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  • sources of play

    Someone asked a question some time back, why does a tool keep cutting, if you stop dialing in the crossfeed, and take extra passes. I just wanted to mention a peculiarity I ran into while turning something on the lathe. I was taking a finishing pass on some 5/8 diameter steel, when the phone rang. Two minutes later, I was back at the lathe, took the same finishing pass again, without dialing in any further. More metal came off. However, if I take the same finishing pass, then do it again right away, no more metal comes off, except atom-sized pieces (haha). So if I wait a minute or so, take the pass again, more metal comes off. This isn't a temperature related thing, as the workpiece was at room temp, before and after the passes- I waited for it to cool down before proceeding with any finishing cuts. And anyway, if the piece cooled, it would be further away from the tool, not closer. This has strictly to do with the passage of a minute or so between cuts. My suggestion, which I welcome any suggestions or comments on, is that the lube on the crosslide feedscrew, and the compound leadscrew, is squished thinner by constant pressure on the threads, as is the case when actually lathing, and recovers in thickness when the pressure is off, within a few tens of seconds. This thickness also squished out over that same period of time when there is again some rearward pressure on the tool. The recovery of thickness would push the tool in just a tad, lose the thickness, the tool comes backward a tad. This would then also be a factor with the way lube. If the compound is not at 90 deg. to the spindle, then it's dovetail ways lube will add to the problem. That's my theory. The difference I'm talking about here is about 1/2 thou. I try to maintain a steady rearward pressure by hand on the toolpost to take up various plays in leadscrew, nut, bearing, and gibs, also on the carriage's play on the ways, and it's rocking motion due to incomplete contact with the rear way, and the error introduced by the tightness of the carriage clamp. I do this as a habit, knowing that finishing cuts depend on all the play being taken up before the final measurements and passes are taken. If I apply forward pressure, I move the tool in about 20 thou, understandably, with all the various plays that have to be there, so the feeds can move. This must be done when cutting from the rear, of course. I don't even want to imagine the additional inaccuracies on lathes without ball or roller bearings and preload. I'm not posing a question here, just seeing if anyone can relate to this, and possibly add their own findings and knowledge to help us all become better machinists.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    darryl:
    Your practice of pushing the toolpost into the work is very unwise! What you are doing is a dangerous practice. Your hand could slip and then you could be in a world of hurt.

    If you do not do this, the play rearward will be taken up by cutting forces and what you describe will not happen. Your actions are causing the problem.

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    • #3
      Thrud, I think you misunderstood what I was saying about hand pressure taking up the play. I see as I reread my post that I used way too many words and it's somewhat confusing to me now, re-reading it. Where I said rearward, what I meant was away from the workpiece. What I do as habit is to pull the toolpost away from the work, when cutting in the regular fashion, and push it away from the work when cutting from the back, with the spindle rotating backwards. If my hand was to slip, it would come away from the spinning piece, and not towards it. The tool would probably dig in, and maybe gouge the work beyond tolerance, worst case it would rip it out of the chuck and throw it. We're talking about 20 thou or so total, so that's what the cutter would be gouging out if the play were to be suddenly taken up towards the workpiece. 20 thou is still not a ripper of a bite, in most cases. I don't use much pressure either, just enough to feel the play being removed, and only when approaching the finish size, when smaller cuts are the norm. I appreciate your concern over the safety of the practise, and I would advise anyone to refrain from any practise, unless and until they have thought it out, and are comfortable with it. Now here's the rub, how else can you manage the play to do close tolerance work?
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

      Comment


      • #4
        My old Cholchester must have a quarter inch play in the crossfeed. I mean pushing and pulling of the crossfeed. I still have no trouble holding .001 As you make continuos cuts, the play is taken out of the leadscrew until you pull the cutter back. When and if I need to get as close as I can, I back out on the tool and then come in for a touch off and then go for the final cut. This releives any pressure that is working on the tool.
        Michael

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        • #5
          There's something I'm going to try, back out the feed, then dial it in again, make the pass. I can see that dialing in again will squeeze the lube out of the leadscrew and nut to the same level as it would have done initially, and therefore the tool should not be removing any extra 1/2 thou or so. I'll have to try this with, and without a short time delay. Then I'll try not dialling out and back in, but instead, remove the play the way I normally do, but wait the minute or so before taking the pass again. I think then I'll get the answer I'm looking for.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

          Comment


          • #6
            Add a cross slide clamping lever, so you don't need to worry about the cross slide moving.

            You can replace one of the gib screws or (better) drill/tap for another locking screw.



            [This message has been edited by SGW (edited 03-18-2003).]
            ----------
            Try to make a living, not a killing. -- Utah Phillips
            Don't believe everything you know. -- Bumper sticker
            Everybody is ignorant, only on different subjects. -- Will Rogers
            There are lots of people who mistake their imagination for their memory. - Josh Billings
            Law of Logical Argument - Anything is possible if you don't know what you are talking about.
            Don't own anything you have to feed or paint. - Hood River Blackie

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            • #7
              SGW, great idea, locking lever for the crosslide, replaces center gib screw. Ditto for the compound. It's on my list of upgrades for the lathe.

              [This message has been edited by darryl (edited 03-19-2003).]
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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