What drill bit for drilling 1 inch long 5mm hole through 10mm grub screw?

2CV's are mostly 7mm and the odd 9mm. No-one seems to have mentioned tap drilling a bigger hole in hard material, eg 5.2 mm for a 6mm tap.Glad it's fixed now.

I'd only use a 5.2mm drill for M6 when tapping plastic. I use 5.3mm as standard for most metals, but might go to 5.4mm for tough materials like stainless steel.

Andrew

I thought it was meant to be 5 for 6 in everything, shows how much I know! What's the reasoning behind all this, just so the threads don't have to cut in so deeply? Will this not mean theres a bit of wobble with a gap between the shaft of the bolt (the narrowest bit in between threads) and the outer material in the hole, if that makes sense?!

My basic information came from 'Drills, Taps and Dies' by Tubal Cain in the Workshop Practice Series, not from industry. I use the same basic tapping drill sizes irrespective of whether I am hand tapping a couple of holes, or machine tapping hundreds of holes.

There is no point in having a full engagement thread. From a theoretical standpoint the roots and crests of threads are specified with clearances. So even if you have perfectly formed threads the engagement is not full depth.

As Chris correctly points out most taps extrude the thread slightly as well as cutting, so drilling on the basis of 100% depth of thread could be problematic. Again, as pointed out by Chris, drilling for a percentage of full engagement greatly reduces the torque needed to drive the tap. However, there is also the strength of the thread to consider. There are two possible failures; one stripping of the thread, or two, failure of the bolt. Stripping of the thread is determined by the shear strength of the material, and failure of the bolt by the tensile strength of the bolt. Generally the shear strength is higher than the tensile strength, so in most normal combinations the bolt breaks before the thread strips.

This is backed up by experiment. Some years ago I did some trials with M4 threads. The core diameter for M4 is 3.14mm, so you might drill 3.2mm for 'full thread' depth. I drilled and tapped a scrap piece of 6082 aluminium alloy with 3.4, 3.5 and 3.6mm drills, and then hand tapped the holes. I started off with a high tensile (12.9) SHCS in the hole drilled 3.6mm. The bolt broke before the thread stripped.

Any questions?

Regards,

Andrew

The core diameter of an M6 screw is 4.917 mm, so a cross sectional area of 18 mm2

For a full form nut the hole through the middle has an area of 19mm2 and the cross sectional area of the screw (including core and thread) is 28mm2.

The area of overlap of thread on screw and nut is 29-19=18mm2.

So, to a first approximation two turns of thread have the same cross sectional area as the screw core for M6, which is why screws so often shear, rather than strip.

Neil

Measurements from here: **LINK**

When I drill capscrews, I drill quite slow, a 5.1 or 5.2mm drill I would use about 220 to 300rpm range and keep coolant or oil on the drill. If the oil smokes, I slow the drilling speed down.

Neil

Yes. If you would check this M4 thread in the 3.6 mm hole with a go/no-go thread gauge, would it still pass that test?

Remembering from a dim past that I had quite often rejected jobs when the internal threads showed too much truncation. Some customers can be very sensitive...

Regards, HansR.

That would depend on which feature of the thread the go/no-go gauge was intended to measure; what did you have in mind?

Fortunately I don't have to worry about your customers, only mine.

Andrew