Thick glass can substitute for granite

[Syaminab]

Just to mention, on hobby works, light parts, you can use a glass as a granite for measuring parts.

 

[Tony Wells]

One of my early mentors used a tombstone as a surface plate, but I suppose a piece of tempered safety glass would be cheaper and easier to get. I'd stay away from plain window glass. It's wavy and not flat.

 

[Ray C]

+1 on Tony's thoughts about thin glass. For grins, I mic'd a small 6x8 pane and it was off several thou from end to end. Optical quality glass is probably dead-on but would cost about as much as a B-grade slab -I'd go for the slab and a B-grade is cut better than anyone in a home shop could dream of measuring (something like +/- 0.00005"). The granite at Shars is pretty inexpensive but shipping (naturally) is costly due to weight.

As far as they go, I never thought I'd use one much but it ends-up getting used a lot...

 

[Syaminab]

+1 on Tony's thoughts about thin glass. For grins, I mic'd a small 6x8 pane and it was off several thou from end to end. Optical quality glass is probably dead-on but would cost about as much as a B-grade slab -I'd go for the slab and a B-grade is cut better than anyone in a home shop could dream of measuring (something like +/- 0.00005"). The granite at Shars is pretty inexpensive but shipping (naturally) is costly due to weight.

As far as they go, I never thought I'd use one much but it ends-up getting used a lot...

What I like most, you can put it up in a drawer.

 

[7HC]

How about employing actual granite as is used for counter tops in kitchens and bathrooms.
Just for fun I put a straight edge on the one in my bathroom and couldn't get the thinnest of feeler blades under it, nor could I see any light.

It's certainly not as thick as a 'proper' granite block but it does seem pretty flat. How flat does it have to be, to be useable in any serious way?


M

 

[Tony Wells]

All depends on what you want to use it for. For qualifying straightedges and scraped flat surfaces, most likely a counter top would not do, but if you wanted to invest the time and money, and for the learning experience, you could start with three equal pieces and trade lap them as flat as you wanted. But as thin as they are, their use would be limited to work in the 0.0050-0.0100 range at best off the shelf, if I had to guess. For layout work, drilling holes, sawing, etc., plenty good I'd think. Or for working in a small area, like a height gage comparison to a Jo block stack, I'd trust it to 0.0010/0.0020 if within a few inches of the reference.

 

[FarFar]

Maybe one could glue two or three sheets of granite counter top scrap pieces together?

 

[Daver]

So let me make sure I get this. There are two considerations for a plate. The first of course is a truly flat surface. Not eye the edge flat like determining if a 2x4 is twistd, but measured flat to some Nth degree. The second consideration is how thick it is. My assumption here is, the thicker the better. Thicker means less flex, less movement, and less plasticity/flow.

am I correct?
am I missing anything?

so back to point, on OP, would a sink cut-out from a thick granite counter top be a good starting place?

 

[Syaminab]

I find shears granite very cheap. Granite in Mexico is ten times more expensive. I liked the Idea of getting a 8" x 12". More than enough for most common hobby work.

Comming back to thick glass, if you dont have granite stone, you need to have a measure and you cant wait for a deliver, go find yourself a piece of thick glass, run the mic over it and get it as straight as possible and finish the job. Its a handy tip only, not the most precise. Mirror glass is quite stright, give it a try.
Now, for the science, can anyone describe a process we can do in a universal grinder or similar machine, to resurface a top counter granite to make it straight under 0.0001" more than enough for my work and assemblies.
regards

 

[Ray C]

Not only very, very flat but with true granite the corners are very, very square. Nice thing about that is you can check all your t-squares. Granite slabs are very thick too and do not twist. I don't believe they guarantee that the bottom is flat or even -just the top and corners. Side thickness might vary a little. Thin glass twists a lot and typical stuff you get at the hardware store is not very even. Thick glass is probably a little better but, if you can see optical distortions (almost all glass optically distorts) it's because of variations in thickness. It can be used for some hobby purposes but not for critical measurements or checking for flatness (such as in checking chuck backplates). If you took a strip of window glass, put it on a granite and slid a TDI over it (referencing off the granite) it would see it right away. In the thread about "D1-4 spindle and chuck mesurement" I showed how to use granite to check for back flatness. In that case, you're looking for 0.0001" accuracy -and that won't happen with glass.

I don't know about counter top material. I guess it depends on the quality of it. That and glass are certainly a handy tool for some homeshop uses but, if you're looking for +/- 0.001 tolerances or better, it would not be a good choice.

So let me make sure I get this. There are two considerations for a plate. The first of course is a truly flat surface. Not eye the edge flat like determining if a 2x4 is twistd, but measured flat to some Nth degree. The second consideration is how thick it is. My assumption here is, the thicker the better. Thicker means less flex, less movement, and less plasticity/flow.

am I correct?
am I missing anything?

so back to point, on OP, would a sink cut-out from a thick granite counter top be a good starting place?