Hole size to tap for 5mm?

A quick and pretty accurate tap drill size is diameter minus pitch, so for 5 x .9mm, drill 4.1mm.

Works with both inch and metric, even if the drill sizes don't always line up perfectly.

Cheers
Trev
 
I learned long, long time ago (45Years) back in old country that 80% size of hole is the rule (exept fine thread), so 4mm hole is the proper one to drill. Makes it 5/32" in here quite right on the nose! GR8
 
Yeah? Sure on that pitch? .8mm or .9mm?

From Machinery's Handbook 23rd ed.
For ISO Metric threads.
"Hole Size = Basic Major Diameter - (1.08253 X Pitch X Per Cent Full Thread)"

:)

Nope. I didn't do the calcs to see if it came out closer to 4.2 or 4.1 mm. For "not going to be inspected bt NASA" parts, either is gonna work just fine. That's accounting for the likelihood that person drilling the hole can hold it that close to size. Depending on the length of engagement, and a couple other factors, like just how much load there is going to be on the screw, you could go to a lower thread engagement percentage, like 70 or even a bit less(larger hole), to greatly reduce the load on the tap when threading, avoiding risk of breakage. The load on the tap goes down a lot faster than the strength of the resultant screw thread engagement.

Have fun!

Cheers
Trev
 
Like trevj says, in metric threads subtract the pitch from the diameter of the screw right across the board. Unless you are doing high tolerance work requiring a certain thread engagement percentage or pitch diameter. A slightly larger hole in stainless and other tough or hard materials makes tapping easier.
 
Thank you for all the responses.
It's a lower strength application (the 5 x.9 pitch (yes) ) screw that fits into a bushing in the bottom front of a M94 Swede Carbie to partially hold the bayonet plate/ lug onto the stock.
 
well - tired a couple on low grade steel.

5/32 was very tight - almost snapped the tap if I didnt keep it oiled and worked very slow with backing off every turn. Screw would not start in the hole.

11/64 was very good. Maybe a fraction too tight but it is loosening up quickly with a little use.

I think any bigger would be too loose.
 
farshot said:
5/32 was very tight - almost snapped the tap if I didnt keep it oiled and worked very slow with backing off every turn. Screw would not start in the hole.
Click to expand...

This is the correct method for tapping by hand. With lube, and turn back out quarter or half a turn for every turn in to break the chips. You will have to chamfer the hole for a screw to go in nicely. A 5/32 hole, however, is too small (3.9mm) so you're lucky you didn't break the tap! the d of the hole is smaller than the minor diameter of the tap!!
 
mwjones said:
This is the correct method for tapping by hand. With lube, and turn back out quarter or half a turn for every turn in to break the chips. You will have to chamfer the hole for a screw to go in nicely. A 5/32 hole, however, is too small (3.9mm) so you're lucky you didn't break the tap! the d of the hole is smaller than the minor diameter of the tap!!
Click to expand...

No, the minor dia of the tap is still smaller. The 5/32" equals 3,969mm so in metric it is 3,97mm b/c mm are divided in 1/100 to start with so 3,97mm is only about .001" to tight but you can only ream that close, drilling it will be allways oversize. Yes backing off the tap to breake the chips is the proper way to do it. Mind you Europeans don't use just one tap, I remember useing three of them to tap one hole; the coarse one, intermediate and finish one in the end, b/c metric standart calls for 85% or more thread engagement versus 65% in NA standart.
 
11/64 is too large

you want a .165 drill or . 166 I use .166 at my shop. that 11/64 is almost .005 oversize. get as much depth of thread as you can especialy if it is going to take some load.
 
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