The crossfeed nut on the lathe was quite worn, as I found out the first time I tried to face a bar for center drilling.  I had the tool set a little low and when it got the nub that was left the cross-slide jumped quite noticeably.  After I used a file to remove the nub and finished the drilling, I disassembled the cross-slide and inspected the nut.  I could feel the slop just by using my fingers to try and slide the nut on the feedscrew.
    While a new nut is obtainable, for now I decided to put off that expense and modify the existing one.  After some thought I decided to cut off one end of the nut, and use this for adjusting the backlash.  By rotating this as if it was a locknut I could take up the slack.  Figure 1 below shows a drawing of the existing nut, and Figure 2 shows the two pieces after the end was cutoff.

Note:  In the following drawings the threads matching the crossfeed screw and the hole and threads for the mounting screw in the stub are omitted for clarity (and to save several hours of CADing) ;-)

Figure 1Figure 2

Figure 3

    I used my imported metal cutting bandsaw to do the majority of the cutting.  While it does not cut perfectly square, it is better than I can do by hand.  The nut was too small to clamp directly in the saw vise, so I turned a simple mandrel to support it.  To do this and still have the nut available to test the fit of the mandrel, I locked the cross-slide in position by tightening the gib screws and used the topslide to adjust the depth of the cut.  I turned a length long enough for the nut to slide on , and for a threaded portion for a regular nut to hold the part during cutting.  This worked, but did have a drawback.  After the nut had been cut part way the end of the mandrel was also cut off.  At this point I stopped the saw and finished the cut with a hacksaw.

    The two pieces were cut fairly square, but not perfectly square.  I had not expected them to be given that a saw is not a truly precision tool.  The next step should be to use a mill to true up the parts.  Unfortunately I do not presently have a mill, so a compromise solution was used.  They also could have been squared up on the lathe itself, but I could not figure out how to mount the smaller piece and still insure that the screw hole would be perpendicular the machined end.  This part was not all that long, and I do not plan for this to be a permanent fix anyway.  A disadvantage to doing this is that the mounting stub is no longer centered on the nut when you are finished.  This could lead to accelerated wear.

    Assuming that the nut was cast to a fair tolerance originally, I reversed the smaller part and mated its’ original cast surface to the cast surface on the opposite end of the larger piece.  While still not perfectly square, these surfaces were much closer than the cut surfaces.  A little time with a file brought them "close enough" for my purpose.

    Both parts were screwed onto the feedscrew and the smaller piece was rotated (lock nut fashion) until the backlash was gone.  I made this adjustment in the area of the screw that was likely to be the most worn.  I then secured the two parts in the proper orientation with a drop of thick superglue at the top and bottom edges.  The thick glue will not wick into the joint as a thin one would.  This prevented me from accidentally making the nut a permanent part of the screw!  I then ran the assembly back and forth the entire length of the screw to insure that I was not inadvertently creating a bind in the nut travel.  Everything worked smoothly, indicating that the feedscrew, at least, was not all that worn, just the nut.  Figure 3 shows the completed assembly, to this point.

    To hold the parts more permanently in position I then removed the nut from the screw and clamped it with the small cut piece up in my drill press.  I drilled a 1/8 inch hole through the small piece and part way into the larger one.  A piece of 1/8 brass rod was then inserted into the hole and held with a medium strength, thread locking compound.  Before inserting the rod I crimped the rod with a pair of pliers to create a few ridges on the rod.  This made it fit more snugly in the hole.  I left a little of the rod projecting out of the hole so that I can grasp it to remove it later, if necessary.  The last step before reassembly is to file off the corner of the smaller piece that extends above the flat surface of the larger piece.  The corrected nut will now bolt back into position without interference.

    The cross-slide was then reassembled with the smaller cutoff piece facing the rear of the lathe.  This is important!  When I tightened the two parts this forced the large portion of the nut towards the front or operators side of the lathe.  This is the same direction that the cutting force is applied to the nut during normal outside machining.  Thus the larger stronger piece of the assembly is bearing the load and it is already pressed against these same thread surfaces by the smaller locknut.  This arrangement places no strain on the smaller nut.  If the parts were reversed the small section would be taking all the strain.  Not a good setup.  With this modification one consideration must be made, inside turning or facing from the center out puts the strain on the small piece as the forces on the nut are reversed.  Therefore lighter cuts need to be taken so as not to strip these threads.

    The stub on the top of the nut in the drawings is inserted into a hole in the top of the cross-slide and a screw is inserted into the stub that serves to hold the nut in place.  The stub and its’ mating hole serve to both locate the nut properly and to transmit the force between them.  This hole was either always a little large or has worn so, and any movement between them is only being stopped by the frictional force between the screw head and the cross-slide.  To take up this error and further reduce any play I slipped a short section of thin walled brass tube over the stub.  This eliminated the possible movement between the parts.  This tubing is available at most hobby stores that cater to the R/C model airplane or car crowds.

    When I do get a replacement nut I am considering installing the present large piece in a similar manner on the new nut.  With a slot replacing the (now) existing pin hole and a screw replacing the pin, this would become a permanent backlash adjuster.

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