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I need to cut a 3mm thick sheet of glass of 40cm x 30cm to 20cm x 20cm, but I don't have a glass cutter. Are there any reliable techniques to cut the glass while being precise and getting a defined cut?

I know one can do it with selective heat and cold, but I don't regard this to be an option due to the lack of good control.

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  • I think water jet machining was the best precise method available for that purpose. Correct me if I'm wrong
    – Fennekin
    Commented Mar 9, 2016 at 17:38
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    Diamond ring is traditional.
    – KalleMP
    Commented Mar 28, 2016 at 15:20
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    @KalleMP It's also a good test if the diamond is fake or not.
    – ott--
    Commented Aug 19, 2016 at 19:00

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A carbide scribe (for writing on steel) makes an admirable substitute glass cutter -- in fact, before wheel cutters were common, hardened steel scribes were used routinely.

In fact, anything that will scratch the glass and can be guided accurately enough will do exactly what a glass cutter does -- create a stress concentration that will cause the glass to break at a predictable location when flexed. A file is hard enough, and a gunsmith's riffler file ought to work (a plain triangular file is used in chemistry labs to cut glass rod and tubing -- just scratch and snap).

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    I once saw a man on a train "cut" the top of a glass coke bottle by using the metal lid. He scratched a line all the way around, then gave it a sharp blow and the top (the part with the screw thread) came away almost completely cleanly. The shape wasn't perfectly circular but he ended up with only two pieces.
    – CJ Dennis
    Commented Jun 26, 2016 at 12:56
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    Most ordinary steel is too soft to scratch glass well, but work hardening can make even low-carbon alloys hard enough to get the job done with some persistence. The harder the steel, the better this works, however.
    – Zeiss Ikon
    Commented Jun 27, 2016 at 11:10
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I think it would depend on whether or not you want a clean(ish) cut or need to simply remove excess glass (a clearance issue for example). I belt sander with a fairly rough grade paper on it will slowly remove small amounts of glass, but even with a glass cutter I have found it difficult to make clean cuts. A glass cutter can be purchased at many local hardware stores near me for a few dollars tops. I would recommend that if any "clean" cut is desired.

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It is possible to cut glass using a string that has been lit on fire, and quenching the fire with a much cooler liquid (water works best).

  • Soak the string (twine is my preference, but I have used yarn) in a liquid fuel that burns (lighter fluid is my choice, but others use acetone or alcohol). Thicker string works better in most cases as you want to string to work like a wick in a candle, to keep the flame in a specific place.
  • Tie the string around the glass.
  • Light the string on fire. Use caution not to burn yourself.
  • The hot glass may change color when it reaches a point where quenching will assure a good break.
  • If the glass is small enough to fully immerse in the quenching liquid, fully immerse the whole piece as quickly as possible. If not, cover as much of the burning area as quickly as possible.
  • You should hear a snapping sound when the glass breaks.

Thicker glass will require more time for the glass to heat before attempting a break.

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  • What kind of string can you use for this? I feel like simple thread would burn into pieces within a second of being lit.
    – Broots Waymb
    Commented Mar 9, 2016 at 21:01
  • I would use twine or yarn. I'll update the answer to include this.
    – Adam Zuckerman
    Commented Mar 9, 2016 at 21:27
  • As long as the string can hold enough liquid, it should be fine, since the fumes of the liquid will burn before the piece of string starts burning. However, I would guess that this technique is not really suitable for a glass plate. Also, can you tell beforehand whether the beak will occur left or right of the string? Since it is not hottest directly under the string I guess it won't break directly underneath, correct?
    – kamuro
    Commented Mar 10, 2016 at 7:36
  • I haven't used this method for plate glass (family has in a pinch), but it would make sense (to me) that it breaks directly on the burn line.
    – Adam Zuckerman
    Commented Mar 10, 2016 at 15:48
  • Just a thought on the type of string, it should be a natural fiber like cotton, manmade fiber is going to tend to melt and stick to your glass and may prove very difficult to clean off.
    – Jon
    Commented Mar 10, 2016 at 21:18
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I learned this one by accident. I had this nice wood and glass coffee table, and a summer dress that needed ironed. I put down a bath towel then laid my dress down on top of it. I am a little OCD so keeping the iron in the border of the dress was done without even thinking about it. When I was done I unplugged the iron and took it back down the hall. After I put it away I heard a loud crash. I knew right away what happened. I still couldn't believe what I seen. There was a hole in my table, the perfect exact shape of my dress. And this was some thick glass.

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I have used the string and lighter fluid method to make drinking glasses out of beer bottles and vases out of whiskey bottles using a 5th. Some times with real thick glass you want to use a rope that will burn longer. I have used strings that were similar to a 250cc motor pull cord size and texture. Soaked it in lighter fluid and tied tight. I also have used my Dremel and diamond bits to score glass.

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