Everything You Need To Know To Find The Best Inspection Surface Plate

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

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Surface Plates are solid flat plates used to mimic a perfect 2D plane as accurately as possible to facilitate other measurements being made on top that plane. Typical uses are precision inspection (for quality control, for example), marking out (putting precise markings to guide machine work), and tooling setup.

Many of the most common measurements are 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 World War II.  Before that time, most surface plates were metal.  Granite surface plates were 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.

Granite has a number of advantages over cast iron.  It's more durable and won't rust.  These advantages are why granite surface plates stayed popular even post-World War II.

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 therefore more wear resistance than black granite.  Quartz is more than twice as resistant to wear as the other minerals in granite.  However, Pink Granite isn't as strong as Black Granite and so it is normally desirable for a pink granite surface plate to be thicker.  Black Granite has superior stiffness, vibration damping, and machinability.

Before granite surface plates 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, cast iron surface plates may be used as a tool to help lap (resurface) a granite surface plate.

Surface Plate Grades and Precision

Surface Plate Grades determine their flatness.  Standard 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.".
• 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 granite surface plate in the largest size that you can afford and that fits comfortably in your shop.  This grade is intended for inspection work (quality control), and so covers the broadest range of use.

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

All surface plates must be calibrated regularly to ensure their accuracy has not been compromised by chipping, warping, or excessive wear.

The flatness of a surface plate is defined by two parallel planes-the base plane and the roof plane.  The distance between the two planes is the overall flatness.

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.

Reducing Wear, Cleaning, and Extending Life of Granite Surface Plates

Reducing wear is all about keeping granite surface plates clean.  Airborne abrasive dust is the greatest source of wear, and it will also tend to embed in work pieces and the contact surfaces of gages.

Cover your plate to protect it from dust and damage.  Rotate it periodically so that wear isn't always in the same place.  Wherever possible use carbide pads on gauging as it is much less likely to have embedded abrasive dust.

Surface Plates should also be kept as clean as possible.  Swarf and other debris 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 granite surface plates.

Ideally, clean your plate at the beginning and end of the day with appropriate cleaner.  Note that the evaporation of the cleaner can cause chilling, which may distort the surface.  Allow an hour for small plates to normalize and 2 hours for large plates.

Depending on how porous the granite in your plate is, retaining water from cleaning can be an issue that makes precision tooling more likely to rust.

Surface Plate Calibration

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.

Here's a fascinating video that walks through the process of testing and calibrating a surface plate in the field:

A process called lapping can be used to increase the flatness of a plate as needed.

Don't Use Your Surface Plate as a Counter!

Surface Plates are not counters.  Don't set anything on the surface plate other than gages and the pieces being measured.

Say you let someone put a Coke can on a Surface Plate.  That can may have dribbled so there's a little Coke on the bottom of the can.  But Coke is acidic and it will eat into your Surface Plate, damaging it.

Placing random things on Surface Plates can also transfer harmful debris that upsets delicate precision and abrades the Surface 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.

Ledges and Threaded Metal Inserts

Ledges are a part of the surface plate and must be specified when you order your plate.  Ledges are used to facilitate work clamping.

Threaded metal inserts may be embedded in the granite to facilitate clamping.  Keep the metal below the level of the granite to facilitate accuracy.

Surface Plate Stands

Proper support is essential for surface plates to minimize warping.  Granite Surface Plates should be supported at 3 points, ideally located 20% of the length in from the ends of the plates.  A precision surface can only be supported accurately by 3 points, not 4 or more.

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:

Temperature Controlled Room

It doesn't do you much good to specify a laboratory grade surface plate and then keep it in a room that isn't temperature controlled.  For the highest degree of accuracy and repeatability, the temperature must be consistent.  That's why so many shops keep their surface plates in a temperature controlled inspection room.

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 Cleaner

For more information, please visit Inspection Surface Plate.

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 (Sine Bars and Sine Plates) 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.

Granite is the material of choice for machine bases and metrology components for a variety of reasons including its machinability, inertness, ultimate flatness, thermal stability, rigidity, vibration and rust resistance, long lasting properties, ease of cleaning and mobility. It is also offered in many sizes along with a range of accessories.

The following primer provides helpful tips for working with a reputable supplier to provide you with a precision granite inspection solution that answers your needs.

