Portass Model S - lead screw nut stripped (5/8'' square thread)

I have just bought a Portass Model S, in good overall condition apart from a bodged repair to the lead screw nut. A threaded bit of brass joined onto the existing nut by a flat plate. The repair piece is badly worn and probably not even the correct thread. As such there is a lot of backlash.

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I have identified the thread as left hand square thread 5/8'' - 8TPI (1/8'' pitch).

This seems to be an outdated thread, I can only find Acme screw of these sizes.

My question is should I make a new lead screw with Acme rod and bush the nut, then thread it to suit.

Or is there a metric equivalent that would maintain the lathes thread cutting abilities?

I'm struggling to find a UK supplier of Acme rod, but I have found a tap for rethreading the nut.

Thanks.

Simon,

You can get 5/8" x 8tpi Acme rod from Kingston Engineering in Hull, but their standard length is 5 feet. No idea of price.

This thread is the same as Myford ML7s up to April 1973, when they changed to 3/4" dia, so you may be able to pick up a decent used one. Length probably OK for yours.. But if a standard bed screw is too short, don't be tempted by the screw off a long-bed ML7, they were always 3/4" I think.

Simon,

​Another supplier for you to choose from is Halifax Rack and Screw

www.halifaxrs.com

Regards
​Brian

Looks familiar! Mine had a stripped leadscrew thread too. Made a round bush to suit from a bit of bronze, cut the thread and loctited in place. Worked out just fine. Well about as fine as anything to do with a Portass S can.

Pretty much first my lathe job, and certainly first screw cutting, I'd ever attempted. Was an 18 or 19 year old ASO trainee lab rat at RARDE in those days. Not officially supposed to use the departmental Harrison L5 but a couple of lunch times under supervision from "Big Den" got the job done nicely.

Also had a broken casting over where the front spindle bearing had been overtightened. First go at cast iron welding too. Back around 1972 / 73 going rate was three packets of twenty and a couple of pints. Came with a writing cabinet full of tooling, accessories and bits varying from sub dubious to very darn good. Brand new box of 12 square Stellite blanks 2 1/2" x1/4" anyone. Unique indicator. Um. Maybe not. Still working my way through some of it. But not with the Portass which is long, long gone with no regrets but reasonable profit.

Clive.

Thanks for info on suppliers, will give them a phone.

I did consider cutting a thread to match, but I've never done it, nor do I have access to a suitable internal thread cutting tool. Been spoiled in work with changeable tip tools so don't know how to grind one. Also the screw itself isn't in the greatest condition so probably best to change it to Acme.

My change wheel bracket has been welded at some stage. I didn't get much with mine, but it has a four jaw chuck and only missing one 20T change wheel. I picked it up locally for £60, used to using Tos Trencin machines so will have to manage expectations as to what this machine will be capable of!

Another Portass "S" owner here - also with a stripped nut quite early on. In fact, my machine came with a recent replacement but the metal was clearly too thin and it didn't last long.

I managed a pretty successful bodge to repair mine, one that lasted many years. I drilled and tapped 2 (possibly 3?) radial holes in the remains of the nut on the same pitch as the leadscrew and made up some brass screws with a reduced nose that fitted neatly into the leadscrew. This avoided the need to attempt an internal square thread, something I suspect would tax even a very experienced operator. I vaguely recall replacing them at one point but they got a lot of use, so that would seem fair enough. This could get you going in no time and like me you may find it lasts for many years.

Given that these machines have no half nuts, perhaps a modern solution would be to get a Chinese rolled ballscrew / ballnut machined to fit your machine. It would probably work out cheaper than a pukka acme jobby.

Murray

Simon, don't know where you live, but HPC supply acme lead scres and nuts in 5/8 x 8tpi

**LINK**

Good Luck and welcome

Bill

Make a delrin nut thread from your screw and fit into the old nut after turning out the old thread?

Phoned the suppliers for a price, cheapest was coming in about £80-£90, then I'd have to buy a tap on top of that. Not prepared to spend that much on this lathe.

Muzzer's way of repairing it sounds good, will have a look tonight.

not done it yet - How would I make a Delrin nut? Only way I've seen it done is to make a tap out of the lead screw.