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  • Chatter help

    Long story short someone brought me a piece of 30" long gun barrel to turn from a 4" od into a 2-3" taper OD.

    The material is stupid hard and seems to smoke most any HSS bit I throw at it. I have been cutting with carbide at a fairly low speed ( roughly 300 rpm) .025 DOC

    The material cuts beautifully for the first 1/3 of the barrel then begins to chatter. Normally I would say its because I dont have a follow rest but I cant believe that a material this beefy is flexing under such light cutting pressure and causing the chatter.

    Any ideas what could be going on.

  • #2
    Yes it will flex due to the length. I would reduce depth of cut (maybe .010 cut) and increase or decrease feedrate. The RPM may need fine tuning up or down. With chatter I would try slower first, but there may be a sweet spot higher in there somewhere. This is the kind of situation where infinitely adjustable spindle speeds can save the day. For example, with a VFD you can fine tune the cut while it's running and get the harmonics tuned. If you only have a few gear choices, it's harder to do.
    Kansas City area

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    • #3
      Well try it with a steady rest and see if that changes anything.

      Dave

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      • #4
        Chatter problem on a lathe??? Yep, been there, done that before. Try this
        As small as possible nose radius on the tool bit, while using positive top rake.
        Slow down the speed, increase the feed, while in keeping a smooth cut, use light cuts.
        Now this may sound crazy, but it works, so don't knock it till after you have tried it. Turn your tool bit upside down and run the machine in reverse. The tool bit still needs to be on center, use carbide.
        Lay a 2 X 4 board on top of the workpiece (holding it by hand will do) while you are doing a cut.
        These are my most major tricks that has really helped me out with this problem.
        The board (2 X 4) trick and the upside down, in reverse trick seems to do the most good.

        Your friend, Doggie
        Last edited by Doggie; 02-08-2015, 01:42 AM.

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        • #5
          That's one hell of a barrel! What's to for if I may ask? I've seen a 3" OD barrel before, but not 4". This was for a 30 caliber, yours may be for something completely different.

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          • #6
            Originally posted by rws View Post
            That's one hell of a barrel! What's to for if I may ask? I've seen a 3" OD barrel before, but not 4". This was for a 30 caliber, yours may be for something completely different.
            Yeah it is stupid big. Its a 40 mm ID barrel from who knows what. Internal bore is rifled and everything. This college kid bought it from some surplus / salvage place and wants to turn it into a mini howitzer kinda looking thing. Originally it was about 40" long as we cut it down to fit in my lathe. It took both of us to lift the thing and get it setup in my power hack.

            I must say thank god for my power hacksaw. It earned its keep the other day cutting that stuff.

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            • #7
              Originally posted by becksmachine View Post
              Well try it with a steady rest and see if that changes anything.

              Dave
              Unfortunately I dont have one.. ughhhhhh

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              • #8
                Originally posted by Toolguy View Post
                Yes it will flex due to the length. I would reduce depth of cut (maybe .010 cut) and increase or decrease feedrate. The RPM may need fine tuning up or down. With chatter I would try slower first, but there may be a sweet spot higher in there somewhere. This is the kind of situation where infinitely adjustable spindle speeds can save the day. For example, with a VFD you can fine tune the cut while it's running and get the harmonics tuned. If you only have a few gear choices, it's harder to do.
                Okay ill give adjusting the speed and feed a try.

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                • #9
                  If you were trying to turn with HSS at 300 rpm, that's crazy fast and you would expect to burn off the tool's nose. At 4" that's 314 sfm. HSS should be under 150. Also if the barrel was heat treated you're likely to run into varying degrees of hardness, which will change your cutting conditions as you move down the barrel. Like others said, try turning at lower sfm, say 100, even with carbide. Or try negative rake.
                  Last edited by Forestgnome; 02-08-2015, 11:51 AM.

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                  • #10
                    Originally posted by Doggie View Post
                    Chatter problem on a lathe??? Yep, been there, done that before. Try this
                    As small as possible nose radius on the tool bit, while using positive top rake.
                    Slow down the speed, increase the feed, while in keeping a smooth cut, use light cuts.
                    Now this may sound crazy, but it works, so don't knock it till after you have tried it. Turn your tool bit upside down and run the machine in reverse. The tool bit still needs to be on center, use carbide.
                    Lay a 2 X 4 board on top of the workpiece (holding it by hand will do) while you are doing a cut.
                    These are my most major tricks that has really helped me out with this problem.
                    The board (2 X 4) trick and the upside down, in reverse trick seems to do the most good.

