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Old 13th April 2010, 08:10 PM
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gingea1pom gingea1pom is offline
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Default 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
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Old 13th April 2010, 08:30 PM
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If my failing memory is still functioning a 7mm hole works just fine for std M8 which is 8x1.25
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Old 13th April 2010, 08:34 PM
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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.

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

Cheers Ginge
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Old 13th April 2010, 08:38 PM
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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).
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Old 13th April 2010, 08:39 PM
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http://www.engineersedge.com/manufac...rill-chart.htm
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Old 13th April 2010, 08:55 PM
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Cheers men,

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

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.

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Old 13th April 2010, 09:01 PM
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If you are going past Bracknell in the near future bring em along and I can drill em and tap em for you.
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Old 13th April 2010, 09:13 PM
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Enoch,

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

Cheers Ginge
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Old 13th April 2010, 09:39 PM
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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
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Old 13th April 2010, 09:54 PM
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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
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