Preparing a granite surface plate

blue_luke said:
Hmmmmm....


I 'need' the comfort of buying something certified, but just out of curiosity, how flat are the granite found at countertop or toombstone fabricator ?
Also, again out of curiosity, I had some belgian, french and austrian colleagues at one point or another in my career and contrary to us north-american as where we refer to a 'surface plate, or a 'granite', they refered to a 'marble plate' or simply a 'marble'
Is it just because granite is more common in America, and marble more comon in Europe, or this is just a semantic quirp?

Luc
Click to expand...
When I had quartz countertops installed in my kitchen, I had some concerns regarding flatness and called several manufacturers requesting a flatness spec. Only one was able to give me a spec and that was 1/8" over about 12 ft. Natural stone may be better but I have my doubts. Bear in mind that we are talking about using a product for something it was not intended for so all bets are off.

Headstones are finished smooth for a mirror-like reflection but again, consider the intended use. I would suspect that they are ground with a diamond process similar to blanchard grinding and polished for a reflective finish

As far a marble is concerned, it is much softer than granite and it is also etched by any acidic product. Granite is fairly impervious to most common chemicals. It is composed primarily of quartz and feldspar (aluminum oxide). This makes it hard and thus less susceptible to wear.

I would use the countertop or headstone for setups in the same manner that I would set up on a benchtop. They should be OK for coarse work but for any precision setups or measurements, I would want to qualify them first.
 
When the water carrier of ammonia evaporates, the ammonia is released into the air, and leaves no residue. Far better than soap. Since Ammonia is a base it doesn't react with the carbonates [correction] oxides in the granite. If you spilled a tiny dot of bleach on your granite surface plate it would cut a hole many thou deep.

-Anything acidic needs to say a far way from granite.
-never use any kind of soap on granite if you want accurate readings.
-lanolin based cleaners leave a small residue. I've heard that it doesn't affect readings, but in my experience the film left is readable in the tenths. (just my 2 cents worth)
 
Last edited:
Some machinists are 'good enough' machinists - if all you ever do is machine to the nearest .003 or .004 then anything goes. If you machine regularly to tenths and measure in parts of tenths, then taking care of your primary references is necessary.

An easy example is to grind to .0002 to square and size. A dirty, decrepit surface plate just won't cut it. If you only care to the nearest thou, kindly disregard my advice.
 
Granite has no carbonates in it. I think you are thinking about marble and limestone, not granite.

Granite is mostly quartz and feldspar. Granite is more resistant to chemical attack than marble. Here are a couple of videos of acid being put on each. By the way, bleach shouldn't have a big effect on granite. But if I had a nice granite surface plate, I would not be putting any harsh cleaners on it including bleach which is a strong base (alkaline).


 
You're right. I did have my geology course cross-wired. The acids attack the oxides of potassium, sodium and calcium, rendering the matrix unstable, particularly in fine-grained granites preferred by surface plate manufacturers. Having any acid on a granite surface is going to etch it badly.

Sorry I misspoke! :eek 2:
 
Thanks you all, this is very interesting!
Just to place you all in context, I am French-Canadian so my first language is French.
Most books and information I have read is written in french, but my command of the English idiom is quite good. It is just that sometimes a few expressions are very confusing. Just to give you an example, check the difference of meaning between 'deception' in English and French! Written just with the same ortograph, completely different meaning!
So anyway, I have checked again with some French (as from France) and Belgian fellows, and to them a 'marble' is a reference plate made most of the time of cast iron or .... granite! I don't know the historical background to this but I guess it closes this aspect of the discussion. It is just an expression, so it seems that if you talk of a 'granite, or a 'marble' any machinist will understand what you are taking about, simply a 'reference plate'
It does'nt have to be always complicated!! :)
 
You must log in or register to reply here.
Back
Top