Long Stringy Chips on Thread Whirler

[SWYooper]

Hello!

I work in a swiss shop doing mostly medical stuff, and have run into an issue we haven't been able to figure out so maybe one of you can help.

I'm thread whirling a screw with a #6-80 thread on a Citizen L20. Thread length ranges from 1.5" to just over 4". The thread whirler is producing long, skinny chips that stay wrapped around the screw over half the time, even with high pressure where it is supposed to be. I have whirled tens of thousands of bone screws and thousands of various other screws with plenty of different threads and have never run into this problem. None of the on-machine cleaning methods have proven 100% successful. The issue is the pickup will pack these onto the thread up against the head of the screw so tightly that it takes 5 minutes to unwind them and then it's a crap shoot as to whether the thread is still good.

The chips are odd. They look wrinkled on the surface. ID edge is clean, OD edge is ragged almost like a feather's edge and almost looks torn rather than cut. I've measured the inserts, they can easily handle the depth of cut.

We have tried, individually or in combinations: increasing whirler RPM (inserts didn't like it), decreasing main spindle rotation (still chips, unacceptable cycle times as this controls the effective feed), PSI setting of high pressure (no change), thread angle (no change -- but insert angle is only 1.78deg, not lots of wiggle room), new lot of inserts (no change).

The insert guys are as lost as we are as the only other time they heard of this issue was someone rotating their whirler the wrong way. We've even had Switzerland call us to check up, so at least we aren't the only ones scratching our heads.

Material is .312" TI6AL-4V ELI and we have a 5mm extended nose guide bushing. We were having issues with .250" bars doing the same and thought it might be a rigidity issue but apparently not.

My bosses and I are at a loss. I'm making parts, but spending far too much time cleaning chips off and this is a mid-length screw. The longer ones have even more issues with chips. Could the 3 degree cutting edge relief not be enough?

 

[michiganbuck]

Wrinkle chips are often from the required force to penetrate the material and stressed up to a certain force, then at penetration the force is reduced for a time, perhaps only a millisecond.

Could be the edge prep of insert, sharpness require, rake attitude, clearance required. insert not seated on tool holder pocket, space behind insert, some factor of tool holder not giving support. some part of the machine not well supported., Give in the bearings or fixture.

Qt [Could the 3-degree cutting edge relief not be enough]

Yes, the feed rate can diminish the clearance ...Think of a drill for example with X feed rate. now draw that spiral to see the angle of decent. Cutting edge clearance is diminished that amount.

Hypothetical test example: a turning tool bit with zero clearance under the cutting edge. Force is built up to stress the tool/machine to bend everything until the bent over condition makes minimum clearance angle to the cutting action so the cutting action is achieved for a millisecond then it goes back to straining everything again.

 

[LockNut]

Those chips don't look like they are from a whirler. They look like they might be from the turning operation. But, the only whirling I have done were on dedicated whirlers, not whirling in a Swiss screw machine. Our whirler heads had 4 to 6 inserts so would not have left ring shaped chips like that. But I have seen chips like that from the turning ops on bone screws. Those look like they are rings that are compressed together. A dull turning insert would push the chip wrap up against the head of the screw or collet and compress them into an impossible to remove mess.

Paul

 

[michiganbuck]

QT: compressed together. Agree to that...

You should have the ability to grind clearance below the cutting lip .. unless it is a diamond cutter. titanium, likely diamond.
Can You tilt down the insert holder to achieve perhaps 5* or 6* primary just for a test?

*Might you trig out the required lowering below center to make attitude of more clearance,

 

[FredC]

That is a heavy looking chip for a #6 80. Is that a typo and maybe #6-18?
Turning tool now oversize?
Since you are used to turning titanium, could you have a bad batch?

 

[SWYooper]

Thank you for the replies all!

First off, definitely not a typo, it's a #6-80 thread and those chips are absolutely from the whirler, there is no turning operation at this point. Whirler goes from bar stock to threaded part in one go. Bad batch of material is possible but unlikely as I just ran a couple thousand of a different screw from the same lot (we use LOTS of this particular size and order large batches).

If by "compressed together" you mean the wrinkle pattern I could see that, they do look and feel pretty solid along the length though.

