Probably the simplest and most overlooked precision measurement tool of all is the lowly granite surface plate. Poor thing, it is just a really flat rock. Yet it is the very foundation for most inspection operations.
It is possible to accurately measure tools, dies, gages, jigs and fixture without a granite surface plate, but not really very practical. The only real alternative is some kind of steel or cast iron plate that is ground and lapped flat.
The problem with metal is rust, magnetism and dings. These all can and will cause poor measurement. It is almost impossible to keep cast iron from rusting, and steel is not much better. Both are magnetic and can and will change the reading of a dial indicator.
Steel is especially prone to dings, or raised surfaces from having tools dropped on the surface. It is just a matter of time before things get dropped on the surface plate. Another problem is that the cast iron and steel tend
to attract dust and are more difficult to keep clean than granite.
The Metrology Handbook, Second Edition
An authoritative book, The Metrology Handbook, Second Edition provides a foundation for understanding basic metrology and calibration principles and practices.
What is the recommended type of granite?
It is generally accepted that black granite is superior to other types of granite. While the lighter shades of granite may contain more quartz, the remaining composition tends to wear out more quickly than black granite. This can cause abrasion on the parts being inspected.
Granite Surface Plate Advantages
|Uniform hardness||Thermal stability||Accurate under a load|
|Low porosity||Easy to keep clean||Does not glare|
|Easy to resurface||Does not rust||Does not warp|
Granite is universally accepted as the material of choice for surface plates, and as such, form the foundation for all precision inspection in a toolmaking shop of any type.
Micro precision machining, tool and die making, injection mold making, aerospace precision machining and CNC precision machining are some of the industries that rely completely on the granite surface plate.
Basic Metrology for ISO 9000 Certification
An authoritative book, Basic Metrology for ISO 9000 Certification
gives a nice introduction to ISO 9001 requirements for calibration of test and measuring instruments.”
Care of surface plates
Your humble surface plate is very low maintenance. All it demands is some respect and cleaning. There are numerous quality cleaners on the market, and simple cleaners work quite well.
Care must be taken to keep it clean, however, for the obvious reason of abrasion. Often there is grinding grit floating about in a typical manufacturing environment, and it just loves to settle on the surface plate. This grit will quickly damage the surface and cause premature wear, with the result of inaccurate measurements.
Leading manufacturers of granite surface plates
- LS Starrett
- Rock Of Ages