Precision Matters
Manufacturing facilities as well as calibration labs rely on surfaces to be precise. At a minimum, surface plates should exceed federal specification ASME B89.3.7. Supplier engineers should have completed a stringent training program and also participate in annual proficiency testing in order to consistently produce high quality products.

A NIST traceable calibration certificate supplied with the surface plate is an important benchmark for quality. And if required, an A2LA accredited certificate should be issued for any calibrations that are within the ISO/ IEC scope.

Initial Inspection

Before taking ownership of a surface plate, review the final inspection process with your supplier. It is recommended that plates are inspected with an autocollimator or electronic levels and Repeat-O-Meter. Some suppliers offer surface plates that far exceed what is “required”. For example, black and pink plates might be engineered to support a loading weight of 100 pounds per square foot, loaded in the center of the plate, which is twice that required by federal specification. This means that the designated load may be placed in the center of the plate without deflecting the overall accuracy more than 50%.

In surface plates, flatness is very important; however, repeat reading is equally important. The flatness and repeat readings of some of highest quality plates are unilateral, not bilateral. The term unilateral accuracy means that all points on the work surface are contained between two parallel planes, separated by a distance greater than the distance specified for each size and grade. The term bilateral accuracy means twice (+ or – the accuracy stated) as much flatness deviation may exist. Values specified calibration certificates are recommended to be the TIR (total indicator reading).

Inspection in your Factory

Once you have the plate in production, it is important to periodically check that your plate is properly calibrated. Federal Spec ASME B89.3.7 dictates the proper methods for determining the overall flatness of a granite surface plate. In section 4.5.9, it states, “A…system of diagonals, perimeter and center lines may be calibrated on plates, and especially on small ones, by using a precision calibrated straight edge, adjustable supports and a suitably mounted indicator…”. This is a basic description of the Planekator.

A Planekator kit consists of a granite straight edge, a set of supports (one fixed height and one adjustable), a calibrated .” dial indicator with indicator carriage and a storage case. The straight edge is supplied with a correction tape.
When selecting a Planekator, consider the size of the plate(s) you will be inspecting. The straight edge should be at least equal to the full width of the plate and at least equal to 50% of the length of the plate. For example, a 36” Planekator straight edge can be used to calibrate any surface up to 36”x72”.

A Repeat-O-Meter can be used to visually scan the work surface of the plate. The range of the indicator readings shall not exceed the values shown in Table II of section 3.3.3 in Federal Specification ASME B89.3.7. The Repeat-O-Meter performs a similar function when a height gage and gage block are used in a calibration process, but is more dependable and easier to use for this application.

In a short amount of time, you can check the repeat measurement accuracy of a surface plate by moving the Repeat-O-Meter over the work surface, noting the full indicator range (F.I.R.). In just a few more minutes, by zeroing the R-O-M at the center along a diagonal or centerline and then recording readings at 5” intervals, you can chart the readings from the plate. The difference between the greatest plus and minus readings is the maximum measurement error possible on the plate.

Also, calibration and certification services for both surface plates and accessories are helpful to maintain precision by certifying their condition and resurfacing if necessary. If you have old granite products that are in very poor condition, some suppliers can grind and lap your old products, making them appear like new and calibrate them to A2LA (for perpendicularity, parallelism, flatness and repeat reading).

Metrology Accessories

Knowing the surface plate accessories that are available will also help you and your supplier create the optimal plate design. Here are some options:  

Master Square: Master squares are ideal for inspecting the X, Y and Z axes of machine tools. This product is very stable and rigid, providing the ultimate in squareness references. Master squares have 5 finished faces (1 large face and 4 edges) finished flat, square and parallel to either laboratory grade AA (.” per 6”) or inspection grade A (.” per 6”). All six faces can also be finished.

Master Angle: These triangular shaped master angles are ideal for checking X, Y and Z axes of machine bases or parts. They are very rigid and stable and come with either 2 or 3 finished faces. The 2-face master angle has 2 perpendicular edges finished flat and square. The 3-face master angle has a large triangular face finished flat and square to those two edges. Master angles are provided in laboratory grade AA (.” per 6”) or inspection grade A (.” per 6”).

Angle Plate: Angle plates provide a convenient and practical means of clamping and holding work in a vertical position. Their excellent finish and flatness make them very accurate and compatible for use with granite surface plates. The angle plates are available with either 2 or 4 finished faces. The 2-face angle plate has the bottom and the adjacent square face finished flat and square to one another. The 4-face angle plate is similar to the 2-face, but has the two adjacent sides finished flat and square to the other two faces, as well as being parallel to each other. The laboratory grade AA has a tolerance of .” per 6” and the inspection grade A has a tolerance of .” per 6”.

