Mounting rails on glass - how flat is glass?

[Swiss]

Hi all,

I was thinking about using a thick sheet of 12mm (0.5") float glass as a base for a small milling machine, because I thought it would probably be quite flat and rigid. Does anyone know how flat float glass actually is?

many thanks

Hugo

[jhowelb]

To my knowledge, glass has these characteristics. Flat, hard and brittle. The third one probably disqualifies it for your purposes.

[Swiss]

Flat, hard and brittle

Sure it's brittle. But that doesn't mean it's not strong enough to use. It depends how much force is applied to it. 12mm glass is actually very strong.

Now, does anyone actually know how flat glass is?

Hugo

[martinw]

Now, does anyone actually know how flat glass is?

Hugo

Dear Hugo,

Erghh, no, I do not know how flat glass is... but I know a man who might.

Ask Pilkingtons, the UK company that invented the float glass process. They used to have a technical helpline that was very good.

Best wishes,

Martin

[martinw]

As flat as the Earth, Larry?

Best wishes to Igalla and Geof

Martin

[TheSlickster]

Flat as a pancake

The Slickster

[Swiss]

Thanks Martin. Good idea to ask Pilkingtons.

I would hope that it's as flat as the earth, but then maybe it warps
or something as it cools.

I guess that if glass is that flat, then why to people buy specially ground flat granite to use as a measuringg surface?

Hugo

[ger21]

I guess that if glass is that flat, then why to people buy specially ground flat granite to use as a measuring surface?

Hugo

Just a gues, but...

1) Can you get 4" thick float glass? (thin glass is pretty flexible)

2) Drop something on it.

[Swiss]

1) Can you get 4" thick float glass? (thin glass is pretty flexible)

I was only thinking of 0.5" glass. A 2'x2' piece isn't going to flex very much, especially if it's supported from underneath. I was thinking about bonding it to something stronger, maybe some steel or something. The only thing was that I was worried about different coefficients of thermal expansion.

2) Drop something on it.

I think glass is a lot stronger than people give it credit for. Half inch thick glass is going to be pretty hard to break.

Hugo

[Khalid]

A little ware idea..if we use Acrylic sheet instead of glass???

[ger21]

I was only thinking of 0.5" glass. A 2'x2' piece isn't going to flex very much, especially if it's supported from underneath. I was thinking about bonding it to something stronger, maybe some steel or something. The only thing was that I was worried about different coefficients of thermal expansion.

It's flatness will be determined by what it's mounted to, due to gravity and it's flexibility.

[Geof]

I was only thinking of 0.5" glass. A 2'x2' piece isn't going to flex very much, especially if it's supported from underneath. I was thinking about bonding it to something stronger, maybe some steel or something. The only thing was that I was worried about different coefficients of thermal expansion......Hugo

This will be a big problem. If you bond the steel and glass rigidly you will create the equivalent of a bimetallic strip; when the temperature rises it will bend one direction and when the temperature drops it will bend the other way. Glass is not very elastic, and cannot handle tensile stresses very well, so when the glass is trying to contract more than the steel as it drops in temperature it will probably crack, maybe with a loud bang.