Grub screw hole size for threading!
 

Here we go then.

I am planning to drill the holes for the grub screws in my Sierra uprights.

I vaguely remember from a fitter machinist / bench fitting lesson 18 years ago that there is a formula for working out the size of hole required when tapping a thread.

I am planning on using these (M8 Grub screws).

Therefore (Oh cleaver people) what size hole should I be drilling.

Thank you in advance.

Next If I bring my uprights to Stoneleigh could someone bring a M8 Tap to tap the thread in my new holes.

Standing by.

Cheers Ginge

If my failing memory is still functioning a 7mm hole works just fine for std M8 which is 8x1.25

Nice one,

If it supposed to be an odd size like 7.2mm just say as I have a rare selection of drill bits.

Just need someone with an M8 tap now.:rolleyes: :D

I appreciate it might be a little far for you!!!!

Cheers Ginge

Just Google for "tap drill sizes". It's a 6.8 mm drill for M8. In general, the formula is D - 1.08 * p, but some lists recommend D - p. (D -- outer diameter, p -- pitch).

Cheers men,

Thanks TT, obviously I could have done that myself but didn’t so other forum members would benefit.:o

I know it is probably imposable to see from the picture and I should really give them a bell in the morning but does the grub screw in the picture look course or fine.

I am hoping course as it can only be one hole size opposed to the three hole sizes for different pitches of fine thread.

This kit car building is a mine field

If you are going past Bracknell in the near future bring em along and I can drill em and tap em for you.
Dave.

Enoch,

What are you up to on Thursday;:D I have a free day, if it’s not too much bother.

Cheers Ginge

Hi Ginge,

Build is looking good on your build thread.

They look like course threads on the grub screws, Depth of thread = 0.866 of the Pitch of the thread so 8-(0.866x1.25)=7.02mm so a 7mm drill would be fine for the M8 thread. Plus the cast of the upright taps easy.

Keep it up....AndyH

You can get a cheap tap and die set that goes from m4 to m12 for about £25 it will even have the wrenches in

No good for everyday use but fine for the odd bit whilst building your car

Spud, it's not that simple. Both ends of the triangular profile are beveled, and the nominal diameter of the thread is not measured at the tips of the triangles (Dmaj+H/8), but the actual height of the external thread (Dmaj), so the math is a bit more complex. The drill diameter is Dmin here.



More info on wikipedia, as always: http://en.wikipedia.org/wiki/ISO_metric_screw_thread

OH MY GOD

That diagram / drawing is awesome if not a little mind bending.

6.8mm then!!

Get your self a copy of the 'Zeus' book. A brilliant pocket book full of all sorts of useful engineering info. Your overalls shouldn't be without a copy!

I know Alga but that calculation has worked fine for me so far, for bigger threads i just use diameter less Pitch and its worked okay, except for M18 fine thread which needs a starter hole of 16.5mm then 16mm.

Wikipedia is a really good page, equation you refer to is:






Too Early....AndyH

Just hammer a nail in there Ginge, that'll hold it...:D

Ginge, I have an MOT first thing in the morning, I need to spend a few hours in the office and expect to be home at about 3pm. I will PM you my phone number so you can call and see if I am free before coming over.
Dave.

sounds perfect to me