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Granite Surface Plates:Your Quality Foundation

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Y our surface plate, that big, flat rock sitting in your temperature-controlled inspection lab, is the foundation upon which many--if not all--of your dimensional measurements stand. Of course, it’s also well guarded by your inspection lab manager. Selecting and caring for a granite surface plate is key to ensuring that your products’ dimensional specifications are as accurate as possible.

Lay a Tombstone on Its Back

Because they already had the equipment and expertise for working with stone, it’s no surprise that granite tombstone and memorial makers were the first granite surface plate manufacturers. Herman Stone, Rahn Granite Surface Plate Co. and Velsey were the first and later Rock of Ages, which still produces surface plates mostly for the OEM market.

“During World War II, iron was hard to get and they needed surface plates, so someone went to a stonemason and asked if they could make a really flat tombstone,” explains Don Rahn, former president and owner of Rahn Granite Surface Plate Co. and son of granite surface plate pioneer Rudolf Rahn.

That’s how a stonemason by the name of Velsey came to be the creator of the first granite surface plate, says Rahn. Velsey (no one seems to recall his first name) eventually left his employer, Gregory Stone, and started his own company, The Surface Plate Co. (later Velsey Granite), creating granite surface plates. It’s still possible to find Herman Stone and Velsey surface plates in use.

One of Velsey’s first employees was Ivan Rahn, Don Rahn’s brother. Ivan eventually got his father involved, and the two started Rahn Granite Surface Plate Co. in 1943. Although Velsey had come up with the first surface plate, he had no accurate means to ascertain its flatness. As an engineer, Rudolf Rahn developed a crude method for measuring flatness and, later, repeatability. This eventually led to his invention of the Planekator for flatness and the Repeat-O-Meter, both of which are still used to gauge flatness and repeatability.

Rudolf Rahn was chairman of a commission of representatives from other granite manufacturers responsible for the inclusion of repeat measurement specifications in the federal specification for surface plates. Don Rahn currently sits on the board that is reviewing a new surface plate standard, ASME B-89.3.7.

As with many old businesses, the originators of granite surface places have merged or consolidated. Velsey Granite eventually folded. Herman Stone Co. was purchased by The L.S. Starrett Co. Recently, Rahn Granite Surface Plate Co. was purchased by Tru-Stone. Rock of Ages, which began in 1953, still produces precision granite products.

Due to its physical properties, such as surface hardness and low thermal expansion, granite is the material of choice for surface plates when tolerances are critical.

The granite for surface plates is usually quarried from large pits or cut from the sides of mountains. The rock is selected based upon quartz content and the history of the material that has been taken from the quarry site. Nevertheless, the usability of a particular piece of rock isn’t really known until it has been brought back to the facility and cut, when serious flaws such as cracks, which could weaken the surface plate, can be detected.

The most popular standard surface plate sizes are 18 x 24 in., 24 x 36 in., 36 x 48 in. and 48 x 72 in. The major surface-plate manufacturers usually stock these sizes but can custom-build any size plate. The L.S. Starrett Co. has built surface plates in excess of 40 feet, and Rock of Ages Corp. and Tru-Stone Technologies have a friendly feud over who has built the largest, each having produced surface plates in excess of 50 feet.

Once the blocks have been rough-cut, they’re placed on a mount, where they remain through the entire manufacturing process (through delivery in some cases). They will rest on the same mounting points once installed. They’re diamond-machined to an initial flatness and then hand- or machine-lapped to a finished flatness and repeatability tolerance. Between each lapping process, the surface plate is allowed to stabilize. It’s then measured for flatness and repeatability before continuing with lapping.

Flatness and repeatability are the two key specifications that define a surface plate’s accuracy. Federal specification GGG-P-463c lays out the specification for each grade of surface plate. Roughly, these two specifications can be defined as follows:

Flatness. This specification means that all points on the surface of the plate will lie between two parallel planes separated by the flatness tolerance. Put another way, if you look at the surface plate from the edge, the difference between the lowest point on the plate and the highest point on the plate will be no more than the flatness specification. Manufacturers use three standard grades of flatness defined by the federal specification:

Depending on the manufacturer, this specification may be shown as total indicated reading or as a plus/minus tolerance. Both mean the same thing. A surface plate with a TIR of 250 uin. is no different from a surface plate with a flatness of +/- 125 uin.

