Thread: Granite references: importing or not

Hi Bryan
possibly a suggestion from left field, but depending upon what you need the straight edge / reference plate for, an ornamental stone mason (head stone maker) or kitchen bench top supplier may be an option.
Some have CNC milling machines and should be able to make a granite surface plate or straight edge

Anything from a stomemason etc will not be of the required accuracy..... With reference surfaces we are talking about low micron accuracy..... For example my surface plate spec sheet lists the maximum deviation as 3.2 microns...

Originally Posted by .RC.

Anything from a stomemason etc will not be of the required accuracy..... With reference surfaces we are talking about low micron accuracy..... For example my surface plate spec sheet lists the maximum deviation as 3.2 microns...

accepted, but there are ways of making three nearly flat surfaces flat -- I recall a post some time ago which outlined how -- and maybe a stone mason with the appropriate gear can get to the degree of flatness you're after.
It seems to me to be at least worth a call given the cost of shipping a largish plate from the US.

You can make three equally-sized square cast iron surfaces flat by scraping them. It is a process described by Whitworth back in the 1830's. There isn't a similar process for stone...they have to be precision lapped to dimension like gauge blocks. The machines that do that are an order of magnitude (at least) more accurate than the tombstone/kitchen polishing units.

There are cheaper ways of shipping items...any of the many car importing mobs will take small heavy items for a reasonable fee.

Having said that, I too am seeking a larger surface plate...something like 600 X 800 if such a size is available. But I am hesitant to trust a heavy object to a chain of handlers and expect no damage.

Greg

I am hoping I'm not the only one, but what is a granite surface plate used for, and how?

Obviously it's not used to lap other parts flat, it would wear away fairly quickly.

Never seen one, never used one, so dunno.

Ken

There is a blurb going on at the moment on another forum, mainly to do with checking for flatness etc.

Tips on buying used granite surface plates - The Home Shop Machinist & Machinist's Workshop Magazine's BBS

Ken

Ken, its just a shop flat reference. Some shops do their marking out one them, or inspection work. The advantages over cast are the stability and that when impacted they chip instead of making a crater with a raised rim like cast does. (This preserves the flatness)

I use mine strictly as a spotting master for scraping...for checking that my levels and straightedges are flat, and for checking small machine components like lathe cross and compound slides.

Greg

Originally Posted by Greg Q

. There isn't a similar process for stone...they have to be precision lapped to dimension like gauge blocks.

Why is that Greg? I dont get it. Can't you spot granite with granite?
Go easy on me.

Stuart

Originally Posted by Greg Q

Some shops do their marking out one them, or inspection work.

And some shops seem to use them as lunch table come welding bench(not that granite would be much good for a welding bench)

Yeah, you could spot granite with granite, but then what? You still need to be able to spot lap (if there is such a thing) to be analogous to scraping. The quarries that do surface plates use a massive grinder that is purpose built.

I read that there are, or were, services that could re-lap a plate on site to restore its accuracy. The last time I checked with a calibration lab here they wanted $600 to drive over and spend an hour certifying my plate. No touch-up service available. Luckily I found a Talyvel the next week for stupid cheap on ebay Israel.

Originally Posted by Stustoys

And some shops seem to use them as lunch table come welding bench(not that granite would be much good for a welding bench)

I went to view some plates at a Bosch auction some weeks ago. They were gouged so badly I thought it was sabotage. About like using a fine micrometer for a welding clamp.

Originally Posted by Greg Q

I went to view some plates at a Bosch auction some weeks ago. They were gouged so badly I thought it was sabotage. About like using a fine micrometer for a welding clamp.

Yeah I've seen a few. It's put me off buying second hand. But then the only choice is Chinese(although according to Kens thread they are within spec). Just how could you do that by accident? I have a slab of granite in the shed and some spare carbide, I might try it tomorrow and just see how hard it is to gouge.

Couldnt you spot lap with a stick and silicon carbide or diamond paste? Granted its going to take awhile.

Stuart

Greg,

Thanks for your explanation, and being lenient with me.

Ken

Originally Posted by Stustoys

Yeah I've seen a few. It's put me off buying second hand. But then the only choice is Chinese(although according to Kens thread they are within spec). Just how could you do that by accident? I have a slab of granite in the shed and some spare carbide, I might try it tomorrow and just see how hard it is to gouge.

Couldnt you spot lap with a stick and silicon carbide or diamond paste? Granted its going to take awhile.

Stuart

You're a better man than I am, Ghunga Din