Calibrating the Leadscrew

My ML7 arrived with no handwheel on the end of the leadscrew. Later models, and the Super 7, have a handwheel with a graduated collar. The lead screw is 8 tpi so the collar needs 125 divisions to give 1/1000 inch movements of the carriage.

L.C. Mason, in 'The Amateurs Lathe', describes making a leadscrew graduated collar in general terms and last week I got around to making one. I have an upcoming project that will need precise carriage movement so I decided to make a quick one and make another more elaborate one with a zero-able collar etc at a later date.

For this one, I grabbed an aluminum roller from a printer and cut off a chunk about 15mm long. I faced it in the 3 jaw. I then cut a peice of 2mm aluminum plate and drilled the 2 parts in three places. The ring was tapped M4 to take some bolts. I wanted to put countersunk bolts in but didn't have any to hand. I then held the ring in the 3 jaw again and trimmed the bit of plate round, and bored it to 11mm to match the end of the leadscrew.
Then, I mounted a bit of BMS in the chuck and turned the end down to 11mm. The end was dirlled and tapped M6, and using a cupped washer I could then hold the plate on the end with a bolt. This allowed me to face the other side of the ring till the ring was about 8mm thick, and to do a final skim over the outside to get a good surface.

I then mounted a 75 wheel on the spindle, a 30 meshing with that, and a 50 keyed to the 30. Now, indexing 2.5 times around every tooth on the 50 gives 125 divisions (Having already made an indexing dofus some time back). I wrapped a rope around the chuck about 5 times then fed it through a pulley before attaching a bit of railway line to it (about 5 kg). This puts constant pressure on the gear train, and prevents one indexing in the wrong direction.

I mounted a thread cutting tool sideways in the toolpost and set it carefully on center. I set the carraige depth stop for about 3mm of travel and started indexing the short ticks. After doing all of them I reset the depth stop about 2mm deeper and indexed around again doing every 5th tick. Finally, I reset the depth stop again and indexed around every 10th tooth for the major ticks. (Do this when you're not tired! you will be when you're done).

Finally, I took a cleanup pass across the surface to remove the sharp bits thrown up by the tool, and mounted it on the lathe. since the Nylock nut on the end of the leadscrew also sets the end float, you cannot tighten it to hold the collar! To solve this, I drilled the plate an the slotted sleeve on the leadscrew to take a 2mm steel pin (pin from a pop rivet). This prevents it from slipping without turning the leadscrew. I suggest a bit of locktight in the holes as mine is a bit loose and I can feel the play. I will disassemble it and Locktight it before using it in earnest.

Here are some pictures of it complete. Sadly, I could not have the digital camera handy while making it.

leadscrew/ls-01out.jpg

General view from right hand end of lathe. You can see the clusty looking 3 M4 bolts holding the plate to the ring. It's temporary....

Last edited February 20 2013 09:53:41.
leadscrew/ls-02in.jpg

View from the left showing how the existing sleeve protrudes inside the ring. when I make the fancy version, I will probably remake this sleeve entirely, and clean up the bearing arrangment at the same time as the end is fairly worn.

Last edited February 20 2013 09:53:42.
leadscrew/ls-03grad.jpg

Closeup of the graduations. You can see that going over the same lines several times at differing depths is visible with my setup. If you don't screw the indexer home in the tooth of the wheel exactly the same each time, it goes out just enough to see.

Last edited February 20 2013 09:53:42.
leadscrew/ls-04num.jpg

After numbering. The numbers are 1.5mm high and were put on mostly straight by using a bit of plate as a backing. I held the plate against the back surface of the disk (disk held in vice), and held the punch against the plate. The spacing was the real problem and I confess I just guessed and got on with it. (-:

I also applied some stud lock to the pin that locks the graduated disk to the spacer on the end of the leadscrew and appears to have fixed the little bit of play that was evident there.

This device proved it's worth even before being numbered as I used it to part off 4 plastic spacers 1.5mm thick. Very handy as I was using a rear parting toolpost so could not use the topslide as one might otherwise do.

Last edited February 20 2013 09:53:43.

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Last modified: February 20 2013 07:52:42.