Questions About Designing Threads and Press Fits

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Akya2120

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I am trying to design an electronic cigarette mod. Pretty much just a flashlight with a momentary switch and a heating coil rather than a LED or mono-filament bulb.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 was thinking I could use a 3/4" tap for the threaded hole as this is quite close to 19mm. I would like to use my lathe to make the threads but I feel a tap will be easier as I am inexperienced with this. The battery tube will be stainless steel and the top cap will be copper. The atomizer connection is a 7mm x .5 thread. Again I plan to use a tap for this, I am also not sure if this small of a hole would be possible for me to thread with my lathe but I would imagine so.

The bottom cap will be a press fit. It will be a copper bottom cap pressed into the stainless steel battery tube. What is the procedure for press fitting? How close of a tolerance do I need for it to press in and hold properly?

Thanks in advance for the help guys! I look forward to learning how to use my lathe and maybe one day getting a mill. I wish I had the money for CNC but it's just not there.
 
0.5mm may be enough clearance for the battery, but you might be happier with 1mm, especially if there is a lot of variation in roundness. Bore it out, then try the fit, and remember, it's easier to make holes bigger than smaller ;)

As far as threading a tube with a tap... I have done it, but you tend to deform the tube (the threads are visible on the outside). Single pointing will likely give you a better result. You probably want a finer thread pitch than even a 3/4" fine will give you anyway. If you have spare material, practice first, as internal threading is tricky. Actually, if you haven't done much single point threading, I would suggest doing the "plug" side first. As far as the 7x0.5, I would use a tap if you can find one. Enco has them for about $6.

There are a variety of different press fits, and they are classified by the force required to assemble the parts. I'm guessing you are going to want a light or medium press fit, something between 0.000 and 0.001" interference. That should be a decent fit for anything up to about 2in in diameter. At the higher end of range I gave, you will need a small press to assemble.
 
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I have not done any threading with my lathe. Single point threading is what I want. I just need to learn the threading procedures. I don't have a thread count indicator(I believe that is what it is called). So I will have to do it the "old school" way where I never disengage the half nut and just back off reverse the feed direction. I'm not sure how I would get my tool inside the tube that I am threading... Can I make threads on any size tube or does it have to be a specific size? Thanks for the quick response :D
 
Basically, within a few limits, you can put just about any thread you want. If you want a 3/4-10, that will require a given wall thickness, determined by the strength of material and thread height (or depth, to some people). There are Unified National Standards for all of this, which is why you will see 3/4-10 as UNC, 3/4-16 as UNF. and 3/4-20 UNEF. But if you want a proprietary thread, nothing would stop you from making a 3/4-12, for example. Or 3/4-24. Single point threading allows for this type of thing at a minimum cost, but to purchase non-standard taps and dies is pretty expensive. You can get them made in virtually anything you want, provided you will spend the money. IN general, on thin tubing, you probably want to go with a finer pitch, due to its reduced height (depth).

Don't be too stingy with the clearance on the bore for the battery. When they die, they can swell, and if left to corrode can be nearly impossible to remove.

Press fitting dissimilar materials can be "sticky", if you'll pardon the pun. Some materials love to gall and seize. Around the sizes you are talking about, a light press to very light would be sufficient.

If you need the detailed dimensions for a particular thread you want to try, I can furnish a datasheet from a nice piece of software I have. Or if you have Machinery's Handbook, all of the standards you should consider are there.
 
MSC has 3/4-40 taps for $54... but remember that the minor diameter of the thread will be less than 3/4. They also have a 7/8-40 tap.
 
Oh, and to address the threading of the ID. If you can't/don't end up tapping it, and do thread it on the lathe, you have a few options. You can buy a solid carbide preground threading bar, suited for small hole work. They come dear, and I can't say I'd recommend them for a first-timer. You can buy inserted threading bars, again with carbide, but replaceable inserts. i have one that will get into the range you are working that uses a single point insert that I love. Initially higher than the solid carbide, but cheaper in the long run. Another option is a HSS tool bit you can grind yourself, including resharpening. Two basic approaches to this. A small round bar that is cross-drilled to accept a reclaimed end mill or centerdrill converted to a threading tool, or simply spend some quality time at the grinder shaping one out of a square HSS blank. Or there is another way. Some have very successfully used a lathe to turn a small disc on the end of a piece of drill rod to the correct profile and then relieved it properly and ground a cutting edge on it. This would have to be hardened and tempered.

I'm sure I've not covered them all, just the more common methods that popped into my tired brain this evening.
 
[To answer your first question.
You can tap any size thread on your lathe if it has enough power to tun it and a holding device big enouh to take it.
I have closed my shop and I may have a 3/4 10 tap out there I'll look tomorrow if I get a chance.

We made many press fits and we had a rule of thumb we followed. The hole should be .001" per inch smaller than the shaft.
In other words to press fit a 1' SHAFT INTO A HOLE THE HOLE SHOULD BE 1.OOOO" AND THE SHAFT SHOULD BE 1.001" tHE HOLE CAN NOT HAVE A SHARP EDGE AS IT MAY SHAVE THE SHAFT AND THE PRESS WOULD BE GONE.
iF YOU USE TWO DIFFERENT MATERIALS AND APPLY HEAT TO THEM THE DIFFERENCE IN THE COEFICIENT OF EXPANSION MAY MAKE THE OUTSIDE PIECE EXPAND MORE AND COME LOSE.
I should have books on all these subjects. Ill try to find them and offer them for sale in the forum.
 
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.
 
Tony Wells said:
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.
Click to expand...

Tony you are absolutely 100% correct , I really do have to learn how to type.
In reality press fits, shrink fits and threading get very complicated when you are starting out or have little experience.
There are facts and parameters that apply and can not be ignored or time and material will not only be wasted but you may also damage tools and machinery.
There was one book I issued to everyone that worked for me. It was The Machinist Handbook. (IMO) no machinist pro or amature should be without it.

I'll send you a PM when the books are for sale. It's KINDA LIKE SELLING MY CHILDREN
 
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