Help turning 17-4 with +0 -.0002 tolerance

[MCImes]

Hello,
I have to make some shafts in 17-4 stainless. They are .125" diameter by 2.8" long. The only dificult feature is a tip .078 dia by .100 long with a small chamfer. The feature itself is not hard but the tolerance is +0 -.0002. Its a medical part so the customer is fairly *****y about it being nuts on. Im used to turning shafts with that tolerance (in 303) and its usually not an issue but the 17-4 is a curve ball to me. Last time I ran this job I had a 50% scrap rate which (obviously) I would like to improve.

I crank down basically as hard as I can on the bushing but my stock was not ground so I was still getting .0002 out of round so I was riding both sides of tolerance. I will have the same stock this time so that wont help any.

I tried turning the tip using a .004 rad Mitsubishi CCMT TiN coated turn tool at first but I had slightly better luck with a .050 groove medium/high rake tool. I think I should have some Iscar .002 rad w/wiper inserts coming in before the job if you think that would work well.

I recently have had good luck by doing multiple plunges and a short dwell (like .05 sec @5000rpm) with the .050 groove tool and was thinking of trying that.

I have relatively little experience with 17-4. Whats the secret to it? Do you have an insert you know works well? Should I try to get a .001ish rad insert? What do you use for speed or feed? Any other process thoughts?

Thanks

[txcncman]

Are you turning too fast and getting bar whip/oscillation? You gave a lot of data, but not all data.

[SirDenisNayland]

The biggest issue with that tolerance is going to be who measures it.. give the same part to a few different guys and they'll all come up with different measurements spanning a larger tolerance than you have. Been through that nihtmare myself. Part measures nuts on for me but customer qc measures four tenths under size.... With my micrometer... Sigh.

[MCImes]

I tried 4k and 5k for rpms. didnt matter which it was. still the same result.
I doubt there is any bar whip as this feature on the front of the part so at max there is .099 sticking out of the bushing...+1mm for clearance I guess...

As for measuring I use a pressure mic from inspection and the customer didnt complain about the last batch, or at least I never heard about it.

Thanks

[txcncman]

Take a spring pass with a different feed rate? I am all out of ideas. I usually do not check lathe parts down the the tenth, but when I have, I have not noticed out of roundness like you are getting.

[MCImes]

I forgot to add that. I have a spring pass in the program following the same tool path.

As for out of round do you mean on a regular lathe or a Swiss? a regular lathe you're right. You should have near zero out of round.

On a swiss about 50% of the out of round from the stock is transferred to the part. In the past Ive had wonky stock that I had a hell of a time with just to get <.001 circularity. The 17-4 isnt nearly that bad but out of round is just the way swiss is with non ground stock.

Also, if you run swiss, what kind of parts do you run that you dont check down to the tenth?

[SirDenisNayland]

I forgot to add that. I have a spring pass in the program following the same tool path.

As for out of round do you mean on a regular lathe or a Swiss? a regular lathe you're right. You should have near zero out of round.

On a swiss about 50% of the out of round from the stock is transferred to the part. In the past Ive had wonky stock that I had a hell of a time with just to get <.001 circularity. The 17-4 isnt nearly that bad but out of round is just the way swiss is with non ground stock.

Also, if you run swiss, what kind of parts do you run that you dont check down to the tenth?

I disagree.. swiss should always have the highest roundness.. unless you're talking about a chucker, then maybe I could agree.

Sharper inserts is all I can say. Or go harder on your guide. I know you said you're already solid as a rock with it, but one man's solid as a rock is another man's breast implant filling

[danrudolph]

Sounds like good bar would go a long way to help your problem.

Call Schmoltz and Bickenbach Home: SCHMOLZ-BICKENBACH.US and see if you can get a sample bar to try, maybe a 6' that can ship UPS. They normally have a $250 minimum, but sounds like you are spending that much on your scrap.

This is the ONLY stainless we use, all bars are normally within .0002 of nominal size and very round and very consistent. And the bars come chamfered, which is just nice.

I would never crank down on my GB like that, that sounds like a bad practice to me, probably giving you flex in the bar as the Z feeds out. Good bar, light GB setting, my 2 cents.

[elsanch]

what kind of bushing are you using? i would have a midland type bushing. a bit longer support.

[TheKid09]

You may have covered this, or stated it, and i over shot it while reading...but what type of machine, and if its a swiss, are you using a stationary or rotary guide bushing??

Rotary...from at least my experience, .0001-.0002" run-out has always been about the norm for me on ground material. On a Stationary, ive been able to hold .00005-.00008" consistantly, measured at stable temp with a laser of course..haha..

[DCogswell]

You may have covered this, or stated it, and i over shot it while reading...but what type of machine, and if its a swiss, are you using a stationary or rotary guide bushing??

Rotary...from at least my experience, .0001-.0002" run-out has always been about the norm for me on ground material. On a Stationary, ive been able to hold .00005-.00008" consistantly, measured at stable temp with a laser of course..haha..

I agree with the above statement.

To hold that tolerance you'll need the following:

1. Stationary Guide Bushing
2. Ground Material
3. Turning tool as close as possible to the bushing. If you can't get the turning tool within .010 then you should custom grind the shank to bring the insert edge close to the bushing.
4. Feeds n Speeds need to be dead on for the insert you're using. Need to reduce heat soak into the material
5. Spring or rough passes will only create more opportunities for problems. With your DOC at .0235" you do not need a rough pass.
6. Consider re-chucking because you have about 3 inches of distance between the head stock and the bushing and could be causing some whipping.
7. Spindle liners to reduce whipping behind the head stock
8. Laser mic
9. Gage r&r study between your shop floor and your customer

Good luck, let us know if you made any progress.

-Dave

[MCImes]

So I ran this job last week and it actually went very well. I have had a lot of luck lately using a .050 groove tool and plunge cutting so I tried this approach on this job and it went great. I used a Kennametal T20G1603R125 .050 groove tool (with TiN coating) for my plunge tool and it worked great.



Some notes on the setup:
-I used the normal rotary guide bushing.

-I tightened it REALLY tight. much tighter than 1/8" stock shoulf be able to take but it didnt taco once on me. To help relieve pressure on the bar between the main spindle and bushing I made all of my Z+ moves as a G1 G98 Z### F60. so there were no rapid Z+ moves.

-There is 1mm bushing face to Z0 on my machine.

-I faced the part with my normal front turn tool and added an oversize chamfer which allowed me to take 2 plunge cuts wit the above mentioned .050 tool. The cuts were both .045 and I figure it was somewhat important to keep the depth of cut fairly constant.

-With the .050 tool I used 6k rpm and .0005 ipr with a .03 second dwell at the final diameter to cut the .0785-.0002 tip.

-The material was not ground but it was very cylindrical. Within .0002 which was nice. way to go EMJ.

-I used my maximum G300 distance of 2.35" with no porblems.

-Checked parts with an Etalon pressure mic calibrated to Meyer class XX gage pins (nominal and low limit)

-Didnt have a spindle liner. The bar was straight enough that it didnt whip much at all.




-I assume everything checked out because I havent heard anything about it and in my shop no news is good news. Most parts were within .0001. A couple bars rode the limits with .0002 out of round but it was consistent enough that I could move it a tenth and stay in tolerance. considering the machine is from 1996 im fairly impressed how well it repeated over a 330 part order. I only threw out a few parts.

-I have been using that Kennametal insert for getting finishes <20 rms on 12l14 (which I was having trouble holding with a .004 and .008r iscar inserts, different chip breakers.) and now turning 17-4 like a charm. I am a huge fan of it. Go plunge cuts!

Hope some of that helps!