Inserted cross slide feed nuts

[Andy CarlsonParticipant @andycarlson18141]

@Hopper: yes, hole measuring… tricky. The nut goes in for about 3mm easily so the fat end is somewhere around 507 thou. I have resolved to make another plug to measure the thin end but right now there is a nut in the way and the lathe occupied by a rather blank looking tap. 'All over the place'… not so sure. Assuming for a moment a parallel hole at the 'fat diameter' and the nut goes fully to one side of the taper then that's 1.5 thou in 14mm (nice mixed units there) which I work out as 1 in 367. There is of course an unknown amount of distortion and a bit of material being scraped off so the pressing is definitely still on the suspect list.

TBH after trying an approach involving doing all of the accurate work in the lathe chuck and finding that it didn't work out too well I'm becoming more convinced that I need to do things in a similar way to the way that Faircut did them… and given the eccentricity in the original nut it seems that 'in situ' was definitely how it was done.

@Nigel: I've struggled to find much concrete info about square threads (as opposde to Acme or modified square). The stuff I've found is fairly vague and hand wavy and says that the screw should be made to size and the nut should have about a thou of clearance. I've gone to plus two thou on the major and minor diameter because it should make no difference to the backlash but your post is the first time I've seen 10 thou quoted. The major diameter is not a big issue – it's just a matter of leaving enough meat in the surrounding nut. 10 thou on the minor seems like a lot – that's a quarter of the depth of the thread.

[Andy CarlsonParticipant @andycarlson18141]

Well that went fairly badly.

I've made a tap, all bar the square on the back end anyway.

Two issues… Firstly both corners chipped off the thread cutting tool – one after about 10 thou of cut and the other towards the end. I cut a bit deeper which should hopefully put the untidy part of the profile in a place where it will do no harm.

The bigger and more puzzling issue is that my 0.0406 thou tool managed to cut wider grooves somehow. The threads measure a tad under 1mm rather than a tad over. I'm not sure why so that does not bode well for being able to correct the problem.

In spite of the issues I decided to finish off the tap and cut the flutes using a 3mm ball end mill in the Potts Spindle. There was a 40T gear on the lathe spindle so I decided that four flutes seemed like a good number. The square for the tap wrench is still to do because the poor little Unimat motor was getting rather warm.

The tap almost certainly won't work for an unworn feed screw but hopefully I will at least be able to find out if the tap approach can produce a cross slide that actually works, albeit still with a worn feed screw.

[Andy CarlsonParticipant @andycarlson18141]

A quick note to say that the saga continues on my lathe thread…

https://www.model-engineer.co.uk/forums/postings.asp?th=144221&p=5#2638709

Edited By Andy Carlson on 11/09/2020 23:41:27

[Ady1Participant @ady1]

I stuck aluminium into my Drummond cross slide and leadscrew nuts and it's been fine so far

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