tru-stone technologies, tru-stone, trustone, trustone technologies, tru stone, tru stone technologies, truestone, truestone technologies, true stone technologies, true stone, granite, precision granite, custom granite, custom precision granite, waite park, st. cloud, st cloud, saint cloud, minnesota, mn, central minnesota, united states, Granite specifications, Granite specs, Stainless steel inserts , Threaded inserts, Insert torque, Academy black, Imperial black, Zimbabwe black, Sierra gray, Tru-Clean, Tru clean, Lab grade, Laboratory grade, Inspection grade, Tool room grade, Toolroom grade, Grade AA, Grade A, Grade B, Metrology, Inspection plates metrology, measurement, custom, calibration, quality, standards, precision, iso 9000, iso9000, iso-9000, engineering, granite engineering, tru-stone, tru-stone technologies, precision granite, custom precision granite, flatness, flatness standards, metrology products, metrology accessory, metrology accessories, Motion system base, Granite machine base, Vacuum table, Wafer chuck, Vaccum Granite surface plates, Surface plates, Federal spec, Federal spec ggg-p-463c, Stands, Surface plate stands, Surface plate tables, Surface plate support,Block and wedge, Drawer stands, Cabinet stands, Granite cleaner, Surface plate cleaner, Waterless cleaner, Granite repair kits, Surface plate covers, Granite metrology accessories Angle plate, Master angle, Granite reference masters, Master square, Straight edge, Parallels, v-blocks, vee blocks, universal right angle, granite cube, bench comparators, granite stands, step blocks, custom granite, lvdt, planekator, rahn, repeat-o-meter, granite calibration, ceramic calibration, granite certification, ISO 9001, ISO 17025, A2LA accredited, Granite resurfacing, Granite reconditioning, Granite lapping, Long form cert, 100 pound loading, unilateral tolerance, bilateral tolerance repeat reading, repeat measurement, precision granite, ceramic, porous ceramic, dense ceramic, carbon fiber, machine bases, NIST traceable, Charcoal black, Pink granite, Black diabase, Impala, Vibration damping, Vacuum chuck, Granite assembly, Tru-vac , Air lift part transfer, Vacuum positioning, Mechanical clamping, Vacuum zone
Tru-Stone Home Page
Benefits of Granite
Standard Metrology Products
OEM and Custom Applications
Tru-Vac Vacuum & Air-lift Technology Solutions
Tru-Stone Technical Assistance
Frequently Asked Questions
Tru-Stone Employment Page
Contact Tru-Stone Technologies
Frequently Asked Questions
  1. What color of granite is best?
  2. Is there an industry standard for surface plate accuracy?
  3. What are the three grades of accuracy, and how are they defined?
  4. How can I reduce wear, and extend the life of my surface plate?
  5. How often should I clean my surface plate?
  6. How often should a surface plate be calibrated?
  7. Why do the calibrations performed on my surface plate seem to vary?
  8. What does 'Repeat Measurement' mean? Isn't it the same as flatness?
  9. Which is more important: flatness or repeat measurements?
  10. Can tighter flatness tolerances than Laboratory Grade AA be achieved?
  11. Why granite? Is it better than steel or cast iron for precision surfaces?
  12. How should my surface plate be supported? Does it need to be level?
  13. Can granite surface plates be relapped on-site?
  14. If my granite surface plate or inspection accessory is badly worn or pitted, can it be salvaged? Will Tru-Stone fix any brand plate?
  15. Why are black plates thinner than granite plates of the same size?
  16. Can my granite parallels, angle plates, and master squares be reworked on-site?
  17. Can Tru-Stone calibrate and resurface my steel or cast iron surface plates?
  18. Can Tru-Stone calibrate and resurface my ceramic angles or parallels?
  19. Can a plate with steel inserts be resurfaced?
  20. What is 'Diabase and why does Tru-Stone use it exclusively for the measuring accessories'? Is it different from granite?
  21. I need fastening points on my surface plate. Can threaded holes be added to a surface plate?
  22. Isn't there a danger of pulling inserts out of the plate?

