Machinist

[Lawrence London <lfljvenaura@gmail.com>]

Precision Granite FAQs

Frequently Asked Questions about Starrett Precision Granite.

Precision Granite FAQs
http://www.starrett.com/metrology/metrology-products/precision-granite/precision-granite-faq

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, granite’s color is directly related to 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
e) Stability
f) Porosity

 

To talk more about the granite that is right for your project, please call (800) 959-0517 or email engineering@tru-stone.com.

 

2) Is there an industry standard for surface plate accuracy?

A) Most manufacturers use ASME B89.3.7-2013 or Federal Specification GGG-P-463c (Granite Surface Plates) as a basis for their specifications. You can access the ASME spec at asme.org  or the 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 that exceed the requirements of this specification. In addition to flatness, ASME B89.3.7-2013  & 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.

To further address surface plate flatness specifications, please call (800) 959-0517 or email engineering@tru-stone.com.

 

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.

To learn more, please contact us at (800) 959-0517 or sales@tru-stone.com.

 

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 Starrett liquid or Starrett 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.

We would gladly talk you through this process. Feel free to contact us at (800) 959-0517 or engineering@tru-stone.com.

 

6) How often should a surface plate be calibrated?

A) This depends on the plate usage and environment. We recommend that a new plate or precision granite accessory receive a full recalibration within one year of purchase. If the plate will see 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 determined, the calibration interval may be extended or shortened as allowed or required by your internal quality system.

Questions? Please contact us at (800) 959-0517 or sales@tru-stone.com.

 

7) Why do the calibrations performed on my surface plate seem to vary?

A) There are several possible causes for variations between calibrations:

• The surface was washed with a hot or cold solution prior to calibration, and was not allowed sufficient time to normalize
• The plate is improperly supported (See Q.16)
• Temperature change
• Drafts
• Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface
• Variations in the vertical temperature gradient between winter and summer (If at all possible, know the vertical gradient temperature at the time the calibration is performed.)
• Plate not allowed sufficient time to normalize after shipment
• Improper use of inspection equipment or use of non-calibrated equipment
• Surface change resulting from wear

If this answer did not sufficiently answer your question, please contact us at (800) 959-0517 or sales@tru-stone.com.

 

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 of 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 ASME B89.3.7-2013 or Federal Specification GGG-P-463c.

Ask us about our guarantee of Repeat Measurement. Call (800) 959-0517 or email sales@tru-stone .com.

 

9) Which is more important: flatness or repeat measurements?

A) Both are critical to ensure 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 that are used to verify repeatability are NOT designed to check overall flatness. When set to zero on a perfectly curved surface, they 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 that meets both the flatness specification AND the repeat measurement specification truly meets the requirements of ASME B89.3.7-2013  or Federal Specification GGG-P-463c.

Ask us about or flatness specification and repeat measurement promise by calling (800) 959-0517 or emailing sales@tru-stone.com.

 

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. Learn more on our Why Granite? Page or contact us at (800) 959-0517 or sales @tru-stone.com.

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.

Learn more by calling us at (800) 959-0517.

 

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 optimum 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!

For assistance, please call (800) 959-0517 or email sales@tru-stone.com.

 

14) If my granite surface plate or inspection accessory is badly worn or pitted, can it be salvaged? Will Tru-Stone fix any brand of 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.

To talk more about salvaging your current surface plate, give us a call at (800) 959-0517 or email sales@tru-stone.com.

 

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, and is not an accurate predictor of stiffness, hardness, or wear resistance. In fact, many types of black granite and diabase are very soft and not suitable for surface plate applications.

Make sure you’re getting the right material. Call (800) 959-0517 or email engineering@tru-stone.com today.

 

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.

Alternatively, you can call (800) 959-0517 or email sales@tru-stone.com to see what we can do you for you.

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.

Alternatively, you can call (800) 959-0517 or email sales@tru-stone.com to see what we can do you for you.

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.

Call (800) 959-0517 or email sales@tru-stone.com today.

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.

Call (800) 959-0517 or email sales@tru-stone.com today.

 

20) 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.

Call us at (800) 959-0517 or email sales@tru-stone.com to talk more.

 

21) 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 an 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.

Question? Call us at (800) 959-0517 or email quality@tru-stone.com.