Grub screw hole size for threading!

[gingea1pom]

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 (http://cgi.ebay.co.uk/M8-X-20-SOCKET-SET-GRUB-SCREW-BOLT-GRADE-14-9-X-20_W0QQitemZ300388570116QQcmdZViewItemQQptZUK_DIY_ Material_Nails_Fixing_MJ?hash=item45f08dd404) (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

[Land Locked]

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

[gingea1pom]

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

[alga]

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).

[twinturbo]

http://www.engineersedge.com/manufacturing/metric-iso-tap-drill-chart.htm

[gingea1pom]

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

[Enoch]

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

[gingea1pom]

Enoch,

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

Cheers Ginge

[spud69]

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

[mark]

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

[alga]

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.

http://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/ISO_and_UTS_Thread_Dimensions.svg/400px-ISO_and_UTS_Thread_Dimensions.svg.png

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

[gingea1pom]

OH MY GOD

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

6.8mm then!!

[davidimurray]

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!

[spud69]

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:
http://upload.wikimedia.org/math/7/3/7/7378efae951ad6694da7029f3e9ac41f.png

http://upload.wikimedia.org/math/8/c/4/8c4a401dc20d97965549fea8abfd7906.png

http://upload.wikimedia.org/math/7/5/f/75f058c5b7aa9f313037878d34778a59.png

Too Early....AndyH

[dogwood]

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

[Enoch]

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.

[gingea1pom]

sounds perfect to me