Imperial thread on Colchester metric lathe issue

[Saxguy]

I have a Colchester Student 1800 lathe with metric setup but which also does screw cutting for either metric or imperial. When I tried to set up for a ACME thread cut yesterday, I geared up for a 5TPI cut and did a test run. Easy peasy. But, when I went back to take a second cut (just with a highlighter for a cutter, which I often find is a cheap way to do test runs), I missed the thread.



I tried again, as I though my “old” reactions might have missed the lead screw thread dial number, but no, multiple tries give same results.



Now, the actual small lead screw/indicator dial is referenced (on it) as being metric, and the 1-4 numbers and their corresponding hit/miss graduations are all therefore calibrated with metric in mind.

So, I used the 20T gear (in the dial) and decided to hit the same number each time. It didn’t work as my second cut as mentioned was about an eighth away from the first!



After a few attempts I decided to do a “metric” cut just to see if something was loose, but this worked flawlessly. Did a course 5mm pitch and repeated it without issue using the dial on the lead screw. Went back to imperial, and no dice.



I don’t know if its age, excess of Christmas spirit (and actual spirit), or what, but for the life of me, cant see the obvious mistake I must be making.



Answers on a postcard please…..

 

[Winegrower]

They say that for cutting imperial on a metric lead screw, or metric on imperial, don’t ever disengage the half nut. There are some hacks around this, but that’s for later.

 

[pdentrem]

Yeap, keep the halfnuts engaged. Stop, back up the tool, reverse the lathe to clear the part, move tool forward and repeat.
Pierre

 

[Saxguy]

Bugger! Shows how little I've actually done as far as imperial threading goes (for ages). My last two lathes were imperial (back in the UK). Here everything is metric, and I guess I've never since needed to do any imperial other than using dies. Just threw me a bit, as this lathe has a do-all gearbox, unlike my last two, and I supposed that I could somehow use the lead screw dial as normal, even when cutting "inch". Doh!!!!
Thanks guys.

 

[LCtikka]

When lead screw is different to thread type being cut. You can disengage the nuts, the dial runs on, stop machine, back tool out, put in reverse, dial goes backwards, re-engage nuts at same position, go back to start of thread, repeat & take more cuts.
LC

 

[graham-xrf]

On any thread cut, any lathe, any units, if you take a cut, and then discover the tool is not in sync with the thread on a second cut, causing a messed up thread, then you know it was because the saddle half-nut was engaged into the leadscrew somewhat out of step compared to the first pass.

One way to be sure is to keep the lead screw engaged the whole time. This requires you drive the saddle back by running the lathe in reverse.
That way can be inconvenient. Letting the lathe stop, then disengaging the half-nut, and rolling back to near the start position and re-engaging requires you do that with the leadscrew in sync relative to the saddle as before, which you do with the aid of the turns indicator. With the imperial lathes, so long as the turns per inch was an even number, then any of the marks would be OK, but if an odd number, then you had to make it match to correspond with the way it was at the start.

With metric lathes, where threads are described by mm pitch, I think re-engaging with the threads indicator exactly as it was at the start pass may be the only way.

Please forgive if you already know this. I put it here for other readers who might be going at this for the first time. This is not something I have great experience at. My first attempts turned out OK, though I did do a "dry run", just slightly marking the Sharpie ink with the tool. I am sure that if I got it wrong, folks at HM will tell us which is the right way. There are bound to be YT videos all about this.