Ensuring precision with granite surface plates

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 shops to select the right granite inspection product to meet their needs.

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. Surface plate suppliers should have completed a stringent training program and participate in annual proficiency testing to consistently produce high-quality products.

A NIST traceable calibration certificate supplied with the surface plate is an important quality benchmark. And if required, an A2LA accredited certificate should be issued for any calibrations that are within the ISO/IEC 17025 scope.

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 lbs. per square foot (PSF), loaded in the centre of the plate, which is twice as much as what is required by federal specification. This means that the designated load may be placed in the centre of the plate without deflecting the overall accuracy more than 50 per cent.

In surface plates, flatness and repeat reading are equally important. The flatness and repeat readings of some of the 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 (± the accuracy stated) as much flatness deviation may exist. Values specified calibration certificates are recommended to be the total indicator reading (TIR).

Once a plate is in production, it is important to check periodically that it is properly calibrated. 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 centre 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 0.000020-in. 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 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 per cent of the length of the plate. For example, a 36-in. Planekator straight edge can be used to calibrate any surface up to 36 by 72 in.

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 ASME B89.3.7. The Repeat-O-Meter performs a similar function when a height gauge and gauge block are used in a calibration process, but it is more dependable and easier to use for this application.

Manufacturing facilities as well as calibration labs rely on surfaces to be precise.

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 (FIR). In just a few more minutes, zero the device at the centre along a diagonal or centreline and then record readings at 5-in. intervals to 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 help maintain precision by certifying their condition and resurfacing if necessary. If you have old granite products that are in poor condition, some suppliers can grind and lap old products to make them appear like new, and calibrate them to A2LA for perpendicularity, parallelism, flatness, and repeat reading.

Knowing the surface plate accessories that are available will also help you and 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 five finished faces (one large face and four edges) finished flat, square, and parallel to either Laboratory Grade AA (0.000025 in. per 6 in.) or Inspection Grade A (0.000050 in. per 6 in.).

Master Angle. 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 two or three finished faces. The two-face master angle has two perpendicular edges finished flat and square. The three-face master angle has a large triangular face finished flat and square to those two edges. Master angles are provided in Laboratory Grade AA (0.000025 in. per 6 in.) or Inspection Grade A (0.000050 in. per 6 in.).

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 two or four finished faces. The two-face angle plate’s bottom and adjacent square face are finished flat and square to one another. The four-face angle plate is similar to the two-face but the two adjacent sides are finished flat and square to the other two faces and are parallel to each other. The Laboratory Grade AA has a tolerance of 0.000025 in. per 6 in. and the Inspection Grade A has a tolerance of 0.000050 in. per 6 in.

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 0.000025 in. per 12 in. and Inspection Grade A accuracy is 0.000050 in. per 12 in. Lifting holes are provided on sizes 48 in. 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 two or four finished faces. Two-face parallels have the two 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 (0.000025 in. per 12 in.) or Inspection Grade A (0.000050 in. per 12 in.).

V-Blocks. V-Blocks are ideal for supporting or holding cylindrical pieces during manufacturing or inspection. They are provided in matched pairs and have five finished faces. V-blocks have a nominal 90-degree V, centred with and parallel to the bottom and two sides, and square to the ends. V-Blocks come in either Laboratory Grade AA (± 0.000050 in. per 6 in.) or Inspection Grade A (± 0.00010 in. per 6 in.).

Custom value-added assembly applications may require precision steel rails, encoders, ball screws, and more.

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).

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 in., 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 comprises 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 by 12 in., 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 to 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.

When the 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 can 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 systems is critical for a range of industries, including semiconductor companies for both wafer production and wafer inspection. Suppliers can provide options from small stones with submicron 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 systems 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. Porous or dense ceramic, carbon fiber, and even metals can meet unique foundation needs for precision assemblies.

On top of 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 vibration-damping devices. Having this assembly done at the supplier factory provides accountability for accuracy and performance

The article was supplied by The L.S. Starrett Co., 121 Crescent St., Athol, MA 01331, www.starrett.com/precision-granite.