want a small vise to hold the work on the plate in a repeatable and consistent manner, without rocking back and forth a few degrees off nadir.

As Harold said, some of the variation in thickness likely was due to stress relief. I did machine both sides- but only one side at a time. Not alternatingly, as Harold suggested.

The shell mill I finally used, cleaned things up well and pretty well corrected the height variation. Still left a bit of height variation on one edge of the work- due to travel limits of the table. I cleaned these up with a bit of hand filing, as in the old days. So the surface is at least smooth now.

Glenn
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rkcarguy
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Re: Fly Cutter Problems

Post by rkcarguy »

If finish was important over flatness, we used to purposely run the head slightly out of tram (~.0005) in the X-axis direction and fly cut with the "deep side" so there was no "crosshatch".
Large diameter tools push the limits of stability of these size of machines no matter genuine BP or copy, we used to rough work on a "beater" and then finish machine everything on our newest Bridgeport. Even then, table sag can make the finish vary when you start working further away from center.
Or, we would use a 3/4" diameter roughing mill and then just take a very light finish pass with a fly cutter to clean it up.
I love the indexable carbide face mills that take the round inserts, they machine wonderfully and you get a lot of life out of the inserts being able to rotate them many times. When carbide started to be mainstream, we pretty much reserved single point fly cutting tools for non-ferrous and plastic only.
Having the right profile on your single edge fly cutter is critical especially in steel, and it's super picky almost to the point where I had to grind and test cut until I found the magic profile. It seemed to like a small radius, and required some good clearance on both corners of the tip as to not drag chips or push up a burr. I used to go back and use the TIG welders tungsten sharpener diamond grinding tool to finish up the radius on my fly cut bits because it made all the difference in the world when we had a customer that wanted those nice rows of shiny fly cut finish across the face of their part.
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