Turning a 4" hole into a 5"/6"

[Ben Blaukopf]

Lots of questions tonight...

The old cooker hood in my kitchen has a 4" hole at the right height, and about as far left as I can possibly have the new hole if I'm having a chimney hood.

Modern cooker hoods appears to want 5", or more usually, 6" holes.

Is it possible, using a core drill, to drill over the the top of an existing hole? Or is it going to wander so far that I end up with a complete mess? Obviously I'd need to offset the centre at the same time, so that the left edge of the hole didn't move any further left.

Never used a core drill, no idea if this is going to work or not. The alternative is to find a hood that takes a 5" hole and use the 5"-4" reducer I have hanging around in the garage.

Ben

[dennis]

You probably want to fill the hole so the pilot stays inline.

Failing that you may getaway with mounting a thickish board over the hole and drilling through it so it stays in line.

[Ben Blaukopf]

That was my initial thought, but it's a cavity wall, so a bit of a pain to fill.

Makes sense...

Ben

[Phil L]

This is impossible with a core drill.

I have and it won't.

Or make the hole larger with a hammer and chisel, like in the days before core drills - it's only the exterior brickwork that matters anyway as inside can be plastered up easily enough, and outside, the fitting on the extractor is often 2 or 3 inches larger than the diameter of the liner, and if it's not, you can patch it up with mortar and/or pieces of brick you remove....this is assuming the house is less than 10 years old, if it's any older, the chances are that it's been patched up in places anyway by now.

[Alex]

Almost impossible with a core drill ,it needs a pilot hole to guide the core,use a sds drill with a 5mm to stitch drill the new side of the hole and chisel out the remains,it was the common approach before diamond core drills

[Duracell Bunny]

Yes, easily done with a core drill & pilot. I usually do it by hot melt glueing a sheet of plywood over the existing hole, then use the core drill on that. The thickness of the ply is more than adequate to stabilise the core drill once the core is out of the ply.

Use a thickness of ply to suit the job, access & your strength. In a door, you can get away with 3 ply, in a ceiling you need much thicker.

[Dave Fawthrop]

|!Lots of questions tonight... |! |!The old cooker hood in my kitchen has a 4" hole at the right height, and |! about as far left as I can possibly have the new hole if I'm having a |!chimney hood. |! |!Modern cooker hoods appears to want 5", or more usually, 6" holes.

Get a 5 inch to 4 inch adaptor, that is what our local Builders merchant offered me.

[Lurch]

On Mon, 16 Apr 2007 13:18:48 +1000, Duracell Bunny mused:

Are you on about a normal holesaw for plasterboard and the like and not a dry diamond core? I have never seen a dry diamond core drill used in a door or ceiling before.

[Lurch]

On Mon, 16 Apr 2007 06:57:14 +0100, Dave Fawthrop mused:

If the extractor wants a 5" hole then reducing it to 4" will impair the performance. I did the same thing on one job and I had complaints the cooker hood wasn't too great. Whether that was because it was just crap (doubtful as it was some huge AEG thing that was as big as the range cooker) or because of the vent diameter I don't know.

[Ben Blaukopf]

Presumably if it wants a 5" hole, sticking a 4" in will reduce the flow by at least 36%. In practice, the flow reduction will be greater, because flow speed varies according to distance from the edge of the pipe. Possiby not much greater though - I've no particular inclination to work it out.

It's pretty academic, because the vast majority of hoods want a 6" hole.

[Duracell Bunny]

Doesn't matter, the principle is the same ... you use a new surface to guide the core drill.

[FKruger]

I hired a 6 inch core driller but ended up stich drilling the circumference because the core was taking ages and almost killing me up that ladder. My SDS drill took a couple of minutes and the untidyness is hidden by the grill thingy.

[John Stumbles]

With a good drilling machine with a mechanical gear box for low revs and electronic variable speed control, and thick leather gloves to guide the core by hand you may be able to start it in the right place to drill a concentric hole over the existing one. The trick is to angle it so you're only trying to start at one point otherwise it jumps around horribly. Otherwise some sort of mechanical support to guide the core may be worth while. A couple of battens banged in by masonry pins to make a V-shaped guide would probably do (again you need to start the core on the angle) or to be really posh a bit of 18mm board with a hole cut to the new size, fixed over the existing hole, would be the biz.

[normanwisdom]

In my experience a core drill need a pilot drill only to start it off and keep it in place. After that the pilot becomes useless - and if in a morse taper it can keep coming loose, or wander off. Once the core drill has cut a reasonable slot it should stay roughly on line. So start through a board perhaps, as has been suggested. Also you need a very big drill with an effective clutch or torque limiter or you risk breaking your wrists.

cheers Jacob

[ARWadsworth]

Same in my experience.

After that the pilot becomes useless - and if in

Usually down the cavity:)

Also true in my experience. Also my apprentice has informed me that since he started courting the core drilling is getting harder as he no longer has the same strength in his right arm.

Adam

[Lurch]

On Tue, 17 Apr 2007 18:20:03 +1000, Duracell Bunny mused:

Yes, but you would really want a decent thinckness guide for a core drill though. Depends on what your wall is made from and how well the core drill will stay in it with minimal guidance.

[Ben Blaukopf]

Well the guy who put the boiler in was moaning how hard my bricks were.

So I reckon I'll stitch drill it. It'll be cheaper that way anyway, as I won't need to hire anything.

Ben