Turning 4140 steel

[maximusek]

Hi,

I must turn bar stock .250" steel 4140 down to .156". I was using Sandvik 55deg turn insert, grade PF4015, machine - swiss. For some reason surface finish looks bad, very bad. I was playing with speed from 4500RPM to 3000RPM at feedrate .001ipr and nothing. Maybe some smart head will put some light on me how to machine that material, I'm looking for feed, speed or maybe I have to buy different insert.
Thank You for any answer Guys

[g-codeguy]

You are turning at about 185 SFM with an insert designed to run MUCH faster on that material. It is tearing instead of cutting. Try a ground insert, either cermet or PVD. Personally haven't used an uncoated insert in so long that I can't recall if one would work or not on that material. Also .001 is pretty slow. Don't know the radius size you are using, but I just finished a job turning .118 dia. for a length of .45 with a 60 point on it at F.002 with an .008R insert in 303 SS.

Not on a Swiss machine either. Had to machine it in sections because of the small diameters and length of the part.

[maximusek]

Hi g-codeguy,
Thanks for an answer. I was using those inserts turning steel 4340 without any problems but with 4140 I had only problems. I agree about insert, according to the producent data I can go lot faster. In the meantime I tried Kennametal 55deg KC5010 grade and everything looks much better (speed and feed is the same, radius corner .016 - Sandvik and Kennametal - sorry about that). I've been told, 4140 is not material I was dreaming to work with but I didn't expect so much difficulties. Anyway thanks again...

[g-codeguy]

I like the KC5010 grade. When Kennametal replaced KC730 with KC7310, I went back to KC730, but we use the 5010 grade on a lot of different types of materials with good success.