Another Acme Thread Question!

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A quick impression, in plaster, wax or plasticine, would soon confirm.

MichaelG.

I,m definitely a convert to plastic nuts , my Dore Westbury MK1 mill leadscrew nut stripped out and rather than try the moulding a new nut from acetal method i just made a whole new nut from acetal billet and got a new 1/2" x 10 TPI ACME tap from RDG tools and it was easy to tap a 40mm length of thread , with bronze unless you can rough the thread out in the lathe before you put the tap up it to finish the last few thou then it will need a lot of force to tap a 5TPI thread just with the tap alone as bronze is not the easiest stuff to tap a thread in , the plastic nut on my mill has lasted 3 yrs now and done a lot of work the backlash is still 10 thou which is the same as when i first made it.

regards Geoff

Stick the internal measuring jaws of a digital/vernier caliper down in the thread "valley" and you will soon see if it's square or Acme by holding it up to the daylight. You can then measure the width of the flat at the bottom of the valley and the width of the flat on the crest of the thread.

Looking at those nuts, you might think about turning up brass cylinders threaded internally and then boring out the old nuts and soldering the newly threaded cylinders in place. Something like common procedure for repairing Drummond lathe half nuts.

Brass will be a lot easier to machine than bronze, which is going to be important when using a 1/4" diameter boring bar with a .100" wide or so Acme/square thread toolbit sticking out the end. You might do well to rough it out with a V tool then finish it off with the final Acme/square tool. To maximize the boring bar diameter, you may have to start with the tool bit not fully sticking out and then move it out once half depth of cut has been reached.

Thaks for all the answers and suggestions guys, a lot to think about. As to the thread being Acme or square - my understanding is (and I amy well be completely wrong on this) that in a square thread the Root and Crest widths would be equal at 1/2 the pitch. Given that the pitch is 0.20" then this figure would be 0.10".

On the other hand, an Acme threads Root and Crest is equal to 0.3707 x pitch, giving a Crest width of 0.0754" and a distance between crests of 0.1246".

I realise that the method od measurement shown below is far from ideal or accurate but I believe to shows enough information to determin that the thread is an Acme one.

The distance betwiin the two yellow arrows is close to 1/8" and the distance between the two blue arrows is significantly less than 1/10". To me this looks like being closer to 0.1246" and 0.0754" than to equal distances of 0.10".

Agreed. It looks more like Acme from the latest picture.

Hello,

Any links / threads / pictures that would help me better understand this idea please ?

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Bill,

It's a simple concept ... Two nuts, each can be a little sloppy-fit on the screw: Then put a spring or an adjuster between them.

To be honest: it's sometimes a bit of a fudge ... It only works well if the thread pitch and form are well-matched on all three components [screw and two nuts]

MichaelG.