My son is crazy about flashlights and spends almost all of his online time looking at lights and specs and spends as much of his money as we allow on buying lights.
He recently designed his own very tiny light and bought the parts to make it, but he ran into trouble trying to tap the 9mm (1mm thick, 7mm inner diameter) brass tube.
The tap he bought is made by Uxcell and it is an M8 x 0.5mm plug tap We’ve tried with motor oil as a lubricant and mounted the tube in a vice since we don’t have a lathe or drill press.
The tap cuts the brass, but just seems to shred it and we can’t get a clean thread. We’ve watched multiple videos about tapping and both tried multiple times.
The Uxcell die he bought for the tube that will screw into the 9mm tube cut just fine using the same technique.
My son thinks this may be an issue with the tap relative to the tube size since he read that in general you subtract the tap pitch from the diameter to get the ideal inner tube diameter which in his case should be 8mm - 0.5mm = 7.5mm. Alternatively, we’re not sure if this is possibly an issue with the quality of the tap, our process, or something else.
I will be happy to spend money on a higher quality tap if there are any recommendations, but wanted to see if there is anything else that we should try first.
If the inner hole your trying to tap is 7mm and your tap is 8mm x.5 you need a 7.5mm hole for the tap to work.
It sounds like the hole is to small and the brass is over loading the flutes causing galling. Brass is fairly soft anyway and should be easy to thread with the proper hole size.
Sometimes like on Titanium I will actually over cut the hole by a few thousands over whats recommended just to make it easier to tap and Titanium likes to gall anway.
Try drilling the hole with a 7.5mm drill bit.
The wall thickness doesn’t support the shear forces in tapping. Firstly, the suggested hole size for an M8 x 0.5 is 7.5mm. Your tube is 7.0mm and thus should be re-bored. Brass is not structurally sound in thin walls, i.e. it will shred when torqued. Either get a starting tap where you have 4 or 5 threads that do the cutting - easing the cutting. Or settle on less thread engagement; say 60% - 7.52mm (.296” 19/64 drill). Could also go to 7.6 mm (~50%) which I would recommend with a plug tap.
Also, some tap profiles don’t help in cutting thin-walled tubing. The spiral taps help in his regard. But make do with what is at hand.
0.5mm thread pitch…? That’s very fine and with fine threads it is imperative to keep the tap straight when cutting. I doubt very much that I could keep that straight if I were using hand tools.
Thanks all for the confirmation. Should have checked that tap size drilling chart first so that was helpful.
Found a friend with a drill press so we will try:
Boring the hole to 7.5mm or 7.6mm
Tapping with the drill press
My son is also working on a redesign with alternate tubes in order to maintain the 1mm wall thickness in case there are still issues.
We’ll post the results later!
It helps to braze the tube to a piece of solid stock which you can put in a vise and not crush the tube. Then cut off the threaded section plus whats you need for length. What cells, hearing aid batteries? They don’t have very much juice in them. You’d be better served using a 10440 cell.
