Drill bit sizes

[Terry Pinnell]

I've just been drilling a few holes in brick for a fixture specifying a size 7 mm masonry drill bit. I found one in my collection marked '7' but as I thought it looked a bit small I measured it with my micrometer. To my surprise the shank diameter was only 5.1 mm. Measuring the drilling section itself was slightly tricky but that too seemed similar.

Am I missing something here, or is the bit just likely to have been engraved incorrectly?

Also, can anyone recommend a simple tool to measure standard drill bits in the popular sizes please. Presumably it would simply be a strip of thickish plastic, drilled with a set of clearly marked holes. Even better if it also has holes for the correct matching rawlplug.

[Jules Richardson]

7/32" maybe? That would work out as 5.5mm, I think.

cheers

Jules

[harry]

The tungsten carbide tip on a masonry drill is significantly bigger than the shank.

Also, they get smaller with use, the sides wear as well as the "front". (So drilling ever smaller holes.)

[Nightjar]

It is a number 7 drill, which has a diameter of 5.1mm.

For a full list of number and letter drill sizes:

Colin Bignell

[charles]

on all my small masonry drills, the shank is smaller than the head. I've just measured a 7mm SDS drill and teh shank is 6.1mm. The head on yours might have worn away

[Dave Plowman (News)]

Older drills were marked using the number system - like No 8 woodscrews, etc. Modern ones are likely to be marked with a number which is mm. But the two ain't the same.

[meow2222]

Drill bit gauge IIRC. Use gently if plastic. Very useful timesaver.

NT

[Terry Pinnell]

Thanks all, much appreciated. Hadn't realised bits used a numbering system like wire guages.

The markings wear off quickly so a guage seems the best approach. Googling 'drill bit gauge' found 'Silverline Drill Gauge (1 - 13mm', which looks what I'm after. And maybe its imperial equivalent (1/16" - 1/2").

[Terry Pinnell]

... although the 1 - 13mm item appears to be marked in imperial measurements!

[Nightjar]

On 23/04/2013 07:50, Terry Pinnell wrote: ...

...

It is an old system and I don't recall when I last used it to order drills larger than about 1mm dia.

Colin Bignell

[Dave Plowman (News)]

The number system sometimes used for masonry drills isn't the same as for 'normal' HSS drills, though.

I've dug out some old masonry drills - not much used since I got an SDS - and one at random is marked 6.5 1/4 No12. It would date from the days of fibre Rawlplugs where you used the same number for drill, plug and screw.

[Terry Pinnell]

I wish that was still the case as I usually find matching all three no simple task!

[Nightjar]

I think the drill number system is the same, apart from, as you note, those sold by Rawlplug for use with their wall plugs.

Colin Bignell

[Andrew Mawson]

The plastic plugs that come all moulded in one chunk say on them the range of screw sizes they cover and the size of drill required. Vast majority of applications are covered by the Red plugs that need a 6 mm hole, and the Brown ones that need a 7 mm hole. I try to standardise on the Brown ones which will take a number 7 to a number 12 screw.

AWEM

[Dave Plowman (News)]

Plastic plugs are usually stated to take a variety of screw sizes with just the one drill - so should be easier. Better makes say the drill size on the strip they're attached to - or even have a hole to check the drill with.

[Scott M]

IMO, brown plugs are way too big for #7 or #8 screws.

I use:

Yellow, 5mm/5.5mm hole (depends on make of plug) for #6/#8 Red, 6mm for #10 Brown, 7mm for anything larger and coach screws & the like.

Scott

[Terry Pinnell]

In practice I frequently end up using trial/error. A major factor is the material. Holes in breeze block and soft wood are always larger than the drill diameter.

And a frequent problem I have is judging whether the plug will fit. It either gets stuck irretrievably because it's too large, or has to be tediously removed after it turns out to be too small.

Also, as the hole, plug and screw respectively become larger as I proceed towards a solution, the hole in the fixture then sometimes needs enlarging too!

[Andy Champ]

To remove a too-small plug - get a long thin screw, and hand-screw it a turn or too into the end of the plug. Then pull. If it won't come out it isn't too small :)

Andy

[Terry Pinnell]

Yes, thanks Andy, that's my usual method too. But it would be good to have a reliable, universal procedure that resulted in getting it right first time ;-)

I reckon the following seems logical and I'll try it next time:

1. Drill the hole (following guidance if there is any).

1. Use a hand vacuum to suck out the remaining dust.

2. Measure the actual diameter by inserting known drill bits until one fits snugly.

1. Use a plug whose widest diameter (apart from any 'lip' at the end) matches the hole. I have a micrometer, so should be able to measure that.

2. Use a screw whose widest diameter is X mm smaller than the hole. But what should be the value of X? And would this vary with the type of plug?

1. Try it - and if I can't tighten it beyond about the 80% depth, replace with a screw of smaller diameter.

[Andy Champ]

My usual method is to get the drill bit that I know matches (No 8?), drill a hole, stick a red plug in it, then put a no. 8 screw in. No vacuuming, no testing. Only thinking is what length of screw.

I only have 3 sizes(1) of masonry bit, so it isn't hard to remember which one.

Andy