# Interesting Information On Surface Plates



## astjp2 (Mar 20, 2015)

I found this on a university's website:
http://www.google.com/url?sa=t&rct=...-4L4Bg&usg=AFQjCNHBSfn5wK5jDVmON_ZMRKTwxxFtuA
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A Full Plate*


Measurement starts from a reference point or plane.  Linear measurement begins at a reference point and ends at the measured point.  For angular measurements the reference point must be a plane.


Since a plane is defined by three points, it can be said that a plane has three reference points that define its flatness.  In reality these points have only length and width.  A three legged stool defines a true plane by the points of contact.  A four legged stool with four equal length legs will reveal the flatness of that plane.  To make a plane usable, it must have thickness to support both the measuring tool and the part being measured.  Planes that provide for useful work are called surface plates.


This reference surface must have known accuracy before it can become the common plane or starting point for angular measurements, height gages and gaging accessories.  The surface plate is as essential for positional accuracy as the length standard is for linear accuracy.


*History*

Ancient Egyptian pyramids and buildings bear testimony to man’s early understanding of flatness as the basic reference plane from which dimensionally accurate heights, angles and squares could be obtained.


The length of the sides of the great pyramid varies no more than 1/20 of one percent from the mean length of 9,069.45 in.  This is remarkable accuracy, considering overall size and the number of blocks of stone involved.


It is believed that the Egyptians, in leveling the foundations for their pyramids and buildings, flooded the area with water using the surface of the water as a reference from which to determine overburden removal or fill-in to make the land flat and parallel to the surface of the water on top of it.  Nature does not provide a flat surface in the raw.  Even the surface of a body of water will tend to be curved by the forces of gravity and atmosphere as the length and width increases.  Flatness can not be taken for granted.  Remember that in ancient Egypt, as opposed to today, the earth was flat!


In an effort to produce flatter surface plates, we are confronted with devising the best method to use on a material that has properties that will preserve the flatness produced. 


In the evolution of methods, gravity has been used to fix a flat surface on material that solidifies after being in a fluid state.  Among these are cement, glass, ceramics, cast iron, and steel.  Each has properties that make them unreliable for extended use.  They lack stability, resistance to wear and corrosion or defy ultra fine surface finishing obtaining greater flatness.


The first machining method that could produce duplicated parallel surfaces on metal was Richard Robert’s invention of the planner in 1817.  Today precision surface grinders and lapping machines do the production work in flattening a surface to known accuracy.


Henry Maudslay, who invented the first screw-cutting lathe in 1797, is credited for being the first to produce flatter surfaces by hand filing and lapping with abrasive particles.  He produced master-reference surface plates by working three plates against each other.


By 1874 Sir Joseph Whitworth had introduced hand scraping instead of abrading to improve the three plate method of producing a flat surface.


Hand scraping permits greater control of material removal from high spots on the plate’s surface.  In the Whitworth method for achieving flat surfaces, three plates were spotted against each other in alternating pairs.  The plates were made of cast iron having a ribbed construction to provide rigidity without excessive weight.  After normalizing in an outside atmosphere for a year to relieve stresses, the plates were machine finished to the accuracy of the machine tool used.


The following is a description of the three plate method for producing flat surfaces.


The three plates are marked 1, 2, 3, for tracking in the process and scraping operation commences.  High spots are seen when bluing from one plate is transferred to the surface of the other plate.  Six steps are involved in scraping these three plates.


§  Step 1 – Plates #1 and #2 are scraped alternately one to the other until they conform to each other.


§  Step 2 – Plate #1 is now the control plate to which plate #3 is scraped.

§  Step 3 – Plate #2 and #3 are alternately scraped one to the other until they conform to each other.

§  Step 4 – Plate #2 is the control plate to which plate #1 is scraped.

§  Step 5 – Plates #1 and #3 are alternately scraped one to the other until they conform to each other.

§  Step 6 – Plate #3 is the control plate to which plate #2 is scraped.


Now each of the three plates conforms to each other in relative flatness.  To increase the degree of flatness, the above steps must be repeated until the desired flatness is attained.



Demands for greater accuracy during World War II caused extensive research for better surface plate material, more accurate machine tools and instruments to measure flatness.


To solve the material deficiencies of cast iron and steel plates, manufacturers rediscovered what the Egyptians knew 5,000 years ago that black granite was ideal material.  Black granite is a form of original rock produced by nature.  Composed of gabbros and basalt (or diorites) it is nature’s most enduring material.  It is a material that combines wear, shock and chemical resistance; plus hardness and stability, together with machinability, to permit low-cost manufacturing into surface plates.


*Plate Accuracy*

Cast iron and steel, as a material for surface plates, has been outmoded by granite.  Granite surface plates having flatness within millionths of an inch can be produced at far less cost than metal plates.  In preserving this flatness, granite plates do not corrode or rust.  They are not subject to contact interference because they do not burr or gall or crater.  To assure greater measurement accuracy, granite surface plates are nonmagnetic and have exceptional thermal stability.  To resist wear, granite plates are harder than metal plates.  They are easier to clean and the non-reflective surface is easy on the eyes.


Since perfect flatness is unobtainable, the problem is solved by knowing how flat a surface plate is.  For this purpose, manufactures calibrate the flatness of their surface plates by using electronic indicators capable of measuring to .000010 in. or with autocollimator.  These measuring tools permit precise reading of critical areas at critical points on the surface of a plate.  By plotting these readings as a graph, the user knows where and how much the surface plate is out of flatness.


