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Back lash what is acceptable ?

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  • #16
    Slight errors in the pitch of a lathe cross slide leadscrew are not usually much bother, as people usually measure the work just before getting to size and work from that point.
    The 118 scale markings per turn using the 3mm pitch leadscrew is actually 0.118110236", the difference is the error per inch or 0.0011" in ten inches.


    • #17
      Originally posted by JoeLee View Post
      So if you have a worn screw and nut assy. does that effect the dial reading ??? Example, if you move in .005 according to the dial did the CS actually move .005. I've never really checked it that way because I have DRO's on my lathe. I'm guessing it doesn't because the pitch doesn't change.

      The pitch DOES change.

      It changes at the points f transition from relatively un-worn, to worn. The amount of the change depends on the amount of wear, but you can guess at it as being a significant portion of the backlash amount.

      In the evenly worn areas, the pitch is what it is supposed to be, just shifted over a bit. In the un-worn areas, the pitch obviously did not change.

      It is just when you move over the transition area that a change occurs. It is not so much a pitch change, as it is a gradual shift from the unworn area over to the worn area. The carriage will not move the expected amount when the nut passes over the transition.
      CNC machines only go through the motions


      • #18
        Yes, the pitch does change, between the unworn areas of the thread, and the worn ones, but as you are most likely, on a lathe at least, using the feedscrew to apply 'cut', so maybe moving 50 or 100 thou at a time, the effects of pitch change over such a short length won't be significant, in fact to most of us, won't even be measurable. It might over a longer length, if, for some reason, you are using the dial to measure, say 2" of movement on the cross slide. It can be an issue with leadscrews when you are cutting a long thread, and some of the travel uses the well worn area near the headstock, and some of it uses the less worn area near the tailstock.
        Its not something that keeps me awake at nights worrying about it.
        'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger


        • #19
          Slk001 asked what I did to remove the .040 of backlash
          I noticed the bolt that holds the nut to the cross slide was moving when I turned the dial I removed the bolt and checked the hole diameter and compared it to the bolt then made a bushing to compensate the bolt diameter from the hole and tightened it
          But I have a question why do they use a brass nut with a steel screw would a steel nut be better then a brass one or am I out in left field whit that thought ?
          My thinking is the brass would wear quicker then steel


          • #20
            Originally posted by RB140 View Post
            But I have a question why do they use a brass nut with a steel screw would a steel nut be better then a brass one or am I out in left field whit that thought ?
            My thinking is the brass would wear quicker then steel
            It's a bronze nut (at least the original was). Steel on steel will gall, while bronze on steel will wear quite well.


            • #21
              I was just going to make that distinction between brass and bronze. Brass is probably more commonly found, but bronze is better in terms of wear.

              All of these things are implicated in the loss of absolute accuracy in lead screw systems. If the nut can move relative to the table it's moving- as it appears to have done in this case- then there's a complex source of slop. The nut can be rocking on the lead screw as well as being axially loose in its mounting hole in the table. That will be compounded even further when you are operating on different portions of the lead screw. The easiest play item to deal with is probably the bearings retaining the lead screw. If using a pair of thrust bearings, you can take that out completely if you add in some pre-load. With plain bearings, I find that I can get down to about 2 thou, which quickly increases and stabilizes to around 10 to 12 thou. That's less than a quarter turn of the dial, and is quickly compensated for automatically by the operator, as JT suggested. In the beginning I worried about that, but now I don't. What I worry about is whether the table- or cross slide or compound- moves by the dialed in amount when I'm trying to hit a diameter. That should remain pretty close regardless of where on the lead screw I'm working.

              The rest of the play in the lead screw system obviously needs to be taken up, but that's fine too unless it's spongy- or simply too much, as RB140 experienced. In a case where the cutting action is taking control of the play out of your hands, any play becomes a potential problem. 40 thou play can become a real game changer, and even an acceptable amount like 15 thou can ruin your day, as well as the workpiece and maybe the tooling.

              If this is a mill, you'll get quite an education when climb milling. On my mill, I've had the experience of the cutting action pulling the play out in one direction- while I turn the dial to allow the table to move, rather than cause the table to move. I tend to keep the gibs a little snug so I don't have to deal with this. Of course I'd rather have zero play- and even two thou of play would often be too much.

              Back to the lathe- I find that my cross slide likes to sit more towards me with the play taken out, so for most lathe operations it's all good. When boring it's a different story, as the slide still wants to move towards me to sit in that no-play position. I could make pass after pass, and still keep cutting as the slide migrates. Keeping the gibs snug is the 'cure', but I'd rather be able to take advantage of an absolute positioning system. On the road to CNC-
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


              • #22
                Originally posted by The Metal Butcher View Post
                Dad's Rockwell had about 20 thou. We just made a new screw and nut today. We'll see how it is when it's all assembled.
                That is nearly new ! ! ! ! ! ! ! !



                • #23
                  Originally posted by Doozer View Post

                  That is nearly new ! ! ! ! ! ! ! !

                  Agreed. May have been as much as 40, I don't remember on those tiny dials. He said it is now 14 hard stop to hard stop with the cross-slide locked. I guess it's a bit better. Whatever the man wants...
                  21" Royersford Excelsior CamelBack Drillpress Restoration
                  1943 Sidney 16x54 Lathe Restoration


                  • #24
                    Thank you all for the great information and story's
                    I think I am going to see if I can get a new cross slide screw and nut from the manufacture then try to recut the screw a few thou smaller to take out any warn spots and single point a new bronze nut to fit
                    If my thinking is correct a smaller diameter screw will not affect the amount of travel as per the dial markings that way ill have a back up
                    I know trying to recut the screw is not an easy task to set up but I have done this once before and had good results


                    • #25
                      In the USA, there are lots of suppliers of ACME imperial threaded leadscrew blanks and a variety of nuts. Metric trapezoidal blanks and nuts should also be easy from places such as ebay.
                      Don't forget, the axial play also contributes to the endfloat, check the thrust bearings on the leadscrew.
                      Last edited by old mart; 06-06-2022, 03:08 PM.