Problems with 4140

[HSPMInc]

Hi guys, I've been lurking around here for a bit and I finally have a question I need help with.

I'm turning a pin out of non heat treated 4140. There's a section on the back of the pin that I have to back turn with a .004 radius tool and I can't get it to repeat. The tolerances on this part are + or - .0002. This shouldn't be a problem but I'm all over the road. I'm repeating within .001 but I need to tighten it up. Any ideas? What's the recommended RPM and Feed for 4140? I'm at 5000 rpm and .001 in/rev of feed now. I've moved feed and rpm around but it's not helping. It's 1/4" matl, turning down to .1585.

Thanks for any help.

[HSPMInc]

I should mention that I'm turning the part down to .188 with the main turn before I back turn down to .1585, and the .1585 is the +or- .0002

[MikeMc]

OK....first of all, what is the condition of the material you are turning? Centerless ground? Cold drawn? What is the OD tolerance of the bar itself?
Next what condition is the guide bushing in? New, used, over used? Next about the tooling, what type, grade, size? You asked about speeds and feeds, but how can I give a speed orfeed without knowing the specifics about the tooling? Lat thing I would ask is about themachine itself, although they are all amazing, what is its condition? Has it been properly maintained?

I realize its a lot of info, but its also very complex question. In my experience, its almost always a very simple solution, you just haven't found it yet!

[HSPMInc]

It was Cold Drawn, Annealed. The OD of the bars is pretty good. No big lumps or anything. The guide bushing is well used but smooth and free of any build up or dings. It's my 1/4 GB so it gets used plenty and I never have problems with it. The Insert is a .002 radius Kyocera(Ceratip) back turn tool. It's PVD coated PR930. Info at http://americas.kyocera.com/kicc/pdf...s_p167-202.pdf SABS-40F tool Holder. As for the Machine it's a 12-15 year old Star SST-16 but it's been WELL taken care of. For a machine this old it's in PRIMO condition. The only time we ever have troubles with finish/tolerance is in 4140 which leads me to believe I'm doing something wrong. Even front turning with a .008 or .016 radius, 55 degree diamond shaped insert I have troubles getting a decent surface finish. Anything else I can turn just fine. With 64 or better finish.

And I agree it's probably a simple solution. But I'm stumped. I was actually able to finish the parts by taking a rough pass and leaving .004 of matl and then turning that off at 5000rpm and .002 in/rev of feed. They were in tolerance but the finish wasn't great. There's just no reason I shouldn't be able to get a nice finish in 4140.

[HuFlungDung]

I've only turned a bit of annealed 4140 (on manual lathes), but I wasn't particularly impressed by its lack of machinability...kind of like C1018 with frequent built up edge problems. That could be causing your dimensional variation because you cannot get enough rpm to get the SFM up where it should be for finishing in hopes of getting around BUE.

Are diamond coated tools supposed to be good for use on steel? I heard not.

Heat treated stress relieved is much nicer to machine, but I've never asked if it is available in such small diameters.

[MikeMc]

It may come down to the machine just not being tough enough to handle the material itself. I have a C-16 with 3/8 square tooling, it would do brass, aluminum, and small steel parts just fine. When I tried to do a 1/2" 304 part in it (that we ran A LOT in our L-20s) the machine just coundn't hold the finish or the tight tolerances. Back turning is another issue, I just can't fine a really good reliable tool to do it in steel for some reason. When the tolerances get tight on a back-turn in steel, I tend to quote for grinding the part after its turned.

[marryfoxgirl]

It looks like youre using Ajax to render the buttons. That feature is currently not supported. I dont believe jQuery is the issue though, as many users have successfully installed the button with jQuery present throughout their pages.
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[marryfoxgirl]

It looks like youre using Ajax to render the buttons. That feature is currently not supported. I dont believe jQuery is the issue though, as many users have successfully installed the button with jQuery present throughout their pages.
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Website I developed for quick payday loans company.

[BobWarfield]

PVD = Physical vapor deposition, not diamond?

BW

[PixMan]

It looks like youre using Ajax to render the buttons. That feature is currently not supported. I dont believe jQuery is the issue though, as many users have successfully installed the button with jQuery present throughout their pages.
_______________________________________
Website I developed for quick payday loans company.

Absolute SPAM. Must be banned.