Cutting a 125mm hole in glass window?

[Gibbo]

I'm a bit stumped on how to go about cutting a 125mm diameter hole in a single pane glass window to install a vent for a rangehood duct. Does anyone have any experience with this? The window is in situ and is an old style box frame construction. I was trying to avoid having to pull the window apart but if all else fails I guess I could get a new sheet cut to fit and the hole cut by a glazier.

Cheers,
Gibbo.

[Gumby]

Could you perhaps find a 125mm diameter rock ?

[Bodgy]

Good luck mate!

Go the glazier route.

[Ashore]

Replace the pane with perspex , cutting the hole in that is easy, just so long as the vent dosen't get too hot , but I doubt if it would.

Rgds

[KRH]

Hi

Replacement glass with the hole cut by the glazer might be a fair deal less expensive that the perspex solution. Also, perspex may look a bit crappy after a while.

[Gibbo]

Thanks guys. Failing finding a 125mm rock that's round enough it looks like I'm going to have to replace it one way or the other. I'll get some prices. Thanks again.

[Bob Willson]

Try this first.
Draw the hole that you need to cut out on the window itself.
Go outside and then with an oiled glass cutter scribe around the hole ONCE ONLY.
Tape over the scribed marks so that the class doesn't drop outside.
Go back inside and gently tap along the marks until the glass falls clear.
Even if this should fail you won't have lost anything by trying.

[HappyHammer]

First, purchase a black wool balaclava with only an opening for the eyes so that the bit in front of your mouth goes moist and uncomfortable, then get a black sucker with attached compass containing a diamond at one end then wait until it's dark sneak up on the window apply sucker, apply diamond to window in circular fashion and gently remove circle of glass without waking anyone up.

Once you have successfully done this on your neighbours window you should be able to repeat the process in daylight on your own window.

HH.

[HappyHammer]

Or what Bob said....

HH.

[silentC]

This is not an easy job even for a glazier. They will charge a bit because they will probably break one or two before they get it right. DAMHIK.

There's no way you will do it in-situ. You need to do it on a bench and you need a circle cutter. You have to scribe the outside diameter first (I used to do both sides) then you scribe smaller radius circles every half an inch or so. Then you tap near the centre until you can break out a bit. Then you work your way out to the outside. It's make or break and one slip and you get to start again.

They probably have a machine that does it now.

The other problem is if there is the slightest nick in the edge of the hole, you will probably end up with a broken pane down the track. My advice to people who wanted to do this was to put in a bit of ply instead of the glass, even if it's just a strip the width of the hole with glass either side. It's much cheaper and if the window is ever broken, you don't have to go through the pain again.

[Bob Willson]

What silentC said about glaziers is more or less correct but I have seen my brother do this many times. He was a photographer and many of his clients wanted a circular hole cut into the glass for display. His failure rate (once he got the technique correct) was very low. (About one in ten.)

Only scribe the glass from one side. Tap along the scribed line. Any nicks in the edge of the glass can be taken out with a suitable abrasive.
Glass paper maybe?

[silentC]

I could more or less do them on demand after a bit of practice. It's still a very difficult thing to do by hand.

The reason I would scribe the outside diameter on both sides of the glass is that one of the most difficult things about the job is getting the glass to crack through on that line without jagging. The theory was that scribing both sides of the glass then lightly tapping from both sides gave the crack somewhere to go: path of least resistance. The main reason for failures was a small fracture somewhere along the outside of the circle and when you pick the pane up, the crack widens and suddenly you have two peices. Doing it this way more or less garaunteed success.

The second cricitical thing was not to try and remove the whole circle in one peice. They do it on the movies but it is not possible in realtiy. You have to break away the waste from inside out. This was the other main cause of failure: the waste has no where to go and small cracks spread and often go beyond your scribed line. By scribing lots of ever diminishing circles, it helped to contain the cracks within the waste bits.

Believe me I spent a lot of time on this, picking the brains of an old guy who worked in the hardware shop opposite who was a glazier. There may be other ways, but this one worked most of the time. I don't know how many broke later on, but I'd be surprised if none ever did.

These were always for exhaust fans (like in motel rooms) and so the hole was much larger than 125mm. In my opinion, a smaller hole would be harder. Also don't forget there will be movement to deal with, so I would make the hole large than it needs to be and caulk it or cover it with a flange.

[Bob Willson]

Oh well, either way, you are only going to get one go at it.

[Gumby]

Too much knowledge is a dangerous thing. I still prefer the rock.

[Gibbo]

Wow! Dangerous indeed. It sounds like I'm gonna make a mess! I might have a go following your instructions and when I stuff it up, go the section of ply option. Thanks very much for your help guys.