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Turning 304 SS - feed rate and depth?

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  • Turning 304 SS - feed rate and depth?

    I have a question about depth of cut for roughing 304 SS.

    I'm making a handful of muzzle brakes, starting with a 1" bar of 304 stainless, turning them down to .850"-.750" depending on the application. I've made a couple brakes so far with good results, and getting a nice finish, but I'm wondering if I can push it a bit harder for roughing without burning up the tool bit.I'm using one of those 1/2" shank tools with the TT style carbide inserts, on a 14x39 Grizzly lathe. Turning at 400 rpm, feeding .010"/rev, at .010" depth of cut, with a final pass at .005"/rev at .005-.010" DOC.

    This lathe doesn't have any other feed rates (except thread pitches), but how much heavier can I go on depth of cut before it burns up my carbide inserts? I'm using a C5 insert coated with something gold colored (don't remember what).

  • #2
    Your material removal rate involves both feed and depth of cut. That said, it sure seems from my experience that a larger depth of cut balanced with a lower feed roughs material better than a faster feed balanced with a lower depth of cut. SS likes feed, though, to avoid heat/work hardening...

    0.010" DOC @ 0.010"/rev? That is easily handled by my comparatively tiny 8" bench lathe. I would start at at least 0.050" DOC. I expect you'll find your 14x39 lathe handles that without any complaint. So try bumping it up by 40-50 thou' more each time until you note a complaint. I wouldn't be surprised if you get up into the .200" DOC range -- but I may be wrong.

    You have no fear of "burning up" a carbide insert. Even if the SS work hardens on you, carbide plows through that anyway. At some point if the feed isn't satisfactory, you'll get a built up edge (BUE) that'll affect your carbide insert. That means the material isn't flowing over your cutting edge fast enough to create a continuous chip string and (ideally) break off into smaller chips. Instead, work material sort-of "welds" to your cutting tool edge. I would fully expect you're working with a TiN coated insert based on your description.

    The two things that will set your limit are 1) available horsepower 2) rigidity of the machine and/or workpiece.
    Last edited by Arthur.Marks; 02-09-2015, 08:53 PM.

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    • #3
      I've made a lot of jet ski driveshafts out of stainless and carbide worked ok. Then I tried ceramet aka ceramic inserts. . The cost two or three times more than carbide but last ten times longer. Thus are far cheaper.

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      • #4
        Thanks guys. Arthur, I'll give it a shot tonight. The machine is fairly rigid, at least in comparison to the little SB9 I've been working on, so that hasn't been an issue yet.

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        • #5
          At ten and ten you should have lots of room to increase both feed and DOC. With insert tooling you can keep pushing both till the lathe starts to complain, then back off the DOC a bit. With 304 I like to keep the feed up because the last thing you want is to be teasing the material and allowing it to work harden. On a small manual machine numbers give you a good place to start but you should let the sound and feel of the lathe tell you when you've found the sweet spot...
          Keith
          __________________________
          Just one project too many--that's what finally got him...

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          • #6
            I was turning this exact same material today at my place of employment. Had a DNMG 432 insert (55 deg., .031 radius). Was taking .010 DOC, at .035/rev. At 2" diameter, I think the RPM was a tad shy of 900?

            The bad part is, DOC was limited due to the part itself, extending out of the chuck three feet. I don't reckon Haas TL's are built as solid as they look.

            (Of course, keep in mind, this is with flood coolant - but that really mainly affects your RPM; spritzing with cutting oil or coolant with a spray bottle can help with edge build-up and lubrication...)

            Anyway - don't worry too much about over-feeding. Even feeding greater than the radius won't cause much damage. But if you're limited with the feed, I'd slowly work up your DOC. One thing I learned just today - what's the worst that can really happen? Chip an insert? If you're nervous about the part, put a center in it if you can. Turn a step on the back end to prevent it from moving back in the chuck. Now, all that can happen is your insert break, tool post/holder slowly move off, chuck slip, or lathe stall - but no parts flying into space.

