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N.W. Gean

N.W. Gean

Joined
Oct 10, 2018
Messages
463
Good morning all. I have been looking at granite surface plates lately but my budget has not allowed it. After reading about using plate glass as a surface plate, I decided to stop by my local auto glass shop this morning. They gave me a cutoff sized about 15" x 17" at 1/4" thick. I asked how much for the piece and he said "ah it's a cutoff, you can have it".

Score! It's much flatter than the home depot granite tile I was using. I can post a pic if anyone wants to see, but it's just glass so I don't think there's anything special about it :D
 
I use the grizzly granite plate and it wasn't that expensive, very accurate though. I imagine glass will work fine but if you wanna see a bow in a blade it may be hard figuring out the gap.
 
razor-edge-knives said:
I use the grizzly granite plate and it wasn't that expensive, very accurate though. I imagine glass will work fine but if you wanna see a bow in a blade it may be hard figuring out the gap.
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I can see the bow in my blades pretty easy now. I pretty much blew the budget for the next 3 months on abrasives and handle materials so I'm grateful for a free solution.
 
N.W. Gean said:
I can see the bow in my blades pretty easy now. I pretty much blew the budget for the next 3 months on abrasives and handle materials so I'm grateful for a free solution.
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Sweet! Yeah I just wasn't sure, never used glass before myself but that awesome its working out!!
 
Another place to find something similar would be granite counter top manufacturers. They usually have a bin with free rems/cutouts. Before I got my 4" thick machinists surface plate, I was using a 1 1/2" thick piece that was a sink cut-out.
 
That's true. I passed a grave/headstone place while I was driving to the glass shop, but I wanted to see if the glass shop had anything first.
 
I use 3/4" glass plate for years, still do at times.
The problem is, it will chip over time all around the edge just from normal use.

My granite block will look brand new forever
 
N.W. Gean said:
If there's any flex, I don't see it. There's no free lunch, but I'm not making folders here.
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That's fine, I don't own a surface plate either. However thin panels (relative to their length) flex substantially under moderate pressure and therefore do not make good reference plates. This can be calculated if you know the modulus of elasticity of the material. For example your panel will sag about 0.2mm under its own weight when supported by the short edges only; a central load of two pounds force will cause an additional 0.26mm sag. Perhaps fine or perhaps not depending on your needs.
 
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You make a good point. .2mm is about 8 thousandths of an inch. Not great.

How does the equation change if the glass is sitting on a table where the plate cannot be wobbled? (This is assuming that by "short edges", you mean not fully supporting the plate?)

Math isn't my strong suit, unfortunately.
 
N.W. Gean said:
I wonder why it chips. I'm guessing glass is too fragile for our purposes.
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It chips because glass is very venerable at right angles, in a shop with tools and metal it will find over time show a chip and you have no idea when it happened.
 
You can mitigate some of that chipping by using your belt grinder and a 400 grit belt to round over the edge. Rounded edges are MUCH stronger.
 
N.W. Gean said:
How does the equation change if the glass is sitting on a table where the plate cannot be wobbled? (This is assuming that by "short edges", you mean not fully supporting the plate?)
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Yes, the calculation was for free support of the short edge only with the middle unsupported, for which there exists a simple formula. On a table deflection should be less but I don't know how to calculate an unevenly supported panel on a table that is itself not flat; that might require finite element methods but you would have to precisely quantify the table.
 
It likely works out to something I can't measure with basic tools. I'll just have to see how the plate works out.
 
D Dsuitor That's a great price!

N.W. Gean N.W. Gean Do you have a straightedge you trust? You can use thin paper like a feeler gauge and see if it will slip under the straightedge at various points with it laid across the glass.
 
I'll give the paper & straight edge a shot. What happens if there is a gap?

I have a woodcraft store about 30 miles from me. I'll head up one of these days and get a proper surface plate.
 
N.W. Gean said:
I'll give the paper & straight edge a shot. What happens if there is a gap?

I have a woodcraft store about 30 miles from me. I'll head up one of these days and get a proper surface plate.
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Well you'd know it wasn't flat by at least the thickness of the paper. Bible paper is about one thousandth of an inch.

Try using promo "comeback" for free shipping.
 
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