Minimum Reasonable Thickness for Threading

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Mattaus

Mattaus

Flashlight Enthusiast
Joined
Mar 29, 2011
Messages
1,765
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Brisbane, Australia
Hi all,

Machining noob question time! Please forgive me for any incorrect terminology :D

As usual my mind is wandering and I'm drawing up some flashlight designs while I wait for various other projects to come to fruition. I need to be mindful of what is, and is not, possible when machining parts both by hand and on a CNC lathe.

So as the title suggests, what would you deem to be the smallest reasonable wall thickness for threading, and as an extension of this what length should said threading be?

Referring to my crudely drawn picture below:



The 2mm dimension is what I am talking about when I say wall thickness, and the length is the 44mm. Please note these are not the dimensions I am thinking of using - to be totally honest I'm looking at 0.5mm thickness and 2-4mm length, or thereabouts.

It is probably worth noting these threads are intended for tiny lights - so no joints will be load bearing, and even if they are it will be minimal at best.

Any advice would be greatly appreciated.

Thanks,

- Matt
 
I reside in the US, and tend to use imperial measures. I dabble in locks, so the lengths of a .003, .005, .010 and .020 inch gap is pretty easy for me to visualize. :)

The .5mm is close to .020 inches, and that's fairly thick. I have made threaded pieces with the walls only .010 thick. How thin you get away with depends on what kind of abuse it will take and what material it's made from. 6061 AL bends much easier than the 7075 alloy, for instance.

The length of the thread is really based on how the thread is used. If you are talking about a fastener that holds two pieces of steel together, the threads should be about as deep as the hole is wide. Or is that 1.5 times.... Someone will correct that, I'm sure.

Since we are talking about non load bearing, the the thread length should be at least 4, to give the user a good feeing about it. If there is only 1 or two turns it feels insecure to some folks.

You should also look at lights that you like. Many of the better ones have an un-threaded lead-in to help align the parts. Others have an unthreaded area on the male part that fits closely to a "skirt" on the mating part. This provides mechanical support so the threads are not the point that gets stressed.

Good luck,

Daniel
 
Thanks Daniel. Just to clarify when you say "the thread length should be at least 4" do you mean 4 times as deep as the hole, or a length in millimeters (4mm or 0.16in)? I'ma ssuming the latter.

I was planning on having a lead-in and a 'stop point' which I guess is the same as the skirt you refer to.

I have my favorite EDC light here (a 4Sevens Mini CR2) and it's join uses 5mm of threading, but if you include the very small lead-in and an o-ring the total 'contact' area comes to 9mm in length. The light also uses standard aluminum that is much thinner than 1mm.

- Matt
 
Glad to be of help.

I meant 4 threads long. Much more than 4 will not really increase the strength of the threads by much.

Many lights have threads that are much longer than what is really used. One of my lights has 10 threads and only turns 4 times before it bottoms out.


Which brings me to another point... As you plan your light, figure out where you need bare metal to metal contact and where the threads will bottom out. Make sure that the threads don't pass through the o-rings or it will cut them up.

Daniel
 
Yeah, I'm taking particular care to make sure no threads pass over o-rings at any point. I'll aim for 6 to 8 turns as well - the light is a twisty and I'll want minimum travel as possible between off and on so the threads will be very short anyway.
 
Not certain what alloy SF uses but believe it is 7075-T6. On the E-series lights the bore dimension for an 18650 cell is roughly 18.54mm & the minimum OD is 19.05mm, leaving a wall thickness of .255mm (.010")

I have made threaded pieces with the walls only .010 thick.
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That's what I'd want, at least .255mm. More is always better.

... to be totally honest I'm looking at 0.5mm thickness ...
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Sounds like that should work. FWIW an unmodified 6P has a minimum wall thickness of 1mm.
 
Thanks Barry. From what you guys have said it looks like I'm actually over engineering the light, which lets be honest is never a bad thing :D
 
Mattaus said:
Yeah, I'm taking particular care to make sure no threads pass over o-rings at any point. I'll aim for 6 to 8 turns as well - the light is a twisty and I'll want minimum travel as possible between off and on so the threads will be very short anyway.
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Twisty's are a special case.

To get positive on/off with 1/4 to 1/2 twist of the wrist, you need either coarse threads or multi-start threads. If you use fine threads (24 TPI, for instance) the battery contacts will not be very far from the battery when it's off. 1/2 turn will move the tailcap 1/48 of an inch, so the slightest breakdown of the battery cushion will cause it to turn on unexpectedly. That is why the Surefires with twisties use very coarse threads.

Multi-start will allow a shallow thread and still have a lot of movement per turn.

Daniel
 
Hmm, I didn't think of that at all. It might not be as much of a problem for my light in that I am using no spring in the tube, only a foam spacer. If the spacer breaks down the battery will rattle worst case. If the tube is screwed in tight enough to cause the light to turn on then it's intended to be on. That being said I am looking for ways to get around the foam spacer requirement by using a more robust system involving wave springs.

Food for thought anyway. Thanks again.
 
Mattaus said:
Hmm, I didn't think of that at all. It might not be as much of a problem for my light in that I am using no spring in the tube, only a foam spacer.

Food for thought anyway. Thanks again.
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That's why I mentioned it. :)

May I recommend that you consider using some spring for one end of the battery? I have many twisties and the biggest flaw in many of them is that you can tighten down the battery tube or tail cap too much. One one light I broke the circuit board, on another the battery was compressed enough that it broke the seals and it started to leak.

You can plan around that by very carefully planning your dimensions but batteries vary in size so you will never be able to say with certainty exactly what size the user will be using. A small wave washer on the negative end is usually enough, just make sure it does not fully compress before the threads bottom out.

:) Of course, you can just print a disclaimer about "Don't over tighten" like many manufacturers do. hehe.

Daniel
 
gadget_lover said:
Of course, you can just print a disclaimer about "Don't over tighten" like many manufacturers do.
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I don't cut corners...normally. Besides - I want my light to stand out, not be like all the rest :)

PEU said:
If you are building a light for scuba diving thats not a problem, but for a small light its actually a bad thing
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I'm genuinely interested to know what makes you say this? I see myself as more of a designer and I'll rarely physically machine anything myself. Anyway I can improve the design from a manufacturing and/or user perspective is important.

Thanks,

- Matt
 
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