What do I need to measure how deep to thread?

[Pcmaker]

I'm replacing the drawbar retaining cap on my PM25MV to accomodate a custom drawbar with a 15mm hex head on top.

I'm going to do some internal threading on the new cap and I don't know how much to take out while threading. So far, I have taken out .100

I don't know how much left I need to take out. I can't take the piece out of the lathe and lose my threading position

It's about .750 internal hole with a 1mm thread pitch

 

[JimDawson]

Normally for metric threads, the double depth of the thread is the thread pitch. In this case 1mm or 0.039'' assuming the starting ID (minor diameter) was correct.

If you have the mating part, then test fitting is the best way.

 

[benmychree]

I assume that the mating part is on the mill spindle ---- What I would have done is to make a plug gage by 3 wire measurement (or thread mike) to fit the thread; you could remove the chuck with the part in it, and with another chuck or collet, make the gage, then replace the part and continue the threading until the gage fits.

 

[T Bredehoft]

Drawbar? you've got loose collets, there's your check gauge If it won't screw on, take another .001 and try it again. Repeat.

 

[benmychree]

Drawbar? you've got loose collets, there's your check gauge If it won't screw on, take another .001 and try it again. Repeat.

I think that he is talking about the cap that screws on top of the spindle upper splined end, that retains the drawbar ---

 

[T Bredehoft]

I think that he is talking about the cap that screws on top of the spindle upper splined end,

Mia Culpa, what I get for scanning, not reading. ....

 

[benmychree]

Mia Culpa, what I get for scanning, not reading. ....

Don't feel like the Lone Ranger, I have done that all too often myself!

 

[Pcmaker]

I just "eyed" it and I got lucky. It fits pretty good

.. and now to make the actual drawbar itself

*edit*

Now that I think about it some more, I got extremely... I didn't even square up the threading bar to the work piece. I forgot to.

 

[Pcmaker]

Normally for metric threads, the double depth of the thread is the thread pitch. In this case 1mm or 0.039'' assuming the starting ID (minor diameter) was correct.

If you have the mating part, then test fitting is the best way.

If I'm understanding correctly, with metric threads, the minor diameter of the thread is half the length of the thread pitch? If the pitch is 1mm, the depth is 0.5mm?

 

[JimDawson]

If I'm understanding correctly, with metric threads, the minor diameter of the thread is half the length of the thread pitch? If the pitch is 1mm, the depth is 0.5mm?

Let's say you are making a M8x1 thread, the minor diameter would be 7mm, a M16x1.5 would have a minor diameter of 14.5mm. The Double Depth of the thread is the major diameter - the minor diameter. The Single Depth of the thread is 1/2 of the double depth, this is the amount that you would move the compound when single point threading.