Post by tphase on Sept 10, 2011 6:17:00 GMT -5
If you need accurate holes in concrete, drill pilot holes with a small bit - it will help keep your bigger bit centred
If the blockwork is exposed, I usually try to drill through the cement joint as it's easier - not sure if it would be recommended practice though.
Rawl plugs and anchor bolts sometimes have a recommended bit size on the packaging - I tend to use a bit 1-2mm smaller as it's relatively easy to make the hole bigger if necessary. Mostly I find it's not.
Drilling large holes through a wall in a building (eg a 100mm hole for a vent):
Drill from the inside out - it means more dust inside but you can more accurately position the hole and avoid wiring and plumbing
Use a small (12-15mm) bit long enough to drill a pilot hole all the way through the wall. Angle it down slightly so any leaks will tend to drain out.
The exit wound will be ragged, more so if the wall is plastered - the idea is to have the ragged edge inside the dimensions of the final hole.
From the outside, drill into the wall using the pilot hole as a guide. Don't go too deep in case your angle is off. Through the plaster layer and maybe 50mm into the block should be enough (though I'm open to suggestion). Use the smaller bit as a guide to give you an indication of the angle.
Finish the drilling from the inside. If it's a standard insulated cavity wall, clear the loose bits of block before you get through the insulation. You don't want lumps of block falling into the cavity.
If the blockwork is exposed, I usually try to drill through the cement joint as it's easier - not sure if it would be recommended practice though.
Rawl plugs and anchor bolts sometimes have a recommended bit size on the packaging - I tend to use a bit 1-2mm smaller as it's relatively easy to make the hole bigger if necessary. Mostly I find it's not.
Drilling large holes through a wall in a building (eg a 100mm hole for a vent):
Drill from the inside out - it means more dust inside but you can more accurately position the hole and avoid wiring and plumbing
Use a small (12-15mm) bit long enough to drill a pilot hole all the way through the wall. Angle it down slightly so any leaks will tend to drain out.
The exit wound will be ragged, more so if the wall is plastered - the idea is to have the ragged edge inside the dimensions of the final hole.
From the outside, drill into the wall using the pilot hole as a guide. Don't go too deep in case your angle is off. Through the plaster layer and maybe 50mm into the block should be enough (though I'm open to suggestion). Use the smaller bit as a guide to give you an indication of the angle.
Finish the drilling from the inside. If it's a standard insulated cavity wall, clear the loose bits of block before you get through the insulation. You don't want lumps of block falling into the cavity.