Over Easter will be drilling a large number of holes in concrete (C30 slab) ... these will be 16mm diam x 120mm deep I have bought a nice new 16mm SDS bit for the job. I need the holes to be accurate ... when using SDS drills is it better to put a pilot hole through first or get straight on with full size bit ? Hole wander is the biggest worry.
I always do this with a small one first if I care about precise location. Works for me.
I have done the same, but seem to recall reading (maybe here) that for SDS you should not use a pilot hole, maybe I imagined it.
better to
If these are for your steel plate fixings (thus spacing being critical) I would make up a wooden template some 50mm thick, drill the first hole, pop in an expanding anchor to hold it, then drill the other three holes starting with the diagonal, and when that is drilled pop in a second anchor or pin to stop it moving. Even with a pilot hole, a bit of agregate will send the drill all over the place.
AWEM
AOL. Seems a bloody good idea to me.
Si
Good idea .. and you are right it is for that job.
The big worry when drilling large holes is the drill drift. By starting with a final size drill you will not see any drift.
Mark the hole you are drilling with a cross that will be visible after you have reached the final size.
When drilling the hole, check that you are still within the cross as you gradually enlarge the hole by reasonably larger drills, Learn from this.
After you have done a few holes, you will then be able to move on and drill full size first time. Look and see if the drill is wandering and angle it to push the tip towards the centre of the cross. Do this very early in drilling the hole though. Once you have drilled deep enough that the drill tip has passed under the surface, you will only be able to pull the hole as far as the difference between the drill tip and the main body diameter.
As an afterthought, get some one to view the drill to determine that you are drilling in the right direction. This will take 2 pairs of eyes from
90 degree directions. A couple of try squares on the floor will help the eyes.HTH
Dave
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