one last thread topic, i promise

[cadetchris]

when threading a hole with for example an 8mm unc thread, what size should the hole be drilled? 8mm? 7mm? or 7.5mm?

[Budgie]

7mm maximum 6.5mm for safety sake drills useually cut slightly oversize.better too small than too big

[cadetchris]

thats what i would, an 8mm hole for an 8mm thread would be pointless,

[alex_holden]

UNC is imperial so it won't be an 8mm OD thread. Do a Google search for "UNC tapping drill sizes" or buy a copy of Zeus Charts.
http://www.amazon.co.uk/Precision-Refer ... B0000CLZUO

[alex_holden]

7mm maximum 6.5mm for safety sake drills useually cut slightly oversize.better too small than too big

Drill the hole too small and there's a much higher risk of breaking the tap.

[linearaudio]

The way I find the right size is to try a drill in a nut of the right thread size. if the drill just slips through without waggling, then it is the right size (for tapping, not rolling) the thread.

[Budgie]

6.5mm equates to 1/4" drill size use plenty of tapping grease and take your time slowly,slowly tappy thready.

[linearaudio]

1/3 turn, then back off enough to break the swarf coil that is forming, repeat ad infinitum, or you can end up with a mashed thread as the swarf gets caught up in the flutes!

[bmcecosse]

Oh dear - I feel a disaster coming on..........

[linearaudio]

Haven't you got a fancy bit of Latin to cover that one, Roy?

[Declan_Burns]

For M8 you should be using a 6.8mm drill bit. Here's a useful link which shows all the information you require for both imperial and metric threads. Print it off and keep the copy in a safe place or hang it on the wall in your workshop.

Hope it helps
Regards
Declan

http://www.hahnreiter.de/docs/seite_1-6.pdf

[bmcecosse]

Apparently - the Romans didn't DO disaster! Well done Declan!!