New to threading: Need to make male threaded cap

[va2fsq]

Hi,
I'm new to threading never having done this before. I have a telescope that has a small polar scope that fits into the polar axis of the mount. I want to replace this whole thing with a threaded cap since it sticks out too far.
So the threaded part of the polar scope is 24tpi, and it is 1.016 inches in diameter. The hole where the cap will go in (female part) is 0.967 ".
So I want to make the part out of Delrin or Brass,
How do I start? Do I just machine the outside diameter of the male part to 1.016 and then make the threads 1.016 - 0.967" = 0.049 deep?
Thanks, Tom

 

[YYCHM]

So, you're measuring OD of the male threaded part and ID of the female threaded part?

Sounds to me you're on the right track.

Aluminum might be another option to and easier to source.

 

[Dabbler]

Brass will be easiest to thread for a beginner.

 

[RobinHood]

How do I start? Do I just machine the outside diameter of the male part to 1.016 and then make the threads 1.016 - 0.967" = 0.049 deep?

Hi Tom,

That is basically correct. Have a look at this website, it’s all about threading. http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx

Your thread depth will actually be 0.049 / 2 = 0.0245. That falls within the tolerance as shown here for a 1”-24TPI UNS, 3A thread.

Not that that really matters since you are making a custom cap to fit your application...

I don’t know if you could test fit your telescope onto the cap while you are cutting the threads in the lathe - might be too big/awkward to try and thread it on.

If you can’t, I would recommend measuring your existing polar scope’s threads “over the wires”. The app above gives you the “best wire size” to use. Pick the closest to them you have, input that info into the calculator and hit “re-compute”. Take an actual measurement and note the result.

Start cutting your threads and as you are getting close (just a visual observation), measure over the wires. Keep increasing the depth of cut until the new threads’ over the wire measurement equals the polar scopes’ measurement (the actual measurement you took before). Deburr, polish with some emery cloth and when you take the new cap out of the lathe, it will fit the telescope perfectly the first time.

 

[Tom Kitta]

It feels this is the thread - https://www.amazon.ca/Right-Hand-Th...NKYZ7Z585RQ&psc=1&refRID=TCTC93JRVNKYZ7Z585RQ

Check all thread parameters on it and make a cut - use all hints above.

 

[PeterT]

Practice on some scrap so you get the general hang of things - incrementing the depth of cut, making sure the threads are clean of burrs, measuring consistently. One of the issues that can arise on parts with short threaded sections like caps is how to terminate the threads cleanly. You typically want a 'relief' groove slightly deeper than the thread minor diameter at the end of the threads which is where you disengage the threading feed (when threading under power). This can be a bit nail biting even at slow rpm. So there are all kinds of recommendations about inverting the tool, beginning the thread in the relief groove & threading 'away' (from headstock to tailstock) vs. conventional TS > HS. But this is a function of your lathe capability & tooling. The other way is to thread conventionally but finish by hand rotation. Or as this video shows entirely by hand. Depending on your part dimensions & material, this video might provide some other methods to consider.

 

[va2fsq]

Thanks for all the info. I got the thread pitch from someone else, but then decided to double check... Turns out it is not 24tpi but somewhere between 32 and 40. I suspect it is metric. So I'm waiting for the metric thread gauge. This may be a problem since my lathe has only imperial threads.
Tom

 

[Dabbler]

By carefully calculating your change gears you can also cut almost any metric thread. Just a lot of work, is all. (and making a custom change gear for your lathe).

 

[YYCHM]

Your thread depth will actually be 0.049 / 2 = 0.0245.

Good catch Rudy! I missed that detail.

Craig