Metric Thread kit

Hi looking for advice , I’ve a late Myford S7 95ish imperial with gearbox .
it came with metric thread kit that I’ve never used although most my threads are metric I’ve always just used or got away with taps& dies .
Thinking of trying doing some manual threading but read that I can fit the kit Temporarily to do metric threads but seems a faff if only Temporarily, so my question is why TEMPORARILY? why once fitted can’t I just leave it or it there a reason or purpose why I cant .

Thanks in advance

Edited By rik arry on 11/06/2023 21:33:28

Could you think it the other way round?

If most of your threads will be metric then the gearing for metric screw-cutting becomes your default so stays there most of the time. Then the "temporary" changes are for the occasional inch-based threads.

The use of "temporary" in the manual probably assumes as it is an Imperial-dimensioned lathe and most were probably sold in the UK, most users will cut mainly inch-based threads anyway.

I cant see any reason you could not leave it on there permanently if metric is your main use.

Rik, Nigel's analysis is correct but look at this thread which is a simple way to cut metric and although there are pitch errors they are so small for most jobs they are good enough. If you really need to be 100% accurate then you can always use the metric kit you have along with the necessary change wheels.

https://www.model-engineer.co.uk/forums/postings.asp?th=125630

John

I expect the "temporary" description is due to the fact that the thread dial is redundant on an inch based leadscrew being used to cut metric threads. It would be expected that you would not want to leave it in that state if you were cutting a lot of threads. However for a non-industry user the extra time taken of having to leave the half nuts engaged and reversing the drive is not going to have a cost. Additionally, if you cannot reverse the motor on the lathe then you will need to hand wind the spindle backwards for each pass. Doing a lot of metric threads would soon become tiresome. There is also the slight risk of a screw on chuck unwinding, not a problem if using in the spindle collets.

Martin C

Agree entirely with Martin.

I’ve yet to come across a lathe that could not be wired for both forward and reverse rotation, but I expect there are a few tinier lathes in this category.

Using a cutter holder which ‘skips’ on the return direction would help considerably if doing a lot of threading that way. I have one, but never used it.

The 'temporary' note made sense when the Myford S7 appeared, and for decades after. In 1952 UK production was dominated by Imperial measure, with Metric available as an option. British industry, foolishly in my opinion, resisted metric, and home workshops were even more conservative. Even today there are Brits who believe Whitworth is the best of all possible screw-threads,

All change! 70 years later, apart from the USA and legacy production, most world manufacturing is metric. Almost all new design is metric. Today Imperial is more likely to be only needed on special occasions, and it's become 'temporary' on most machines.

Model Engineering may be the exception because classic British steam locomotives plans are invariably imperial. In a workshop dedicated to building them, not much call for metric, so it's still 'temporary'.

The only objection to metric being mounted permanently on an Imperial only lathe I can think of is if the conversion needs a permanently mounted 127 toothed gear. They're inconvenient because they take up so much space on the banjo.

Dave

When the metric conversion set is fitted, you lose the fine feed capability of the lathe.

Please see Brian Wood's reply in this thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=133063

Please also see this document:

https://www.haythornthwaite.com/Myford%20Metric%20Conversion.pdf

If you only ever want to cut threads with the lathe, make it permanent; if you want to retain sliding or surfacing auto-feeds, it has to be temporary.