The nut as it arrived.South Bend Cross Slide Screw Repair
5 May 2001
About four weeks ago one of my freshman students asked me come over and look at the lathe that he was using. Our lathes are mostly ancient 13" South Bend models that have had years of hard use. Most of the machines were in place when my co worker was a student in our school in the late sixties. The problem he reported was that he was turning the cross slide crank but that the cross slide was not moving. I grabbed the tool post and gave it a good shake and much to our amazement it had about three inches of free play! NOT GOOD! I disassembled the cross slide and found that the screw and nut were so completely worn that they were simply by passing each other.
Although the politicians in CT are constantly bragging about the funding that education is getting it has been our experience that when we have asked for repair funds they are never available. I can't afford to have a lathe down and set about thinking of a solution. One thing I didn't do was to call South Bend and ask after the price of a new screw and nut. My heart couldn't stand the shock I am sure. A couple of years back we checked on the price of a cross slide screw and hand crank unit for one of our 9" South Bends. It was (as my work partner recalls) $1200!!
Looking in the MSC catalog I found that they have three foot lengths of 5/8 - 8 LH Acme threaded rod for about $30.00 and nuts to match for $20. I called and ordered a length of the rod and three nuts. With their usual dispatch the items were on my back porch in about a day and a half. The rod was as expected but the nuts didn't look at all like the ones pictured in the catalog. Time for some more thinking. I came up with an idea and set about making it happen.
The nut as it arrived.
First step was to cut off the worn out threaded portion of the cross slide shaft. This lathe has a taper attachment so I wanted to keep the out board end of the shaft rather than recreate it from scratch. I then cut the 3' threaded rod into three equal lengths and faced one piece to the proper length to duplicate the old treaded section. I then set up a horizontal mill and machined the required keyslot in the end of the threaded shaft. While I was at it I also put keyslots into the other two pieces of threaded shaft. After all if this worked I could fix two more lathes with worn cross slides. Next step was to drill and ream a 3/16 hole in the end of the threaded rod and neck down the end of the out board end of the shaft to match. I then pressed the two sections together and had a restored cross slide shaft ready to go.
The old thread section and new
Next was adapting the new nut to the cross slide. I designed a part to fit the existing hole in the cross slide. I had a goal of minimum modification to the machine so I was trying to avoid anything I didn't have to do. The new nut had a 15/16 - 16 thread section. They say there is a first time for everything and I am sure that in this is the first time I have ever seen that size thread. Had to look it up in Machinery's Handbook to see if it was a standard size or some odd ball thing. It is standard and I checked MSC for a tap. Wow, $27 for an import or $57 for USA made. Hmmm, not in keeping with my budget. Next step was to post pleas for help in r.c.m. and on several mail lists that I subscribe to and first thing you know I had not one but two taps sent by helpful internet friends. I do love my computer and the internet!
The nut adapter (center)
When I went to assemble the nut and adapter however I didn't have clearance between the bottom of the nut and the saddle so I had to turn down the larger end of the nut (to one inch) and further modify the adapter.
I reassembled everything and found that I seem to be missing one of the ball thrust bearings. As soon as I get a replacement for that item I think I will be down to about .010 of play in the cross slide. Not to shabby for a $30.00 project. How long it will last compared to a factory original replacement remains to be seen of course but as long as it serves our purpose it will be fine I am sure.
15 May 2001
The ball thrust bearings finally arrived yesterday and today I installed them. The final step in this project! Final end play in .015 and backlash is less than .040 on the dial. The next lathe in line has one and one quarter turns of backlash. I have already started disassembling that one for the same treatment. Not too bad for a $35.00 repair.