5C taper problem again

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BadDog
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Re: 5C taper problem again

Post by BadDog » Wed Feb 08, 2017 2:11 pm

Oh, and at those cuts, I would try running it faster. 500 is roughly 100 sfpm, which may be the limit depending. And maybe 2.5 times faster for (most... check mfgr) carbide, probably wide open on most machines for cermet (~1000 sfpm desired = 5000 rpm).

Doing the same with 3/4" aluminum is generally as fast as possible on most machines.
Russ
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Chuck K
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Re: 5C taper problem again

Post by Chuck K » Wed Feb 08, 2017 7:52 pm

The 0.001 cut seems suspect to me. I'm trying to get my mind wrapped around 0.004 taper when taking a 0.001 cut. Either you're taking multiple passes with the piece deflecting until you move closer to the collet or something is seriously out of square. I can't think of any tool other than a shear cutter that would work at that depth.

rexcsmith
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Re: 5C taper problem again

Post by rexcsmith » Sun Feb 12, 2017 11:43 pm

I think I'm onto what's going on, thanks to suggestions from this board. I got a piece of precision stock and made runout measurements. Long story short, the runout is due to the chuck/collet and not the workpiece. I then took the collet out of the chuck. Measured inside the nose end of the chuck (right end) and saw .0005" runout there; the high point corresponding to the position of the high point on the work piece. I then measured the runout at the spindle. Nada. It looks like the chuck needs some machining to fit more squarely to the spindle. I do know someone who can do that, but of course he'll want to verify my analysis. At any rate, I think I've got it.
I assumed that since this was a Monarch 10ee with a Monarch chuck/drawbar assy, that I wouldn't have to worry about such things, but the stuff has been around awhile. Thanks again. rex

johnfreese
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Re: 5C taper problem again

Post by johnfreese » Mon Feb 13, 2017 12:41 am

It sounds like there are two different things going on.
I understand the turned shaft is smaller near the headstock. That is obviously part deflection due to cutting forces. Maybe the collet was not tightened sufficiently.

It you turn a taper when the end is supported by a center I would suspect the tailstock is off line. Do a two collar test to see if that is the case.

Harold_V
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Re: 5C taper problem again

Post by Harold_V » Mon Feb 13, 2017 2:43 am

rexcsmith wrote:I think I'm onto what's going on, thanks to suggestions from this board. I got a piece of precision stock and made runout measurements. Long story short, the runout is due to the chuck/collet and not the workpiece. I then took the collet out of the chuck. Measured inside the nose end of the chuck (right end) and saw .0005" runout there; the high point corresponding to the position of the high point on the work piece. I then measured the runout at the spindle. Nada. It looks like the chuck needs some machining to fit more squarely to the spindle.
I'm beginning to wonder if you are confusing run-out with taper. They are not the same thing, and are not related to one another.

The test you described will determine run-out, but it has nothing to do with taper. It is clear your collet head does not hold parts parallel to the spindle. What's important to know, now, is if the part being machined results in a taper, or not. Do not assume that because the part being held as it is will result in taper. It will not.

Don't alter the collet chuck mounting until you fully understand the problem. The fit you described may not be the problem at all.

Harold
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rexcsmith
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Re: 5C taper problem again

Post by rexcsmith » Mon Feb 13, 2017 8:39 am

I'm actually not turning anything, just making measurements. Back when I did the turning, I observed the problem on around 10 different work-pieces; always made sure they were tight. I tried pushing on the workpiece with my hand during measurement; less than 1 thou measured on the dial.

rexcsmith
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Re: 5C taper problem again

Post by rexcsmith » Mon Feb 13, 2017 8:52 am

It cuts a taper every time, and to the same degree, and from my measurements, I'm pretty convinced the chuck/collet is not holding the workpiece parallel to the spindle. But, I'm not going to cut the collet chuck mount based on my own limited understanding. I know a machinist who deals with such things. He's coming over when he has some free time.

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GlennW
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Re: 5C taper problem again

Post by GlennW » Mon Feb 13, 2017 9:02 am

You are dealing with two separate issues.

This was one of the the points of my previous post with the quotes. One post you are referring to taper, and the next run-out, then thousandths, then tenths.

Having run-out has nothing to do with cutting a taper.

You could offset a piece of bar stock half of it's diameter in a four jaw chuck and still turn a round straight part if the lathe is set up properly.
Glenn

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Russ Hanscom
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Re: 5C taper problem again

Post by Russ Hanscom » Mon Feb 13, 2017 10:19 am

Glen is right.

John Hasler
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Re: 5C taper problem again

Post by John Hasler » Mon Feb 13, 2017 11:18 am

Russ Hanscom wrote:Glen is right.
I agree. You've got two problems. You've found the proximate cause of the runout and that's great, but you still need to account for the taper. The two problems might be unrelated, but make sure of that before you change anything. If there is a single root cause just fixing the runout could make things worse.

rexcsmith
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Re: 5C taper problem again

Post by rexcsmith » Mon Feb 13, 2017 12:06 pm

Are you saying that lack of parallelism between the workpiece and the spindle doesn't cause taper?
I know that one cause of taper can be deflection of the workpiece while cutting.
I've pretty much ruled out deflection for a couple of reasons. The main reason is that I can put the workpiece in my 3 jaw chuck, make the same cuts, and there's no taper. Also, I've done this many times with different depths of cut and taking spring cuts, with the same results.
Are there others causes I should be looking for?
I'm pretty sure the collet isn't moving in the chuck. I've checked that over and over and that thing is drawn in there tight.

John Hasler
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Re: 5C taper problem again

Post by John Hasler » Mon Feb 13, 2017 1:17 pm

> I've pretty much ruled out deflection for a couple of reasons. The main reason is that I can put the workpiece in my 3 jaw chuck, make the same cuts, and there's no taper.

But I bet that if you put your precision bar in that same chuck you will measure two or three thou of runout. The fact that you get taper-free cuts with the three-jaw proves that the cutter is moving parallel to the spindle axis. That eliminates one possible cause of taper.

Now think about what's happening. As the carriage moves the cutter defines a constant-radius cylinder centered on the spindle axis. It is going to whack off any parts of the workpiece that stick out past that cylinder. No matter what the initial shape or position of the workpiece was the only thing that can stop you from making it into an untapered cylinder is it bending or wiggling out of the way.

Can you borrow a different collet chuck to test with? You already know that there is at least one thing wrong with the one you've got.

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