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  • Broken Lead Nut?

    This is the lead nut from the cross slide on my Chekko that I'm trying to resurrect.

    After cleaning off all the gunk from the last 100 yrs I thought it was trashed, but looking closely I can see it was never a complete nut. The mill marks are still visible on the threads on the horizontal face. The vertical face looks broken though.

    Is this broken?? Was there ever more than 270' of thread?

    Click image for larger version  Name:	P1010269.jpg Views:	1 Size:	188.8 KB ID:	1969118



    The thread is not square. It's ACME or trapezoid. 6TPI, with outer dim 0.63" (5/8"). Is this some sort of standard?

    There's about 0.5mm play when I push / pull the lead screw in the thread. The ends of the thread have less play.

    The nut is cast. I was expecting bronze.

    Do I need to make a replacement?

    Your thoughts are much appreciated as always.

    Last edited by Jonesy; 11-07-2021, 06:32 PM.

  • #2
    I'd say it is broken, those marks look like slitting saw marks not mill.

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    • #3
      It looks like it was slit with a bolt thru the missing piece for adjustment. Probably too much tightening produced a broken ear. Needs a replacement if possible. Are parts available? If not, time to become a machinist
      Sarge41

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      • #4
        I agree with sarge - it looks like there was a piece that allowed tightening via a bolt through the hole shown, although the one's I've seen have adjusted (tightened) axially, not radially, presumably to take up back-lash.

        You may want to do a search for "Evannut" on this forum. Find the posts that describe the how-to.
        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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        • #5
          So from the beginning it was slit? And then tightened down onto the lead screw? Being trapezoid, I assume that would take up any play, where-as on my RandA the threads are square.

          If I have the gearing on my little RandA to do 6TPI, and I make a cutting tool, what material should I make it from?

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          • #6
            Bronze is the normal favourite but brass will do. The original CI was not so bad and often used as it is a good wearing match with a steel screw. There is a school of thought that the nut should be a sacrificial wearing element so as not to wear the screw which would introduce dimensional errors if worn unevenly whereas a worn nut only introduces backlash. Hence brass may be the best option. (make 2 while you are at it)
            If using the RandA don't try to screwcut 6tpi under power. Drive the operation by turning the leadscrew by hand not the mandrel.

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            • #7
              I made new lead nuts for my RandA years ago from brass. They didn't hold up too well and the current lead nut is a very botched up affair, made from a block of plastic / nylon material that was part of a 2 foot diameter pulley that held the wire rope on a big crane. It works very well (for my needs). Tiny amount of back lash on the square thread. The lead screw on the RandA is worn massively in the most used areas, so the tiny back lash is as good as I could hope.

              I wonder if this material will suffice for the cross slide on the Chekko.

              I looked up EvanNut but many of the posts are gone. Is it the principle of single point threading a plastic nut, or melting the plastic onto the leadscrew in 2 halves, or casting an epoxy nut around the leadscrew?

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              • #8
                Found it here...

                https://www.denfordata.com/bb/viewtopic.php?f=50&t=3727

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                • #9
                  You were not far off, however. A number of lathes used a literal "half of a nut" on the leadscrew, not a pair of them. The Rivett 608 is one like that. One bronze half-of-a-nut is used to engage the leadscrew.
                  CNC machines only go through the motions

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                  • #10
                    Click image for larger version

Name:	_IGP2132.jpg
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ID:	1969255 The nut certainly looks to me to be broken as already mentioned by being clamped up and the metal being too brittle to tighten down. It might be easier to get hold of a similar pitch ACME length of steel and make a replica of the old screw. Then a new nut could be made to fit where the broken one was.
                    I made a new 6tpi ACME nut for the museum's Smart & Brown model A. It was gunmetal, and made as an inlay to fit using soft solder into the original sliding block. This design was only using about 40% of the nut and a smooth backing support when it was engaged. I sold the spare part and recovered the price of the gunmetal, as the new one will have an indefinite life due to the fitting of telescopic leadscrew protectors.

                    Last edited by old mart; 11-08-2021, 11:54 AM.

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                    • #11
                      Just to note- if there's wear in the middle of the lead screw, the new nut will be loose over the center portion if it's not too tight over the ends of the lead screw. At the same time, if it's snug over the center part, it will be too tight at the ends.

                      When I was experimenting with plastic nuts, I found the best fit to be cast epoxy. I played extensively with tapping the plastic, using a tap made from the lead screw material itself. I wasn't happy with the result, probably because I was using ldpe and hdpe. Acetal or delrin would have been a better choice, but at the time I didn't have any. The epoxy nuts I'm using in one of the machines seem to be holding up, but I did have to run the tap through several times to loosen the fit, which I didn't want to do- but the fit was just too tight and that was my only resort.

                      By the way, a rolled thread lead screw is probably the only type you should consider if you're going to cast or heat mold with. With a turned thread you run a higher risk of not being able to remove the cast or molded nut.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        Thanks for all the advise.

                        To start with I'm going to make a cutting tool and try to cut the thread on my RandA (I think I've got the gears). I've cut 4 pieces of the big pulley and brought them into work to square them up on the mill here. Don't know what material it is but it's tough. If no joy I'll move onto the other options.

                        When I did this for the square thread on the RandA I did make the end of the lead screw into a tap to finish the thread.

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                        • #13
                          My 'plastic' blocks are squared and prepared. Slightly oversize so when I get the threaded hole in the wrong place I can mill them to fit.

                          And, my little RandA does do 6 TPI!!. The lead screw is 8 TPI and I found a 60 which I put on the head spindle and a 45 which I put on the lead screw. A couple of intermediate gears joined them up and gave me a left hand 6 TPI thread.

                          For the cutting tool I've found an old wood chisel which will lend itseld to a bit of grinding. It's not exactly tool steel but it's hard enough to hold an edge cutting plastic... I hope.

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                          • #14
                            The new lead nut is made and fitted.

                            I cut 4 rough blocks from this large wire rope pulley (the only way I can cut this is with a coarse blade on a jigsaw going very slowly) and squared them up on the proper mill at work...

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                            Set up the little RandA for 6 TPI and engaged the backgears...

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                            My cutting tool made from a chisel was a disaster so I made a far superior tool from a fairly hard M8 machine screw, a broken M4 tap, and a square of allu...

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                            I rough marked the hole centre, drilled the inner diameter (12mm) and cut my thread 6TPI (my first attempt, marked with the X, was with the chisel tool, and I cut it right hand instead of left hand (DOH!!!) and the chisel flexed real bad, total disaster)....

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                            • #15
                              Then I cut some edges into the first couple of threads on the lead screw...



                              And tapped the thread to final size...



                              I had drilled the hole (deliberately) too far over so milled to final size on my poor mans mill (my pillar drill)...



                              Final fit (upside down) was very nice...



                              I've not fitted the saddle back on yet as I've now got the apron in pieces, but the Chekko is posisioned and bolted down to the concrete. The wooden floor has been cut away and metal riser blocks under the mounting points bring it up to finished floor level...





                              Previos to this repair I had to make a new bearing nut on the headstock as the one nearest the chuck was snapped in half. This really maxed out the RandA...



                              I know this is all very basic for you guys but for me this is a whole new experience and I am having so much fun getting the old Chekko into a working condition. It's far from a restoration. More a resurrection!
                              Last edited by Jonesy; 11-16-2021, 07:53 PM.

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