Jim Foreman wrote:
> Lacking a chart, the way to get this drill size, you
> take the pitch
> of the threads, double that and subtract that number from the
> size of the
> tap. Since I was raised with and relate to inches, I simply
> convert the
> numbers.
> Take the M 5 X .9 size. That means that the outside
> diameter of the
> screw is 5mm (.200") and the thread pitch is .9mm (.036").
> Double that and
> you have .072". .200 - .72 = .128". Since that is only
> slightly off being
> 1/8", you can use that size drill bit.
Have you actually tried this - running a 5mm tap into a 1/8" hole - or are you theorizing?
Since the true drill size depends on several other factors, like depth of the hole, whether through or blind, the material to be tapped, and the rigidity of your drilling setup, you might get that to go when all of these other factors are in your favor. Normally 1/8" will be too small though. Sticking to fractional inch sizes, you'll need at least 5/32", and sometimes 11/64. I think you err in subtracting the thread pitch from the diameter, when it is the thread depth you want to subtract. Thread depth is always less than thread pitch.
If you drill the hole with a hand drill, it'll usually flop around so much that the hole size you get is noticeably larger than the drill size used. For critical applications, like classic unobtanium parts, drill the hole enough undersized that you are assured that there's no way it can go over; then sneak up on the size by drilling again with a bigger drill.
Also remember that cutting oil for aluminum is different from what you use for steel. Plain kerosene makes a better AL oil than the black sulphur oil commonly sold as cutting oil for steel.
I welcome corrections from real experts; I'm self-taught and I know I get some stuff wrong!
Mark Bulgier