Help with slide rest feedscrew nuts

[measuretwice]

Here's a photo of a lathe I'm working on, a Schaublin 70



The feedscrew nuts are the two round things visible on the slide surface. They are a press fit into the slide rest base.

Why are there two? One idea is its some sort of antibacklash or backlash reduction arrangement (commonly done with two nuts on larger lathes) however its a puzzle. Given they are pressed in, it seems highly probable they'd be drilled and threaded in situ as it would difficult (next to impossible?) to press in while maintaining alignment of the threaded hole. Bored and tapped in situ, how would the two holes be taped out of phase (needed if they are indeed antibacklash)

Or is that just an imagined red herring.....as in maybe there are two nuts just to give a long area of engagement. I'm increasingly thinking that is the case as the other axis's slide has a long (single) nut as well.

Anybody have any insight or knowledge into this?

thanks
Mike

 

[wefalck]

What is also puzzling is that they seem to some kind of iron metal (cast iron?), rather than the usual bronze ...

These nuts are not normally pressfit, but a quite tight sliding fit, allowing the float and thus to accomodate slight misalignment that otherwise would result in excessive wear.

You could contact David Samways at Anglo-Swiss Tools - Anglo-Swiss Tools, who has a lot of experience in reconditioning Schaublin machines, he might have an answer.

 

[measuretwice]

thanks....I sent him a note. Will report back if there is enlightenment

 

[measuretwice]

He didn't know but was inclined to agree its just for addition contact area. I'm going to proceed on that basis. thanks

 

[wefalck]

Mechanically it doesn't really make sense, as only one nut will actually bear.

 

[measuretwice]

Mechanically it doesn't really make sense, as only one nut will actually bear.

agreed, but it was another voice in the direction of not having specific reason for it or clue on how to make it. As they were definitely an interference fit, I can't see how they were threaded unless done in situ no way to maintain alignment on the press fit), which means they are in phase and therefor not anti backlash related. Best I can think of is more area provides greater sampling making accuracy less susceptible if there was an area of very high wear on the screw. I feel like i'm realty stretching to grab a staw on that idea! On my replacement I just used one nut as it would take quite a tap to reach both.

Anyway, new parts are all done. Made a new base and nut. Used the original screw which was 6mm x 20tpi. Using a new H3 tap, I tapped a 1/4 20 hole, then single point chased the thread until it was a snug fit. I then did a bit of lapping on it. Worked out well. Made the new nut a light press fit in the base, then drilled and taped in situ.

Just got the tailstock quill back from being hard chromed. Got it ground today, will lap it tomorrow. light at the end of the tunnel!

 

[L. Vanice]

agreed, but it was another voice in the direction of not having specific reason for it or clue on how to make it. As they were definitely an interference fit, I can't see how they were threaded unless done in situ no way to maintain alignment on the press fit), which means they are in phase and therefor not anti backlash related. Best I can think of is more area provides greater sampling making accuracy less susceptible if there was an area of very high wear on the screw. ...

Hardinge used the same nut design on some of their slide rests. The 1946 model slide rest has two bronze discs pressed into the slide base. It is important that the nuts do not move once they are tapped, so the fit is tight. Yes, two nuts will give longer useful life than one and the space in the slide base for the diameter of the nut (length of thread) limits the thread length in each nut. They are drilled and tapped using bushings in the screw support bracket with the freshly scraped upper slide installed. Milling and scraping a worn dovetail changes the position of the screw and nut, so the drill and tap operation has to be done on new blank nuts. For repair work, Hardinge sold the blank nuts and loaned (against a huge cash deposit) the set of drills, taps and bushings. On this slide rest, the lower screw has a 1/2-20 RH square thread and Hardinge made sets of either two or three taps for roughing and finishing. Hardinge supplied detailed instructions for replacing the nuts. The file size of the photo was too large to attach here.

I have never owned a Schaublin lathe, but I have several Hardinge lathes and have rebuilt some slide rests.

Larry