Benefit of Acme Thread

Recently I was looking at several flashlights, and I saw discussion about Acme-type thread.

For example

It is a thread where the thread is made with trapezoidal shape instead of sharp edges. It seem like many users are interested in Acme thread instead of regular thread for flashlights. Acme thread is usually use for high load bearing uses, which is not required in flashlight.

I wanted to ask if this is something you prefer and like, and why is the reason?

I think it’s a bit less susceptible to cross-threading.

On, off, on, off, on, off, …, every time a battery needs charging, and…

Just looking at it seems it would be smoother and stronger.

If OP is asking what are the benefits, then my take is:

1- Faster (fewer turns) for given travel.
2- Shallower depth for given # of turns (thin-walled tubes).
3- Stronger load bearing without locking (overtightening can be undone).
4- No chance of cross-threading as with standard 60º threads with lower tolerances (the cheaply machined lights).
5- Cleaner look.

By the way - ACME is not the term but TRAPEZOIDAL.
ACME is 29º which is obsolete as WHITWORTH threads (55º).

I noticed some members also inter-change with the term ‘square threads’ which is itself another form (90º - parallel flanks) and some of its derivatives as ‘buttress threads’.

Thank you for the explaination, this is very detailed and I agree with your points!

I think one of the biggest concerns is we’re dealing with aluminum. I don’t know about anyone else but the second I’m screwing with aluminum I’m expecting things to strip.

Also if they’re chunky threads it’s harder to cross-thread, bonus points if it screws on faster.

While most are trapezoidal, I have an Olight and a Night Provision similar to the Folomov 18650s (gray) and possibly more that use 90° cuts so they are indeed square threads. Although the depth and the pitch are greater than the crest (top). So that would be a rectangle which is a variation of a square. You’re going to need a loupe or a magnifying glass to see this on some or most lights.

@ Oli:

Also with my S25S / S10 headlights. Although sold under Sofirn they are outsourced. A nice feature is the smoothness and speed to open / close.

My best observation dealing with these Metric Trapezoidal threads is they are great for flashlights. :smiley:
They maintain reasonable strength when machined fairly loosely. This is a manufacturing benefit, in that allowances for anodizing are less of a concern.
Basically they are machined to fit loose on purpose. Add a little grease and it feels tighter. Also a loose fit will not wear the anodizing on your threads as quickly. :wink:
Here is some easy reading.


Yes, put the tail cap on without a battery in and you can feel the loose tolerance a lot easier. It’s best to put a little tiny bit of pressure on the tail cap against the spring while you’re screwing and unscrewing with a battery in place.