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Precision Granite Surface Plate: Complete Guide [ Cast Iron, Steel, Inspection, and More ]

Precision Granite Surface Plate Cast Iron Steel Inspection

Grade A Black Granite Surface Plate, 18″ x 12″.  About $160 on Amazon…

The purpose of a Surface Plate is to mimic a perfect 2D plane as accurately as possible to facilitate other measurements being made on top that plane.  One of the most common measurements is done with a Height Gauge.  

A Surface Plate with height gauge is an extremely handy thing for any CNC’er or machinist to have on hand, so I recommend getting the set relatively early on. 

Surface Plate Material: Granite, Cast Iron, Steel, or Glass

Surface plates are usually made of granite, and they come in a variety of sizes and grades.  There are cast iron, steel, and glass surface plates available, but most prefer to stick with granite.

Granite came into wide use during the Second World War.  Before that time, most surface plates were metal.  Granite was conceived as a means of saving the metal for more critical applications.  The idea to use granite came about by the happy coincidence of Wallace Herman owning both a monument and a metal shop in Dayton, Ohio.

You can find both black and pink granite used to make surface plates, with black granite being more popular.  Pink granite has more quartz, and is therefore more wear-resistant than black granite.  However, it isn’t as strong and so it is normally desirable for a pink granite surface plate to be thicker.

Before granite became prevalent, Cast Iron was the most common material for surface plates.  It is still popular for certain applications because it is easier to work it to a particular degree of precision than granite.  In fact, a cast iron surface plate may be used as a tool to help lap (resurface) a granite surface plate.

Surface Plate Grades and Precision

Surface Plate Grades include:

  • AA (Most accurate):  These are referred to as laboratory grade.  Their flatness is (40 + diagonal (inches) of surface plate squared / 25) x 0.000001″. 
  • A: These are Inspection Grade.  They’re the AA spec x 2.
  • B: These are Toolroom Grade.  They’re AA x 4.
  • Workshop Grade (Least accurate):  These are not really suitable for CNC applications, so I won’t quote a spec.

Try to purchase an “A” grade surface plate in the largest size that you can afford and that fits comfortably in your shop.  

The ASME (American Society of Mechanical Engineers) has published ASME B89.3.7 — 2013 which is the current standard for granite surface plates.

Standard Surface Plate Sizes

Standard surface plate sizes are 18 x 24 in., 24 x 36 in., 36 x 48 in. and 48 x 72 in, but you can get most any size custom made if need be.  I have an “A” Grade 24 x 36 in A Class granite surface plate for my metrology needs.  It’s a very nice surface plate for a small shop, but larger shops will want even larger surface plates and will likely have more than one.

Surface Plate Calibration and Cleaning

Surface Plates can experience chipping, warping, and just plain wear, so they must be calibrated regularly.  Often, the wear will be localized due to a particular tool, such as a height gauge, being used in the same place constantly.  Surface Plates should also be kept as clean as possible.  Swarf and other degree can easily interfere with accurate measurements.  Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of the plate.

Surface Plate Accessories

There is a huge variety of accessories used with surface plates.  We won’t try to cover them all, just the most commonly used ones.

Surface Plate Stands

A variety of stands are available to support all types of surface plates.  Smaller surface plates can be placed on any bench, but larger plates benefit from proper suspension.  Here’s a typical fabricated stand for the sort of large surface plates found in the inspection and quality departments of many machine shops:

surface plate stand

Surface Plate Covers

If your surface plate is not in continuous use (most are in commercial shops), it’s nice to protect it with a fitted cover:

surface plate cover

Surface Plate Cleaner

Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of the plate.

Height Gauges, Sine Gauges, and Gauge Blocks

Height Gauges, Gauge Blocks, and Sine Gauges are probably the most common measuring accessories used with a surface plate, though there are many more.

This article is part of our Complete Guide to Metrology.

 

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Precision Granite Surface Plate: Complete Guide [ Cast Iron, Steel, Inspection, and More ]
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