12-Inch by 18-Inch by 3-Inch Granite Surface Plate
Granite surface plates belong in every shop and inspection room. They are an indispensable flat surface for all kinds of precision work. They also have many advantages over cast iron surface plates.
- Twice as hard as cast iron.
- Minimal changes in dimension due to temperature changes compared to cast iron
- Parts will not easily wring to the granite
- Free from burrs or protrusions as a result of surface damage (you may have dropped something heavy)
- Insignificant stickiness ensures a high degree of flatness
- Will not damage your parts or instruments
- Will not rust
- Long useful life (ours has been in perfect condition for over 50 years)
Granite plates are available in three grades (the description will tell you where these are best used):
- Laboratory Grade AA
- Inspection Grade A
- Shop Grade 0 (sometimes called Grade B) has ±.0001" accuracy over its surface
Three common accessories will give maximum usefulness to the granite plate.
- Angle Blocks
Granite surface plates are actually made of real granite and they are heavy. You will need a sturdy stand to accomodate them.
Here is some very useful information provided by Mahr-Federal in their April 2016 Gaging Tips newsletter:
- Surface plates provide a broad, smooth, flat reference surface that can be extremely useful for inspecting incoming, in-process, or finished parts. When used in combination with various gages and accessories, such as height gages, gage blocks, angle plates, and squares, they can be used to check a wide range of parameters, including length, flatness, squareness, straightness, angle, feature location, and runout. Surface plates are simple and extremely versatile.
- Surface plates come in a wide range of sizes, from about 12" x 12" to 6' x 12', and can weigh up to 10 tons. Three grades are available, the flatness tolerances for each grade varying with the size of the plate: AA (laboratory grade); A (inspection grade); and B (tool room grade). Many can be ordered with ledges and threaded inserts, both of which make it easier to clamp workpieces or accessories to the surface. Granite is the most common material used: it is harder and denser than steel, has very little internal stress, and is less subject to dimensional change due to temperature variations.
- Any of the checks mentioned above can be done using a height gage and indicator as the measuring instrument and a gage block stack as the zero reference. However, there is a gaging technique that anyone can adapt to improve the overall performance of the measurement to be made. It's one that will improve those measurements when gaging performance to 50 µin or 1 micron is required.