What drill bit size to use?

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#41
Lo_Tse,

The engineering of threaded fastener attachments is a rich field of non-trivial technical depth. Most of the complexities can be greatly simplified for audio equipment sitting on a shelf once built...the tough stuff is cars, aerospace, and mechanical equipment where high speed motion and forces of many g's are the norm. If you get a good tap-drill chart covering metric and inch sizes up to 1/4"/6mm and follow the rules of thumb for clearance hole dimensioning (the clearance hole should be ~120% of the major diameter of the fastener) you will get good results. The fundamentals are disciplined layout of mating parts using relaible measuring tools and squares. The old adage is the best guide: "Measure twice, cut (or drill) once!"
 
#42
I have to apologize to the original poster for overly confusing this thread. Pun not intended.

I found ISO specification 965 which gives the limits of the major diameter of an M3 thread. They are 2.980 max and 2.874 min.
 
#43
Peter Daniel said:
And what exactly are you doing with those 3mm holes? Are you building a rocket? 😉
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Nope, not a rocket. The only rocket I have built was out of balsa. (Though my late company did supply components which NASA certified for space.)

I had what started off as a simple project, involving a few audio connectors mounted to an extruded aluminum panel. What I didn't realize until I got it is the panel is not flat. There is no way (that I know) to use a nut on the M3 screw because the rear surface of the panel at that point is not orthogonal to the axis of the screw.

But I have a plate with threaded holes for M3 screws which will mount on the back of the connectors. As long as the assembly is centered top to bottom the plate should be orthogonal. If the centering is off there will be a cant to the plate.

Of course if I had a milling machine I could just mill the section of the panel flat, but that is for another thread!
 
#47
You might also be able to spotface the rear of the panel at the locations where the screws are to provide flat/square bearing faces for the nuts. The easiest approach is that suggested by vinylkid58 if the panel is thick enough...just tap it.
 
#49
kevinahcc20 said:
You might also be able to spotface the rear of the panel at the locations where the screws are to provide flat/square bearing faces for the nuts. The easiest approach is that suggested by vinylkid58 if the panel is thick enough...just tap it.
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I like the idea of spot facing with an endmill as plan B, if for some reason the plates don't work. I am less keen on tapping the aluminum. Pretty confident the plates will be OK though.

Waiting now for the rest of the parts I need to set up and run my drill. I started this project over six months ago.
 
#50
Conrad Hoffman said:
For something a bit nicer, order a Balax forming tap. You'll need a very specific (and larger) drill size, but the tap will form the threads by deforming the aluminum. No chips at all and very strong threads. The taps are much stronger than cutting taps and last near to forever.
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I had never heard of Balax taps but have been reading up on them (one of the first Google hits was your website, by the way). So far I have not been finding much in the way of part numbers or dealers. Where is a good place to buy them in the U.S.?

I still suffer a prejudice against tapping aluminum. I used to make cameras for a living and for a while we tried diecast aluminum enclosures with tapped holes. The threads never held, or if they did the holes were crooked. The enclosure machining work we contracted and some of the workmanship over the years was terrible.

The reason I am back to thinking about tapping is that I spent a couple hours yesterday fitting things together. The steel plates I have been planning to use don't fit so well when used as intended. They fit very well upside down, but then as I noticed the holes don't line up to the holes in the connectors. I confess I failed geometry in school and have difficulty with concepts such as "left" and "right".

So far I do not understand thread tolerance. The material I am working with is 1/8 inch extruded aluminum. Is this too thin to sucessfully tap? What would be the best tap to use and where could one buy one?
 
#53
1/8" aluminium is thin for a tapped hole.

It will be OK for a weak fixture >= 1/8" diameter.
It will be better for threads from 2mm to 3mm.
1/8" will give a relatively long thread engagement for <1.5mm diameter.

I would suggest that aluminium needs to be about 2.5 times to 3 times thicker than mild steel for the same pull out strength.
 
#56
He already mentioned that he is not building a rocket, so for regular use, 1/8" aluminum is fine for tapping, providing it's done properly.

Tapping is your best option. If you don't want to tap, you may consider using threaded insert as a nut: http://www.amazon.com/4-40-Brass-Th...5?s=industrial&ie=UTF8&qid=1326896605&sr=1-25 With some additional countersinking on the holes, it should adjust enough for curve fitting.
 
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#57
Just giving a figure as people have mentioned areo/mill uses ali parts, if you can achieve a 6mm thread depth in Ali then you know its gonna take some stick. 3mm is often used for rack mounted panels, and provided its not screwed in and out it can also take some force
 
#59
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