I've got a glass box that measures 900mm x 450mm x 450mm.

The width of the glass is about 6mm thick.

If I drill, cut, or grind a small hole in/through one of the faces; will it significantly affect the structural integrity of that piece of glass?

I'm concerned it might become prone to flexing, and cracking; if I pick it up to move (or transport) it, for instance.

Is there some kind of ratio, or formula that I can use to determine a relatively safe size, and adjacent edge distance for an aperture?

If you're wondering: It's basically a small indoor display garden / vivarium type situation, which needs at least a single outlet point through which gravity can be allowed to drain the excess fluids from the soil.

glass box schematic diagram

  • Some people are getting a little over-zealous in the close flag. 3 votes to close as arts/crafts/decorating advice. READ THE QUESTION PEOPLE - it's a question about STRUCTURAL INTEGRITY. It belongs in engineering! ;) – The Evil Greebo Jun 23 '17 at 11:59
  • It's a simple question about drilling a drain into a glass fixture. It's going to be mounted in a new wall as a display feature. – tjt263 Jun 23 '17 at 13:20
  • Exactly - I'm joking about moving it to Engineering - just coming down on the over zealous close votes because your project is artistic - just cause it's pretty doesn't mean questions about it don't fit here. – The Evil Greebo Jun 23 '17 at 13:46
  • I am assuming that this is not intended to carry any kind of load. Why do you wish to drill a hole? – The Evil Greebo Jun 23 '17 at 13:48
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    Agreed. OPs shouldn't have to lie to us. "I've got a 1/4" thick glass... uh, 'counter top'. Where can I drill a hole?" – Mazura Jun 23 '17 at 17:00

While I cannot cite any specific sources for this, I'm going to posit the following answer:

Yes, structural integrity will be affected, but not enough to be concerned about.

The box you describe is not suitable for carrying any kind of load anyway, and as you state above it's for display purposes. Drilling a tiny hole will not cause any cracking - in fact it's a technique used in windshields (particularly aviation windshields used in small planes) to STOP cracks from continuing to spread. (Small hole drilled right at the leading edge of an expanding crack relieves the stresses in a sound manner)

Granted aviation glass is different than what you've got but still - I think you're fine. I'm curious though why drill a hole in it? That will have a tendency to trap moisture.

  • Well, yeah; it is for display purposes, so it has to support the items on display, It generally won't be carrying a load from one place to another; but it will be mostly suspended over a gap, with support under the edges. Also, I have prior experience with this (almost) exact project, but generally a bit smaller, and always with some kind of clear, molded plastic. Drilling a drainage hole will solve the problem; I learned that the hard way. It's just that I have no experience working with glass. – tjt263 Jun 23 '17 at 16:18

Once you successfully (!) get the hole drilled, there's little concern. Not cracking it while you drill it is the hard part.

I'd say ~2 inches in from a corner, or at least an inch on a leading edge. The thickness of the glass will ultimately determine how close you can get - and remain successful.

IIRC, the general rule for the bare minimum distance from an edge, to drill a hole in a structural member, is four times its thickness.

(anecdotal supposition)

I've heard that all panes of glass have a sweet spot. I'd assume it lines up with one of the 1/8th harmonic intervals. So on a 24" pane of glass, trying to drill exactly 3" or 6" in could be trouble. But it probably has more to do with how it's mounted, which means for drilling it, it should probably be on some hard plastic, not e.g., on a towel on a table.

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    Thanks. If anybody can confidently confirm, deny, or elaborate on these figures, please do. – tjt263 Jun 23 '17 at 16:47

There are a great number of aquariums , many with similar dimensions with 1 or 2 holes in the bottom, typically 1.5 inch diameter , with no structural problems. And they carry more weight than your terrarium . Actually , you could just get an aquarium , you wouldn't even need a drawing of it.

  • How are the holes made? Is there a special technique? Are they drilled, blown, ground, re-annealed, etc? I've never seen one like that before. Not much of a market for that sort of thing where I'm from though. Anyway, the box has actually already been built. I'll keep it in mind for next time though, thanks. – tjt263 Jun 24 '17 at 4:54

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