Thick glass can substitute for granite

[David Kirtley]

Most work doesn't require accuracy of less than 0.001" +- 0.0005". Within that range, a surface plate is not that expensive but sheets of glass or cut granite tiles are usable. If they are not flat enough, get three of them and get busy and make them flat enough. It doesn't take that long because you are taking off very little material. Just get some fine grades of diamond paste and a little water. We would flatten stuff that way when I took Optical Mineralogy in college using 1/2" thick glass plates. Glass is a bit easier as you can see the interference figures in the cutting fluid when you put them together. It is pretty amazing how well the plates can stick together.

Don't get too hung up over the accuracy of a shop surface plate anyway. There are just too many sources of contamination. Just because an instrument is calibrated to 5 or 6 digits, doesn't mean they are meaningful digits. Without really good technique and a very clean environment, measurements in tenths (0.0001") and beyond are not really reliable in general shop conditions. That's why they make clean rooms.

 

[David Kirtley]

I find shears granite very cheap. Granite in Mexico is ten times more expensive.

If you order from Rex Supply in Pharr and let them wait for it to come in regular shipment, you can have it picked up in the store without paying shipping. That's how I bought mine. Lots of people come here from Monterrey for shopping that could pick it up and take it back. Just tell the Aduanales that it is a metate.

 

[joe kozak]

I'm thinking not so much high tolerance within a small dimensions but rather a reasonable tolerance over a wide x or y axis.

 

[Ebel440]

A few months ago I bought a black granite floor tile 12x12 just to check out to see if it would be useable. I was in the aisle and it was a loose one so I just grabbed it didn't check it at all in the store. I set it up on a real surface plate to check it for flatness and parrelelism, I stopped checking after I measured one corner +.01. It rocked on the surface plate as well. It wasn't really useable for anything. Its probable some are better and some worse I suppose you would need to check a few to make any real conclusions. But if you don't have a larger surface plate to check it I'd say forget the tiles. I have been meaning to get a sink cut out to check.

 

[Keith Foor]

I am curious to know how the sink cutout checks. I have one myself that I use to sharpen and lap on. I haven't built a table for it yet, due to not wanting to waste the effort if it wasn't really usable. I assume that it's about flat, but I don't really have a method of checking it to any real degree of accuracy at this point. The other thing I use for lapping is a mirror. I wet it and put fine grit wet dry paper on it for sharpening as well. I can typically get a pocket knife far sharper than it would ever reasonably need to be with this method and the nice thing about it is unlike a stone that will wear in the middle, glass with paper on it stays as flat as it was to start with.

 

[Wizard69]

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?

It depends upon what you want to do with it, how much the parts and tools sitting on it weighs and what type of granite it is. A surface plate also needs proper support Under it. Beyond the issues you outlined above some granites are far more wear resistant than others.

A one inch thick piece of granite has its uses but thicker is even better. Just consider huge variety of squares one can buy some are more suitable than others for the job at hand.

 

[TOOLMASTER]

I has used glass for many years...thick table tops are great...the last one I bought cost me 5 bucks at salv army....24x24x 1/2

 

[Wizard69]

I am curious to know how the sink cutout checks. I have one myself that I use to sharpen and lap on. I haven't built a table for it yet, due to not wanting to waste the effort if it wasn't really usable.

You are using it already so obviously it's usable.

I assume that it's about flat, but I don't really have a method of checking it to any real degree of accuracy at this point.

In a home shop at best you can do the three plate method and scrape them in. That can get you very accurate plates but you still won't be able to measure flatness effectively. That would likely require setting up some optical instrumentation and hardware to lap the plates to perfection.

The other thing I use for lapping is a mirror. I wet it and put fine grit wet dry paper on it for sharpening as well. I can typically get a pocket knife far sharper than it would ever reasonably need to be with this method and the nice thing about it is unlike a stone that will wear in the middle, glass with paper on it stays as flat as it was to start with.

Obviously the granite is useful to you already. The real question is how flat do you need in a home shop. I wouldn't be surprised to find some of these granit table tops to be very flat over a limited area. If they where the you would see it via optical distortions in the surface reflections.

 

[Wizard69]

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

Your best bet here is to look at the armature telescope making forums. Normally they are making very precise curves in glass but there is also a need to make flats. I know it can be done, at least in smaller sizes, but you would likely need to build some optical instrumentation to do so. Telescope makers have been DIYing optical parts since Galileo so something should be floating about the net.

If you don't want to go that route you can always hand scrap in surface plates using the three plate method. Back in my youth I meant a guy at a place I was working that did this for a plant he worked in in the 1940's and 50's. This was a machine tool making business and hand scraped surface plates where suitable for the shop floor. I do believe he was talking iron plates at the time but you can scrape in granite. It is possible to get very accurate references this way even if you can't measure them against standards or optically.

 

[Wizard69]

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.

You can also see optical distortions cause be the lack of flatness on highly polished surfaces. This is a contributor to optical distortions seen in plate glass. If you happen to come across a really old house you will find glass panes that are actually wavy on the surface distorting you view of outside.

For a hobbiests the real question how good do you need and over how Wide of an area. A surface plate accurate to 50 million of an inch isn't going to be all that useful for a hit and miss engine. This especially when you consider that these engines, in commercial guise, where the high precision devices to begin with. The same can be said for steam engines.