4140 recomendations?

[rigo430]

I'm going to be making some parts out of 4140 that will mate to reach other. One will be a turned part and the other a milled part. Does anyone have any recommendations for machining them? Both parts are annealed raw stock. Also what is welding going to do to any tight tolerances? should i machine the close fits after welding for the best results?

[The Aspirator]

I really enjoy turning 4140 on my lathe, it gives an amazing surface finish and seems to machine pretty easily. As for welding, IIRC 4140 requires some specific welding techniques to prevent it from cracking during the cool down period, totally do able, but you might want to do a bit of research if you haven't already. In a perfect world, with TIG welding, the metal will expand during welding but then contract back to original size (I think) after cool down. Might want to do a few test pieces first to get the hang of it and learn your mistakes, that's always a good idea. Best of luck!

[rigo430]

Lucky for me i won't be the one who has to weld them that's left upto another guy at the shop. What kind of speeds and feed do you use for a good surface finish since our customer tends to be very picky about that. I've done a test peice in aluminum and it looks good but just want to be able to make sure i don't cause to many scrap pieces trying to get the finish right with the tolerances.

[The Aspirator]

My lathe is nothing special, just a cnc converted Grizzly 0602, however with a 35° insert for steel I usually rough at 10-20ipm and finish at 3-5ipm. I crank the rpm up to my max of 2400rpm, since I've only machined 4140 pieces under 1" dia. Any bigger than 1.5" and I'd probably slow it down to 1200rpm. And use coolant if you can, that leaves an even better surface finish (in pretty much everything except brass that I've tried). Ohh and I usually make the finish pass have a cut depth of 0.005" or less, just so it's not working too hard and only has to shave a tiny bit off, making it nice and smooth.

Your tolerances will depend on your surface finish, how accurate the machine is, and IF they go out of wack after welding. You might want to machine a simple pin and tube in 4140 to your tolerances, get your welder to zap the backside for a while just to heat them up, then after cool down check the tolerances and fit again.