turning 4130, 4140

[Stifler51]

lately ive been turning a bunch of 4130/4140, i havent worked long on the lathe but my question is how do i get a better finish when turning it. my finish is always a cold finish. granted it test fine but looks bad. so do i turn it faster, slower? positive rake, negative? .03, .015 rad tool or what works best?

thanks

info: usually turning 400-600 sfm, 3000rpm max, .01 rough .005 finish, tried both .03 and .015 rad with neg rake

example part ive made: .3125 od, .25 id about 1 3/4 long out of 3/8 4130 bar stock

[RICHARD ZASTROW]

If you're turning .3125" dia. part at 3,000 rpm you haven't reached 400 sfpm, maybe 245 sfpm?

Dick Z

[peaceandcalm]

sry in MM units, use spindle speed 2200, 3000 ok but it might burn the inserts faster..use 0.4 VBMT style inserts for finishing, leave on roughing pass about 0.2 to 0.3 for finish, and feed is 0.12 to 0.15 max..
also use coolant...dont repeat on the finishing pass, do only one pass.
hope that helps.

[PixMan]

If you're using coated carbide inserts, that's a one-pass cut not rough & finish. You'll need to use a smaller nose radius and somewhat upsharp geometry (in your negative rake holder). A .002" (.05mm) edge hone will help with surface finish and tool life, but may push the stock away at the far end. The positive geometry helps prevent that.

What shape & size insert can you use, or what are you currently using? The grade, nose radius and chipbreaker you use determines the speed, feedrate and depth of cut you use.

With a weak geometry such as a VBMT isn't a great choice for this. I'd lean toward a CNMG431, DNMG431 or WNMG431 with a finishing chipbreaker and take it all in one pass. If you were to leave "0.2 to 0.3 for finish" (that's .0078" to .0118") it's well below the .015" nose radius of a 431 insert. That's a formula for bad finish unless you use a much smaller nose radius. Then you don't have a strong insert.

What holder do you have?