Cast iron leadscrew nuts

[Wedg1e]

I'm at work, which is seriously hampering my ability to google stuff - need to make replacement lash adjusters for a mate's mill and they're cast iron. Leaving aside the niceties of single-pointing an internal square thread, I'm trying to determine which variety of cast iron would be the optimum for the purpose. Plenty of places sell it, but I don't want to buy some random length of bar and find it's only suitable for casting into handrails or a new bedroom toy for Ann Widdecombe.
So... grey? White? Grade 250? Any thoughts/advice?

 

[Kram]

The most common is grey/250 I think, should be adequate.

 

[Screwdriver]

Meehanite is the "trade name" for quality, machinable cast iron. Naturally the seller may lie but at least you have trades descriptions etc.

 

[Agroshield]

...Leaving aside the niceties of single-pointing an internal square thread...

That may be just loose typing, but a square thread form is unlikely on a device that requires precision of movement.

Square threads are normally used on more agricultural devices, such as bench vises, with Acme or its metric equivalent used on leadscrews. Acme threading inserts are available (at a price) from all major manufacturers if you are not into grinding your own.

Cast iron bar is available in rectangle and square as well as round if the form factor is more suitable (e.g. College Engineering, M-Machine).

 

[Wedg1e]

Cheers chaps.
Agro, I know it should be an ACME and probably is (it's a Beaver mill, not some Chinesium thing) but from the poor photo my mate sent it didn't look like one, and there's not enough left of the nut thread to tell either way!
Had a word with a machinist we use at work and he suggests using bronze instead, but then also said that given the difficulty of producing an internal ACME for a novice (which both my mate and I are, to be fair) it might be as well to consider a new leadscrew and nut from the likes of Accu as it wouldn't be that (relatively) expensive - the screw would need some work to suit the drive ends etc.
Food for thought...

 

[brightspark]

make it out of bronze and you can buy acme taps and dies from tracy tools ACME TAPS (tracytools.com)

 

[mtt.tr]

What beaver is it, mine had a pair of bronze nuts

 

[Agroshield]

...(it's a Beaver mill, not some Chinesium thing)...

Do you know which model? I can send you pdfs of the manuals if required.

Do you know which axis?

Original Beaver nuts are cast iron and X- and Y- were in two pieces, so they can be adjusted for backlash. They are a little complex to reproduce. With the X-axis, the leadscrew has a keyway along its length for the powerfeed drive and is a bit cruel to the nut.

X- is 1"-5TPI RH and Y- is 1"-5 LH.

Edit: diagram of nuts attached (this is a good representation of the set-up).

 

[MattF]

What beaver is it, mine had a pair of bronze nuts

I'm not going there....

 

[Wedg1e]

Do you know which model? I can send you pdfs of the manuals if required.

Do you know which axis?

Original Beaver nuts are cast iron and X- and Y- were in two pieces, so they can be adjusted for backlash. They are a little complex to reproduce. With the X-axis, the leadscrew has a keyway along its length for the powerfeed drive and is a bit cruel to the nut.

X- is 1"-5TPI RH and Y- is 1"-5 LH.

It's a VBRP Mk2, the X-axis. As you say it's in two parts for lash and has a locknut and locking clamp. He has the manuals as pdfs but thanks anyway. I can see it's RH but hadn't got as far as determining the TPI (I was guessing 6).
I have some 45mm dia. centrifugally-cast leaded bronze (as used on the Boxford collet closer a few months back ) but the bore won't clean up to less than about 25mm which wouldn't leave much thread depth. The adjusting thread is 20tpi but I forget the diameter, 1-3/8" maybe.

 

[Agroshield]

...(I was guessing 6).

It would have to be a sadistic designer to put a 6tpi leadscrew on a milling machine: one turn of the handwheel being 166 2/3 thou.

5 tpi ACME inserts seem only to be available in 22NR (size 4) size, with a 20mm dia. bar as the minimum to hold them. That might be a problem...

 

[RichardM]

I made a square thread "nut" for a Abwood vice out of brass and soldered it into the original, I would have made it from PB but would have had to buy some. Not hard to screw cut in a lathe, ground HSS tooling, acme is supposedly easier than square thread to cut. I just bored to the core diameter and cut to external then advanced the tool in the thread until the screw fitted nicely.

 

[Wedg1e]

It would have to be a sadistic designer to put a 6tpi leadscrew on a milling machine: one turn of the handwheel being 166 2/3 thou.

5 tpi ACME inserts seem only to be available in 22NR (size 4) size, with a 20mm dia. bar as the minimum to hold them. That might be a problem...

Good point.

Roton supply 1" - 5tpi Acme nuts in bronze, thick end of $50 though, I guess you could modify them so one threads into the other for lash adjustment, so it'd be a simple boring/ 20tpi thread job.

 

[Wedg1e]

I made a square thread "nut" for a Abwood vice out of brass and soldered it into the original, I would have made it from PB but would have had to buy some. Not hard to screw cut in a lathe, ground HSS tooling, acme is supposedly easier than square thread to cut. I just bored to the core diameter and cut to external then advanced the tool in the thread until the screw fitted nicely.

I was thinking the same, grind up an HSS bar, conveniently ignoring the trivial matters that I'm as blind as a bat and only have an off-hand grinder that shakes like Shakin' Stevens using a Wacker plate

 

[not done it yet]

That may be just loose typing, but a square thread form is unlikely on a device that requires precision of movement.

Square threads are normally used on more agricultural devices, such as bench vises, with Acme or its metric equivalent used on leadscrews. Acme threading inserts are available (at a price) from all major manufacturers if you are not into grinding your own.

….

Objection, m’Lord. Not so!?!

Virtually all Raglan lathes and mills were equipped with square threaded feed screws - Acme was mostly limited to the lead screw on the lathe (because half-nuts are much easier to engage if the thread form is anything but square). (underlining because lots of people think all feed screws are lead screws)

Square threads are likely stronger than other thread forms and Raglans, while a small ‘niche’ company, supplied high quality machines which were much sought-after by hobbyists - even though costing about twice as much as a myford of that era. They were (and still are) so much more capable than that particular competitor - bigger, better, power feed on both axes, separate feed shaft and lead screw, wider and completely variable speed range, enormously hard wearing ways, etc, etc.

Vises are often fitted with square or buttress threads for strength. Buttress were, again, likely better for ‘quick-release’ mechanisms. The first square thread I remember was the front furrow adjuster on the drawbar of a Fordson plough, so true - very tough and agricultural. Farmers certainly knew what was good quality in those days.

 

[Agroshield]

Objection, m’Lord. Not so!?!

Objection noted.

A counter-example of one is hardly a great weight of evidence.

I was thinking the same, grind up an HSS bar

I did find some solid carbide bars that would do the job, but only in US. Only $68, which I thought was good. Micro 100 is one brand. The house brand of MSC Direct (Accupro, US version) was another one.

e.g. http://internaltool.com/products/lathe/19/acme-threading-tools *, https://www.carbideanddiamondtoolin...Threading.Tool.29.Degree.4.5.6.8.10.14.16.TPI (note similarity of part numbers between the two sites).

If you do go that route, they are specified by the length/depth of thread (e.g. 1" long, 2" long).

* they have a distributor in Iceland, so next time you are buying frozen chips...