Acme thread

Hi Hopper,

I must have missed the square taps. Can you supply a detailed route to them, please?

Cutting external square threads is a relative doddle - even if the cutters may be a little fragile. It’s not like a lead screw in length and the width of the thread is relatively easy to adjust (as long as the cut is too narrow).

I have used cutters ground from 4mm HSS rod held in a home made holder (usually a length of silver steel rod). Clearly more difficult as the diameter decreases and the length increases!

A square HSS cutter in a square hole would be preferable, but I have got by with round ones so far.

Here is what I can source at a fraction of the price and make my own replacements.

Exact same 2mm pitch as specified, only difference is the diameter is 0.5mm larger than myford's.

https://www.mooreinternational.co.uk/ProductGrp/10x2-quality-rh

making the nut is a piece of cake too.

What complicates things is if you need to machine it to the exact 3/8" (9.5mm) diameter that myford use.

If the feedscrew diameter is not an issue, then this is a simple and cost effective replacement.

.

Shaun

This is important ... although sellers all over the world seem to ignore the facts these days:

The ACME thread form is very specific, and has 29° flanks

The Metric equivalent is Trapezoidal, and has 30° flanks

Details of both are readily available.

[sorry for preaching from my soapbox]

MichaelG.

Either way, I dont think its critical for operation but may affect how smooth the handles turn?

May also affect the rate of wear? Seems trapezoidal is easier to find than square thread anyway.

The supplier in the link i posted also stocks rolled ballscrew, ground ballscrew and satellite roller screw.

I have no idea what the difference is between them.

You are, of course, at liberty to make whatever you want, Shaun

... and MooreInternational does look a very good place to source ‘make-from’ materials.

MichaelG.

Shaun , a slightly larger diameter screw would still work the same in regard to the dials if the pitch TPI is the same .(as long as everything else is machined to fit (the nut, dials ,handles etc...)

Edited By mark smith 20 on 27/11/2019 09:37:59

I'm not going to jump into any Square/Acme/Size of Material discussion here Shaun - just wanted to support Hoppers statement (warning) that 'tapping' an Acme tread is not as simple as just buying a single Acme tap and drilling a suitable hole. I think you will find it is very hard (if not impossible) to turn it.

It's either a staged tap (e.g. more than one tap = quite expensive) or an interior thread cutting operation, perhaps using a tap as a finishing touch, if you already have a suitable tap. I've seen YouTubes of folk going straight in with a single Acme tap but I don't know how they do it - because I certainly couldn't when I tried it, albeit on a slightly larger (1/2" x 10tpi) nut.

Of course, if you can purchase a screw & matching nut and modify them to suit your application, then this need not worry you.

Regards,

IanT

Edited By IanT on 27/11/2019 10:22:46

Thanks, thats all I need to know.

I would hate to find the dials were not accurate if i used this feedscrew.

Im actually considering making my own resettable dials anyway, but would need to CNC engrave the markings on the outside accurate enough

OK, ill take note of this. I never realised it was that difficult. I see Geffoery Crockers restoration video on youtube he is simply tapping the holes after drilling and looked fairly straightforward to me.

Either way, i have a friend who can probably do this easily enough on his machine at work.

Also regarding material of choice, i see I can choose the option of stainless or just (mild?) steel.

What is the best option to go for regarding wear?

Stainless has the added benefit that it wont rust.

I think its pretty clear to me now that the metric feedscrews myford solutions sell are simply made from standard 10mm  2mm pitch trapezoidal prethreaded rod.

Edited By Shaun Belcher on 27/11/2019 11:03:04

I made some here for my southbend when i got it. This was the result of only my third attempt ever at screw cutting. I just used leaded steel and used the original nuts . They were a large improvement in back lash etc on the original screws . Still working fine after a few years. I grafted the threaded section into the original parts using loctite and a pin.

Edited By mark smith 20 on 27/11/2019 11:11:36

There is an important difference between ACME/Trapezoidal thread and square thread when used in a cross or topslide as the form can be used to take up wear. If the nut is made with thinner slots & thicker webs so that it won't actually go on the screw, then slit lengthways the two halves can be fitted round the screw but not quite meet together. The section of the web of the nut is a wedge and is gong into the angled slot of the screw thread, as far as it can. When it wears it can be pushed further into the slot of the thread taking up the slack. Some lathes do make use of this feature which although not ideal may be advantageous if you want a minimal backlash setup.

I was surprised by this as I recently had no difficulty in tapping a length of cast iron 1/2" x 10TPI acme. I should add that this was done under power and the tapping hole was slightly oversize. Other materials may well be problematic, of course.

Its interesting to know anyway. Im not sure if its as difficult with brass or bronze?

I made these square thread serial taps:

to make the nuts for the brake shafts on my traction engines:

After I'd made them I cut a trial thread in brass which was easy. In contrast the bronze I used for the actual nuts was a total PITA.

Andrew

Edited By Andrew Johnston on 27/11/2019 22:22:12

The comparisons of ease of tapping suggest one possible advantage of the reviled standard Myford nuts; better to have to replace a relatively cheap component than the expensive leadscrew?

Wow, thats impressive! I dont think that grinding HSS to make taps would be an overly easy job.

Anyway, ive been looking at different taps on the market and they appear to be fairly tapered and long so should in theory make it very simple to cut the thread without a sequential series of taps.

I wouldnt use stainless. Can be a right PITA to machine and no great advantage in this application. Mild steel works just fine, especially with brass nuts. Most of the commercially available threaded bar will have a rolled thread which is left in a work-hardened condition.

If you do use bronze for the nut, make it leaded bronze and not phosphor bronze as the latter will wear non-hardened steel faster. The idea of brass nuts is the same as Myford's orginal Zamak, the nut is sacrificial and wears faster than the leadscrew. So you replace the inexpensive nut, not the expensive leadscrew. Plus its easier to machine.

Don't suppliers of trapezoidal threaded rod also usually supply brass nuts to match? Would be easier, and probably cheaper, to buy the nuts even if you have to machine them down to a sleeve then bore out the original nut and Loctite and pin the sleeve in place.

Edited By Hopper on 28/11/2019 01:04:09

Edited By Hopper on 28/11/2019 01:22:27

Dials will still be accurate. The imperial dials you have have are graduated in 100 graduations. This becomes then 1/100th of the 2mm pitch of your feedscrew or .02mm. Diameter of screw makes no difference.

Model engineers were engraving dials long long long before CNC was invented. You engrave the lines with a sharp lathe tool mounted sideways in the tool post that is racked back and forth by the carriage handwheel, using a carriage stop to control length of the line.

Position of the lines is controlled by a pointer or similar engaging with the correctly set up change gears on the headstock quadrant. To get 100 graduations, you use a 50 tooth gear meshing with a 25 on the spindle, or some similar 2:1 reduction combination of gears such as 20 on the spindle meshing with a 40 which has the 50 tooth gear on the same stud.

You then stamp the numbers on using number stamps and a hammer. A simple jig to hold the dial and punch in the right positions helps make a tidy job.

Many of the old ME books by G H Thomas, LH Sparey, Ian Bradley etc cover the technique. Thomas in the most detail. His "Model Engineers Workshop Manual" includes the drwaings and instructions to make resettable dials for a Myford.

Edited By Hopper on 28/11/2019 01:24:27

The supplier sells the nuts but they dont have the correct flange, I would have to machine them down as you say, plus they are expensive, although I can find the same ones overseas for a fraction of the price.

Yes ive seen a youtube video of someone engraving dials in a similar fashion. I just dont have any of the correct gears for my lathe at the moment.

Im still in the process of getting my lathe up to scratch and getting tooling etc.