5C taper problem again

[Harold_V]

The register at the threaded end is what I called the "back bearing" (Hardinge's term) above. A burr there would angle the collet (which as you note excessive clearence there would do as well).

Correct, but it would manifest itself as a loss of parallelism, not taper. In essence, any machining done to the piece would would be at a slight angle, the amount of the offset created by the burr, but it would NOT be tapered. Only when the piece is offset constantly as it rotates would it manifest itself as taper, which I why I suggested that the tailstock may be part of the problem. It may be offset, coincidence, or not, by the amount that is revealing a taper. The slack of the register (assuming there is any) would permit the collet to "wobble", for lack of a better term, as it rotates, always deflecting away from the cut. It stands to reason that it would diminish to the point of not existing as the cut neared the collet, where there was far more support.

I think it would be better not to use the live center for these tests.

Unless it has slop, it isn't adding any problems, even if the point oscillates a small amount. Because the tool creates a load on the center, it, too, should be offset by a given amount and still support the part as it rotates. It would be my recommendation that a DTI be applied to the point, which should then be rotated to determine if it isn't running true. It would be best done with the center under a slight load, to ensure that the thrust bearing is operating in its expected condition.

The OP has clearly stated that he is creating taper. I'm assuming he's not discussing run-out of a part that is installed and a DTI applied. In that case, it might be difficult to determine where the problem lies, as a ding or a mismatch of registers can be the problem. All of my comments relate to a part that is machined in situ.

Harold

[ctwo]

When you measure the diameter after a cut, the end of the bar is .0004" larger than the collet end.

well, that is very close to me, but I take back what I said as that could result from the work pushing away from the tool. Maybe there is flex in the collet/holder?

[GlennW]

This is getting just a bit confusing!

Over 3", the taper is about 4 thousanths; narrower near the collet.

Same result. 0 runout at the collet. 2 thou runout at 3"

It is runout because when you rotate the uncut bar, the dial indicator moves between 0 and .0002" When you measure the diameter after a cut, the end of the bar is .0004" larger than the collet end.

Are we dealing with tenths or thousandths here?

[rexcsmith]

My apologies, I let an extra zero slip in there. I mean 2 thou and 4 thou.
Not getting repeatable readings tonight so I'm going to call it a night.
I ordered some precision bar and I'm going to start afresh with that and see if I can get a solid set of results.

The only thing I've been able to repeat so far with any confidence is that I get a taper when cutting using the 5C collet and not with the chuck.

[John Hasler]

The register at the threaded end is what I called the "back bearing" (Hardinge's term) above. A burr there would angle the collet (which as you note excessive clearence there would do as well).

Correct, but it would manifest itself as a loss of parallelism, not taper. In essence, any machining done to the piece would would be at a slight angle, the amount of the offset created by the burr, but it would NOT be tapered. Only when the piece is offset constantly as it rotates would it manifest itself as taper, which I why I suggested that the tailstock may be part of the problem. It may be offset, coincidence, or not, by the amount that is revealing a taper. The slack of the register (assuming there is any) would permit the collet to "wobble", for lack of a better term, as it rotates, always deflecting away from the cut. It stands to reason that it would diminish to the point of not existing as the cut neared the collet, where there was far more support.

I think it would be better not to use the live center for these tests.

Unless it has slop, it isn't adding any problems, even if the point oscillates a small amount. Because the tool creates a load on the center, it, too, should be offset by a given amount and still support the part as it rotates. It would be my recommendation that a DTI be applied to the point, which should then be rotated to determine if it isn't running true. It would be best done with the center under a slight load, to ensure that the thrust bearing is operating in its expected condition.

The OP has clearly stated that he is creating taper. I'm assuming he's not discussing run-out of a part that is installed and a DTI applied. In that case, it might be difficult to determine where the problem lies, as a ding or a mismatch of registers can be the problem. All of my comments relate to a part that is machined in situ.

Harold

I was discussing the runout on the test bar only, not what happens when he cuts. It's not entirely clear to me under what circumstances he's getting taper. Test bar DTI measurements should be done without the live center.

[Harold_V]

I was discussing the runout on the test bar only, not what happens when he cuts. It's not entirely clear to me under what circumstances he's getting taper. Test bar DTI measurements should be done without the live center.

