4340 machineability

[MrWhoopee]

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So HSS is an option.....
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Yes. There was a time, not long ago, when there was no better option.

 

[john.oliver35]

I second JRaut's comment - keep turning up the speed using carbide inserts for 4140. I made a few parts with some on my 10" Boxford and once I got the speed over approx 500sfm the surface finish magically transitioned from '1018 - like' to really nice. Poured on some oil for coolant and the smoke just rolls off

 

[mksj]

I have machined a bit of 4140 but only with carbide inserts (CCMT 32.51), SFM was in the 500 SFM range but you need adequate feed to get a better surface finish and this is also a factor of the nose radius and DOC. My original feed was around 0.004"/r and the finish was poor, moving up to 0.008"/r there was a significant improvement. Since the steel is fairly tough my take is that you are pushing the steel and not cutting it if you do not remove enough material as you are cutting. A sharper insert may be better for lighter cuts, but they may not hold up very well. The swarf coming off when cutting is very nasty and difficult to break.

HR4140 Initial feed rate was too low, surface finished improved midway when I doubled the feed rate.


Finished pass with higher feed