Inserted Angle Plate: Inserted angle plates are similar to the standard angle plate with the addition of metal discs inserted into one side. Inserted angle plates also have a main gauging face for magnetic chucking purposes and threaded inserts for clamping purposes.

Straight Edge:  Straight edges have a single long, narrow face finished flat. Laboratory grade AA accuracy is .” per 12” and inspection grade A accuracy is .” per 12”. Lifting holes are provided on sizes 48” or larger.

Parallels: These precision tools can be used to elevate work above the surface of a plate to enable quick and easy inspection of piece parts with shoulders or steps, along with many other uses. Parallels are provided in matched pairs with either 2 or 4 finished faces. Two-face parallels have the 2 narrow faces finished flat and parallel. Four-face parallels have the wide faces finished flat and parallel, as well as the two narrow faces. Parallels are available in laboratory grade AA (.” per 12”) or inspection grade A (.” per 12”).

V-Blocks: V-Blocks are ideal for supporting or holding cylindrical pieces during manufacturing or inspection. They are provided in matched pairs and have 5 finished faces. V-blocks have a nominal 90-degree “V”, centered with and parallel to the bottom and two sides and square to the ends. V-Blocks come in either laboratory grade AA (+/- .” per 6”) or inspection grade A (+/- .” per 6”).

Bench Comparators: The bench comparator is a highly accurate inspection tool used for checking roundness, diameters, lengths, steps, flatness, angles and parallelism. Bench comparators have a stainless steel post and indicator holder (lug mount or stem mount).

Surface Plate Support

To ensure accurate readings, granite surface plates must be properly supported on three or four points as specified in ASME B89.3.7. The standard work height for stands is generally 36” and it is recommended to use floor locks for all castered stands. There are several options for supporting the plate.

Tubular Steel Surface Plate Stands are constructed from welded square steel tubing for exceptional strength and durability. Steel crossbeams are located at the proper support points to ensure maximum surface plate accuracy. Cabinet Support Stands are built with square steel tubing and heavy gauge steel for durability. Drawer stands may be available as well.

Also, if using a block and wedge system, a large steel stand to support the granite base is unnecessary. This system is comprised of three granite blocks, cut and honed to provide the desired work height. An adjustable wedge is placed between the top of the block and the bottom of the surface plate. This system provides maximum support with minimal cost. Standard blocks are 12”x12”, with a height based on the thickness of the surface plate.

For maximum life, it is also recommended to use a high-quality cover to protect the surface plate when it is not in use and regularly clean the plate with granite surface plate cleaner. Keeping the precision surface plate clean and in the best condition can lengthen your calibration intervals and help ensure that your readings are accurate.

SIDEBAR - Custom Configurations

When your application calls for a plate with custom shapes, threaded inserts, slots or other machining, you’ll want to thoughtfully and carefully select your supplier to guide you through the customization process. Identify a supplier with engineers who have the knowledge to understand your precision base needs, and add the value of their experience to optimize designs for both performance and cost.

Working with a supplier with comprehensive and custom capabilities for bases and other custom granite solutions is critical for a range of industries including semi-conductor companies, for both wafer production and wafer inspection. Suppliers can provide solutions from small stones with sub-micron accuracies to large systems requiring Class 100-cleanroom assembly and value-added assembly, to custom frames, stands and fixturing, and cleanroom packaging. Also, custom granite inspection solutions can encompass CMM bases, calibration laboratories, flat panel inspection, and assemblies used in precision automation applications.

Starting from a new concept or mature design, small part or large assembly, simple requirements or complex needs, experienced professionals can help provide a high-quality, efficient design. A broad knowledge of materials, precision finishing and assembly methods combined with state-of-the-art factory equipment will all work together to result in the best finished product. In addition to using granite for a custom surface, utilizing other materials can be beneficial. Materials including porous or dense ceramic, carbon fiber and even metals also provide solutions to unique foundation needs for precision assemblies.

In addition to collaborating on the design and build of your machine foundation, working with a supplier that is skilled at value-added assembly offers advantages. Next level assembly includes adding bearing rails, encoder rails, screw drives, stages, and/ or vibration damping devices. Having this assembly done at the supplier factory provides accountability for accuracy and performance.

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