                    Your friend, Doggie
                    I havent tried it in reverse... ill see if that helps.

                    Comment


                    • #11
                      You can't use a steady rest when cutting a taper full length of a piece. You also can't use a follower rest cutting a taper on a piece. So, both of those aids are out of the question.

                      I have not tried using a cutter upside down, many have and say it works. I have used a 2x4 to apply pressure against the work to control chatter and it works. As mentioned by others you have to experiment with rpm, feed and depth of cut to find the sweet spot. That sweet spot may change as it travels the length of the work. The 2x4 is the best control element. You may also want to try using different grades of carbide or types of carbide. You may even have to go to boron.

                      Do you know a company with an alloy gun to analyze what the metal is? If so give them a visit with the barrel.
                      It's only ink and paper

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                      • #12
                        The best inserts I have ever used for killing chatter in a long ratio turning application are the Valenite TNMM431-ER PN1. They have a radically upsharp chipbreaker that lowers cutting forces dramatically and I have turned shafts (hollow rolls, actually) over 4 feet long without a follower rest.

                        Here's all I could find that look similar, but you would want the smaller "1" radius on the tip. These are a "2", which is 1/32" rather than 1/32".

                        http://www.ebay.com/itm/5-VALENITE-C...item418ec58de4

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                        • #13
                          Chatter is related to the force on the tool, and the looseness of the setup, including flex of the work.

                          Reducing the flex of the machine by pulling the tool as close to the toolpost as possible, not extending the compound, etc will help.

                          Changing speed, changing depth of cut, reducing tool nose radius, all can reduce force and thus flex and chatter.

                          For this case, with a hollow part, plugging the ends and filling the part with loose sand may do a lot to cut its vibrations. I assume you are doing this between centers of some sort.....

                          You can use a steady rest, doing the taper in sections, moving the steady, and blending any slight irregularities with polishing paper.

                          Long term, changing from single phase to 3 phase will do a LOT towards reducing chatter, AND making all the remedies that people recommend actually work. No help right now, of course.

                          Originally posted by Carld View Post
                          You can't use a steady rest when cutting a taper full length of a piece. You also can't use a follower rest cutting a taper on a piece. So, both of those aids are out of the question.
                          Not in every case it isn't out of the question......

                          Not that it may do this problem any good at this point...., but any taper you can cut in one go you can obviously do with a follower on the original surface, running thin to thick end. After that, it's tapered, and you would be correct.

                          Any taper you can cut IN GENERAL you can often cut with a follower because you cut in multiple passes... You can set it to the OD, and cut in the "thinning" direction, so you start out with the follower on the original OD, and run off it. That may get you what you need, because the follower may still be effective for the middle of the piece, where the problem usually is.
                          Last edited by J Tiers; 02-08-2015, 12:38 PM.
                          CNC machines only go through the motions.

                          Ideas expressed may be mine, or from anyone else in the universe.
                          Not responsible for clerical errors. Or those made by lay people either.
                          Number formats and units may be chosen at random depending on what day it is.
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                          Generalizations are understood to be "often" true, but not true in every case.

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                          • #14
                            Been trying several suggestions and am making a lot of progress. I slowed down the RPM so im no longer burning up my HSS and have bumped up the feed rate. This cleared 75% Kinda funny I never thought about the SFM issue because I rarely turn anything this large of a DIA. I guess I need to read up on speeds and feeds.

                            Im still getting occasional chatter but I think I have another issue with my center setup. Gonna check into this today or tomorrow.
                            Last edited by Axkiker; 02-08-2015, 01:19 PM.

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                            • #15
                              Originally posted by Axkiker View Post
                              Been trying several suggestions and am making a lot of progress. I slowed down the RPM so im no longer burning up my HSS and have bumped up the feed rate. This cleared 75% Kinda funny I never thought about the SFM issue because I rarely turn anything this large of a DIA. I guess I need to read up on speeds and feeds.

                              Im still getting occasional chatter but I think I have another issue with my center setup. Gonna check into this today or tomorrow.
                              Yeah, that sfm thing kind of sneaks up on you as you increase diameter. Seems like it's turning too slow if you're used to adjusting by the seat of your pants. It's interesting to watch the old movies of machinists turning really large diameters of several feet using HSS tooling.

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