Good luck tapping a 5/8 square thread by hand. A lot of torque required!

Can you not screwcut the thread in the lathe, making up a stepped repair sleeve , splitting it in twain and soldering the halves into recesses machined into the original half nuts?

This is commonly done on M_Type Drummond/Myfords. Details are in the Files section of the Drummond/Myford groups on groups.io and FB.

Unlikely to find an off-the-shelf tap.  40tpi square lead-screws were going out of fashion in the 1930s and Myford switched to 8tpi ACME after WW2.

I’d ask the usual tap/die suppliers to see if they will do a special.  No idea what the cost might be: in my young day, specials were “if you have to ask you can’t afford it“.  These days CNC makes small production runs much cheaper, maybe!   RDG first because of their Myford connection.

I’d take Cedric’s advice,  maybe varied because I find it easier to accurately cut external threads than internal:

• remove most of the metal by screw-cutting in the lathe and leave somewhat oversize.
• make a tap out of Silver-steel.   The tap, being external, is easier to measure accurately.  It need not be particularly strong and sharp because the lathe does all the hard work and ensures 40tpi is cut straight and accurate.  The tap is only required to finish to size.  Only needs to be good enough for one job.

The tap step is unnecessary if the internal thread is easily tested for fit by offering up it’s external mate.   Often awkward – in this case the lead-screw would have to be removed from the lathe, and may be an inconveniently long pattern.

Dave

Never heard of a lathe with a 40tpi leadscrew. That’s a micrometer thread. Far too fine for a leadscrew on anything but a micro jewellers lathe perhaps?

I have screwcut 8tpi square thread halfnuts with no need for a die to finish it off with. The joy of square threads vs Acme or BSW etc is there is no angle to mate up between male and female .

So you grind the cutting tool to exactly the width you want the groove in the thread to be. IE say nominal plus one thou for clearance. For 8tpi grind it, and finish with an oilstone   to about .063″ . Then make your depth of cut a few thou deeper. Extra clearance on the oD makes no problem at all. It is a non contact surface. Likewise bore the hole to start with a few thou oversize for clearance.

I cut my thread like this in a bobbin -shaped piece of brass then split it lengthways with a hacksaw, cleaned it up with a file, dropped it on the leadscrew of the lathe used to cut it, and perfect fit. With a square thread it can’t be otherwise if you machine to the dimensions as described. I never trial fitted the nut on the leadscrew before hacksawing it in half and all was well.

Miles,

Many people have found that heat-formed half-nuts made from delrin (acetal) perform well and are not too difficult to make. The idea was put forward by a guy named Evan on the Home Shop Machinist forum, so they have become known as Evan nuts. I’ve never needed to try making them, but it could be worth investigating.

Here’s a link: https://bbs.homeshopmachinist.net/forum/general/2012856-please-tell-me-about-the-evan-nut

George