Threaded boxes numbers

[C Edward Moore]

As the new owner of a threading jig I find that there are numbers that I need to know in order to get the results I want. Today I watched the DIY Network show about threaded boxes that was on at noon and was disappointed in what was provided and what was not provided. There was a relief groove cut in the lid that was visible, but it was never mentioned. A transfer from the outside diameter of the threads on the bottom was made to the top. If the top was then hollowed to that dimension, the resulting threads would have been totally outside the the bottom threads and would not have met. There is a number that should have been subtracted from that dimension to determine the inside diameter of the top before threads are cut. I was disappointed by the sloppiness and incompleteness of this presentation.

So my question becomes "Where can one find the numbers one needs to make a threaded box?". Trial, ERROR, and a growing scrap pile may eventually lead to what I seek, but I am certain that someone somewhere has published an article on this and has the numbers worked out accurately.

 

[Nick Stagg]

Threaded box numbers

Ed,
I don't know what threading jig you have. I have the Klein Jig so I can give you those numbers.
After hollowing the lid and making sure the sides are parralel the "step" on the inside of the lid should be 1/16' deep and wide and then chamfer it to 45 degrees so you leave a gradual thread when you start.
For the base measurement: measure the inside diameter of the threads on the lid, do this in three places to be sure, and add 3/64" . This then becomes the size of the male thread. make a tenon about 1/8" long on the base a little oversized from the 3/64" measurement and hollow the base and finish the inside to completion. Now recheck the tenon size on the base, sometimes wood movement after hollowing will change it, and extend it to the length you want for the thread. You should now be ready to thread the base.
Hope this helps, Nick

 

[C Edward Moore]

Thanks, Nick.

I have done a little trigonometry for the 16 tpi cut by a 60 degree cutter. The depth of the thread is the square root of 3 divided by 32, which comes out to .0541265 and since we are dealing with diameters, twice that amount is .108253. So, if we add 3/64 or .046875 to the inside diameter of the top, this leaves .0613781 play or about 1/16 of an inch.

So, the question becomes "Is 1/16 of an inch the appropriate amount of play between threaded surfaces in small boxes?" I will admit that this seems a little loose to me.

Would 1/32 (.03125) of an inch be too tight or too loose?

Tomorrow I guess it's back to the trial and error. I am aware that if a box ovals a little then the tolerances are affected. My hope is to get a good test done where I add 1/16 or .0625 to the top threads diameter and see how it goes.

 

[Nick Stagg]

Ed,

1/16" would be too much play, I doubt that the lid would stay on. I would be thinking in terms of something less than 1/64". You need some sideways movement, but too much won't allow the lid to tighten down and it will act like a "stripped" thread.

Nick