The surface plate may be the unsung hero of the precision measurement community. Surface plates are the foundation of so many types of measurement and provide a context on which those measurements can take place. A surface plate is the flat plane that is used as a horizontal reference point in the measurement process, and can be used for setting up tools, inspection purposes, or transferring designs from one part to another (also known as marking out). Functioning as a baseline, the surface plate is of great importance. Therefore, the material it is made out of, its tolerance, its size, and its flatness are of equal importance.
Surface plates may be made out of granite, metal, cast iron, or glass. Today we are going to focus on granite surface plates and how they are optimally suited for precision measurement purposes. The first granite surface plate was created by Mr. Wallace Herman of Dayton, Ohio in World War II. Mr. Herman used granite to replace the typically used metal because of a lack of metal availability since much of it was being used toward the war effort at the time. The most commonly used type of granite for surface plates is black granite, however quartz-containing granite, known as pink granite, may also be used. Black granite, also known as diabase, is prized for its capability to bear heavy loads. Black granite will typically be thinner when compared to pink granite because it is made of the stone alone. Alternatively, pink granite, while not as load bearing as its black counterpart, is incredibly resistant to wear and is better when parts need to be moved across the surface. As the thickness of a pink granite surface plate increases, so will its ability to bear heavier loads. Both types of granite are great for dampening vibrations, improving machinability, and maintaining stiffness.
A well-made granite surface plate is used generally as a zero reference point or a standard reference point. Surface plates may be utilized through the actual manufacturing process, or during the final inspection of a part or product. Surface plates play a vital role in the process of quality control. A height gage that has been zeroed on a granite surface plate can be used to check the flatness of other parts, automotive and airline manufacturers use granite surface plates to asses for warped areas on a chassis or crankshaft, and more. Specifications for granite surface plates have changed over the years. Most recently, the American Society of Mechanical Engineers (ASME) released standard specifications in 2013 including definitions, certification, and recertification regulations. Granite surface plates come in three different grades, which are defined by different flatness tolerances. These include: 1) Laboratory Grade AA which means the plate has a flatness tolerance equal to (40 + diagonal squared/25) x .000001″ (unilateral), 2) Inspection Grade A which has a flatness tolerance equal to Laboratory Grade AA times two, and 3) Tool Room Grade B which has a flatness tolerance equal to Laboratory Grade AA times four.
Maintenance of a granite surface plate is very important. While a small chip may not impact the functionality of the whole plate, you will want to take care to avoid damaging this useful tool. First and foremost, do not use your granite surface plate as a table or workbench. Other tools can cause damage if they come into contact with the plate, and any residue or grease from food or coffee can impact the functionality of the surface plate. When using your granite surface plate, be careful when placing parts or other tools on it, for any excess force may nick the surface. Any granite surface plate should be regularly inspected to assess flatness and tolerance. Given the size of the average granite surface plate, you may find spots that are out of tolerance requiring recalibration or replacement.
The granite surface place has been and continues to be an invaluable source of flatness and stability in the precision measurement world. Whether you are working with a black granite surface plate or a pink granite surface plate, you can do incredible measurements with this tool.