Repeatability. The repeatability of surface plates is measured with a repeat measuring gage, sometimes called a Repeat-O-Meter, after the gage invented by Rudolf Rahn, a pioneer in granite surface plates (See the sidebar above). This instrument simulates placing a gage block and height gage on the surface plate. The gage is placed at the center of the surface plate and zeroed. As the repeat measuring meter is slid across the plate, the indicator will show any local deviation in the plate’s flatness. Repeatability measurements are taken across the entire surface to ensure that there are no local peaks or valleys that fall outside the repeatability specification. This specification is much tighter than that of flatness and also varies with the diagonal of the plate. For a plate with a diagonal of between 30 and 60 inches:

For example, to fully meet federal specifications, a laboratory grade AA 48 x 60 in. surface plate would require an overall flatness of within 280 uin. but must not have a localized variation of more than 45 uin.--as measured by a repeat measuring meter.

Not all granite is the same. Each type of granite has particular physical characteristics, three of which are important for surface plates: hardness (affects wear resistance), stiffness (the amount of deflection under load) and density. The physical characteristics of granite vary by composition and not necessarily by color. You can have two granites of the same color with distinctly different physical characteristics.

“Each company that sells granite has its own home brand,” says Dennis Ethen, Tru-Stone’s key accounts manager. “California has Academy Black granite, we have Charcoal Black, and Starrett has Crystal Pink. If you’re looking at a surface plate, the quartz content is important. The higher the quartz content, the better the plate.” Quartz gives the surface plate its hardness and resistance to wear.

Because granite is a natural material, there’s no precise definition for the rock you’re getting. Tru-Stone’s black and pink granites fall between 22-percent and 28-percent quartz content, according to third-party analysis. Rock of Ages’ Barre Gray and Salisbury Pink granites have a quartz content of between 24 and 32 percent, says Michael Caputo, director of Rock of Ages’ Precision Granite Products division. A 1965 university analysis determined that Crystal Pink granite from Starrett’s quarry has a 32-percent quartz content, according to Jimmy Coalson, sales manager for the granite division of Starrett.

The material to be used for an application is dependent on the size of the surface plate, the weight of what will be placed on it, and what kind of wear it needs to have. The vendors can determine what will work best for the client. Some types of granite are stiffer than others, meaning that a surface plate of a certain loading strength can be thinner when made from that type of granite than with another type.

“If you’re using a surface plate in a traditional application, sliding gages and parts on the surface, you want a hard and wear-resistant material,” says Caputo. Each surface plate manufacturer has a granite that fits the bill for this type of application.

If weight or thickness is a consideration, a stiffer material might be the ticket, says Caputo. This could be a stiffer granite or even a nongranite stone such as gabbro. Gabbro is much denser than granite but has no quartz and so is less wear-resistant. It’s also less porous and more thermally stable. Its high density allows it to be used in surface plates that are thinner and weigh less than granite plates of the same strength. It’s also more expensive. Gabbro is often used in high-precision surface plates when wear resistance is less of an issue (e.g., CMM bases, where parts aren’t moved around the plate).

“The first thing you need to determine is what kind of quality you need,” says Coalson. “Are you going to be in a lab or out on the floor? How big is the part? What kind of money do you have to spend?” Coalson points out that a lot of foreign imports are taking business away from U.S. surface plate manufacturers.

The others agree with Coalson that imports (mostly Chinese) are hurting business. Some Chinese imports might cost about half as much as similarly sized NIST-traceable plates manufactured in the United States. Although they’re inexpensive, these plates are often not traceable nor accurate, say these experts. Nor are they fixable, add both Coalson and Ethen. Rahn Granite Surface Plate Co., Starrett and Tru-Stone have all purchased Chinese surface plates with the thought that if they could be calibrated they might provide an inexpensive source of surface plate blanks. Neither company was successful in calibrating the plates.

“They say they’re traceable to the federal specification, but they aren’t,” says Coalson. “We bought some and we could never get them flat.” The main problem is that the plates are not sold with mounts, says Coalson. Because the mounts represent an integral factor of plate accuracy and there’s no way to know the mounting conditions in which they were produced, there’s no real way to calibrate them. “But people are buying them like crazy,” he says.

Not all foreign surface plates have this problem, and some are well manufactured and traceable, says Caputo. Some large CMM manufacturers purchase Chinese-made granite surface plates for use with their products. The key is to be aware of what is out there and be sure that you’re truly getting what you want.

