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Topher

Machining bullet proof glass

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I can't find anywhere on the internet where they describe in detail how bullet proof glass is cut/machined .. specifically a diameter 30mm hole through a 30mm thick glass? any help greatly appreciated.

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I don't specifically know, but I do know that bullet proof glass usually has layers, or a layer, of tempered glass. Tempered glass can't be cut so any holes would have to be drilled before it's tempered -- like with a diamond dust bit.

 

EDIT... when I say "can't be cut" I mean that if you try it will shatter

Edited by Iggy

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I can't find anywhere on the internet where they describe in detail how bullet proof glass is cut/machined .. specifically a diameter 30mm hole through a 30mm thick glass? any help greatly appreciated.

Perhaps it would be best if you let us know why you want such information. If there's no info anywhere on the web, maybe another approach is needed.

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I could drill D=10mm holes in a Champagne bottle, rather easily.

 

I took a drill bit for concrete and made it seriously sharp by grinding.

 

As well, but maybe it wasn't necessary, I made a pool around the future hole, with modelling clay, and filled it with water, to cool the glass and the bit, and absorb the vibrations.

 

I even got thin glass chips. It doesn't take very long. Moderate cutting speed. Small vertical home drilling machine, feed by hand.

 

Mind your health. Cloth around the glass?

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The problem is the material properties of the tempered glass, as Iggy highlights, and as far as I remember bullet proof glass is layers of tempered glass (hard) and some sort of resin (absorbs the energy of the bullet).

 

 

The tempered glass is cut to shape while still malleable and then cooled fast, the result is the outside forming a hard crust as the inside is still cooling and trying to shrink. This means that the outside is in compression whilst the inside is in tension.

 

Now, glass has a fair large compressive strength but almost no tensile strength.As a result the outside will be hard but as soon as your bit tip enters the core and creates somewhere for a crack to propagate, the glass will shatter as the interior is in tension, which is where the glass is at it's weakest.

 

 

That said, the world is full of clever people so it wouldn't surprise me if somebody has created some exotic method of machining tempered glass. Putting the glass under pressure might work I guess, that way you increase the compressive stresses throughout the glass (hydrostatic pressure will create pure compressive stress), which might mean that you don't have glass interior under tension that would otherwise shatter.

 

Hope this helps/ makes some sense...

 

 

Ed

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It would depend on the type of bullet proof glass. If it's a machinable type then I would use a carbide or diamond dust hole saw.

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It's an assignment question, it asks, ''How to cut and/or machine bullet proof glass. For example a diameter 30mm hole through 30mm thick glass.'' I've not seen anything on tempered glass before only polycarbonate layered with normal glass, so I guess I want to know how would you make this cut into polycarbonate and glass layers? or would you make the cuts before lamination? And with what drill bit etc.

 

EDIT: ''And with what drill bit etc.''

Edited by Topher

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It's an assignment question, it asks, ''How to cut and/or machine bullet proof glass. For example a diameter 30mm hole through 30mm thick glass.'' I've not seen anything on tempered glass before only polycarbonate layered with normal glass, so I guess I want to know how would you make this cut into polycarbonate and glass layers? or would you make the cuts before lamination? And with what drill bit etc.

 

EDIT: ''And with what drill bit etc.''

 

If you assume that the glass isn't tempered (or that it's annealed to where it won't shatter) then a diamond dust drill bit is the way to go. There are other recommendations for a glass-drilling method here: http://www.ehow.com/how_7525770_drill-hole-glass-plate.html

 

The idea behind the diamond bit is that diamond is harder than glass... perhaps that is the point of the question.

Edited by Iggy

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Im going to guess that your doing an AD of engineering at mt gravatt tafe brisbane. Im getting through the exact same assignment. haha

 

tongue.gif

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I would have assumed the use of a laser, CO2. Google suggests the same, unless you wish to anneal the glass first. I don't think the idea of annealing applies in this case. Bullet proof glass is layered and the plastic layer would probably melt.

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