I now understand. However, that isn't likely to expose the problem, for it may be due to loading of the cut, causing the collet to deflect at the rear. Considering it's likely a constant amount of clearance, it makes sense that that can be the cause of the resulting taper.
I'd like to see this machine in person. What's happening may be obvious, although I've seen issues with machines that were never resolved, one of which was on a four head Excello boring mill. Didn't matter how closely the heads were dialed, concentricity was always out of tolerance. We also had a #1 B&S universal cylindrical grinder that was unstable, with taper ever changing. Both cases we're talking a few tenths, but that's far more than may be acceptable, as it was in our case.

Harold

[GlennW]

A couple more questions:

What material are you cutting for your taper testing?

What tool bit are you using? (got a pic?)

Spindle speed? I find spindle speed and taper quite relative.

What does the surface finish look like?

My theory, from the info provided, is that the tool bit isn't doing it's job and the bar is simply deflecting while being cut causing the taper.

When held with a chuck the bar has lots of support, but a collet has a very minimal contact length in comparison which would allow for more deflection with a cutter that isn't cutting properly.

3" stick out on a 3/4" bar held with a collet isn't the ideal setup for checking for taper.

[John Hasler]

I was discussing the runout on the test bar only, not what happens when he cuts. It's not entirely clear to me under what circumstances he's getting taper. Test bar DTI measurements should be done without the live center.

I now understand. However, that isn't likely to expose the problem, for it may be due to loading of the cut, causing the collet to deflect at the rear. Considering it's likely a constant amount of clearance, it makes sense that that can be the cause of the resulting taper.
I'd like to see this machine in person. What's happening may be obvious, although I've seen issues with machines that were never resolved, one of which was on a four head Excello boring mill. Didn't matter how closely the heads were dialed, concentricity was always out of tolerance. We also had a #1 B&S universal cylindrical grinder that was unstable, with taper ever changing. Both cases we're talking a few tenths, but that's far more than may be acceptable, as it was in our case.

Harold

Deflection doesn't explain the test bar runout, but it is not clear under what conditions that was observed. I'd like to isolate the variables by working on one thing at a time. The live center can't do anyhting but muddy the waters at this point.

rexcsmith: Please do the DTI measurements again with a precision bar and no live center. Write down everything you do. Check and record runout at the collet and at the end. Mark the high spot on the bar and on the collet. Rotate the bar 90 degrees and check and record runout at both ends again, noting whether or not the high spot moved when you rotated the bar.

Next apply firm pressure to the bar. Don't use a hammer, but lean on it pretty hard (just don't bend it). Now measure runout again (not while applying pressure) and see if the high spot moved or if the runout changed significantly. Also apply moderate pressure with the DTI contacting the bar near the far end and see how much it deflects.

I also suggest that you reach into the collet holder as far as you can and feel around for anything suspicious. Do you have an inside micrometer or bore gage with which you could measure the back bearing?

[rexcsmith]

Harold; I'll follow your procedure as soon as I get the precision bar. Thanks

Glenn, I'm cutting hard steel; rockwell 30. However, I get the same with mild steel and drill rod.
Using a HSS cutter I ground myself. Works great when using a chuck. Cuts a very nice finish with either a chuck or a collet. Cuts are only 1 thou or less.
500 rpm, lubed

What is the ideal setup?

thanks, rex

[rexcsmith]

BTW, same results with carbide inserted cutter.

[GlennW]

Try aluminum and a sharp HSS cutter.

That should eliminate deflection.

If that cuts straight, we'll move on from there.

[BadDog]

For most bits, 1 thou depth of cut is not enough, much less 0.001 off the diameter. And that is even more true for most carbide bits. Some can reliably make those cuts, but not all. Most just want to rub (and push) at that depth.

And I would follow Glenn's advice. Try switching to aluminum with a very sharp low pressure grind HSS tool and see what you get. For very small cuts like you are doing, I would try a significant positive side rake with moderate positive back rake, little or no lead angle, with an almost nonexistent nose radius and honed razor sharp. Lousy chip control (not that it maters at 0.001 DoC) but should skim off with very little pressure. For steel and making those very light cuts look up "shear bit", or cermet inserts designed for the task.