Surface Plate Question.

[ddickey]

Starrett Master Pink18" x 24" x 6" thick has been sitting on two 1x4's for twenty years.
What happens to these surface plates when they are supposed to sit on three points and end up like this? Do they actually move or what?

 

[benmychree]

Maybe set on 3 points and let is sit another 20 years?

 

[Dabbler]

I have asked Mitutouyo, Starrett and Precision Granite. They are all in agreement that there is a slight movement in 2-3 months, depending on temperature and humidity, and then no more. The plate I was inquiring about was 36" X48" and had been set on the 4 corners for 40 years.

When I measured it it was 5 tenths depressed in the center. Local deviation was not measurable.

I hope this helps

 

[Braeden P]

surface plates are like rubber that is why they are thick Robin Renzetti posted on Instagram he was measuring a vee block and when he pushed down on the plate with one finger it moved the indicator it was a really fine resolution thats why it could pick that up.

 

[Dabbler]

Deflection due to the weight of the plate is to be avoided, This is why we place the plate on 3 or 4 Bessel points. When placing heavy objects on a plate, one tries to position them near one of the support points.

The deviation that Renzetti was measuring was in the millionths of an inch. My standard indicators only go to 50 millionths per division, and I wouldn't even see that kind of deflection.

So @Braeden P From a physicist's point of view, the entire world is rubber, just some things are harder than others. Granite is quite rigid for it's weight, which is shy we use it for surface plates.

 

[ddickey]

It's sitting at $55 with 8 hours left.

 

[Braeden P]

Deflection due to the weight of the plate is to be avoided, This is why we place the plate on 3 or 4 Bessel points. When placing heavy objects on a plate, one tries to position them near one of the support points.

The deviation that Renzetti was measuring was in the millionths of an inch. My standard indicators only go to 50 millionths per division, and I wouldn't even see that kind of deflection.

So @Braeden P From a physicist's point of view, the entire world is rubber, just some things are harder than others. Granite is quite rigid for it's weight, which is shy we use it for surface plates.

yeah he does crazy stuff and deflection is really little nothing a hobbyist should worry about.

 

[jwmelvin]

Granite is quite rigid for it's weight, which is why we use it for surface plates.

So, as an example, Aluminum and granite have about the same density of 2.7 g/cm^3. Aluminum has an elastic modulus of 69 GPa while granite’s is 40 GPa. So aluminum is more rigid for its weight than granite. (Cast iron seem to have a wide range for modulus, between 60-240 GPa, but gray CI is listed at 90 GPa and density about 7 g/cm^3. That puts its modulus/density ratio a little below that of granite).

I think granite is a good surface-plate material at least because:

(1) it is quite hard, so doesn’t wear much;

(2) it is brittle, so damage does not cause deformation of the surface (just a void in the surface); and

(3) its coefficient of thermal expansion is pretty low (about half of aluminum, and a little less than cast iron)

 

[middle.road]

It's sitting at $55 with 8 hours left.

That's a fine price.
They've been going for $200+ the last few years down here in the SE. I still have (2) Master's and (1) Crystal from last year's auction.
24x36's have been at over $300/350
Last week in an auction near Huntsville, a black 18x24x4 (Ledge) went for $150
The big arse gray 12" thick - looked like 48x60 went for $575.00

 

[ddickey]

It sold for $65. I was watching a Hermann Schmidt mag chuck (sold for $375) and when I clicked back to the surface plate it just timed out. Oh well. There will be more.