Third slide for threading on a watchmaker lathe

[threesixesinarow]

This is my version of K. N. Harris’ “screwcutting device” shown in Model Engineer July 8, 1948. It takes the place of the toolpost on a plain lathe to allow feeding the tool independent of the cross slide in order to cut just with the leading edge while using a threading attachment that drives the topslide.



His was designed for a 90mm center height lathe. Mine is for a Unimat-sized machine with half the swing of his Boley and even less clearance over the topslide so I had to modify some of the original details.



The least obvious of these is the key for the barrel which in mine is a staple-shaped piece of 0.056” piano wire flattened in the middle and held in two close fitting holes in the bottom.

The overall length of my version is about 3-5/16” and the body is 1-3/4 x 1-1/2 x 9/16”. The 3/8” barrel has about 1/2” travel and is split at the end to clamp tools instead of using a set screw.

I use 3/16” round tools in an eccentric 1/4” adaptor sleeve that allows some vertical adjustment.

[DIYer]

Thanks threesixesinarow! We've added your Watchmakers Lathe Threading Slide to our Lathe Accessories category,
as well as to your builder page: threesixesinarow's Homemade Tools. Your receipt:

Watchmakers Lathe Threading Slide by threesixesinarow

tags: lathe

[Toolmaker51]

What am I missing?.....Surmising this allows retraction at end of cut, is it spring loaded? The depth of cut screw and knob are clear enough.
As interest towards single point threading is broad, various approaches are utilized to mount the attachment, set tool height, advance tool bit, and clear thread for next pass. Examples of spring-loading have designs of both persuasions; to push slide in or snap it back, with a locking arrangement independent of depth control.
I have a large lathe. A small, secondary top slide makes sense for pitch and diameter combinations about .312/ 8mm or less. Different submissions on HMT.net all have merit; as I sort which one to build. My inclination would employ a lever operated eccentric.

[threesixesinarow]

I don’t think you missed anything - it’s simple because it only brings back something the screwcutting attachment prevented. It’s a compact design, though, and self-contained with the eccentric holder.

Here is a Unimat toolpost next to it for scale. I think with most of the screwcutting methods for it that has to do straight in feeding, too.



The tool only retracts with the cross slide, but I doubt more than a turn will ever be necessary on this lathe, and since the top slide has to be returned at the end of the cut anyways, reversing the spindle and using a clapper-like swing up tool would work as well as anything.

Some other things aren’t clear in the drawing: the feed screw pushes against a collar that’s pinned inside the bore in the body, and the second knurled ring is just for adjusting the dial, which is otherwise held in place by friction with a bent leaf spring that fits in a groove on the knurled takeup nut.


Stevohdee’s quick bevel tool is more elegant and adaptable and his design removes the need for a microscopic key.

[threesixesinarow]

Pattern slide - cam feed, spring retract