I always use a drill bit .5 less than what the hole should be, purely because the drill wanders so much when putting it into plaster. So 5.5 would be good for me.
Many brown plugs including Plasplugs state 7mm holes... Sometimes it is handy to adjust the hole up or down a bit depending on what the wall is made from and the gague of screw you are using.
I keep a stock of 3mm masonry bits. I find it easier to centre the hole accurately that way, and you can then use the bigger sizes to enlarge the hole, even when they're relatively blunt. It also gives you an idea of what you're drilling into before committing yourself to a particular hole size. 5mm can easily become 6mm or 7mm on a bad surface.
As others have said, smaller hole, at least to begin with, in softer materials or for tighter fit. Plenty of advice like this around (e.g. "use 7mm drill for 8mm plugs"). A damn site easier to drill too small and enlarge...
Supplied fixings (e.g. special bolts) that need odd sizes.
Drilling through a hole in, for example, some sort of bracket with a small-ish hole. (Probably drill through that hole and enlarge with the brackets removed - but nice to be *able* to use the largest possible size drill.)
Sequential drilling - i.e. start with small hole and increase in steps. Nice to be able to choose intermediate sizes.
Just sometimes, the actual drill lengths can make a difference - use a smaller diamater/shorter drill because you can get in there. Not strictly related to diameter but they do tend to go together.
My take - get one cheap-ish general set and buy specific half-way-decent additions/replacements as appropriate. Cost of a set is pretty acceptable these days.
(I lent my cheap SDS to someone - and it came back with a Titan set from Screwfix added to the few odds and ends I had accumulated. He needed some bits but had no use for them thereafter. I have used many of them. Lucky me. :-) )
Seems they've gone up since I bought them! Couldn't find another supplier at the time though...and there was no other easy option...white painted old brickwork.
I invariably use a 5.5mm SDS drill for red plugs - but I have trouble with my brown plugs. It says to use 7 or 8 mm depending on the material. However I find that, with brick, the plugs tend to shatter when hammered into a 7mm hole - but are like a prick in a shirt-sleeve if I use 8mm.
Anyone know whether you can get a 7.5mm SDS drill - I don't recall ever seeing one?
Ah, handy tip that sounds counter intuitive but actually works quite nicely:
When you need to get a plug into a slightly undersized hole, push it in as far as it goes, and then turn the screw into it a few turns. Now tap the screw with a hammer and the whole plug will slide into place (and not the screw just sink into the plug as you might expect). Once set at the right depth, just unscrew the screw.
Handy for setting a plug below the tiles on a tiles surface as well to make sure you don't crack one on tightening the screw.
Recently I noticed a new (I think) product called Fill & Fix from Fischer (of plug fame).
Having read what is is (polyurethane glue, I'd guess), and how it is used (bung it in the hole and let it set, then screw into it), I thought I would try my own version.
I had made a few holes in ordinary fairly modern bricks. And the plugs I wanted to use were not gripping as much as I would wish. (Using the next smaller drill would have put me in precisely the situation described - too small a hole.) So I smeared the plugs with some polyurethane glue, sprayed the hole with a touch of water and inserted the plugs. Stood back and let it set before driving screws in.
Their version is not supposed to require a plug - but I can't see how it is kept in the hole until it sets. Perhaps it is extra thick (gloopy scale not IQ)?
The plugs are held securely. The screws seem to be holding just fine. I would have reservations about doing this with a heavy duty load but for light to medium - seems to work well.
I used to have some hot melt wall plug sticks at one time. You used a glue gun with a longer nozzel on it to to pump the molten plug into the hole. Leave it a minute to set and then screw straight into the plug formed. They could work ok on very crumbly walls.
Drill the hole with a 7mm bit. Then widen out using the 8mm bit without hammer mode. You get a more accurate 8mm hole, which is somewhat smaller then the hole you get by going in with the 8mm bit in one go. (Depends on the nature of the brick too.)
I was doing this just yesterday!
Other method I use for this is to turn a screw round backwards with the head on the flange of the plug, and tap the point with a hammer to recess the head of the plug. Need a screw head which will just go into the hole.
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