I bought a set of these from ARC and have started using them eventually. **LINK**

However I am a bit confused.

If you want a clearance hole for a M12 thread I would have though 12mm would be good as the thread dia is about 11.8mm?

However the pilot dia on the M12 is 13.5mm.

Is this not too big?

I agree, I like tight tolerance but officially M12 clearance is 13.5mm

One of the reasons for a large clearance hole is that there is an internal radius where the underside of the screw head meets the shank so a tight hole would mean the radius would hit the corner and stop the full underside of the head seating down to take the load.

J

One answer is to use a kit comprising a shank with two added parts. One to fit your hole and the other to mill the flat surface. Then you can make adaptors of mild steel to match any hole, and not be constrained to make an existing hole 'too big' just to tidy up a counterbore. Even then, the range of diameters of the cutters on offer is limited, so eventually you will learn how to make the cutters as well.

Or not.

Cheers, Tim

I used to specify 7.2mm clearance for M6, with a hole spacing tolerance of +/- 0.5mm on greehouse components. It kept the reject rate down and didn't affect the building's geometry as the screws were used to secure parts together, not locate them.

Brian

The size of the corresponding counterbore also seems overgenerous and results in a rather ugly appearance.

PS Plus one for Tim Stevens' custom counterbores.

Edited By ega on 29/10/2017 18:52:58

The relatively large clearance around the capscrew assumes the cap screw head can be at top tolerance size and if an accurate location is required dowels would be used. The aesthetics of the counter bore is not normally an engineering consideration.

I have a set of these from Arc...

http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Counterbores/21pc-HSS-Interchangeable-Pilot-Counterbore-Set-Metric

I have only used one so far but at least I have them, & as Tim says you can make pilots to suit your needs.

George.

It may be something to do with difficulties of producing matching pairs of equally spaced radially drilled holes, lets say on collar fittings and in similar other situations.

With 0.2 mm tolerance these would certainly not match without extreme caution in their manufacture.

Previous discussion on this subject.

Why is the pilot diameter of a counterbore so large?

Link to save you all looking

Many thanks guys.