Looking again at your photo djswain is correct the pitch does not appear to match your gauge, try a 19 TPI gauge, this is used on British pipe threads but the diameter you quoted does not match any standard BSP thread.
Where in the U.K. are you located ?
To late to investigate further tonight but if it’s clearly an imperial pitch 20 TPI why measure the diameter in metric ? Your measurements equate to 0.7409” which suggests it’s 3/4 x 20 albeit 0.009” undersize, possibly a UN special I.e. none preferred size.
Denn, Only just noticed this post, did you get the machine sorted? Peter above is correct regarding the feed/screw cutting lock system and this sounds to me to be the most likely cause of the problem so a strip down of the apron will be needed.
Do you have a manual? I have the original that came...
Basil, I just spotted this site ACME Leadscrew | ABSSAC
They seem to do a suitable Acme threaded bar you could use to fabricate a new lead screw from, no prices but worth a call I recon !
John
Alex, point taken, however as you point out Mazak is great for complex castings which this item is and you will find it is more wear resistant than you imagine. For a great many years Delapena used this material for replacement shoes on honing mandrel’s and I can vouch for their longevity...
Basil, have a look at this thread on another forum for supplies of Acme threaded bar UK Supplier of Imperial Acme Threaded Bar? | Model Engineer
As for the half nuts they are as far as I am aware made from Mazak , your do look well worn so I would bite the bullet and purchase replacements, if...
Almega, lots of good advice on here but here's my three penne'th use HSS ground thread taps, yes they are a higher price but worth the money -- they CUT ! one source I use is MSC and Lyndon taps, excellent value when on offer !
Lubricants for mild steel / black iron [CRS & HRS ??] use coconut...
Well my on board computer was programmed in imperial so still use thous and tenths ! Plus all my kit is imperial. However I have a close friend in the gunmaking industry in Italy and asked him to clarify the metric terms , the list is below:-
Metric Micron 0.001mm
0.001
1 millimetre = 0.03937"...
Hi Baron,
Don't want to clog up or hi-jack Bazmark's thread but if your interested the adaptor blank I used came from Myford [25 years back ??] I think they still do them
Main point is they are much larger diameter blank and longer than any others you see many of which will be far to small or...
Hi Baron, use mine quite a bit on a Super 7 here's a pic with the mounting adaptor No 2 morse, yes the dies are expensive but if you have a large batch to do its worth it, however the often come up on flee bay but you need to mack sure they are good.
John
Hi my member name was my first Ariel -- still have it along with several others in varying stages of re-build hope to finish some before I'm too old to ride !! I'm now passed the sell by date !
My interests are all things mechanical, time served engineer then after 18 years went self employed as...