Bolt sizes...
Turbo Mini Forums

... daft beginner type question, but...

I'm going to have a go (for the first time) at drilling a hole and tapping a thread into in. I've got some alloy to practice on.

What size holes do I need to drill for the following size bolts?

M14 x 1.5
M12 x 1.5
M10 x 1.5

etc

I'm guessing the 1.5 is the thread size. Does the number after the M represent the hole size in mm or am I being a bit optomistic?

Thanks

You won't need either.
You need imperial mate :)

I never remember, so I use a vernier and measure the bottom of the threads on the tap.

I guess I can Google it.. Oh yes I can!

Oh wouldn't you fucking believe it..

http://en.wikipedia.org/wiki/Tap_and_die

Edited by Vegard on 18th Mar, 2009.

Edited by apbellamy on 18th Mar, 2009.

Metric threads require metric taps....

The number after the "M" is the nominal outside diameter of the bolt/stud and the X whatever is the pitch of the threads in millimetres. This is where metric differs, imperial has the "pitch" in TPI (threads per inch) whereas metric just gives the measurement from one thread to the next. There are several sets of standard pitches, the most common being ISO metric coarse. Only your M10 is the common size, M12 would normally be X1.75 and M14 normally X2.0

Because the M size is based on outside diameter, the drill for tapping is smaller.

A lot of use use a little set of tables called Zeus although the sizes can be found on the internet as well - I can't find my Zeus at the moment so use here
http://www.engineersedge.com/manufacturing...drill-chart.htm

You will see your M10 in the left (standard) table and the other two in the "fine" table.

EDIT - typo

EDIT 2 - one other little ruse with metric, because all metric threads use a 60 degree included angle, within reason you can simply say the tapping drill size is the big number minus the little one, ie, M10 X1.5 tapping drill = (10 - 1.5)= 8.5mm. Basic maths on a 30 degree triangle !!! Where the quoted drill size is slightly different is because of standardisation of drill sizes.

Edited by Rod S on 18th Mar, 2009.

Schrödinger's cat - so which one am I ???

As above. this is what you need. http://www.amazon.co.uk/Zeus-Precision-Cha...37370276&sr=8-1

A happy little book that makes you look like a proper engineer when you get it out.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Metric tapping sizes are very easy to work out- take the base diameter, subtract the pitch, and that is your tapping drill size. For e.g. M8 standard is 8mm dia by 1.25mm pitch. So your tapping drill size is 8 minus 1.25 = 6.75mm. In reality there isn't such a thing so you use 6.8mm.
M10 = 10x1.5, tapping drill 8.5mm
M12 = 12x1.75, tapping drill 10.2mm
M6 = 6x1, tapping drill 5mm

For imperial stuff you need a Zeus book or similar, although I normally just google.

Thanks Rod. I've got that clear in my head now.

EDIT: And everybody else. I am now slightly less simple...

Edited by apbellamy on 18th Mar, 2009.