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Help centering large stock in lathe

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  • Help centering large stock in lathe

    I have a 10" long 3" diameter steel round that I need to machine. I center smaller work in the 4 jaw so know all about that. This is the first heavier chunk I've dealt with, and it has a ton of overhang. I blued the end, and center drilled it on the drill press hoping it would be close enough for the tailstcok dead center, but after getting the stock centered pretty well at the chuck, it was way out at the tail stock.

    I'm guessing I need a steady rest. Then dial in at the chuck, move toward the staedy and dial in, repeat until satisfied, then center drill with tailstock, remove steady and machine away?

    Any way I can fudge this procedure for this project.. a rest would set me back $200.

    Thanks,

    Mike

  • #2
    If you bothered to mark the end with a good center finder, and drill a center hole at the correct location, then use the tailstock to support the tailstock end *while* you clamp it in the chuck, then it has to stay aligned.

    also, if you ever planed to use that center drilled hole, the workpeice has to be aligned to the hole, Not the OD of the stock.
    After some turning, the OD of the stock will align to the hole.
    (how accurately you drilled that hole determins how much stock you have to waste before getting the work aligned)
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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    • #3
      Right. It was off more than I cared to 'correct'. And the tailstock is centered for those wondering.

      One end of the stock isn't super square, so drilling it on the drill press was a pretty hoakey affair.

      Is the standard way of doing this to centerdrill, then mount in the lathe, or use a steady and use the tailstock. Seems like it would be tricky to get the hole as centered as you could with the tailstock.


      st

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      • #4
        You should have center drilled the work piece in the lathe after you centered it in the chuck.

        The rule of thumb I was taught is 3:1 length to dia for overhang. In other words your 3" dia workpiece could extend up to 9" out of the chuck before needing additional support. You should be well within this as you probably have at least 2" inside the jaws, unless you are using a very small 4 jaw.

        Flip the workpiece end for end in the chuck and center it, then drill your center hole. Keep in mind your stock will probably be slightly out of round. If this is the case use the four point method. Indicate it so that jaws 2 and 4 are the same and jaws 1 and 3 are the same. Note that there may be differences between these two sets of jaws.
        Last edited by Dr Stan; 04-13-2010, 06:12 PM.

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        • #5
          Yes, follow Dr Stan and you should have no trouble. Peter
          The difficult done right away. the impossible takes a little time.

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          • #6
            It's not JUST about diameter. You also have to consider machine (spindle, headstock, etc.). If that's not a 16" Monarch (ok, overkill, but to make a point), then I think he will find turning a 9" overhang is going to be an experience.

            I might set the center in the drill press or mill, using layout to find center on both ends (square cut or not, but better square as possible), and turn between centers.

            Or, I've done somewhat similar by mounting in a chuck, making sure far end has minimal runout, and drill center at very low speed. Now the center is pretty much on axis and unlikely to walk it out of the chuck, then proceed. Obviously, as Stan said, you can flip as needed for both ends.

            Details depend on what I wanted to end up with (regarding material removal, important surfaces, tolerances, etc), but even on my big 17x60 lathe, for anything other than trivial/light/slow work, I don't think I would turn a 3 x 10 without support.
            Last edited by BadDog; 04-13-2010, 05:02 PM.
            Russ
            Master Floor Sweeper

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            • #7
              Hang on a moment - the 3:1 rule is fine, but what about reality? Most HSM's haven't got a lathe (or chuck) with a 76mm (3") bore, so all of that stock is being held by the chuck jaws. The jaws on my jawed chucks range from around 1 to just under 2" high, so having a piece of material 3" in diameter and 10" long held by say 1 1/2" of chuck jaw doesn't sound all that good to me.
              This 'rule' is a guideline for how much material can protrude from a chuck and not experience significant deflection. I've had a stub of diameter 30mm material held in a chuck (10mm in the chuck and 50mm protruding - certainly meets the 3:1 rule) that came loose while turning due to cutting forces because the chuck was not as tight as it could have been. It totally wrecked the carbide insert tool I was using at the time.
              The safer way of doing things is with a centre or a steady. A steady doesn't have to be a flash thing that came with your lathe. For a one off use (hard)wood can be used to provide support. The important thing is that it is supporting the rotating material at the right centre height.

              Michael

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              • #8
                True-ing up

                Originally posted by softtail
                I have a 10" long 3" diameter steel round that I need to machine. I center smaller work in the 4 jaw so know all about that. This is the first heavier chunk I've dealt with, and it has a ton of overhang. I blued the end, and center drilled it on the drill press hoping it would be close enough for the tailstcok dead center, but after getting the stock centered pretty well at the chuck, it was way out at the tail stock.

