Cutting Glass

There was an example of glass cutting on the Repair Shop, specialist glazer was brought in to cut some panes for a Victorian gas lamp. He scored the glass with a single stroke of a diamond no lubricant. He simply held the glass in his fingers either side of the score and with a slight twist of the wrists it split cleanly.

Richard

All depends on the glass - the thinner the glass the easier it snaps. I use some 2mm glass in my fused-glass jewellery, and it snaps as easily as a chocolate bar - 2mm picture-frame glass is equally cooperative.

Modern 'stained-glass' glass tends to be 3mm - again, fairly easy to work with.

Given that this is the op's first attempt, and he's only got one chance to get it right, he probably needs all the help he can get!

It's probably just me getting old & crabby, but the more often I see anything on TV that I know a little about (like stained-glass on the Repair Shop), the more often I find myself shouting "You don't do it like that!" at the TV. These programs are produced for entertainment, not education....

How thick is the glass?

There are some very cheap large mirrors[1] around where the glass is only around 1mm thick and nearly impossible to lift from a supporting backing without it bending and cracking. I have the T shirt when I thought it a good idea to remove a mirror in order to paint the frame.

I was shown how to do this with quarry tiles and used it when fitting the same. Score the glazed top surface with a glass cutter in the same way as glass and then use a very small light weight hammer

More so when they completely edit out the boring bits that in a process are essential for producing a decent finished article or even edit out bits that the "experts" don't want the DIYer to know.

I find that I get annoyed when labour costs are not included so that £50 profit would be more like a £200 loss even at minimum wage. Worse is when that ream of gold leaf comes "free" because it was left over from another job or the sand blasting and powder coating of the ocean liner was done free as a small favour to the presenter.

Repair Shop doesn't charge for repairs. Unlike many of the classic car progs where they claim to have made a profit.

The original poster said 4mm

The sort of glass cutters we use in stained-glass tend to have a weighted brass 'head' (the opposite end to the cutter) - and this can be used to start the score running.

If you're doing a straight 'edge-to-edge' cut then you shouldn't need to do any more than tap at the start and end of the score - and the section in-between will follow the line of the score, if the score has been done properly.

If you have a sinuous score then tapping at strategic points along the curve can ensure that the glass behaves - rather than cracking off in random directions where it wants to..

I had never heard of that. I love the acoustic aspect. Quarry tiles were a right bastard in the days before tile cutter machines.

Wasn't sure if that would work well with 4.5 mm thick glass, I guess it should and might be a bit more controlled that applied pressure. The thickest glass I've cut would be 3 mm the lift and underside tap worked well.

It took me three attempts to get a half decent edge and taking off about a foot of glass. I bought a 'professional diamond glass cutter' and it was crap. It became blunt very quickly. I ended up using a new, tungsten carbide tipped two fluted end mill, which gave a deep score.

Strangely, a lighter score (so it's only just visible on the surface of the glass) works better, and you're more likely to get an accurate break.

I had it described to me once, that the glass is like a toy balloon - the outer layers (top & bottom) are the rubber - the main body of the glass is the remainder of the thickness.

All you're doing with the glass 'cutter' is to create a line of weakness by cutting into the outer skin. Then, with the correct amount of force / angle, you'll persuade the main body of the glass to break along the line of the score..

Scoring the line too deeply causes the surface glass to splinter (you can see fine glass-dust along the score-line) and then the core is likely to run off in random directions.

When I've taught people who to do stained-glass, we spend an amount of time with sheets of horticultural (greenhouse) glass - learning just how much pressure is required when scoring. Straight lines first, then arcs and onto snakey-lines! Different colours and types of glass require different pressures - after a while you instinctively adjust the scoring pressure to suit the piece of glass you're cutting.

Maybe but it only takes a single scratch to divert the cut, particulary if the angle between the scratch and score is small.

This is true. Very many years ago I went on a course on properties of advanced ceramics, and one of the practical experiments we did was intended to demonstrate the significance of flaws on the strength of such materials. We were each given a box of glass microscope slides, and measured the modulus of rupture of half of them by three-point bending. We then had the etch the surfaces of the other half with HF, wash and dry the slides and repeat the strength measurement. Even though the surface had gone from smooth and shiny to matt and pitted, the etched slides were stronger than the originals, because the HF had attacked and dissolved out the microcracks which were the principal cause of weakness.

The stress intensity at the tip of a microcrack is very high, due to its almost atomic scale radius; opening it up by etching increases that radius and reduces the stress. The fine flaw produced by a glass-cutter is in effect just a long flaw, and fine, even invisible scratches will divert the line of fracture. Old glass, such as the greenhouse glass I referred to in an earlier post, is full of invisible microcracks and almost impossible to cut by conventional methods. I used a diamond wheel tile cutter with great success when cutting down panes of old greenhouse glass for re-purposing.

Can it be annealed by heating, and if so, at what temperature?

No idea. Annealing will relax any stresses in the glass, but I don't know if it heals microcracks. Don't know what temperature either - I was going to guess 600C or thereabouts, but wiki says upper 400's.

IIRC "flame polishing" as carried out by glassblowers aims to take the surface just to dull red heat, which ties in with your 600C. I don't think "annealing" would deal with the cracks.

Changing the subject, I was amused to see on TV last night some firemen taking the roof off a crashed car with what looked like an bog standard cordless sabre saw. Through the door pillars and straight across the windscreen as well.

I assume they'd identified the car as diesel or battery-powered. The pincer-type cutters were designed to cut into metal where a sabre or other saw might cause sparks which could ignite any petrol around.