Hi:
We just received our order of a dozen 1/2 inch thick tempered glass shelves for built-in bookcases, and despite accurate measurements, they’re all scraping wood on the ends (gee, no one mentioned the 1/8 inch variance/allowance when I ordered these [expensive] shelves…).
The glass guy says I can grind some length off the ends okay, but that just doesn’t seem right to me–or at least I have some trepidation about it with what understanding I have of tempered glass.
If this is possible, I have access to a large lapidary grinder/wet stone, so it’s not like I’d be trying to do this with my angle-grinder.
Opinions?
Thanks.
Replies
As far as I'm aware you can NOT cut, sandblast, scratch, or grind tempered glass without having it shatter on you.Tempered glass is mainly used for security and as a safety glass in showers, patios, or any place that might get someone hurt if they broke the glass.
To reverse the tempering so that you could cut, grind, sanblast, etc. you have to run the glass through an annealing cycle in the kiln. This relieves the external stress in the glass.
Uhmmm....
There are kitty cats stretching up the sides of my porch windows that say you can sandblast already-tempered glass. I know nothing about grinding it, however.
Why would 1/2" glass need tempering anyway? I can't imagine its breaking unless the person likes to practice his hammer-toss wind-up in the living room.ET·ERITIS·SICUT·DII·SCIENTES·BONUM·ET·MALUM
We used tempered glass because these particular shelves are located in the therapy room of a private clinical psych. office. The good doctor says she's had a patient totally lose control a time or two in her career--so while she specifically wanted glass shelves on which to display a few worldly artifacts, she also didn't want to have to imagine the remotest possibility of a psychotic patient cutting themself on all that glass.
It looks as though we'll just live with these shelves. They fit--just really, really snugly, whereas they were intended to be readily adjustable.
This may be where you take a belt sander with a 40 grit belt and take an eighth off the inside wall of the carcass, sand to grit, then refinish.
Ouch.
Er, or just peel the maple veneer off the sides...
belt sanding tempered glass
I’ve got thirty years in the art glass business. Soooo, yes I have sandblasted into tempered glass to the depth that it can be in-filled with paint and I have sanded the edges of flat polished tempered glass and it stayed stable. However, I used a 220 belt with a finger polisher to do it. And (this is a big “And”) I wore safety glasses, gloves, a long-sleeved shirt, covered the glass both sides with a protective mask and cleared everyone out of the room. Why? Because even though I’ve never had it blow-up on me while sanding I’ve always thought my odds were 50/50 at best. As well, because you did mention that you weren’t aware of the tolerances allowed within the industry when ordering glass I guess I would be concerned about you trying to do any serious belt sanding on your shelves. But hey! If you’re still game to have a go…COVER UP FIRST!
Sacrifice one of the glass maidens to the lapidary grinding machine !
I think you will find that the tempered will tolerate edge sanding, but an impact or fracture on the edge is what generates the "shatter".
.............................Iron Helix
You'll likely get away with grinding the ends with fine lapidary equipment - the risk is that when that 15th heavy book or big sterling figurine is added the the shelf in a few years, bango. Likewise, splint's kittycats would mean that the door would take less of an impact prior to shattering. (no kids at home = probably no risk, etc.)
As a kid tried to grind a notch (found out that's a big non-no) on a bench grinder, got about 1/16" and blooie!
Following copied from PPG web site (the wording indicates this obviously this had lots of input from the corporate lawyers): http://www.ppg.com/gls_ppgglass/architect/fabric.htm
"PPG Industries has historically recommended
that heat treated glass, whether it be heat
strengthened or tempered, not be further
fabricated after heat treatment. This
recommendation is supported by ASTM
C1048 (Standard Specification for Heat
Treated Flat Glass), the FGMA Glazing
Manual, and the GTA Engineering Standards
Manual.
PPG TSR 95-1, Sandblasting of Tempered
Glass, provides PPG's position that the
strength of tempered glass that has been
sandblasted after tempering will be
substantially less than identical glass that is
sandblasted before tempering. Similar
strength reductions would be expected in
tempered and heat strengthened glass that has
been subject to other types of fabrication,
such as etching, edging, or V-grooving, after
heat treatment.
While cutting, notching, and drilling are
fabrication operations that obviously cannot
be performed on tempered or heat
strengthened glass, there are other fabrication
operations also not recommended by PPG
after the glass is heat strengthened or
tempered. These operations include, but are
not limited to, grinding, edging, etching,
sandblasting, and V-grooving. These types of
glass fabrication, performed after tempering
or heat strengthening, significantly decrease
the ability of the glass to resist both applied
mechanical and thermal stresses which occur
in-service when compared to similarly
fabricated glass that has been heat treated
after fabrication. In addition, fabrication
operations performed after the glass has been
heat treated may negatively affect
performance in safety glazing and other
applications by altering the break pattern,
impact resistance, frequency of breakage, and
expected in-service performance."
DON'T DO IT!
I am a glass etcher by trade, and can tell you that PPG is totally correct.
Now, that being said, let's expand a bit. The residual stress induced in the glass is not uniform. The edges and corners are not stressed like the surfaces and the interior. Cannot be. But - you do not know exacxtly how deeply the unaffected glass goes from the edge. It will vary from sheet to sheet. You start grinding, and you take away the margins of unaffected glass, getting closer to the highly stressed sections. Cut into one of those, and BANG, you have a pile of itty bitty hexagons. If you ever get a chance, examine a piece of tempered glass that has been in service for a long time. The surfaces have scratches, but they do not go deep enough to get into the highly stressed areas. If they did, it would cause the glass to shatter and defeat the purpose of the tempering. There is a finite non-zero thickness of surface that may be violated w/o the glass shattering. But you never know how thick it is.
A dilemma all glass etchers face when doing shower doors or any other large surface with complex, high value etching/frosting is whether to do it before or after the glass is tempered. Glass treatment facilities will not warrant their work beyond the cost of the glass. Your etching value added is your problem. You can safely surface frost a piece of tempered glass. It is a decision you, as an etcher, must make - do you want to accept the liability of etching a piece of treated glass? I have accidentally grabbed a piece of tempered glass out of my scrap bin to do a practice etching. I had etched a significant distance into the glass in blissful ignorance. Suddenly my hand was empty - no glass. I heard nothing, felt nothing, saw nothing (Sorta like Sgt Schultz) but there was a pile of itty bitty hexagons on the floor of my cabinet. And my point is... you have no idea how deeply you can safely go w/o destroying the glass.
It it was me, I'd be looking for a way to turn a tolerance error into a design feature. You could add wood to the sides and route in a slot so it looks like you got glass that long on purpose. Maybe you can figure out a way to hide a rope light in the back of that addition of wood that will make the end of the glass glow. Maybe an "OO! Cool!" would excuse the change.
I'm afraid that the designer that specified those glass shelves meant for the thing to be clean and simple or worse yet, adjustable. I'm also afraid if you're building this for a customer that can afford 1/2" thick tempered glass bookshelves, they are going to be pretty set in their idea of what they want and may turn up their noses at attempts to add design features. I feel for you. Best of luck!
Need for tempered glass
My dog, 1/2 german Shephard, 1/2 Jindo, h as broken 3 panes of glass in my English Tudor style home trying to get at the mailman. Trust me, some times you need tempered glass.