No matter whom you buy a surface plate from, be sure of the following:

The plate is certified to GGG-P-463c for both flatness and repeatability.

The plate is supplied with a mount and/or existing support pads permanently fixed to the surface plate.

Final calibration measurements are traceable to NIST or a comparable standards body.

If you consider buying foreign granite, be sure it’s of comparable quality to what you can find domestically.

Does the manufacturer guarantee quality materials and workmanship?

There may be other considerations, particularly if your company is registered to a quality management system standard such as ISO 9001:2000 or ISO/TS 16949. For example:

Are the surface plate company’s lab practices accredited by a third party such as A2LA?

Is the surface plate company registered to a quality management system standard such as ISO 9001:2000 or ISO/TS 16949?

Because granite surface plates appear thick and rugged, they’re often not treated as precision instruments. The result can be excessive wear on the plate, leading to inaccurate measurements or expensive relapping. When caring for your granite surface plate, consider:

Support. Whether large or small, all surface plates must be supported at the same three points used for support during its manufacture. From the user’s perspective, if you don’t mount the plate using the same three points, the plate will settle differently and possibly be out of tolerance. Three supports are used because it’s physically the only way to repeatably support any static object without the weight shifting from one leg to another.

At times, the manufacturer may use something other than a three-point support. In that case, there’s a jackscrew located on each support. The screws are tightened with a torque wrench until each jack carries an equal load. This process is done during manufacture, setup and as part of routine calibration. The supports for a three-point mount don’t need to be calibrated.

Care. Laboratory-grade plates should be kept in a controlled environment of between 68° and 72° F and 40-percent humidity.

Keep the plate clean. Even airborne dust can eventually cause wear. It collects on the plate and gage surfaces, and as those surfaces are rubbed together, the dust abrades the surface plate. Clean the surface plate at the beginning and the end of a shift using a cleaner recommended by surface plate vendors. If possible, cover the plate when not in use.

Don’t set anything on the surface plate other than gages and the pieces to be measured. It’s not a kitchen counter, says Coalson. “People set cans of Coke on the table, and a little runs over the side,” leaving microscopic pits as the acids dissolve minerals in the plate, he explains. “It seeps into the granite and it goes right into the pores. I’ve been into a place where there were six people eating lunch on a $4,000 surface plate.”

Use. If your measurements involve using a small part of the table, don’t use the same spot all the time. Over time, constantly using one corner of the table will wear down that area. Sure, it may take several years, but if you’ve invested in something, why not go the extra mile and change the location of your workstation on the table?

Don’t place loads on the table higher than what it’s rated for, says Ethen. “If the load is higher than rated, the plate won’t come back,” he explains. “It can bend from millionths of an inch to thousandths.”

The surface plate should be calibrated upon receipt after it has had time to normalize to the temperature of its new location, between 48 hours and a week, depending on the size of the plate. After that, the surface plate should be calibrated at least once a year or even every six months if it receives heavy use. When feasible and depending on its location, it’s a good idea to rotate the surface plate. This alleviates the problem of only certain areas of the plate being used. If it can’t be rotated, try to change the location of workstations on the plate so that one area doesn’t receive the majority of use.

A monthly inspection using a repeat measuring gage is advisable. Some labs do this every day or every week, depending on how they use the plate. The instrument is relatively inexpensive (about $500) and easy to use, and the test takes only a few minutes to perform. Repeatability inspections may point out areas of the table that receive more use than others, indicating that the table needs to be periodically rotated. It may also show that employees are using the surface plate as a counter. Craig Howell, owner of CPM Labs, notes that low spots often emerge at the part of the plate that is nearest the door due to technicians throwing their papers, briefcases, coffee mugs or lunchboxes on the surface plate as they enter the room.

Remember, it may be flat, but it isn’t a table. Treat it like the precision instrument it is. The accuracy of your measurements rests on it.

Dirk Dusharme is Quality Digest’s technology editor. Letters to the editor about this article can be e-mailed to letters@qualitydigest.com. Our thanks to Rahn Precision, Rock of Ages, Starrett and Tru-Stone for their help in this story. We also extend our condolences to Don Rahn whose brother Ivan Rahn, one of the pioneers in granite surface plates, died during the writing of this piece.