[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.

[LeeWay]

Exactly! Different materials react differently not only to torque, pressures and exteme stresses of machining, but most importantly, temperatures. Glass may be within range of hardened steel with regard to movement at temps, but that certainly bares/bears investigating.

[Swiss]

OK, so forget gluing glass to steel for now. What if I glue two sheets of glass together, with glass spacers inbetween, to make a really stiff structure? Like the Y-axis slide on this machine: http://www.cnczone.com/forums/showth...t=15835&page=2 I'm sure it would be both strong and stiff enough.

The question is: Would it be flat enough? I'm going to ring Pilkington glass on Monday to find out.

Hugo

[ger21]

OK, so forget gluing glass to steel for now. What if I glue two sheets of glass together, with glass spacers inbetween, to make a really stiff structure?...............

The question is: Would it be flat enough?

It'll only be as flat as what it's resting on when you glue it together. Also, if you're glue has any thickness, it'll throw off the flatness.

[cam1]

I've been following this with some intrest, but what is the motivation here? did you inherit this glass, or get a deal on it? Why not just make it out of reinforced concrete?
The way things can be made flat is by shimming. This is an accepted method to mount and align things.This alleviates the requirement for extreme flatness.
It all sounds a bit wonky to me.

regards

[Swiss]

what is the motivation here?

Curiosity partly. I've read quite a few threads where people discuss trying to make really flat beds by pouring epoxy. I was just wondering if using glass would save a lot of effort. It's readily available, inexpensive and pretty flat. It's very hard, doesn't dent. All of these seem like perfect attributes for a machine bed.

BTW, i'm only talking about small machines here. 2' x 2' base. So not one of those massive room-sized machines. I think glass would be totally inappropriate for those.

Hugo

[Swiss]

It'll only be as flat as what it's resting on when you glue it together. Also, if you're glue has any thickness, it'll throw off the flatness.

Surely the glue will conform to the shape of the glass, not the other way round?

I get the feeling that everyong here thinks glass is some kind of wobbly jelly. I was under the impression it didn't bend much at all. - I'm talking about a fairly thick plate of glass here.

Hugo

[Swiss]

Some calculations done using the Elastic Beam Deflection Calculator ( http://www.aps.anl.gov/asd/me/Calcul...sticBeam2.html ) give the following results:

For a 600mm x 600mm x 12mm glass plate, supported only on two opposite edges, with a 100N load in the centre, the deflection is 72 microns.

With a 24mm plate, the deflection goes down to 9 microns.

With two 12mm plates with a 24mm gap, the deflection is just over 1 micron.

So, I'm pretty sure that glass would be stiff enough.

Hugo

[Geof]

....I get the feeling that everyong here thinks glass is some kind of wobbly jelly. I was under the impression it didn't bend much at all. - I'm talking about a fairly thick plate of glass here.

Hugo

On the contrary, I know that glass is a distinctly non-wobbly jelly. Normal glass does not bend much...before it breaks that is. Tempered glass can be used as a spring (almost). I mentioned above that glass cannot handle much tensile stress. All glass has micro-cracks on the surface and these are what reduce the ability of glass to take a tensile load at the surface that is why normal glass does not bend much before it breaks. Internally, where there is no surface, therefore no micro-cracks, glass can take a tensile load approaching that of high carbon steel I think. Tempered glass can carry a significant load and bend a considerable amount because the tempering procedure puts a high tensile stress in the core with a corresponding compressive stress in the surface. When tempered glass is loaded and bent the compressive stress on the outer side of the bend is reduced, but provided it is still compressive the glass will not break. What is happening is that the internal tensile stress increases as a result of the bend. But if tempered glass has the surface damaged deep enough to enter the tensile stressed region then it shatters.