Hypothetical test example: a turning tool bit with zero clearance under the cutting edge. Force is built up to stress the tool/machine to bend everything until the bent over condition makes minimum clearance angle to the cutting action so the cutting action is achieved for a millisecond then it goes back to straining everything again.

The boss and I just spent the last bit examining one of the inserts and it seems like this could be the answer. We were discussing how it looks like the relief isn't quite enough as there is more wear in that area than there should be. And yes, we do have the ability to grind clearance below the cutting lip, but we have a 9-insert whirler and what we don't have is a fixture to hold them all so they can be ground identically. Besides, we have 28 other brand new inserts that would need to be modified. It might be worth trying while we wait for a new batch with a greater relief angle.

 

[SWYooper]

I'm also getting the expected mill chips from the whirling, the stringy ones are just an added bonus.

MichiganBuck: I'm confident with the setup and tooling, as far as the machine and whirler go and even the setting of the inserts. I'm meticulous with these setups because I've seen some tiny details missed create huge issues. I don't have the ability to tilt the holder to create more relief, if that is what you were suggesting. It is mounted on a B-axis so I can change the thread angle, but only by about 0.2* either direction before the thread goes wonky. Our inserts are coated carbide, same as we have been (successfully) using with our other whirl applications.

 

[michiganbuck]

Qt: I don't have the ability to tilt the holder to create more relief, if that is what you were suggesting.

What about putting the edge below center as that will do the same thing..yes doing that also change the top rake angle.
Perhaps change tread form geometry attitude a tad.

 

[Tonytn36]

Remember when dealing with relief angles that a certain percentage of the material being cut actually deforms and passes around the cutting edge and springs back on the other side. Now, this is typically only a few microns, but it can be enough to cause issues if there isn't enough primary relief.

 

[SWYooper]

More information: I whirled about half an inch of threads, then did a rapid move off the material to try and see what the cut looked like. Pics below are what I got. It's not what I thought it was, the chip is being formed on the trailing edge of the insert. In pic "A", you can see that the point of the insert (minor of the thread) is where the chip begins and in pic "B" you can see more of the chip formed, and eventually broken on the ragged edge.

We're waiting to hear back from the insert vendor to see what they have to say, this isn't something they have seen before. Seeing how the chip is being formed has helped me realize why changing up RPM had no effect, it looks like the material there is being pushed rather than cut. Perhaps a more aggressive geometry is needed to break this particular chip?

 

[FredC]

Cutting edge way above center?

 

[SWYooper]

Conclusion, just so this isn't yet another "problem with no solution" thread.

Turns out the insert geometry was incorrect. The trailing edge wiper was not wide enough and rather than shearing the chip off it was folding the chip over, which led to the feathered edge and wrinkled appearance. It also explains why RPM changes had no effect.

It took a bit of convincing that it wasn't something wrong with my setup or program, but when the insert manufacturer's engineers finally got wind of the issue and were able to replicate they had a new set made that day for their own testing, and we had a new set of inserts in-hand in 3 days to prove them out in a production environment. With the wider trailing edge wiper, the 6-80 screws ran as they should: producing nothing but mill chips.

So in the future if any of y'all run into this and you are certain you aren't doing anything wildly wrong, consider that the inserts themselves might be the problem.

 

[TeachMePlease]

Conclusion, just so this isn't yet another "problem with no solution" thread.

Turns out the insert geometry was incorrect. The trailing edge wiper was not wide enough and rather than shearing the chip off it was folding the chip over, which led to the feathered edge and wrinkled appearance. It also explains why RPM changes had no effect.

It took a bit of convincing that it wasn't something wrong with my setup or program, but when the insert manufacturer's engineers finally got wind of the issue and were able to replicate they had a new set made that day for their own testing, and we had a new set of inserts in-hand in 3 days to prove them out in a production environment. With the wider trailing edge wiper, the 6-80 screws ran as they should: producing nothing but mill chips.

So in the future if any of y'all run into this and you are certain you aren't doing anything wildly wrong, consider that the inserts themselves might be the problem.

If your whirling insert guys were calling from Switzerland... There's your problem. You want the guys that call from Germany.

Glad they got it sorted eventually.

 

[SWYooper]

If your whirling insert guys were calling from Switzerland... There's your problem. You want the guys that call from Germany.

Glad they got it sorted eventually.

Not going to name names because they were good about it, but....... noted.