Surface Plates

Surface Plates are often made of granite. Why don't they make them from harder rock ?

Reply to
Cliff
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Granite is a hard rock. Andrew VK3BFA.

Reply to
Andrew VK3BFA

Granite is a pretty generic term referring to a rock formed of an assortment of minerals. It can have a very wide range of hardness based on the specific mineral composition and the size of individual mineral crystals.

I'm sure there is a specific definition of whether a rock is granite or something else, but that definition encompasses a wide range of "granites."

I would expect its selection is that because of its composition it does not experience a wide range of expansion and contraction over a relatively long time period by human standards (but not by geological standards). Or it could simple be because of price.

Reply to
Bob La Londe

It's a fairly easy stone to work, and it has the advantage of being available in large chunks that have no stratification or large-scale segmentation. So it's pretty much homogeneous, at the relevant scale.

It only came into use during WWII, when there was a shortage of good iron and steel. Maybe things would be different if the originator started with marble or something else, but granite has pretty decent properties for a surface plate.

At the very extreme, high end of metrology, they use iron or steel.

Reply to
Ed Huntress

Because there was already an ancient industry making flat granite for various monuments and buildings. Including grave monuments.

Paul

Reply to
co_farmer

There are harder ones. People used to "machine" granite with them, often by hand.

Reply to
Cliff

They "machined" granite with harder rock for the Pyramids & in Central America IIRC.

Reply to
Cliff

Paul sez: "Because there was already an ancient industry making flat granite for various monuments and buildings. Including grave monuments."

Close, but no cigar ! Nice observation but . . . . there is a vast amount of difference in the precison required between making construction-grade granite and surface plates.

Bob Swinney

Paul

Reply to
Robert Swinney

There are many grades of granite and the industry relies on two generic concepts. The so called Pink granite that is full of quartz crystals and other pink minerals. The other is a black.

Either will grind down or shave any iron or steel placed upon and moved.

Harder rock would be only homogeneous - Basalt or exotic mineral that no one could buy.

Mart> >> Surface Plates are often made of granite.

Reply to
Martin H. Eastburn

Recalling it was Herman Stone that first made and marketed them during wwII wartime because iron was expensive what with it's importance for the war effort.

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Reply to
PrecisionmachinisT

Why would one pick a material which is significantly more expensive to fabricate and has no significant advantages?

Dan

Reply to
dcaster

Coefficient of thermal expansion is about 1/3 that of cast iron. Of course we all keep our surface plates at a constant temperature so that's not a problem. I used to work in a governemt research lab with an engineering metrology department. They had a BIG surface plate. I can't remember whether it was granite or CI. The thing they hated was temperature change. They didn't car what temperature their room was as long as it stayed there. A chang of 5C put them out of action for a whole day while everything settled down. The original reason for using Granite was the scarcity of iron during the war.

John

Reply to
John

Could be. I've read that granite is preferred because dings don't throw up a lip around a crater like metal.

Reply to
Jim Stewart

difference in the

I wonder about that, if there is no morter or cement.

Think of the downward force in something like the great pyramid ... without pretty good flatness each block only contacts the one below or above on about

3 points .. which might lead to failure ... huge PSI loads ...
Reply to
Cliff

Harder rocks are not uncommon. Ganites are of volcanic origin IIRC.

Reply to
Cliff

Harder = more resistant to wear for starters.

Reply to
Cliff

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I have not found a lot of worn out granite surface plates around here. Granite surface plates are already significantly harder than cast iron ones. What harder rock would you choose?

Dan

Reply to
dcaster

What harder rock would you choose?

I'd choose Cliff's head, although the supply is somewhat limited.

Reply to
Garlicdude

Don't know what "ganite" is, but granite is plutonic, not volcanic. Volcanic is extruded onto the surface, plutonic is where magma upwells into other rock formations, but never reaches the surface. Being buried deep underground, it cools much more slowly, allowing for the formation of larger, more well-defined mineral crystals within the rock. Lots greater pressure there, as well.

Granite, from a geologist's POV, is quartz, feldspar, and mica (usually the black variety - biotite). Countertop makers are a *bit* more loose in their definition of granite.

Metal content: quartz is made of silicon, feldspars of aluminum, and the darkness in biotite is from magnesium or iron (usually).

Joe

Reply to
Joe

I've seen them recalibrated & the surface "adjusted" by material removal to compenste for wear, sag & etc.

I have no idea. Not a geologist .

Reply to
Cliff

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