1) What color of granite is best?
A) It is important to note that color alone is not an indication of the physical qualities of the stone. In general, the color of granite is determined by the presence or absence of minerals which may have no bearing on the qualities that make good surface plate material. There are pink, gray, and black granites that are excellent for surface plates, as well as pink, gray, and black granites that are totally unsuitable for precision applications. The critical characteristics of granite, as they pertain to its use as a surface plate material have nothing to do with color, and are as follows:

a) Stiffness (deflection under load - indicated by Modulus of Elasticity).
b) Hardness
c) Density
d) Wear resistance


2) Is there an industry standard for surface plate accuracy?
A) Most manufacturers use Federal Specification GGG-P-463c (Granite Surface Plates) as a basis for their specifications.

You can view a copy of GGG-P-463c here.


3) How is surface plate flatness defined and specified?
A) Flatness can be considered as all points on the surface being contained within two parallel planes, the base plane and the roof plane. The measurement of distance between the planes is the overall flatness of the surface. This flatness measurement commonly carries a tolerance and may include a grade designation.

The flatness tolerances for three standard grades are defined in the federal specification as determined by the following formula:

Laboratory Grade AA = (40 + diagonal squared/25) x .000001" (unilateral)
Inspection Grade A = Laboratory Grade AA x 2
Tool Room Grade B = Laboratory Grade AA x 4.

For standard sized surface plates, we guarantee flatness tolerances which exceed the requirements of this specification. In addition to flatness, Federal Specification GGG-P-463c address topics including: repeat measurement accuracy, material properties of surface plate granites, surface finish, support point location, stiffness, acceptable methods of inspection, installation of threaded inserts, etc.

Tru-Stone granite surface plates and granite inspection plates meet or exceed all of the requirements set forth in this specification. At present, there is no defining specification for granite angle plates, parallels, or master squares. Our standard tolerances (listed in our catalog) for these items are the tightest in the precision granite industry.


4) How can I reduce wear, and extend the life of my surface plate?
A) First, it is important to keep the plate clean. Airborne abrasive dust is usually the greatest source of wear and tear on a plate, as it tends to embed in work pieces and the contact surfaces of gages. Second, cover your plate to protect it from dust and damage. Wear life can be extended by covering the plate when not in use, by rotating the plate periodically so that a single area does not receive excessive use, and by replacing steel contact pads on gauging with carbide pads. Also, avoid setting food or soft drinks on the plate. Note that many soft drinks contain either carbonic or phosphoric acid, which can dissolve the softer minerals and leave small pits in the surface.

5) How often should I clean my surface plate?
A) This depends on how the plate is being used. If possible, we recommend cleaning the plate at the beginning of the day (or work shift) and again at the end. If the plate becomes soiled, particularly with oily or sticky fluids, it should probably be cleaned immediately.

Clean the plate regularly with Tru-Clean or Rahn Waterless surface plate cleaner. The choice of cleaning solutions is important. If a volatile solvent is used (acetone, lacquer thinner, alcohol, etc.) the evaporation will chill the surface, and distort it. In this case, it is necessary to allow the plate to normalize before using it or measurement errors will occur.

The amount of time required for the plate to normalize will vary with the size of the plate, and the amount of chilling. An hour should be sufficient for smaller plates. Two hours may be needed for larger plates. If a water-based cleaner is used, there will also be some evaporative chilling.

The plate will also retain the water, and this could cause rusting of metal parts in contact with the surface. Some cleaners will also leave a sticky residue after they dry, which will attract airborne dust, and actually increase wear, rather than decreasing it.


6) How often should a surface plate be calibrated?
A) This too depends on the plate usage and environment. We recommend that a new plate or precision granite accessory receive a full recalibration within 1 year of purchase. If the plate is going to receive heavy use, it may be advisable to shorten this interval to six months. Monthly inspection for repeat measurement errors using a Repeat-o-Meter, or similar device will show any developing wear spots and only takes a few minutes to perform. After the results of the first recalibration are known, the calibration interval may be extended, or shortened as allowed or required by your internal quality system.

