Minimum wall thickness after tapping 18mm x 1.5 into a .875 (OD) diameter tube.

[rons]

It was asked before and I was looking at a old thread yesterday. Searched for it today and can't find it. There was no definite answer. Just sort of do what looks right.
The background;

Made an adapter for a paint gun cup connection. The paint gun has a 18MM x 1.5 external thread. The paint cup bushing has a 18MM x 1.5 internal thread.
There is also a metal seal connection at the top of the paint gun connection and the inside of the paint cup.
They advertise that no plastic seals are used because the machining of the gun is so tight.




The adapter accepts 7-16-14 threaded cups. Like the plastic and metal ones made by Devilbiss.



The last pic is a part that started out with diameter = ,875. The straight turned down section has a diameter = .775
The 18MM x 1.5 tap outside diameter is .715. The thread length is .325. The overall length of part is .875.

What I call the "seal area" is the part of the connection that stops paint/solvent from leaking.
The original seal area is a circular ring at the top of the metal tube on the gun. I tried to duplicate that but could not
make the internal step very accurate. So I use a cylindrical HDPE (white) piece that is forced down by my bushing part.
The intent of the force is to make the seal area inside the bottom of the gun tube.

I know this stuff is made using chrome plated brass but I've seen that peel off, including in my collection.

The part (last pic)still feels heavy like a socket. I wanted to reduce weight and diameter so I measure the original large
paint cup bushing. It is set in that plastic and I made the diameter to be .775. Now I am trying to learn what limit I can push
it and still have a working piece that is light as possible.

I think that a set of table can be made for minimum thread wall thickness using variables like pitch, overall diameter,
type of metal, length and height of thread, etc.

From calculations:




tan 30 = (P / 2) / H

H = .866 P

H = .866 x 1.5mm = .0511

Thread height = .625 x H = .0319

The outside cylinder surface to the inside thread will vary from (.775 - .715) / 2 = .0300 and .0300 + .0319 = .0619.
I'm looking for the limit dimension. (And for something to do)

 

[Bill D]

There is a standard for nut size across the flats for each standard bolt size. I would think that would be a good minimum diameter to use. If the nut size is too large a standard socket can not fit into recess pocket to reach it. Of course they do make heavy nuts of. a some different standard larger sizes.
You use has no real pressure and very minor load on the threads so you can safely go below minimum.
Bill D

 

[Bill D]

a 7/16 nut is a 21/32 wrench size.
The British call out bolt size by head size not shank diameter. That is how I learned metric sizes on cars. On my 1977 Datsun 10,12,14mm covered 90% of the wrench sizes

 

[rons]

There is a standard for nut size across the flats for each standard bolt size. I would think that would be a good minimum diameter to use. If the nut size is too large a standard socket can not fit into recess pocket to reach it. Of course they do make heavy nuts of. a some different standard larger sizes.
You use has no real pressure and very minor load on the threads so you can safely go below minimum.
Bill D

A way I do it is cut a 15 bevel on the 6 points until a circle is created on the side of the nut. When the circle reaches the middle of the flats then I stop. Do both sides.
But the dimension is way past what a common nut would be.

Some people collect coins, I collect stuff like this old indicator light housing. It's 27 TPI (.9407 mm).
The wall thickness of the left-side piece is .066 and it is chrome plate brass.
The right-side is .049 and it is stainless steel.
The stainless side is thinner because of stronger material.

My part is .065 thick and 1.5mm per thread. I could shave it down a little. But since the thread height is a little more for a larger pitch thread,
I would not want to go less than .050. Up to this point the design has just been about what looks right.

Any mechanical engineers know where these dimensions come from (like on the indicator light) ?

 

[rons]

Final{
Tap a thick wall cylinder with tap (.715 diameter).
Turn down outside of cylinder to .750.

.018 is the thickness between a spiral groove on the inside thread to the outside surface.