Wooden Worm Screw

[midnightlunchbox]

I need to make a worm screw for a project but I'm not sure how to mark it out. I need the outside diameter to be 25mm with a 4tpi thread. I've looked on the web but I'm not finding anything that is easy to grasp. Can anybody help?

Cheers
Craig

 

[J-G]

I need to make a worm screw for a project but I'm not sure how to mark it out. I need the outside diameter to be 25mm with a 4tpi thread. I've looked on the web but I'm not finding anything that is easy to grasp. Can anybody help?

It might seem a little odd to have a metric O/D with an imperial pitch but the O/D doesn't matter that much as far as layout is concerned and it may well be easier to measure 1/64" or 1/32" rather than .375mm or .75mm.

The first thing to do - after you have the work turned to whatever diameter you decide - is to mark lines along the length every 22.5° (16 lines) or 45° (8 lines) depending upon how accurate you want it to be. Obviously using 16 lines would be better but will be more tedious.

Then, starting at the tail-stock end (for a right-hand thread) make a mark 1/64" (I'll assume 16 lines) along line one, 1/32" along line 2 (turning the work anti-clockwise), 3/64 along line 3, 1/16 along line 4 etc. etc. increasing the distance by 1/64" for each line until you have covered the length that you want. It's then just a matter of 'joining the dots'.

If you decide to use 8 lines then make the increments 1/32".

 

[midnightlunchbox]

That makes perfect sense, thanks for that. So I would be right in thinking if I wanted to alter the threads pitch I just increase or decrease the sizes along the longitudinal lines? Right, I think I'm sorted, it's quite simple once it's explained.

Cheers
Craig

 

[J-G]

That makes perfect sense, thanks for that. So I would be right in thinking if I wanted to alter the threads pitch I just increase or decrease the sizes along the longitudinal lines?

Correct.

Right, I think I'm sorted, it's quite simple once it's explained.

You've understood the basic principle - now you can experiment with increasing the number of lines and/or increment.

Should you want to make a larger diameter item then more line might be needed (to make a smooth curve) simply make the increment 1 / number of lines x pitch.

 

[dickm]

... or make yourself a thread chaser, and do a lot of practice with it