                I'm guessing I need a steady rest. Then dial in at the chuck, move toward the staedy and dial in, repeat until satisfied, then center drill with tailstock, remove steady and machine away?

                Any way I can fudge this procedure for this project.. a rest would set me back $200.

                Thanks,

                Mike
                Mike.

                There is no need to get unduly concerned.

                Put the job in the chuck - preferably 4-jaw - and set it up to be concentric/"true" first at the chuck and then use a bronze or "soft" (cast iron will do) hammer and tap it into line/concentricity at the tail-stock end.

                Note: "true up" by moving the chuck spindle by hand - not under power - at this stage - and use a good dial indicator.

                Re-check both ends and when OK, finally tighten - but don't over-tighten - the chuck jaws.

                Set your top slide around to the right (clock-wise) by 60 degrees.

                Put a small boring bar in your tool post, set it to centre height, and then bore the previously "out" centre-drilled hole "out" so that you have a "true-ed up" 60 degree tapered hole. If you need to drill it any to complete the 60 degree boring, get as far as you can and use a drill that will follow the "trued" section and you will be OK.

                Keep the boring bar - which will be small - as rigid and as sharp as you can - and take it slow and easy.

                Do the boring of the 60 degree hole at slow speed, take it carefully, clamp your saddle and cross-slide - use a good cutting/tapping oil - and take your time.

                Job done.

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                • #9
                  According to what I have read the way to get the centre hole accurately positioned is to dot punch both ends as near as possible then mount between centres so you can rotate the work by hand and detect any off centre error. If there are errors you can move the dot punch by tapping sideways with the punch and retry. Once you have a dot punch in the true centre then take it to the drill press and drill the end(s).

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                  • #10
                    Thanks all.

                    This is on a Boxford VSL 10" machine, so fairly large workpiece. No way in hell I'm not at least using a dead cener at the tailstock.. maybe a steady if things don't pan out.

                    I realize that once I get cutting, things will true up, but it was out quite far.. too far for me to feel comfortable putting it under power.

                    I've blued and center punched the other end.. I'll try to to a better job center drilling it and use some of the tips here.

                    Fun stuff!

                    Thanks,

                    Mike

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                    • #11
                      Alternative

                      Allan, if you have a mill that has enough "head room" just use a 3-jaw chuck clamped to the mill table, put the job in it, centre the job under the mill spindle by using a good dial indicator and the mill "X" and "Y" slides.

                      Clamp the slides, recheck, if OK remove the indicator and put a centre drill in the collets (for concentricity and stiffness) - although a drill chuck might be OK too - and then drill the centre hole.

                      This pic shows the general principles except that I've used a drill chuck:


                      I can set it up pretty accurately by eye using a "wiggler":


                      If you have a co-axial indicator, that will work fine as well:


                      I've used a bent nail or bit of welding rod etc. held onto the spindle with "Blue-Tack" (like plasticine) before and it worked well enough.

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                      • #12
                        Some questions not being asked:
                        How big is your chuck?
                        What's the spindel bore?

                        It's no problem (at least with a 160mm chuck) to clamp the part and center bore it in the lathe unsupported (if you don't rev up like mad).
                        But it is the right procedure to first center drill in the drill press, move the part to the lathe. Clamp the part in the chuck, but don't push it to the buttom, just clamp half slightly. Then, with the tailstock push the part a bit further left, tighten a bit more, push a bit more and the tighten the chuck finally. Readjust tailstock pressure and off you go. Takes longer reading than doing. :-)


                        Nick
                        Last edited by MuellerNick; 04-13-2010, 06:24 PM.

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                        • #13
                          He's using a 10" Boxford, so the chuck is probably only a 6". However, as long as he uses the 4 jaw and not the 3 jaw he should be in good shape. The holding power of the 4 jaw when used properly is considerably higher than that of the 3 jaw.

                          Indicating it near the chuck and again at the free end is an excellent suggestion. Just keep in mind you'll have to repeat the procedure several times to get the stock trued up.

                          Once you have a good center hole you can add the live or dead center to help support the work. Center drilling does not produce much in the way of radial forces, so you're not going to force it off center as long as the chuck is properly tightened.

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                          • #14
                            Indicating it near the chuck and again at the free end is an excellent suggestion. Just keep in mind you'll have to repeat the procedure several times to get the stock trued up.
                            If he centers in the lathe, there is no need to initially center at the chuck side. Just get the tailstock side true, center drill and after that (supported by the tailstock) care for the chuck side.


                            Nick

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                            • #15
                              6" 4 jaw. Through hole not even close enough to matter. Will report back.

                              st

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