Granite how to straight and flat

Drroaster · Sep 3, 2014

I picked up a 24x24x6 table on a stand I don't have any tools to go with it. Started wondering when I need it how would I know how accurate it is? I have a strait edge I used to use on heads to see if they needed surfaced. But what is the true procedure to get to a good starting point? I have looked through the archived post didn't find anything that explained a good way to go. Any help would be greatly appreciated
Boyd

The real Leigh · Sep 3, 2014

If it has no obvious damage, it is likely quite adequate for home/hobbyist usage.
I seriously doubt that you have the tools required to evaluate its surface condition.

Gouges are generally not a problem unless they're really large.
Any points projecting above the surface must be removed.

It should be supported on three points arranged as a triangle, with the base and apex each about 6" in from the edges.
If it's on its original stand I expect that requirement is satisfied.

If it's to be used for any commercial work it needs to be certified, which you cannot do yourself anyway.
There are people who drive around doing this kind of work. Not expensive for a plate that small.

- Leigh

Atomkinder · Sep 3, 2014

There is proper metrology equipment for this process. I would suggest finding a metrology shop near you and just taking it to them. I bought a 9"x12" DoAll Grade B surface plate, heavily used, and had it resurfaced and calibrated to Grade A for $35 (flat within 40 millionths). I don't know of any good way to check a reference surface (although I'm certainly no expert) without that equipment.

Drroaster · Sep 3, 2014

Thanks guys the stand is a welded one I didnt know about the three point but I will check it next time I'm in the shop. It is smooth and has no scratches but there was oil on it I did rub it down and used a rasor blade to clean off the buildup but other than that it sets in front of the mill thought it would be useful some day

The real Leigh · Sep 3, 2014

Here's the correct location of the three "Airy" support points for any surface plate, per the Federal standard.
Note that the values shown are for a 12x18 plate used as an example.
The proper spacing is 20% to 25% of the plate length and width, whatever those may be.

I too have a 24x24x6 plate, and find it has a much larger usable work area than my 12x18 plate.

- Leigh

Drroaster · Sep 3, 2014

Thanks I will assume that I then use a level to get it there and then all I need it a digital caliper with a base. It will be a while

The real Leigh · Sep 3, 2014

Surface plates need not be "precisely" level, just level enough that the tools won't roll off.

The level condition (or lack thereof) has no bearing on the accuracy of the plate unless it changes the support points.

- Leigh

specfab · Sep 6, 2014

Regarding the 3-point support: Depending on who made the plate, and how diligent their manufacturing process is, you may find (on the bottom surface) markings indicating the 3=point support that was used for doing the lapping of the plate at the factory. Ideally, your support for the plate will employ those same points for best accuracy of the existing plate conditions. I acquired a 36x36x6 thk DoAll plate some months ago, and the support points are marked on the bottom with 2" dia red painted dots. Very handy.

Drroaster · Sep 7, 2014

Ok went into the shop and first thing I did was to look in fact the frame made of angle iron and the plate is resting in it. As spec fab noted there are 3 pads protruding from the bottom . No I am adding adjusting studs to support the plate properly. Thanks to all for there input.
Boyd

specfab · Sep 8, 2014

For what it's worth, here is what I did with mine (if the attachments load...). The white painted angle iron frame is what came with the plate; I added 6" 700-lb rated casters on 1/2" thk mounting plates, and the rest of what is shown that is unpainted. This does allow reasonably easy moving of the plate and stand for reconfiguring space as needed (only about 600 SF in the shop).

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