A SIMPLE EXTERNALLY MOUNTED LEAD SCREW AND GRADUATED DIAL
FOR THE TAIG MICRO LATHE
One of my favorite features on a small lathe such as the Sherline as well as many other small modelmakers lathes is a hand operated lead screw to advance the carriage. These are always located at the tail stock end and allow you to advance the carriage very evenly during a long turning cut. The problem with these units, is that you are stuck with the slow lead screw advance even when you dont need or want to use it. A rack and pinion carriage allows you to move the carriage say, 5 inches to the right by turning the crank a few quick turns where the lead screw system would require dozens of turns. Could get monotonous really fast!
The Taig micro lathe sports a rack and pinion carriage crank. Great for moving that baby quickly. But not so good for those times when you need a super steady advance rate. Right there and then, I decided that I had to have a way of advancing the carriage with the smoothness of a lead screw but not be made a slave to it when I didnt need or want to use it. I would have to somehow make a half nut affair that one could disconnect when not needed. Not an impossible feat for the home shop, but one that I would rather not tackle if I didnt have to. I came up with the idea of an externally mounted lead screw that would simply bear against the side of the carriage, pushing it toward the head stock as the lead screw is turned. This way you could use it say, to take one last finishing cut, but when not being used, could be kept screwed all the way out of the way when not needed.
Once the idea was conceived, it was time to put it to the test with some sort of practical design. I wanted to also be able to advance the carriage at specific rates so the pitch of the screw became an important factor of the design. I chose a 20 pitch thread not only because it is conveniently available in the form of threaded rod but it also gave the perfect rate of advance per turn. The heart of this simple little unit is a threaded block through which the lead screw passes. The height at which the lead screw passes is 1-1/4" from its bottom surface. The dimensions of the block are not important so long as the longitudinal hole is approximately 1-1/4" from the lower edge and sufficiently long to provide plenty of longitudinal support to the screw. The block I made was made from 1" thick material and it happens to measure 1-3/4" in height and it is 2 1/2" long. I could have simply drilled and tapped the block to 1/4-20 I chose but that would have resulted in quite a lot of backlash in the system. Instead, I made a couple of threaded, hard brass bushings that I then inserted into a larger through bore that I drilled and reamed on the block. The result is a block that is only threaded at the extreme ends. One of the bushings is a hard press fit while the remaining one is a mild press fit. Now I can adjust the fit of threaded rod by rotating the "loose" bushing until I have taken up all the backlash. Too much and the fit will be too tight. Rotate it the opposite direction and you will introduce slop until the thread fits tight again. The lead screw itself is a very simple affair. One end of the rod is faced and tapered not quite to a sharp point. I left a flat spot of about 1/16" diameter. I inserted the lead screw into the threaded / bushed block about half way in and sat the block on top on the lathe mounting board so it is flush against the lathe base and parallel to the bed. The tapered end will be bearing against the cast surface of the right side of the carriage. Once you see where the point of the lead screw will contact the carriage, mark it with a marker. I crazy glued a small sheet brass square to that location to act as a bearing surface for the tip of the screw. This will protect the softer aluminum surface of the carriage as you use the screw. All I did next was to permanently glue the lead screw block on the mounting board surface with either crazy glue or epoxy. Now all I needed was a crank to operate the screw. I made that out of a slice of aluminum which I threaded to fit the outer end of the screw. I also drilled and threaded two 180 degree spaced holes for two small brass crank handles. After engraving it with fifty equally space marks (thats another story), I glued it in position after screwing it into the threaded rod end and toped the whole thing with a brass acorn nut just for looks. This type of assembly may seem a bit unorthodox but believe me when I say that it work!
The resulting setup turned out great! It operates very smooth and the resulting carriage advance is as even as it can be. This translates to super fine final surfaces on your turned pieces. You can also measure actual lengths of cut for certain jobs such as creating shoulders.
As a foot note, the actual direct carriage advance per full turn of the carriage happens to be .060" per turn. Not the most convenient number to have to use but quite usable if you have to. I will be engraving that dial with 60 marks so I can take direct carriage measurements during non lead screw advances.