Acme threads

[Popcorn]

I've got myself a nice little Lathe, the tailstock didn't auto eject so I thought I'd have a go at fixing this. It appeared to be the tailstock thread wasn't long enough to hit the morse taper and eject it. So drilled and taped it and fitted a cap head screw, worked great.

After a few goes at ejecting various morse tapers I have it stopped working. Anyway on closer inspection it had pulled the threaded bung out of the tailstock spindle.
It had 2 thread holes at either side of the bung with missing grub screws. No problem I thought so grabbed a tap the correct size
and cleaned the threads out, found a couple of cap head screws in m odds and sods box and Loctited them in. Hacksawed the extra off and faced them in my lathe (first job).
In my haste I'd made a big mistake, I never noticed the screws actually protrude into the acme thread of the tailstock. So it won't screw back in.

Yes I know and I do feel swear word stupid...........
So 2 questions can you get taps in acme type thread form ? If I could run a tap down it I'm petty sure that would sort it out.
If not can anyone around my way East Yorks chase the thread through on a lathe recutting the thread ?
Or is my only option to drill out the screws and try again.

 

[frank horton]

assuming the new screw has protruded thu eatch thread groove......
I little time with a Dremel will sort that prob out.....just go easy it's not a race.....
If I had a tap of the correct size, no way would I use it for that job....well at least after the main part of the screw has been removed...
just using said tap for a clean up.....
how is the thread anyway, worn or stiff....

 

[OJCar]

can you get taps in acme type thread form ?

https://www.tracytools.com/product-category/taps-and-dies/acme-taps/

 

[Popcorn]

It hardly protrudes into the thread at all really, I reckon if you forced it in it would go through. But as you can see if you look at the first thread on the left hand side it is damaging it.
@frank horton do you think the cap head screw would be too hard for a tap and would damage it ?

Thanks for the link @OJCar I'd totally forgotten about tracy tools in my search for taps

 

[6ply]

What size tap are you after?

 

[Popcorn]

1/2" x 8 TPI ACME Left hand thread as well as pic above

 

[Agroshield]

1/2"-8 is not 'standard' ACME so it may be difficule to find a tap.

External ACME Thread General Purpose Size Table Chart

External general purpose ACME Thread size table chart There are three classes of General Purpose ACME threads, 2G, 3G, and 4G each class provides some clearance on all standard size diameters for free movement, and are used in assemblies with the internal thread rigidly fixed and movement of the...

www.engineersedge.com

 

[Popcorn]

Thanks for that @Agroshield, so it might not be a ACME thread it's a old lathe from the forties. Seldon L10 or Challenger when sold over here.
I assumed it was a ACME thread but doesn't look like it from Agroshield post above.
Does anyone have a better idea of the Thread form ?

 

[Agroshield]

I assumed it was a ACME thread...

Sorry, I was unclear. It is highly likely ACME form, just non-standard (extra coarse) pitch for its diameter. Taps are more likely to be found in a standard diameter/pitch combination (see RDG and Tracy Tools for typical examples).

A lot of leadscrews are like that - they need to be small diameter to fit the space, but coarse pitch so you are not cranking for a month.

 

[RichardM]

Is the nut brass or PB? what O/D? Are those cut off screws showing the original thread?
Are you going to keep the lathe?

 

[E T]

1pc ACME 1/2-8 HSS Left Hand ACME Thread tap | eBay

There are a variety of sizes available but they are not all listed.

www.ebay.com

 

[E T]

https://www.amazon.com/Aceteel-ACME-Trapezoidal-Thread-Left/dp/B0B46VS9KX

 

[Popcorn]

@RichardM Yep keeping the Lathe, even thought about getting a new insert made but where's the fun in that.
Thanks @E T I've found a few places selling them now, don't really want the hassle of buying from the states.

 

[Agroshield]

...don't really want the hassle of buying from the states.

Both the ones above are direct from China, just listed in dollars. Have a quick look at Ali as you might find one there.

ACME-Trapezoidal is a special thread form that is neither one nor the other.

 

[Popcorn]

Should of known that everything comes from china now a days.
So I don't want the ACME-Trapezoidal form just ACME @Agroshield ?
Found these on UK Amazon:

Aceteel Acme 1/2-8 Trapezoidal Tap, Acme 1/2"-8 HSS Trapezoidal Thread Tap Left Hand : Amazon.co.uk: DIY & Tools

Shop Aceteel Acme 1/2-8 Trapezoidal Tap, Acme 1/2"-8 HSS Trapezoidal Thread Tap Left Hand. Free delivery on eligible orders of £20 or more.

www.amazon.co.uk

 

[Agroshield]

So I don't want the ACME-Trapezoidal form...

I think that is just enthusiastic marketing words by a non-technical and non-English speaker so the listing has a better chance of being found by a search engine.

All you can do is hope that the technical people who make the tap appreciate the difference and make it correctly. As the size is imperial, the thread form is probably also ACME.

 

[Popcorn]

Thanks @Agroshield ordered one so we shall see, 26August to the 6th sept delivery estimate. So guess it is on a Junk on it's way over