holesaws

[Fred]

Hi,

I bought a plastic bulkhead light fitting [1] from Screwfix and tried to drill a 20mm hole to pass the cable through. I thought that a hole saw would be the most appropriate way to do this but halfway through the hole saw got stuck! What was I doing wrong? Is this particularly tough plastic, or do I have a blunt/poor quality hole saw or should I have used something completely different all together?

Talking of holesaws, is there a logic behind the sizes they come in? Written in metric the sequence 35, 38, 40, 44, 51, 57, 64, 70, 76, 86,

102, 127 doesn't seem to follow any pattern. Does it make more sense when in imperial?

They don't even seem to match common pipe sizes (I realise you would want the hole a couple of mm larger than the pipe). I suppose 35 and

44 saws could be used with 32 and 40 mm waste pipe but 127mm seems too big if you wanted to fit 110mm soil pipe through it.

I would like to cut a 100mm hole to fit some ducting; would a 102mm hole be too fit a fit? I notice there is a 152 mm hole saw, presumably for 150mm ducting but again it only leaves one millimetre either side, so there's little room to manouvre.

Thanks, Fred

[1]

[The Medway Handyman]

How thick was this plastic?

I'd like to know that as well.

[John Rumm]

You need to take care not to drill too fast if the plastic is a thermoplastic - otherwise it can melt and stick to the saw.

But generally speaking, that is how I drill backboxes when I need a 20mm hole...

Probably a mixture of imperial and handy real world sizes...

51, 76, 102, 127 map to 2, 3, 4, & 5" nicely.

The smaller ones are reasonable clearance sizes for common pipes.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Fred saying something like:

I tend to wiggle the holesaw a bit in plastic - the size isn't usually critical, so a bit of wiggle keeps it cutting.

Plenty of clearance is better than too little, but yes, some of the sizes are a bit odd.

[ARWadsworth]

I chose a core drill size of 117mm when I bought my last bit. I also have a

102mm hole cutter which is just a little bit on the small side and a 114mm one which is fine when ducting through plasterboard/soffits etc.

Adam

[Dave Plowman (News)]

Assuming you have access to both sides, a Q-max cutter is the best answer. Hole saws never give a good edge and can melt some plastics if you're not careful. A Q-max gives a near perfect hole of exactly the stated size.

[Fred]

I'm guessing but perhaps 3mm?

[Fred]

Wouldn't it be more logical if the width was "pipe size + a constant" The current system has 35 which is +3, 44 which is +4, but 102 and 152 are only +2. Why does the offset change (though I suppose there probably is no real difference between 1mm either side of the pipe and

1.5mm or 2mm either side), and why is there no 110 width for soil stacks?

[Fred]

I'm looking to make a hole for some 4" ducting, so perhaps I will look for a 114mm holesaw rather than a 102mm then? Thanks.

[Rod]

They approximate to the following diameters in inches:

1 3/8 1 1/2 1 9/16 1 3/4 2 2 1/4 2 1/2 2 3/4 3 3 3/8 4 5

I'd always been perplexed as well so bunged the sizes into Excel and came up with these figures. Looks like there are a few "missing" - maybe they are the sizes you see in other sets?

[John Rumm]

Probably because 110 is quite variable - especially if you need to accommodate a joint - so 127 is usually the best bet for those unless you are sure you only need to get the pipe through.

[Fred]

Hadn't thought of that - thanks.

[Fred]

Thanks for doling that. I had a suspicion that they would make sense in imperial and was going to get around to converting them; thanks for saving me the time. Thanks again.

[js.b1]

For 20(25)mm holes you are using the wrong tool.

Buy a step-drill off Ebay, about =A310, works wonderfully up to about

6-8mm thick plastic. Perfect almost machined hole.