Quality Scan: Caring for Granite Surface Plates
By Jimmy Coalson
Surface Plate Specialist
Granite Surface Plate Div.
L.S. Starrett Co.
Mt. Airy, NC
During WW II, Wallace Herman, the owner of a monument and metalworking shop in Dayton, OH, required a flat table for checking parts. Unfortunately, military production had claimed the metal formerly used to make surface plates. Herman deduced that granite could be used to produce a flat surface, and he then manufactured the first granite surface plate.
Granite types are black or Starrett Crystal Pink. Black granite is the harder material; its modulus of elasticity is 9.5 (106), while that of Crystal Pink is 3.8 (106). Because of the 32% quartz content of Crystal Pink, it outlasts black granite by a ratio of 5:1, and Crystal Pink's quartz content permits instruments to slide more easily without any risk of plate wear. At any specific weight, a black plate will be thinner than a Crystal Pink plate. Black granite is used primarily for its load-bearing capabilities.
A granite surface plate is a precision piece of equipment, and must be properly installed and maintained. Be sure the granite surface plate is always used on its support system. Typically a hard-rubber pad is attached to the bottom of the plate; this system forms a nondistortable three-point support. Pads are installed during manufacturing, and the plate rests on them throughout lapping, inspection, and shipping. They are a critical factor in surface plate accuracy, and must not be removed. Plates as large as 6 X 12' (1.8 X 3.7 m) are furnished with this nondistortable support system. When mounting the plate on a stand, be sure only the pads rest on the stand. Never support the plate by its ledges, edges, or corners; doing so voids the accuracy guarantee.
Plates larger than 6 X 12' are supported at six or more points. The supports consist of granite pedestals and leveling wedges. Plate size determines the number of support points and their positioning, while plate thickness and required plate working height determine the height of the granite pedestals--or even if they're used.
Use nylon slings when lifting the granite. If you use a forklift to move it, place protective padding between the metal forks and the surface plate.
Once set up, plates don't require extensive care and maintenance. Keep the surface clean to maintain accurate tool readings and extend the life of the plate.
For Pete's sake, don't use granite surface plates as workbenches or lunch tables! Dropping wrenches or hammers on plates can chip and nick the surface, and spilling coffee or drinks on granite can cause permanent stains and eat away the surface finish. Grease from food can soak into the pores of a granite plate and cause a permanent stain. Please keep in mind that Crystal Pink is very porous.
A surface plate is a zero-reference with many uses. When employing a height gage on the plate you must zero that gage anywhere on the plate with a gage block. Remove the gage block and you can check any of your parts to see if they are flat. Place the workpiece you're measuring on the plate gently. A sudden jolt or blow to the plate with a heavy metal object can chip or nick the surface. When inspecting workpieces, especially small parts, try to use different areas of the surface plate. Using the same spot over and over will cause wear at that location. When using threaded inserts in a granite surface plate, use the minimum required torque on hold-down bolts. Don't exceed the limits on the caution label attached to the plate that came with the threaded inserts.
Surface plates should be checked for wear on a regular basis using a repeat-reading gage with a manual indicator. Generally, long before a surface plate has worn beyond specifications for overall flatness, it will show worn or wavy spots. The gage will detect these error-causing areas. Also note that actual calibration of overall flatness traceable to NIST can be performed periodically using an autocollimator.
When inspection indicates that the overall accuracy of a surface plate is out of tolerance, the plate should be relapped to restore it to its original accuracy. A plate should be calibrated on an established cycle, whether it be one, two, or three years, depending on frequency of use.
This article was first published in the May 2004 edition of Manufacturing Engineering magazine.