7) Why do the calibrations performed on my surface plate seem to vary?
A) There are several possible causes for variations between calibrations:
  1. The surface was washed with a hot or cold solution prior to calibration, and was not allowed sufficient time to normalize.
  2. The plate is improperly supported. (See Q.16)
  3. Temperature change.
  4. Drafts.
  5. Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface.
  6. Variations in the vertical temperature gradient between between winter and summer. (If at all possible, know the vertical gradient temperature at the time the calibration is performed.)
  7. Plate not allowed sufficient time to normalize after shipment.
  8. Improper use of inspection equipment or use of non-calibrated equipment.
  9. Surface change resulting from wear.

8) What does 'Repeat Measurement' mean? Isn't it the same as flatness?
A) A repeat measurement is a measurement of local flatness areas. The Repeat Measurement specification states that a measurement taken anywhere on the surface of a plate will repeat within the stated tolerance. Controlling local area flatness tighter than overall flatness guarantees a gradual change in surface flatness profile thereby minimizing local errors.

Most manufacturers, including imported brands, adhere to the Federal Specification overall flatness tolerances but many overlook the repeat measurements. Many of the low value or budget plates available in the market today will not guarantee repeat measurements. A manufacturer who does not guarantee repeat measurements is NOT producing plates that meet the requirements of Federal Specification GGG-P-463c.


9) Which is more important: flatness or repeat measurements?
A) Both are critical to insure a precision surface for accurate measurements. Flatness specification alone is not sufficient to guarantee measurement accuracy. Take as an example, a 36 X 48 Inspection Grade A surface plate, which meets ONLY the flatness specification of .000300" If the piece being checked bridges several peaks, and the gage being used is in a low spot, the measurement error could be the full tolerance in one area, 000300"! Actually, it can be much higher if the gage is resting on the slope of an incline.

Errors of .000600"-.000800" are possible, depending upon the severity of the slope, and the arm length of the gage being used. If this plate had a Repeat Measurement specification of .000050"F.I.R. then the measurement error would be less than .000050" regardless of where the measurement is taken on the plate. Another problem, which usually arises when an untrained technician attempts to resurface a plate on-site, is the use of Repeat Measurements alone to certify a plate.

The instruments which are used to verify repeatability are NOT designed to check overall flatness. When set to zero on a perfectly curved surface will continue to read zero, whether that surface is perfectly flat, or perfectly concave or convex 1/2"! They simply verify the uniformity of the surface, not the flatness. Only a plate which meets both the flatness specification AND the repeat measurement specification truly meets the requirements of Federal Specification GGG-P-463c.


10) Can tighter flatness tolerances than Laboratory Grade AA be achieved?
A) Yes, but they can only be guaranteed for a specific vertical temperature gradient. The effects of thermal expansion on the plate could easily cause a change in accuracy greater than the tolerance if there is a change in the gradient. In some cases, if the tolerance is tight enough, the heat absorbed from overhead lighting can cause enough of a gradient change over several hours.

Granite has a coefficient of thermal expansion of approximately .0000035 inches per inch per 1°F. As an example: A 36" x 48" x 8" surface plate has an accuracy of .000075" (1/2 of Grade AA) at a gradient of 0°F, the top and bottom are the same temperature. If the top of the plate warms up to the point where it is 1°F warmer than the bottom, the accuracy would change to .000275" convex!!! Therefore, ordering a plate with a tolerance tighter than Laboratory Grade AA should only be considered if there is adequate climate control.


11) Why granite? Is it better than steel or cast iron for precision surfaces?
A) The answer is 'yes' for almost every application. The advantages of granite include: No rust or corrosion, almost immune to warping, no compensating hump when nicked, longer wear life, smoother action, greater precision, virtually non-magnetic, low co-efficient of thermal expansion, and low maintenance cost.

