# What do I need to measure how deep to thread?



## Pcmaker (Nov 25, 2019)

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


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## JimDawson (Nov 25, 2019)

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.


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## benmychree (Nov 25, 2019)

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.


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## T Bredehoft (Nov 25, 2019)

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.


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## benmychree (Nov 25, 2019)

T Bredehoft said:


> 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 ---


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## T Bredehoft (Nov 25, 2019)

benmychree said:


> 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. ....


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## benmychree (Nov 25, 2019)

T Bredehoft said:


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


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


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## Pcmaker (Nov 25, 2019)

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.


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## Pcmaker (Nov 26, 2019)

JimDawson said:


> 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?


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## JimDawson (Nov 26, 2019)

Pcmaker said:


> 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.


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## Pcmaker (Nov 29, 2019)

is there a chart that tells you how much to take off in inches when threading, organized by thread pitch? I figure that would be very helpful


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## T Bredehoft (Nov 30, 2019)

Depth of thread equals 1/2 of thread pitch. (Only app;lies in 60º threads)

16 pitch thread is one thread every .0625 inches, half of that is .03125.

20 pitch equal one thread every .050 inches, half of that is .025.

And so on.,


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## savarin (Apr 9, 2021)

So does this mean the depth of an M1 thread is 0.5mm?


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## benmychree (Apr 9, 2021)

The basic fact is that one cuts a thread until it nearly comes up sharp, then try a nut or gage on it until it fits, having lightly filed the OD to remove burrs.


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## Flyinfool (Apr 9, 2021)

That 1/2 of the pitch for depth is on one side of the thread. or the actual distance the cross slide must move the tool. Remember most lathes have the cross slide dial calibrated in amount removed from the diameter not the distane the tool is moving.


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## Winegrower (Apr 9, 2021)

The book number for a sharp V is 0.866 * 1/tpi.     This never seems to be deep enough for me.


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## savarin (Apr 9, 2021)

The reason I'm asking is that I will be attempting some quad start M1 threads to screw two tubes together smoothly so I need to know how much extra metal on the outer dia of one tube the thread needs so there will be enough metal to cut the thread.
If I cut the female thread first then I am assuming the male needs to be 1mm larger dia for the thread to be deep enough.


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## savarin (Apr 10, 2021)

successfully cut both male and female quad start threads but my calcs regarding the diameters didnt go well.
The threads were rough as they were single pointed with the hand crank in aluminium.
My tiny internal threading tool flexed a bit the the female thread tapers a tad.
I then had the thought of using a M6x1 tap as a cutting tool mounted in the tool post and cleaning up the threads.
This worked so well I believe it would be even better to fully cut the threads using the tap.
I will try again as I have a M12x1 tap that wont flex quite so much
As I want a quad start the geometry works spot on.
Now to refine my calcs on the tube diameters again and have another go.


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