To determine overall area flatness even more precisely, an absolute standard, the wave length of light is used in the form of a laser interferometric surface contour projector.  This measuring tool permits viewing and photographing areas up to five inches wide by forty-eight inches long at one time.  The photograph pictures the surface as a series of interference bands which can be read to accuracy within a few millionths.  Deviations from straight, horizontal or vertical bands denote high or low points on the surface plate.


Surface plates are made to three grades of accuracy.  Grade AA is used for laboratory work where greatest accuracy is required.  These plates are made to a flatness tolerance of ±25 millionths of an inch per two foot square area.  Grade A is an inspection quality grade surface plate having a flatness tolerance of ±50 millionths per two foot square area.  Grade B is a surface plate having a flatness tolerance of ±100 millionths per two square foot area.  It is used in the shop for tool room work. 


Accuracy of surface plates is given as a bilateral tolerance.  Bilaterally is the tolerance above and below a perfectly flat plane.  This accuracy is expressed at “No point on the work surface shall vary from a mean plane thereof by more than the amount specified.  Measurements of accuracy should be not be made nearer the short and long edges than 3% of the width and length or in no case closer than ½ in. from edge.”



*Surface Plate Design*

Granite surface plate design has followed that of metal surface plates in having ledges for clamps and surfaces that are square or rectangular in shape.  Today granite surface plates are made in any shape desired.  Standard sizes range from the small 8 x 12 in. to those 72 x 144 in.  To obtain a surface area of much larger size several plates are linked together by proper alignment to each other. 


The geometry of many parts, fixtures and measuring instruments require their being clamped to the surface plate during measuring operation.  To provide this facility, surface plates were originally furnished with two or four ledges.  These ledges were formed by undercutting the sides with diamond saws.  Ledge thickness was one-half the thickness of the plate itself so that the ledge would support whatever was clamped to the edge of the plate by means of a C-clamp.  Unfortunately, when manufactures of granite plates originally began making them, they related plate accuracies to the number of ledges.  For example, 4-ledge plates were made with laboratory grade, 2-ledge plates with inspection grade and plates without ledges to shop-grade accuracy.  Users for years purchased the more expensive 2- and 4- ledged plates without need for the ledges because they required a higher degree of accuracy than shop grade.


Ledges are not only expensive, but a great cause for inaccuracy.  Experimentation and research reveal that no-ledge plates retain their accuracy better than ledged plates.  Deflection caused by thermal expansion of unequal lengths of top and bottom portion of ledged surface plates can result in inaccuracies from deflection.  Even a bulky and thick ledge will deflect under heavy load or clamping pressures and cause inaccuracies.  Clamping on ledges restricts the gaging area to edges of the plate which causes excessive, concentrated wear.  Based on these findings, no-ledge plates are now being manufactured to the highest degree of accuracy attainable.  Users are now benefiting from no-ledge plates in several ways.  First, they can purchase the plates at a lower price because of the ledge elimination.  Second, the plate can be made more accurately by the manufacture and the accuracy is retained longer.  Third, the use of inserts for clamping distributes the wear more uniformly over the surface of the plate and provides greater versatility and usefulness.


*Surface Plate Maintenance*

Good housekeeping is a basic practice essential to taking precision measurements.  The accuracy of the measurement being made is in direct proportion to the degree of cleanliness in the shop or metrology lab.


Temperature and dirt will displace measurement accuracy.  The surface plate presents the biggest target for contaminants.  Its broad surface is a natural for collecting dust, dirt and oil.  For this reason it must be cleaned before taking any measurements.  Use a non-water base cleaner to spray or cover the plate surface.  Rub small areas at a time with a damp cloth or disposable wiper. As they say turn the cloth frequently as the plate is wiped clean.  Using a non-water base cleaner will help seal the fine silken texture of a granite plate.  The sealing process prevents moisture absorption which can catalyze corrosion of parts or gages left on a plate overnight or longer.


Utilize the full surface of a plate so that wear is distributed and not concentrated in one area.  One of the most common abuses of surface plates is a stool or chair.  Placement of sitting appendages will concentrate wear in one area of a plate, human nature being what it is.


Surface plate covers should be used religiously for protection.  Covers should hold their shape and not absorb moisture like wooden covers.  Soft covers can cushion shock and prevent damage by an accidental dropping.


The condition of the accurate plane is an integral factor in the measurement being made.


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## cazclocker (Mar 21, 2015)

That's an interesting read! Thanks for sharing.
...Doug


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## thomas s (Mar 22, 2015)

Yes good read thanks for posting. thomas s


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## RJSakowski (Mar 22, 2015)

An excellent article! All you would ever want to know about surface plates and then some!


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## astjp2 (Mar 30, 2015)

One of interest for scraping information, I felt it was somewhat biased towards a few things but a fairly good read.  Tim
http://www.schsm.org/SCRAPING.pdf


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## astjp2 (May 10, 2015)

from: http://www.tru-stone.com/pages/faq.asp




*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.




*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.




*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.



*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.




*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.



*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 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.



*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.




*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.




*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.




*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.



*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.




*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!




*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.



*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.




*A)* No. The specialized equipment and training necessary to rework these items requires that they be returned to the factory for calibration and rework.



*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.



*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.



*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.



*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.



*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.



*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.


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