Best halfnut material: LG2 bronze or 306 brass?

I am giving my dad's old 1936 Drummond M-Type lathe a bit of a freshen up and want to make a new threaded halfnut insert for the leadscrew .Locally (Back o' Sticksville, Oz) I have available to me either bronze to BS 1400 LG2 spec or straight brass 360.

The LG2 bronze is about 5 per cent tin, 5 per cent lead and 5 per cent zinc and is recommended for use with shafting of about 250 Brinell. It is equivalent to SAE 40 bronze. I am unable to get a higher leaded bronze (6-8%) equal to SAE660, which I would prefer, and which is ok with Brinell 160 shafting.

I want to put minimal wear on the old leadscrew, which is still in pretty good nick, about .004" wear on the thread width on the worst part. But I have no way to tell what hardness the leadscrew is.

So, would I be better off using 360 brass? The nut might wear out quicker but it would create less wear on the leadscrew, I am thinking. And I can make more nuts easier than a new leadscrew./

It is doing my head in looking at specs for brass, bronze, gun-metal aka red brass etc. So I was wondering what those with experience in halfnut replacement on Myford/Drummond or other lathes might recommend.

Thanks

Hopper

Edited By Hopper on 24/04/2013 15:15:11

Swarf might embed in brass easier than in bronze, causing more wear...

Neil

I recently made one from Acetal(Delrin) rod by following the post on another forum:

**LINK**

I've yet to make a sliding mechanism for it and build it in to the apron of my lathe. Hopefully this will give zero backlash and wear on the leadscrew.

I made some half nuts out of cast iron for an old pool special lathe with no problems.

M series owner who burned through two original halfnuts in a year

(Drummond leadscrew nuts are made of  brass putty)

Current one is an aluminium shell placed in the original cup and still going well

If you are really worried about leadscrew wear you can always check threadcutting distances with a DRO if it's genuinely critical

i.e. 16 turns of headstock = 1 inch on leadscrew/carriage (16 tpi etc)

They couldn't define wear parameters back in the old days, like we can now, so they erred on the soft as possible on some half nuts

The real world sez you will have a load of wear at the headstock end and hardly anything at the tailstock end on a 60 year old M series

The same goes for your carriage and bed

gl

Edited By Ady1 on 25/04/2013 00:36:14

Thanks for the input guys. I had not even thought about the plastic option, might be a bit too modern for the old girl though! And yes cast iron could be a good option too.

Addy, my M has surprisingly little wear for its age. I think it was maybe refurbished over the years, as leadscrew has only .oo4" wear on the carriage end high-use area. I have only clocked the tops of the flat ways on the bed, but only .001" measureable wear there. And spindle runnout is .0002" since I nipped up the bearing adjuster rings as per factory procedure. Chuck jaws need setting up and regrinding though, as the runnout there is .004" and belmouthed like 1960s trousers.

I think the lathe may have belonged to a model engineer before my old man bought it. It has had a thread chaser dial and brass swarf guards added to the carriage and graduations added to a flat machined on the leadscrew handwheel. All the sort of stuff that used to be in ME magazine back in the day.

Although, the secondhand dealer who sold it to Dad in the UK in the 1950s told him the lathe had been used to make aircraft parts during the war. So who knows? Salesman's patter or historic relic?

But my son and I are having a lot of fun sorting the old thing out and it does a lovely job, cuts to size nicely and gives a good finish with the right set up. Just all the sort of "running gear" like half nuts, chuck, countershaft bearings, motor bearings that seem to be mostly worn. Plus missing a few change gears and the backgear guard.

Some are takling/writing of wear at the headstock end on the lead screw. There was a bit a few years back in ME on the subject, the cure was to take the lead screw off, and turn it end for end, and refit it, a bit of fiddling about required. Ian S C

Ian I reckon mine has had a new leadscrew fitted at some stage, not a lot of wear on that part. Just the halfnuts are worn down to .010 thick threads. I like the cutting and reversing leadscrew idea though, if i ever wear the thing out. Carriage is almost never at the far end of the bed on these lathes so it measures up like new there.