Acme thread

I've read about a metal with a very low melting point ie less than boiling water Somebody will know the stuff I mean. Probably used 50 years ago and outlawed nowadays ?

Bill

maybe bismuth or cadmium alloy?

Michael W

Woods metal or Cerrobend

Having introduced it in a molten stage to a blind bore with a thread you want to measure, how do you expect to get it out for measurement if you can't unscrew it?


​Plumbers merchants stock it, it is used as a fill material in pipe bending which can be melted out again after the bending. Melting point is about 100 degrees C

Brian

I thought Cerrobend had a melting point of around 70ºC?

Andrew

Andrew,

I knew it was at hot water temperatures, but how hot I wasn't sure so opted for boiling to be sure. My point about extraction is still valid though.

Brian

Would this work ?

Grease the blind hole with something

Cut / grind a piece of dowel in half to make a D shaped piece of dowel the same diameter as the hole

Insert dowel into blind bore and fill up other half of hole with your molten material.

When cooled pull out the half dowel with pliers and you now have some room to wiggle the newly cast materila loose and you have a "half cast" that you can measure the thread on.

There may be some molten material get behind the dowel but these remnants would surely be fairly weak and broken with a bit of wiggling.

Bill

I've already given my solution on the previous page, the material was free and it can be used for any kind of impression taking.

I used it to make copies of the church key, a great big Victorian thing with complex wards that would be impossible to separate having poured metal round it; it will also make an archive copy of a magnetic particle indication of a crack if so needed.

I have also screwed bits of soft timber into threaded holes to determine the pitch, not very informative on thread angle if it were needed but a faithful impression will capture that detail too.

Brian

I am needing to machine some new feed screws and nuts on my ML7 too.

Just stumbled across this thread so thought that I would ask a question.

Myford claim its 10TPI for the imperial feedscrews, but only say the metric ones are 3/8" 2mm pitch.

I'm assuming that they are 8TPI?

Either way, I thought that it would be a good time to convert the lathe to metric when replacing the feedscrews, just need to get the dials from myford.

What changewheel do I need for cutting such threads?

I don't have any that came with my lathe but I've got a friend who is an engineer and can probably machine me one on his lathe if I know the right specs. I also have no idea what type of tap to get for cutting out the feedscrew nuts.

8 tpi would be close to 3mm pitch (actually 3.18). Please do not assume anything - you may well be wrong, so always check it out properly. 2mm pitch is just that - there is no need to try to convert to imperial (it would actually be 12.7 tpi!).

While not standard myford, I would make them in square thread, not acme, although at only 3/8” diameter that may be a little more awkward. Cutters are easily ground in the home workshop for both inside and outside threads.

Myford’s competitor (‘just down the road&rsquo used square threads for feed screws - but 1/2” in diameter. Nothing at all wrong with their feed screws (their lathes were of much higher quality than those of myford). I am currently cutting a new nut in square format for one of those. One only really needs ACME thread for lead screws, that need to be engaged and disengaged with half nuts.

One could always cut the screw easily enough and then make the feed screw nut in acetal. Probably better to cut a piece of extra screw for that, including clearance, as acetal will leave no thread clearance when moulded by the ‘heat and squeeze’ method.

I think what this means is that the outside diameter of the screw is 3/8" but the pitch that is cut on it is 2mm.

Basically a metricated version of the imperial screw, the only change being the distance between the thread crests. It means all the stuff at the handle end will fit unchanged.

.

25.4/8 = ?

Most certainly not 2mm.

.

By internationally agreed definition ... actually 3.175

MichaelG.

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P.S. ... Yes, there’s a lot of detail in 5 microns ...

https://www.nature.com/articles/s41524-019-0202-3/figures/1

Edited By Michael Gilligan on 24/11/2019 10:29:12

I got the 8tpi figure somwhere else on another myford thread in a forum (excuse the pun lol)

I didnt think it sounded right.

Anyway, ive done a bit more research and yes, its supposed to be a square thread, and I believe this is what Myford use.

I also found a blog where someone machined their own feedscrews and nuts. You need a moving steady and the right screwcutting gears and ground cutting tooling etc.

Quite a lot of work and hard to do a nice job, he had to do many slow passes on the lathe.

He made his shaft 9mm but 3.8" is closer to 9.5mm

Does shaft diameter affect TPI?

If not, I think ive found the answer.

I can find many retailers who stock square threaded shaft in 10mm diameter, 2mm pitch.

Should be able to find the correct tap to make my own nuts also.

If you look at the aftermarket feedscrews that myford solutions sell, they look simply like they are made from cut down prethreaded rod, with the end machined down and threaded to accept the handle.

They differ in that they dont have the unthreaded "lip" at the end of the shaft, I thought this may have been required, so not sure why they machined them like this, unless they had to do away with it by using prethreaded metric rod.

See photos:

Their feedscrews also are bronze, not the crappy zamac alloy that myford uses, but I will machine my own if this will work with 10mm rod if it does not affect the dial calibration.

I think you will find Myford uses Acme thread (or maybe trapezoidal for metric?) and not square. But square will certainly work for you if that is what you can buy in threaded rod.

Tracy Tools and others can supply square and Acme thread taps. Many on eBay too. Can be a bit hard to tap large chunky threads like that from scratch so it is common to rough it out by screwcutting in the lathe, even with a steep-sided V tool and finish with the tap. Or buy a set of progressive taps.

I make all my leadscrew nuts from brass, not bronze. Less wear on the leadscrew that way. And easier to machine.

Im quite lost to what thread Myford uses, but most sources ive seen say that myford used square thread on all their lathes.

Anyway, from what i understand, acme was the term used for trapezoidal thread in the days of imperial?

As far as bronze goes, I think alot of people call brass bronze and vice versa. Bronze is supposed to look more copper in colour isint it?

The nuts in the photo certainly look brass in colour than the coppery bronze colour.

Anyway, would a 0.5mm increase in diameter over the original myford imperial feedscrew change the TPI and affect the dial readings? Or does it stay constant irrespective of the diameter if the thread pitch is the same?

For the sake of clarity [hopefully not for further confusion] regarding the metric screw threads

The aftermarket versions supplied by MyfordSolutions are very clearly stated to be 2mm pitch.

**LINK**

https://myfordsolutions.com/collections/ml7-carriage-assembly

Click the thumbnails for individual descriptions:

” Each rotation of the spindle gives 2 mm displacement. “

MichaelG.

Myford also state 2mm pitch for their metric feedscrews.

What im saying is I can buy the prethreaded rod 10mm in diameter with a 2mm pitch.

Myford say theirs is 3/8" diameter which is about 9.5mm diameter.

Is there any issues with accuracy if i use a slightly larger diameter feedscrew? e.g, change the TPI rate?

Please note, that I also would be machining my own nut to match.

Edited By Shaun Belcher on 27/11/2019 08:52:51

Edited By Shaun Belcher on 27/11/2019 08:53:10

Edited By Shaun Belcher on 27/11/2019 08:54:25