Using Granite Countertop as surface plate????

[Falcon67]

AFAIK, there is no DIY method for checking a surface plate. If you are concerned, you would take it to a calibration house for testing and/or refurbish and optionally a traceability cert. I used to have it done when I was in QA for the company 3'x4' plate and it takes the guy a few hours to do with specialized equipment. Not cheap either, but then you don't really want to cart an expensive 3'x4'x6" piece of granite down the street in a pickup either. Typically to get any kind of decent cal on something, you need a source 10x better to check against. 100x is even better. So for a .0002 tool room grade plate, you'd need equipment repeatable to at least .00002 - not typically home shop type equipment. Most affordable home shop gauge blocks don't get there and if you are not in a temp controlled environment, they wouldn't stay there anyway. I'd think a steel plate would move around more than that between morning and noon in a typical home shop.

Not much else there - You buy a plate from Enco and it comes with a non NIST traceable cert to .0002. You either believe it and file it, or take it somewhere and let them check it. As said above - if you need the accuracy, go get it. Otherwise, probably good 'nuff.

[J Tiers]

There IS a way to survey-check a flat..... if you have a sensitive indicator. I have a Federal that has a range of only about 5 thou, and direct reads to 0.0001, which is decent for the purpose.

An indicator of less sensitivity than 0.0005 per div isn't going to tell much, other than finding serious problems.

You mount the indicator on the arm of a "surface gage", or other similar FLAT bottomed base.

Reach the arm out so the indicator is registering on the surface of the plate at a distance. You can zero if you want, it doesn't matter.

Now slide the assembly around and swing the arm and indicator to allow the indicator to cover all of the plate, being sure the base or surface gage remains flat down on the surface. if you see variations (other than quick dips such as in chips/divots etc) as you move around, the plate is "out" an amount similar to what you see on the indicator. Not necessarily the *exact* same, due to an assortment of effects.

If you see very little variation, that's better than a lot.

Typically, the middle would be worn down more than the edges and corners, so sweeps edge-to-edge with the base in a corner would give a good indication.

There is a commercial instrument of a similar type action for exactly this purpose. The Connoley scraping book gives some ways to test also, but requires tools you typically won't have.

[BigBob]

Thanks, what you describe is pretty much what I did. I appreciate your suggestions.
Bob

[BigBob]

I have tried all the internet searches I can think of to find the source of the cast surface plate I have. So, I would appreciate it if anyone knows anything about the plate. It says stamped on one end top Made by Jois.Mach & Reb. It is 10 x 15" with about a 4 inch ribbed section underneath. I assume this means Jois machine and rebuild, New York City.

Bob

[Rosco-P]

I have tried all the internet searches I can think of to find the source of the cast surface plate I have. So, I would appreciate it if anyone knows anything about the plate. It says stamped on one end top Made by Jois.Mach & Reb. It is 10 x 15" with about a 4 inch ribbed section underneath. I assume this means Jois machine and rebuild, New York City.

Bob

Jois Macine? Likely as not, a company long out of business. What did you expect to learn from them? Many CI surfaces plates were made and sold by Brown and Sharpe. A few guys have sent their Enco/chinee granite plates out for testing, lapping, certification and received passing marks. But if you don't do so, it's little more than granite pizza cooking stone. If the stone doesn't have a known, traceable level of accuracy, how can you use it for a reference?

[BigBob]

I knew B&S and others made them and about surface plate standards and accuracies from previous posts, replies and then researching this.
I also have a granite surface plate to use when needed.
I am just curious about the history of this old plate, as I am regarding some of other older machinery and tooling.
Thus, if anyone knows anything about this company, even if long out of business or the techniques vintage companies used to make and measure them, I would appreciate the information.
Bob

[ROGERC]

I got to this forum from some interest in metrology, and the Wenzel Group, which specializes in metrology on an industrial scale.

While your desire to use a 1" countertop as a substrate is understandable from a cost standpoint, the more mass you have in the piece, under any circumstance, the better. If you are interested in precision (name the degree you desire to achieve), good old mass, homogeneity of the material (relates to thermal and physical stability) and the thickness, which relates to all of the above, will play into the equation.

The problem with 'gluing 2 pieces of 1" together to achieve a thickness of 2" (or whatever) is that you now have a non-homogenious mass, thermally and dimensionally. None of the physical properties will transfer evenly throughout the 'broken'/reconstituted mass.

I was looking into metrology and the flatness of a granite surface when I happened on the forum.

To put this discussion in perspective, and borrowing from some of the prose from the Wenzel group, they start with a 35-ton piece of granite and cut it into slabs of whatever size they wish for the ultimate purpose at hand.

https://www.wenzel-group.com/en/company ... zel-group/

This is the home page, but you can look for a variety of topics explained in detail in the links within the website.

You might also look on YouTube where I have just found several sections (you may already have been there) about the science of: roundness, flatness, measurement, etc. The scary thing is how much time in the past 2-3 days I have spent watching these videos both at home and at the gym on the treadmill.

I have a background in Physics and Astronomy (studied in college) and a variety of other scientific subjects, electronics, some chemistry, studied as a layman. I worked in and owned a camera store for about 50 years. Recently retired and have some time now and interest in all this stuff.

[Bill_Cook]

Before I acquired a pink Starret I used the mill table.