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            • #7
              I read horror stories before getting into 304. I've recently done #8-32 blind tapping in it and reached the conclusion that it doesn't give that superman's kneecap work hardening that many seem to fear. Sometimes in drilling I could feel a minor amount of hardening, but it would punch on through with a little more pressure.

              If I go in with a sharp tool and keep enough coolant on it life is good. Once it starts to heat, then chips start to weld and it knocks the tip right off my HSS tool every time. Once in a fit of laziness, I used a parting tool and compressed air to carve a shoulder for a 5/16-18 thread out of a chunk of 1/2" hex. The next time I found the right tool and went much faster. I never managed to discolor HSS tools...just saw chips coming off at a "hey, that looks different than before" angle then I'd hear the CLICK where the tip would break off the tool. I caught it few times before that and realized there was a bit of chips welded to the tip of the tool. Sometimes I could just knock it off with a screw driver and others I had to grind it off. It really acts like a heavier grumpier cousin of aluminum.

              The short version is I basically settled on straw-colored chips. Silver meant I was taking too long and blue meant it was eating tools.

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              • #8
                Originally posted by 2ManyHobbies View Post
                The short version is I basically settled on straw-colored chips. Silver meant I was taking too long and blue meant it was eating tools.
                This is mainly why I asked; I'm getting golden-brown chips (really long coils) right now.

                I can't increase the feed on this lathe, unless I just use a thread pitch, I think 48T is the finest it does. I'll run the depth of cut in and see what it can do.

                Some of you mentioned coolant; I'm not setup for much in the way of coolant right now. I've been brushing Relion (sp?) cutting oil on the part every other pass.

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                • #9
                  I make parts for my motorcycle project from 304 and my swarfs are golden-brown too.

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                  • #10
                    OK, did some turning last night. .040" doc worked fine, other than occasional issues with pushing the workpiece back into the chuck. I try not to crank down on the chuck too hard, but it was necessary in this case. .030" doc didn't have that issue, and cut nicely. Chips were still a nice golden brown.

                    Thanks for the help guys.

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                    • #11
                      Great. .040 DOC is pretty good, you got a fairly stout lathe for a hobbier. Tip - if you have issues with material pushing back, turn a short step on the end you plan to chuck on, if possible. That will prevent any kind of push-back. And then, also if possible, putting a center in the part basically sandwiches it between the chuck and tailstock - ain't going anywhere.

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                      • #12
                        Depending upon how much power your lathe actually has and what tooling you use, turning 304 can be easy and somewhat fun. My lathe is a Taiwanese Victor 1640 with 5 Taiwanese horsepower spindle motor, and it wasn't even working hard for this cut is the slightly nastier 316 stainless steel. Negative rake CNMG432-M6 grade 8525 (discontinued) and light straw colored chips. Still hot as hell though!

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                        • #13
                          Originally posted by PixMan View Post
                          Depending upon how much power your lathe actually has and what tooling you use, turning 304 can be easy and somewhat fun. My lathe is a Taiwanese Victor 1640 with 5 Taiwanese horsepower spindle motor, and it wasn't even working hard for this cut is the slightly nastier 316 stainless steel. Negative rake CNMG432-M6 grade 8525 (discontinued) and light straw colored chips. Still hot as hell though!

                          Wow those are wonderful chips for 304/316

                          I'm roughing 304 right now on my little Tormach lathe and getting chips out the door!

                          What depth of cut is this?
                          feed? diameter?

                          What brand was that insert?!

                          Thanks in advance for any info


                          Paul

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                          • #14
                            If you view the video on YouTube (click the logo in lower right while it's playing), you can read the comments I posted with it that give the information.Moreover, those inserts used in that video are no longer available. Others which work just as good or better have taken their place. However, that doesn't matter because you don't have that kind of tooling or machine.

                            There are inserts I can suggest but would need to know what toolholders you have, and how much power the Tormach lathe has available.

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