Why are there sizes missing in drills and flat bits?

[Geoff_S]

I mean with things like a flat bit set I am looking at. For example, in the set there is a 6mm and 8mm but no 7mm. And when I look to see if the 7mm can be bought separately, no, it cannot.
Its the same with 10mm and 12mm, no 11mm option.

It's not a show stopper, after all I can use a rasp, and I have had to on the odd occasion.

I just wondered why these sizes are missing. Oh yes, and 15mm and 21mm as well, missing! And so on.

 

[Myfordman]

It is possibly linked to the (common) availability of metric bolts.
M7 is not uncommon on cars but rarly used in general engineering - 15 and 21 mm bolts are similarly rare.
No such problems with lip and spur bits from Wealden for example - available in 1mm steps

 

[NazNomad]

I wondered this same thing yesterday, trying to get a 15mm dowel to fit tightly in a 16mm hole.

 

[Geoff_S]

It is possibly linked to the (common) availability of metric bolts.
M7 is not uncommon on cars but rarly used in general engineering - 15 and 21 mm bolts are similarly rare.
No such problems with lip and spur bits from Wealden for example - available in 1mm steps

Good link I shall use that, thanks. But still missing 17mm, 19mm and 21mm onwards? So strange.

 

[Phil Pascoe]

it surprising how many ranges of augers don't include a 19mm.
Ford brake calipers used to have 7mm allen bolts - try finding a set of allen keys with a 7mm in it.
Socket sets often used to miss out either the 11mm or the 12mm - with no consistency whatsoever.

 

[Sheffield Tony]

8mm + grinder = 7mm. Assuming you don't want the fancy sort with spurs.

 

[Eric The Viking]

M7 was also the size for the battery clamp on the MG/Rover 200 series. I think they were just using up as much stores stock as they could - utterly bonkers idea. I remember modifying my daughter's car's battery tray to 8mm (like most of the rest of the sane world) after the bolt rotted.

I did procure a few 7mm bolts, but the threaded hole (and plate - the tray itself was mostly plastic) was too far gone. Those bolts were roughly 50x the price of M8 in the same length.

So imagine: you go to charge the battery indoors in the winter, and somehow manage to lose the bolt: car completely immobilized, and most nut+bolt factors simply laugh at you.

 

[Racers]

Flat bits can be reduced in size with the help of a grinder or diamond stone, I did one last weekend.

Pete

 

[MikeG.]

They don't even need that, Pete. A file will do it nicely. I've often filed mine to different sizes (just remember to mark them!!). A file is all you need to sharpen them, too (we are talking spade bits, aren't we?). They're made of mild steel.

 

[Phil Pascoe]

http://www.namrick.co.uk/acatalog/Home_ ... 12_25.html

Eric - now you mention it, yes. Look at the difference between 7mm and 8mm.

 

[AndyT]

There's a sound mathematical basis for this, which I think is embodied in a specific standard, though I don't know which one. There are similar series of "preferred sizes" for all sorts of things. Put crudely, as things get bigger, the intervals get bigger, but you can still hit any required value within a selected percentage of accuracy.

Edit: try this for more maths and more detail

https://sizes.com/numbers/preferred_numbers.htm

 

[bourbon]

I remember Ford using 7mm Allan bolts. Way back when I was fit enough to get under a car (and back up again!). And having the same problems sourcing said size. BUT, and this is the whole reason for it. The Main dealers have them! So Joe Bloggs then goes to them. It still happens, Unique bolt heads, sizes etc. All to make money for the car companies. My father worked for Lucas in the 70's/ 80's.Thats how I know.