7x16 Mini Lathe [page 11]
Tuesday, May 2 2017
Compound slide gib lock
I saw this youtube video about improving the rigidity of the compound slide: https://www.youtube.com/watch?v=vugfzye-B_o. I’ve been trying to eliminate chatter when cutting — OK, it’s a Mini Lathe… Ive been trying to minimize chatter — and I know that one source is the compound slide. I don’t want the gibs on the compound slide so tight that it is hard to move the slide. Alas, anything less than that leads to excess chatter. I’m hoping this modification will improve operation.
I didn’t have brass screws nor brass stock so decided to make the needed screws out of Aluminum. Another change from the video is that I wanted to be able to lock the compound slide without using a tool. I turned some 5/16" stock (almost 8mm) down to 4mm for a length of 8mm and used a die to create 4mm x 0.70 threads. And therein lies a story.
About 17 years ago I bought a very cheap metric tap and die set. I was restoring a vintage BMW at the time and wanted something to chase existing threads to remove paint, oxidation, etc. The carbon steel taps and dies worked well enough for that.
I tried using the 4mm x 0.70 die to form the thread in the screws I was making. The die couldn’t cut aluminum. Apparently this if the first time in 17 years I’ve tried. I now have a few reasonable quality/price HSS adjustable dies for some of the common metric threads. I used the 4 mm die in the tailstock die holder I made last week. It cut the aluminum like butter.
I drilled a 4mm hole in the heads of the screws and created the T sections by turning down some 3/16" aluminum rod to get an interference fit. Now I have my no-tool-needed compound slide locking screws.
I removed the compound slide from the lathe and took it mostly apart. I put the slide in my mill vice and used a center finder to locate the middle of the center hole. I locked the Y axis and then moved the X axis + and - 17.5 mm to drill and tap two additional 4 mm holes for my locking screws. Why 17.5mm? Because the holes seem to be 35 mm apart. 17.5mm will put the new holes half way between the exising holes.
I put the compound slide back on the lathe and adjusted the gibs for easy in and out movement with no observable play. It feels good. I then added my locking screws after first inserting a 3 mm ball bearing to sit between the screws and the gib. The locking screws were adjusted so a 1/4 turn of the screws by hand locks the slide.
So far I’m happy with this change. However, I haven’t made any test cuts, yet.