Flat Plate

Federal Specification GGG-P-463c "Granite Surface Plates" defines the three standard grades of tolerance for surface plates as follows: * AA - Laboratory Grade: For high precision operations in consistent temperature gauging rooms and metrology departments * A - Inspection Grade: For general use in quality control areas * B - Toolroom Grade: For shop work and production checking

[Purchase Federal Specifications from Federal Supply Service Bureau; Specification Section; Suite 8100; 470 E. L'Enfant Plaza Southwest; Washington, DC 20407]

Flatness tolerance is a unilateral tolerance, not a plus/minus tolerance. Occasionally, flatness will be defined as: deviation from a mean plane. Tolerances stated in this way are plus & minus, as opposed to unilateral. The flatness tolerances for these grades are defined by the following formula: * Laboratory Grade AA = (40 + diagonal squared/25) x .000001" * Inspection Grade A = Laboratory Grade AA x 2 * Tool Room Grade B = Laboratory Grade AA x 4.

[A new replacement for Federal Specification GGG-P-463c (ASME B89.3.7) is nearing publication It will incorporate most elements of the current specification.]

We use a �B�. We bought a nice box of pins at an auction for like $25 and they threw-in the plate. [Well, they don't literally 'throw it in'. The mutha weighs about 150 pounds. It was good ballast fo the ride home.] It�s granite, maybe 2-foot square on the surface. We got it calibrated [AA Jansson; Waterford, MI] and follow a procedure to maintain the whatever.

I real life, probably the flatness of a Formica[r] bench top is about as accurate as you need to get. You�re just using this for process control. If you end-up in a fight with your: * Board fabricator, their metrology shop will carry more weight than yours. * Customer, their metrology shop will carry more weight than yours.

If you are gearing-up a serious metrology lab, among the high quality sources for your plate are: * Standridge Granite; 9437 Santa Fe Springs Rd; Santa Fe Springs, CA 90670; (562) 946-6334; Fax (562) 941-3804 http://www.standridgegranite.com

* Rahn Precision; 75 Holiday Dr; PO Box 216; Englewood, OH 45322-0216; 888-832-1943; Fax 937-832-0848 http://www.rahngranite.com

* Tru-Stone Technologies; 1101 Prosper Dr; P.O. Box 430; Waite Park, MN 56387; 320-251-7171; Fax 320-259-5073 http://www.tru-stone.com

For additional sources and maybe some used stuff go to: http://www.metrologyworld.com/

Learning about warp: * There is a fair amount of background on warp* in the fine SMTnet Archives. * Coombs' Printed Circuits Handbook [McGraw-Hill Professional; ISBN: 0071350160] has a page or so on the design, fabrication, and assembly processing drivers to warp.

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Maybe you're well ahead of me, but make sure you search the archives for "bow" and "twist" as well as warp.....

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