Drill bit sizing for taps

[Carl Fisher]

I've seen various conversations on this through searching, but seem to find some varying opinions.

Let's take the ever popular 9 x .75 tap.

I found a spreadsheet linked here somewhere that says for that tap you would use a letter Q drill bit which is a touch bigger than 8.4mm according to the drill chart. The other formula I've found for metric in this thread is to take the size and subtract the pitch, so in this case 8.25mm would be correct. That's just over .2mm difference in hole size (assuming no runout). Now to go a step further, a 21/64 falls between these two at a strong 8.3mm

Obviously I can try the 21/64 but before I jump out and buy some various letter bits I don't own or a very specific metric bit, I'd like to hear what others have to say.

Thanks!

 

[Twissy]

I use the subtract the pitch method. For the M9x0.75 I use an 8.2mm drill bit.
Cheers
John

 

[InvisibleMan]

I use the subtract the pitch method. For the M9x0.75 I use an 8.2mm drill bit.
Cheers
John

Me too.

 

[Carl Fisher]

Thanks guys.

So it sounds like an 11.2 would probably be about right for a 12x.75 as well by that formula. Off to hunt down some drill bits to order.

 

[LL Woodworks]

Subtract the pitch method & use the closest bit I have or the best one that works for you. For m9 x 0.75 I use "P" bit

 

[InvisibleMan]

Thanks guys.

So it sounds like an 11.2 would probably be about right for a 12x.75 as well by that formula. Off to hunt down some drill bits to order.

Yes, that's the bit I use for my 12.75.

 

[mredburn]

Try a 7/16 for your M12 tap it will work fine. I would also try the 21/64 on scrap material and see how you like it. Your results will depend on the material you tap. Some materials stretch as they cut and are tighter when your done. Some taps are better made and cut more of the thread depth than others. You may find you have to go to that slightly larger hole to keep the parts from sticking when you try and thread them together.

 

[frank123]

Victor has an 8.25 MM tap that works perfectly for the 9x.75 tap and a 10.25mm for the 11x.75 tap. I thread aluminum, brass, and urethane with those taps.

I haven't used a 12mm tap (preferring a thinner pen body) so I don't have a specific drill bit for that one.

 

[Timebandit]

I use the 21/64 for the 9mm and the 7/16 for the 12mm. I see no need to buy special sized bits that will only be used for these specific taps. These others are close enough that they work fine and i already owned them as most do. There is no right answer here. All drill sizes are theoretical.