as far as i know its impossible to cut. read this to know why en.wikipedia.org/wiki/Toughened_glass

btw, tempered is the same as toughened


Toughened glass must be cut to size or pressed to shape before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Due to the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a point). Using toughened glass can pose a security risk in some situations due to the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the windowsill.

Actually you can "cut" it, and I have helped do it. Expect about 30% (or more) breakage...

BTW, you could always wet grind it off...

The trick is to de-temper it. And the method I learned from an admittedly crazy glazier was to put the work on some dowels on a table so its not touching the work surface, soak a thick string in Naptha or gasoline, place it where you want the cut and light the bad boy. Have an extinguisher handy. When the flame goes out, pull the cut site to the edge of the table, remove the dowels and while putting downward pressure on the piece you are removing, have somebody smack it with a rubber mallet. You will get 1 of 2 things... A good edge, or a pile of shards...

Does NOT work on Laminated glass!!!

Remember:
Fire extinguisher
Gloves
Eye protection

Be careful...

You cannot cut tempered glass.

The ideas given here may work, but my father has owned a glass shop for 30 years and has never has any success cutting tempered glass.
The tension in the glass that causes it to be safer (ie not leaving shards in the sill for a burgler to cut themselves on and sue you) also causes it to do it's little shatter dance with any modifications cutting, grinding or drilling.

Laminated glass (two sheets or glass fused together with plastic between them) can be cut: score bothe sides of the glass, run the break with runners and slide an exacto between them to cut the platic.

The wikipedia article on tempered (toughened) glass is very informative.
en.wikipedia.org/wiki/Toughened_glass

It says it cannot be cut or drilled and must be annealed first. It has an article on annealing glass too, but a quick look tells me it's not a good diy project.

I remember now that the way to tell if glass is tempered is to look at it with polarized sun glasses. You can see the stress patterns caused by the tempering process.

I cut bottles with a hot wire, not sure if thats tempered glass. The way you do it is to take a steel wire heated with electricity to orange hot, place that against the glass to heat it up, and then cool the heated glass all at once with cool water and tap lightly. I'm guessing the quick temp change causes enough contraction to make the break, which turns out to be very clean. You may need to tap it harder to break it since it sounds thicker than bottles. I think finding a 19" piece of hot wire might be tough though.

no you can't cut tempered glass. give it up.