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.
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.
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.”
TABLE: SIZE AND ACCURACY
Work Surface Accuracy