Turned hardened steel

I'm having a problem with tir and taper when turning 4140 prehard hrc 29...

Mori MS850 5hp machine, 1800rpm (max rpm), .006-.008 ipr, cnmg 432 inserts rp/fn/ff, kennemetal 3/4 holder at various lead angles, in aloris post.
Round bar 0.669 sticking 1.250 out of the chuck turned down to 0.500, testing doc ranges between .030 to .080. Any of the doc produces mirror finish.

The problem is...
I'm getting a taper of roughly .001 per .500 length and tir of about .0015 down the entire bar.

With a 2.000 tube of 6061 or 1018 sticking 8 inches out of the chuck, using same speeds as above, I get no measurable taper or runout...

Not sure if I'm using the wrong tooling for the hardened stuff, I have a job coming up with the same size bar stock in 4340 @ hrc 40 and I fear even worse results.
Maybe there is something else causing the problem?

Any help would be greatly appreciated!

Sounds like you need outboard support or several spring passes.

1st thing I would check if you are holding this in a 3 jaw, is if the jaws are bellmouthed at all- 2nd, not sure I would be running 1800 RPM in an engine lathe with probably no coolant. Are you taking a finish pass or ? I don't really understand your TIR comment- if you're just checking runout after a turning pass and you see .0015, that's almost certainly due to your stock moving around in your work holding device. Do you have a bunch of material flopping around behind the chuck? That will also cause the issues you are seeing.

Stepping up to your 40 Rc job- that's going to be nothing like your 28Rc experiment.

Spring passes will involve work hardening.

What is the nose radius of the tool ?

Barry680 said:

I'm having a problem with tir and taper when turning 4140 prehard hrc 29...

Mori MS850 5hp machine, 1800rpm (max rpm), .006-.008 ipr, cnmg 432 inserts rp/fn/ff, kennemetal 3/4 holder at various lead angles, in aloris post.
Round bar 0.669 sticking 1.250 out of the chuck turned down to 0.500, testing doc ranges between .030 to .080. Any of the doc produces mirror finish.

The problem is...
I'm getting a taper of roughly .001 per .500 length and tir of about .0015 down the entire bar.

With a 2.000 tube of 6061 or 1018 sticking 8 inches out of the chuck, using same speeds as above, I get no measurable taper or runout...

Not sure if I'm using the wrong tooling for the hardened stuff, I have a job coming up with the same size bar stock in 4340 @ hrc 40 and I fear even worse results.
Maybe there is something else causing the problem?

Any help would be greatly appreciated!

Click to expand...

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you using .030 or more DOC on relatively hard steel and not expecting taper ??
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reduce DOC and or take .001 finish cut or 2 finish cuts

CNMG imparts a lot of nose pressure (in a harder material) for a boring bar. I'd switch to a T or a D insert capable bar. Or swing the bar to a negative lead angle so you can get better penetration of the radius of the insert, and less dragging of the heel. I do this frequently when the hole size/bar combo permits. It is convenient on a multi-fix toolpost to swing the toolholder 'one notch' (9°) for a quick roughing op.

Edit: somehow got the idea that you were boring holes!

Anyways, yes, I would carefully check the quality of the chuck jaw grip to make sure the part is not squirming around. A 3 jaw chuck tends to allow a 3-lobed surface to be formed if the work is squirming, and this can be difficult to measure correctly and consistently.

If all is tight, then I'd adjust the 'lathe level' at the tailstock end to change the angle of approach of the carriage so as to cut cylinders instead of tapers. This is a fine adjustment, and something that may have to be tweaked from time to time, let's say, if the floor shifts a little bit seasonally. That's why I say all the test bars in the world won't help you get the results you want in real life, you've got to be prepared to adjust the machine to your cutting preference.

The stock seems to be 17 mm. Is it in a 17 mm collet? Anything else will let it deflect too much.

Is the tool driving into the holder, don't laugh it happened to me on some hydraulic cylinder rod, I could not figure it out, till I saw the marks on the top of the tool, lights came on (at last), I drilled the tool with dimples to fit the holder, problem solved, then I broke the tip, one of those days, new tip problem solved, again
Mark