How Does Granite Flat Surface Plate Work?

Grade A Black Granite Surface Plate, 18" x 12".  About $160 on Amazon...

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Surface Plates are solid flat plates used to mimic a perfect 2D plane as accurately as possible to facilitate other measurements being made on top that plane. Typical uses are precision inspection (for quality control, for example), marking out (putting precise markings to guide machine work), and tooling setup.

Many of the most common measurements are done with a Height Gauge.  A Surface Plate with height gauge is an extremely handy thing for any CNC'er or machinist to have on hand, so I recommend getting the set relatively early on.

Surface Plate Material: Granite, Cast Iron, Steel, or Glass

Surface plates are usually made of granite, and they come in a variety of sizes and grades.  There are cast iron, steel, and glass surface plates available, but most prefer to stick with granite.

Granite came into wide use during World War II.  Before that time, most surface plates were metal.  Granite surface plates were conceived as a means of saving the metal for more critical applications.  The idea to use granite came about by the happy coincidence of Wallace Herman owning both a monument and a metal shop in Dayton, Ohio.

Granite has a number of advantages over cast iron.  It's more durable and won't rust.  These advantages are why granite surface plates stayed popular even post-World War II.

You can find both black and pink granite used to make surface plates, with black granite being more popular.  Pink granite has more quartz, and therefore more wear resistance than black granite.  Quartz is more than twice as resistant to wear as the other minerals in granite.  However, Pink Granite isn't as strong as Black Granite and so it is normally desirable for a pink granite surface plate to be thicker.  Black Granite has superior stiffness, vibration damping, and machinability.

Before granite surface plates became prevalent, Cast Iron was the most common material for surface plates.  It is still popular for certain applications because it is easier to work it to a particular degree of precision than granite.  In fact, cast iron surface plates may be used as a tool to help lap (resurface) a granite surface plate.

Surface Plate Grades and Precision

Surface Plate Grades determine their flatness.  Standard grades include:

• AA (Most accurate):  These are referred to as laboratory grade.  Their flatness is (40 + diagonal (inches) of surface plate squared / 25) x 0.".
• A: These are Inspection Grade.  They're the AA spec x 2.
• B: These are Toolroom Grade.  They're AA x 4.
• Workshop Grade (Least accurate):  These are not really suitable for CNC applications, so I won't quote a spec.

Try to purchase an "A" grade granite surface plate in the largest size that you can afford and that fits comfortably in your shop.  This grade is intended for inspection work (quality control), and so covers the broadest range of use.

The ASME (American Society of Mechanical Engineers) has published ASME B89.3.7 - which is the current standard for granite surface plates.

All surface plates must be calibrated regularly to ensure their accuracy has not been compromised by chipping, warping, or excessive wear.

The flatness of a surface plate is defined by two parallel planes-the base plane and the roof plane.  The distance between the two planes is the overall flatness.

Standard Surface Plate Sizes

Standard surface plate sizes are 18 x 24 in., 24 x 36 in., 36 x 48 in. and 48 x 72 in, but you can get most any size custom made if need be.  I have an "A" Grade 24 x 36 in A Class granite surface plate for my metrology needs.  It's a very nice surface plate for a small shop, but larger shops will want even larger surface plates and will likely have more than one.

Reducing Wear, Cleaning, and Extending Life of Granite Surface Plates

Reducing wear is all about keeping granite surface plates clean.  Airborne abrasive dust is the greatest source of wear, and it will also tend to embed in work pieces and the contact surfaces of gages.

Cover your plate to protect it from dust and damage.  Rotate it periodically so that wear isn't always in the same place.  Wherever possible use carbide pads on gauging as it is much less likely to have embedded abrasive dust.

Surface Plates should also be kept as clean as possible.  Swarf and other debris can easily interfere with accurate measurements.  Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of granite surface plates.

Ideally, clean your plate at the beginning and end of the day with appropriate cleaner.  Note that the evaporation of the cleaner can cause chilling, which may distort the surface.  Allow an hour for small plates to normalize and 2 hours for large plates.

Depending on how porous the granite in your plate is, retaining water from cleaning can be an issue that makes precision tooling more likely to rust.

Surface Plate Calibration

Surface Plates can experience chipping, warping, and just plain wear, so they must be calibrated regularly.  Often, the wear will be localized due to a particular tool, such as a height gauge, being used in the same place constantly.

Here's a fascinating video that walks through the process of testing and calibrating a surface plate in the field:

A process called lapping can be used to increase the flatness of a plate as needed.

Don't Use Your Surface Plate as a Counter!

