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Granite Surface Plates

Starrett Granite Plate with Cabinet Base

The importance of a good layout plate cannot be understated. The granite plate is the basis for almost all layout derived measurements. The fixturing will rest on it, as well as the gage used for measurement, and as such, it sets the relationship between the two. While cast iron is used by toolmakers and other skilled trades, granite is most prevalent due to the temperature stability, precision flatness that can be achieved, and durability. Granite comes in many standard grades and configurations,and can also be ordered with custom dimensions and features for use in flatness measurement, specialized fixturing, incorporation into equipment, and as guide ways for precision reference edges.

BuyGage carries granite plates from a number of manufacturers and can configure it to your specifications. While granite has many good properties, it is heavy. Very heavy. Due to this, most granite surface plates require shipping via freight and in the case of the larger plates, will require rigging by a specialized company for movement and setup once at your facility. Please call us and we can help you spec out the best table for your needs and work with you on getting it shipped to your facility and set up.

A few notes about granite plates...

Type
There are 2 types of granite - solid and composite. Solid granite is mined as a single piece and machined and ground to the final size and shape. Composite granite is molded from pieces of granite and a bonding agent (acrylic or other material). While composite granite has gained popularity in molded product (kitchen countertops, etc.), it is not often used for precision layout work.

Solid granite is rock. The most desirable granite is very dense, with closely spaced, homogeneous particles, and no air pockets. The kind used for layout is generally found in gray, black, and pink. The color is a result of the materials composing the granite. Some of the materials are harder than others, creating a surface with better wear characteristics. Like many other things, you get what you pay for. Some people prefer one color over another, but you can buy every color in every grade (see below).

Grade
There are 3 primary grades...

AA - often called lab grade, it has the highest accuracy and is often reserved for high accuracy work and for certification of other gages
A - often called inspection grade, has a mid-level accuracy and is most often used exclusively for inspection
B - often referred to as shop, or working grade, is most often found on the manufacturing floor for continuous use

Support
Base - When installing a granite plate, it is important to ensure it is well supported. Since 3 points define a plane, it is best to have 3 points to rest the granite on (2 corners on one side, and the center of the opposite side). As all standard granite comes in a rectangle, this will leave 2 corners of the plate unsupported. Placing a heavy part on an unsupported corner can cause it to tip, causing damage, injury, or even death. For this reason, we recommend you purchase a solid metal base designed specifically for the size of granite you are purchasing. If you do choose to make your own base, we strongly recommend that you construct the base with 5 points - 3 of them in the configuration mentioned above, as well as 2 "dummy" points just below the granite in the unsupported corners. This will allow the granite to rest on the 3 points, as well as prevent it from tipping if a heavy load is placed on an unsupported corner.

Isolation - Vibration can cause problems in precision layout and must be considered. Factories have machines with moving part (motors, press platens, vibratory feed systems, conveyors, forklift traffic, etc.) and all of these machines create vibration. We have seen an indicator on a height stand fluctuate as much as 0.005" due to a vibratory finish operation over 200' from the layout plate!! A solid steel stand with a granite plate directly on top of it will transmit all of this vibration directly into the workpiece and gaging, which together create a noise "loop". For example - a test indicator is mounted on a height stand, which is resting on the granite that holds a fixture, which in turn holds the part being measured by the indicator. This loop will interact with the vibration and set up harmonics which will cause fluctuations in the indicator. The larger the noise loop and/or the more sensitive the instrument (i.e. 0.00001" probe w/amplifier), the more pronounced the effect.

There are a number of systems to reduce vibration - anti-vibration materials, air mounts (balanced and individual), and active. We urge you to call us to help you define what would work best in your environment. There are very effective systems that can be purchased for very little money which can pay for themselves very quickly. We had a customer who had been doing their "supermic" calibrations on Saturday for over 5 years due to excessive vibration. They were only aware of "active" vibration systems that can cost over $10,000, and we helped them move their gage calibration program to normal working hours for about $400!! This small investment saved them overtime for 2 weekend days a month!