12) How should my surface plate be supported? Does it need to be level?
A) A surface plate should be supported at 3 points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered. Only 3 points can rest solidly on anything but a precision surface.

The plate should be supported at these points during production, and it should be supported only at these three points while in use. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same 3 points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. All Tru-Stone steel stands have support beams designed to line up with the proper support points.

If the plate is properly supported, precise leveling is only necessary if your application calls for it. Leveling is not necessary to maintain the accuracy of a properly supported plate.


13) Can granite surface plates be relapped on-site?
A) Yes, if they are not too badly worn. Our factory setting and equipment allow the optimium conditions for proper plate calibration and rework if necessary. Generally, if a plate is within .001" of the required tolerance, it can be resurfaced on-site. If a plate is worn to the point where it is more than .001" out of tolerance, or if it is badly pitted or nicked, then it will need to be sent to the factory for grinding prior to relapping.

Great care should be exercised in selecting an on-site calibration and resurfacing technician. We urge you to use caution in selecting your calibration service. Ask for accreditation and verify the equipment that the technician will use has a NIST traceable calibration. It takes many years to learn how to properly lap precision granite.

Tru-Stone provides quick turn-around on calibrations performed in our factory. Send your plates in for calibration if possible. Your quality and reputation depend on the accuracy of your measurement instruments including surface plates!


14) If my granite surface plate or inspection accessory is badly worn or pitted, can it be salvaged? Will Tru-Stone fix any brand plate?
A) Yes, but only at our factory. At our plant, we can restore almost any plate to 'like-new' condition, usually for less than half the cost of replacing it. Damaged edges can be cosmetically patched, deep grooves, nicks, and pits can be ground out, and the attached supports can be replaced. In addition, we can modify your plate to increase its versatility by adding solid or threaded steel inserts and cutting slots or clamping lips, per your specifications.

15) Why are black plates thinner than granite plates of the same size?
A) Our black surface plates have a significantly higher density and are up to three times as stiff. Therefore, a plate made of the Tru-Stone black does not need to be as thick as a granite plate of the same size to have equal or greater resistance to deflection. Reduced thickness means less weight and lower shipping costs.

Beware of others who use lower quality black granite in the same thickness. As stated above properties of granite, like wood or metal, vary by material and color is not a accurate predictor of stiffness, hardness wear resistance. In fact, many types of black granite and diabase are very soft and not suitable for surface plate applications.


16) Can my granite parallels, angle plates, and master squares be reworked on-site?
A) No. The specialized equipment and training necessary to rework these items requires that they be returned to the factory for calibration and rework.

17) Can Tru-Stone calibrate and resurface my steel or cast iron surface plates?
A) No. Our equipment and calibration methods are designed for granite and do not work well on metal surfaces. Also, our lapping equipment will not work with metals, which generally requires hand-scraping to achieve the desired tolerances. Since hand-scraping is rapidly becoming a 'lost art', it is often more economical to replace a worn steel or cast iron plate with a new granite plate than it is to have the old plate re-scraped. There are many advantages of granite to the cast iron, as outlined earlier in this FAQ section.

18) Can Tru-Stone calibrate and resurface my ceramic angles or parallels?
A) Yes. Ceramic and granite have similar characteristics, and the methods used to calibrate and lap granite can be used with ceramic items as well. Ceramics are more difficult to lap than granite resulting in a higher cost.

19) Can a plate with steel inserts be resurfaced?
A) Yes, provided that the inserts are recessed below the surface. If steel inserts are flush with, or above the surface plane, they must be spot-faced down before the plate can be lapped. If required, we can provide that service.

20) What is 'Diabase and why does Tru-Stone use it exclusively for the measuring accessories'? Is it different from granite?
A) Diabase is a variety of a stone type known as Gabbro. While it is closely related to granite, it is not the same. Diabase lacks several of the minerals (such as quartz and mica) which give granite its distinctive grain structure. The lack of quartz means that diabase is slightly less resistant to wear than the hardest granites. However, this is offset by the lack of mica, which is very soft, and tends to flake out of granite plates, leaving pits. Diabase is denser than granite, weighing about 190 lbs. per cu. ft. while most granites weigh between 160-170 lbs per cu. ft. It is also less porous and more stable, which means that a smoother and more uniform surface finish can be achieved. In general, we quote diabase as the material for multi-dimensional, very high-accuracy applications that require a very smooth, stable surface like our angle plates, parallels, master squares, and straight edges.

