Questions About Designing Threads and Press Fits

[Tony Wells]

I share your feelings on the Handbook. It should be one of the first tools bought, IMO. Simply too much necessary information in it not to have one (at least) at hand. I keep one in the toolbox and one in the office.

I know how it is with books. It's hard to part with them.

You're quite right about the fitting of parts too. Lots more to it than meets the eye. But then, that's why we have experienced friends and fora like this one. People who don't know can learn. That is, once they know to ask.

 

[onecut jimmy]

Jimmy,
don't forget about the length of engagement. That's a factor for calculating the fit and tonnage required to assemble. That will be in your reference materials as well.

When you get ready to sell, I'd be interested in adding to my library.

I just looked at my books and the first one i picked up was the 1940 Machinist Handbook. That was befor my time. i started my apprentice ship in 1944. My Grandfather was a Blacksmith and My Father was also a machinist.
They taught me to avoid vexation and acquire knowledge.
I think you agree with that philosophy.
The 1940 book isn't much different from the modern one, where the basics are concerened.
I saw your post on carbide vs HSS concerning flaming. I agree .
I would also like to have a discussion on that subject. I think it would be educational for the novice.

 

[darkzero]

The battery I am designing it to accept is an 18650. Obviously it can not be and 18mm diameter tube otherwise there will be no tolerance for the battery to fit. I am planning to make it ~18.5mm

I come from many years of flashlight modding so my recommendation is that an 18.5mm bore is the small side. I usually bore to .735" for 18650s. Ironically 18.65mm is a good bore size for 18650.

I only use AW IMRs & AW protected cells. As you know 18mm is the nominal size of the cell but then you also have the wrap, label, & contact strip if using protected cells. Sure the label can always be removed but cell wrap thickness can be inconsistent & the the larger capacity protected cells tend to run ever so slightly larger should you decide to ever upgrade. And if you plan to be able to use cells not limited to one brand it's better safe than sorry to make the bore slightly larger.

So the 18.7mm bore size is a better size to go with for compatibility. Some 18650 bodies are even as large as 19mm to accomodate almost any 18650 out there but IMO that leaves cells a bit too loose for my preference. Keep in mind that when boring to test fit, even if the cell seems to have a nice free sliding but snug fit, when your device is assembled on one end & is air tight, the battery may no longer slide out as easy as you may want. Also having too snug of a fit there is a greater risk of snags which can tear the cell wrap leading to risk of a short. You might think that's not very likely to happen but for example if you were out and about & dropped the tube by accident denting the end leaving a sharp edge, that could lead to risk of tearing the cell wrap.

 

[Akya2120]

Wow! A lot of information haha. You are absolutely right, the 18mm is nominal. I had read somewhere that most of the larger 18650s measured 18.35mm and nothing really over that. Fifteen hundredths does sound a little small for tolerance though. 18.65mm sounds good to me, but it will probably end up being 18.75 because my lathe is only good to 1/4mms. Still probably more accurate than me though hah. Also it will have ventilation. Some people manage to vent these batteries and though I've never had this issue I don't want to run the risk of pretty much holding an explosive next to my face. Vents will be at the base so if it ever did happen it would go away from me.

So now I need to figure out a threading tool, a thread size, and thread count. Well the OD of the device will be 22mm, or as close to it as I can get. The ID will be 18.75, should the threads be for a 19mm hole? I would like the thread count to be higher, probably 32 or 40. I know TPI but I am not quite sure how the equivalent system works for metric. Let's say 19mm x 1, does the one mean one thread per milimeter of thread length/depth? Since the tube will be stainless steel and the cap will be copper, I would assume I shouldn't go to fine on the threads. Also I want to have the least thread length while still keeping durable. Mainly to keep size down as it will be carried in my pocket. I like the sound of turning the tool on my lathe and relieving it. I have an angle grinder but I've never used it to make a lathe tool. I also need to figure out how to aneal it.

I believe that was the book I ordered on amazon, I will read it when I get it.

As for the larger press fitting it will be about one inch long, maybe a little under, and 19mm in diameter. I will check my design when I get home. How thick should the copper press fit flange be? There will be a second press fitting for a momentary switch. They both need to be strong enough to hold up to droppage. I would like to have a way to remove them if there was ever a problem but I think I would have to ruin the bottom cap.

Thanks so much for all the input guys!