Harrison mill - metric lead screw form

Does anyone know if the leadscrews on the metric version of Harrison's milling machines are ACME or trapezoidal?

Have a look at lathes.co.uk/harrisonmiller/index.html and have a read if you can't find it drop them an email and ask [email protected]

Phil

Edited By Phil Whitley on 04/11/2020 20:21:47

I have heard rumors that a nonstandard thread was used by some lathe manufacturers to save money on tooling. The only sure way to find out your particular thread would be to take the leadscrew to a sympathetic machine shop and get them to check the angle of an unworn section on a shadowgraph.

Lots of leadscrew available from the likes of Automotion Components etc. Change the screw and nut together then all you need to know is pitch/diameter and length/left or right hand. I have found it easy to modify a standard nut to suit the machine, only downside is you will likely need the mill to modify a standard nut. Most leadscrews are available in 1 metre plus lengths. Very cost effective way to go.

Given the era in which the machine was made the thread form is almost certainly ACME. UK and US machine tool builders generally just put a mteric pitch on the same imperial sized blank rather than doing a proper metrification.

Keeping everything except the pitch the same makes sense from a production point of view.

If the screw diameter is imperial its very unlikely that the thred form is metric trapezoidal.

Clive

Thanks for the replies. I'm in the process of converting the machine to CNC with stepper motors and ball screws. I'll post progress to date on here soon.

I want to keep all the original bits so it can be put back to standard should a i or a future owner so desire. The existing bronze nut for the Z axis is very worn and I was thinking about adding a new one to my list of 'projects'. It would be the first lead nut I've tried to single point thread so was wondering if I'd get away with the closest ACME insert (6 tpi is close to 4mm pitch) or if id have to gring a HSS one (i don't have a boring bar that will take HSS bits). Based on the last reply I may chance my luck and buy a 6TPI insert and see what the profile looks like (the existing screw is 1-1/8" dia, 4mm pitch, single start).

You won't have any bother singlepointing a 1 1/8" diameter thread with a 4mm pitch thread, trapezoidal or ACME form. You would best get an `IR16 x 4mm insert (internal trapezoidal). I have started a 1" X 5 ACME which has a starting bore of 0.8" and because I cannot get 5 ACME in IR16, and have to go to IR22, I am having to make a holder for the job. Holders under 20mm don't seem to exist for the 22 size inserts, but you should get the choice of 16 or 20 mm in size 16 for your 4mm pitch.

What pitch leadscrew did the IMPERIAL machine have?

I agree with Clive's assessment in that the original manufacturer would likely have just cut a metric pitch ACME thread on imperial stock. Even to the extent of using the same cutting tool. So rather than looking for the nearest ACME insert to your desired pitch, perhaps look for the same ACME insert that would have been used on the imperial leadscrews.

Thats a good point, can anyone with an imperial machine tell me the pitch of the Z axis?

You would have to be careful mixing the inserts, as even if the pitches were very similar, the different angle of thread and the width of the tip of the insert would not result in a very good thread if you got it wrong. I still think it would be a good idea to take the leadscrew to a machine shop to get the exact angle of thread checked, after lockdown, of course. The difference between 29 and 30 degrees would be difficult even fitting an insert in an unworn part of the leadscrew. There would be no hurry if your immediate concern was fitting ballscrews for CNC use.