21) I need fastening points on my surface plate. Can threaded holes be added to a surface plate?
A) Yes. Steel inserts with the desired thread (english or metric) can be epoxy bonded into the plate at the desired locations. Tru-stone uses CNC machines to provide the tightest insert locations within +/- 0.005”. For less critical inserts, our locational tolerance for threaded inserts is ±.060". Other options include steel T-Bars and dovetail slots machined directly into the granite.

22) Isn't there a danger of pulling epoxied inserts out of the plate?
A) Inserts that are properly bonded using high strength epoxy and good workmanship will withstand a great deal of torsional and shear force. In a recent test, using 3/8"-16 threaded inserts, an independent testing laboratory measured the force required to pull a epoxy-bonded insert from a surface plate. Ten plates were tested. Out of these ten, in nine cases, the granite fractured first. The average load at the point of failure was 10,020 lbs for gray granite and 12,310 lbs for black. In the single case where an insert pulled free of the plate, the load at the point of failure was 12,990 lbs! If a work piece forms a bridge across the insert and extreme torque is applied, it is possible to generate enough force to fracture the granite. Partially for this reason, the Federal Specification gives guidelines for the maximum safe torque that can be applied the epoxy bonded inserts.

Thread Size Torque Rating

.250 7 ft. lbs.

.3125 15 ft. lbs.

.375 20 ft. lbs.

.500 25 ft. lbs.

.625 30 ft. lbs.

These figures are extremely conservative and were based partially on the fact that several surface plate manufacturers use helicoil inserts or inferior grades of epoxy.


Granite surface plates, Surface plates, Federal spec, Federal spec ggg-p-463c, Stands, Surface plate stands, Surface plate tables, Surface plate support,Block and wedge, Drawer stands, Cabinet stands, Granite cleaner, Surface plate cleaner, Waterless cleaner, Granite repair kits, Surface plate covers, Granite metrology accessories, flatness, flatness standards, metrology products, metrology accessory, metrology accessories, Motion system base, Granite machine base, Vacuum table, Wafer chuck, Vaccum, Granite specifications, Granite specs, Stainless steel inserts, Threaded inserts, Insert torque, Academy black, Imperial black, Zimbabwe black, Sierra gray, Tru-Clean, Tru clean, Lab grade, Laboratory grade, Inspection grade, Tool room grade, Toolroom grade, Grade AA, Grade A, Grade B, Metrology, Inspection plates
Home  |  Benefits of Granite  |  Standard Metrology Products  |  OEM/Custom Applications
Tru-Vac  |  Technical Assistance  |  Forms & Literature  |  Employment  |  Contact Us

©Copyright 2005-2014 Starrett - All Right Reserved
Website developed and hosted by KDV Technology & Consulting


Angle plate, Master angle, Granite reference masters, Master square, Straight edge, Parallels, v-blocks, vee blocks, universal right angle, granite cube, bench comparators, granite stands, step blocks, custom granite, lvdt, planekator, rahn, repeat-o-meter, granite calibration, ceramic calibration, granite certification, ISO 9001, ISO 17025, A2LA accredited, Granite resurfacing, Granite reconditioning, Granite lapping, Long form cert, 100 pound loading, unilateral tolerance, bilateral tolerance, repeat reading, repeat measurement, precision granite, ceramic, porous ceramic, dense ceramic, carbon fiber, machine bases, NIST traceable, Charcoal black, Pink granite, Black diabase, Impala, Vibration damping, Vacuum chuck, Granite assembly, Tru-vac , Air lift part transfer, Vacuum positioning, Mechanical clamping, Vacuum zone