My work in inspection, and as a quality management system auditor, provides numerous opportunities to acquire knowledge. It also provides areas of risk for ensuring valid and reliable results. One risk that keeps repeating itself in my audit findings is the treatment of surface plates. Instead of being treated as a precision device, they are treated as an area to stack tools, parts and other debris. The root cause of this treatment is the lack of knowledge about the surface plate and its mistaken identity as just a table. Let’s take a journey of the history and recommended care for the surface plate.

The surface plate was designed and developed as a precision tool by Henry Maudslay and Joseph Whitworth in order to ensure the production of identical tool parts. The first surface plate was composed of cast iron. The first surface plates were sold in 1904 by the Crown Windley company. During World War II, metal was in high demand which resulted in the introduction of using granite, a non-corroding metal that is extremely hard and non-magnetic.

A flatness check of a surface plate. Source: ASQ

Engineering and manufacturing specifications are demanding tighter tolerances, which elevate the importance of accurate measurements. To deliver measurements, each piece of metrology equipment requires a base plate.

Granite is the most common material used for surface plates. The surface plate does require maintenance. The following are recommendations for maintaining these precision instruments:

• Practice proper installation methods
  • Use the support system that came with the surface plate, which includes a hard rubber pad attached to the bottom of the plate. These pads are critical for the accuracy of the surface plate. Some large plates may have more than one support system to assist in controlling vibration.
• Perform regular service and calibration procedures, including regular inspection of the surface for dents, scratches
  • Resurfacing may be required if damage risks reliable and accurate measurements with a compromised table (not properly flat)
  • To verify flatness, periodic inspections can be made on the surface using a repeat reading gage with a manual indicator (move the gage across and identify if there are any deviations more than 0.000025 inches for AA plate, 0.00005 inches for A plate and 0.00001 inches for B grade plate)
• Maintain a clean table
  • Use granite cleaners to clean plates prior to use, and after. (Using a dirty surface plate will result in inaccuracies in tool readings and cause the plate to wear faster)
  • A cover should be utilized to prevent debris from settling on the table
  • Only use soft cloths to prevent scratching the surface
  • Do not use metal straps of chain for large or heavy parts
  • Do not drop the plates when positioning them on the table
• Rotate table or table location
  • Rotate your position around the table to minimize wearing the table in one location
  • Do not lift or move a table by the granite (keep forks or lifting device away from the granite), be sure you are under the rubber pad.
• Do not use surface plates for storage
  • Surface plates must not be used for storing tools, food, and parts
  • coffee will stain plates
  • food will contaminate the surface with a grease residue
  • storing on plates will accelerate the wear of the surface and promote uncertainty of measurements
  • A cover should be utilized to prevent debris from settling on the table
• Avoid overload
  • Ensure you have the correct size of surface plate for the size of the parts and measuring equipment you need to use
  • Ensure even distribution across the surface plate to prevent issues with accuracy and serious damage
• Use threaded insert correctly
  • A surface plate may have threaded inserts to hold fixture gages or other tooling
  • The bolts should be torqued at a minimum setting
  • Do not ignore the warning labels included with these plate types

A surface plate on a CMM (r) and other surface plate devices (l). Source: ASQ

Along with checking the flatness of parts, surface plates can identify part warpage. The surface plate is the “0” reference point. If a height gage is used on the plate, the inspector can place the gage anywhere on the plate and zero the gage. This can be completed in conjunction with a gage block. The gage block is removed, and small parts can be accurately measured. This is also a technique to verify small parts for flatness.

The surface plate is an underrated metrology tool. It has been more than 80 years since granite was identified as an alternative to cast iron. Granite surface plates have become foundation for many standards in metrology. The coordinate measuring machine (CMM) does represent the acceptance of granite as the only material used in this vital area of metrology. Hopefully, you were able to obtain knowledge and insight on why I feel it is critical to properly maintain surface plates. Valid and reliable results must be acquired in order to make proper decisions on the disposition of parts.