Surface Plates are not counters.  Don't set anything on the surface plate other than gages and the pieces being measured.

Say you let someone put a Coke can on a Surface Plate.  That can may have dribbled so there's a little Coke on the bottom of the can.  But Coke is acidic and it will eat into your Surface Plate, damaging it.

Placing random things on Surface Plates can also transfer harmful debris that upsets delicate precision and abrades the Surface Plate.

Surface Plate Accessories

There is a huge variety of accessories used with surface plates.  We won't try to cover them all, just the most commonly used ones.

Ledges and Threaded Metal Inserts

Ledges are a part of the surface plate and must be specified when you order your plate.  Ledges are used to facilitate work clamping.

Threaded metal inserts may be embedded in the granite to facilitate clamping.  Keep the metal below the level of the granite to facilitate accuracy.

Surface Plate Stands

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Proper support is essential for surface plates to minimize warping.  Granite Surface Plates should be supported at 3 points, ideally located 20% of the length in from the ends of the plates.  A precision surface can only be supported accurately by 3 points, not 4 or more.

A variety of stands are available to support all types of surface plates.  Smaller surface plates can be placed on any bench, but larger plates benefit from proper suspension.

Here's a typical fabricated stand for the sort of large surface plates found in the inspection and quality departments of many machine shops:

Temperature Controlled Room

It doesn't do you much good to specify a laboratory grade surface plate and then keep it in a room that isn't temperature controlled.  For the highest degree of accuracy and repeatability, the temperature must be consistent.  That's why so many shops keep their surface plates in a temperature controlled inspection room.

Surface Plate Covers

If your surface plate is not in continuous use (most are in commercial shops), it's nice to protect it with a fitted cover:

Surface Plate Cleaner

Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of the plate.

Height Gauges, Sine Gauges, and Gauge Blocks

Height Gauges, Gauge Blocks, and Sine Gauges (Sine Bars and Sine Plates) are probably the most common measuring accessories used with a surface plate, though there are many more.

This article is part of our Complete Guide to Metrology.

A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup.[1] The surface plate is often used as the baseline for all measurements to a workpiece, therefore one primary surface is finished extremely flat with tolerances below 11.5 μm or 0. mm per mm for a grade 0 plate.[2] Surface plates are a common tool in the manufacturing industry and are often fitted with mounting points so that it can be an integrated structural element of a machine such as a coordinate-measuring machine, precision optical assembly, or other high precision scientific & industrial machine. Plates are typically square or rectangular, although they may be cut to any shape.

There are varying grades used to describe the accuracy of some metrology equipment such as: AA, A, B, and Workshop grade. While workshop grade is the least accurate, all grades of surface plates are held to a high degree of flatness.

Surface plates must be calibrated regularly to ensure that chipping, warping, or wear has not occurred. A common problem is wear to particular areas, such as that caused by the frequent use of a tool in one place (such as a height gauge), that causes an uneven surface and reduces overall accuracy of the plate, this may be greatly accelerated if abrasive dust is present. Tools and workpieces may also cause damage when dropped on the surface plate. Also, damage can be caused when swarf and other debris have not been removed. This will result in erroneous measurements. Damage to the plate can be corrected only by resurfacing, which requires specialised techniques and equipment depending on the grade of the plate.

Unlike most mechanical precision instruments, surface plates do not derive their precision from more-precise standards. Instead they originate precision by application of the principle of "automatic generation of gages". In this process, three approximately flat surfaces are progressively refined to precise flatness by manually rubbing them against each other in pairs with colouring matter in between, and then hand-scraping the high points. Any errors of flatness are removed by this scraping, since the only stable, mutually conjugate surface shape is a plane.

The importance of the high-precision surface plate was first recognised by Henry Maudslay around . He originated the systems of scraping a cast-iron plate to flatness, rubbing marking blue between pairs of plates to highlight imperfections, and of working plates in sets of three to guarantee flatness by avoiding matching concave and convex pairs.

Joseph Whitworth, born in , had been an apprentice with Maudslay from but had left by the time he started his own business in . He described this process to the British Association in in his paper "On producing True Planes or Surfaces on Metals"'as he related during his chairman's address in at the inaugural meeting of the British Institute of Mechanical Engineers in Glasgow.[4][5] His paper, and his past work for Maudslay, has led to some writers claiming Whitworth as the originator of the surface plate scraping technique, not Maudslay.

Before the Second World War, metal was the standard material used for surface plates, however, the war efforts of various countries put a strain on the availability of metal. A monument and metal shop owner (Wallace Herman) in Dayton, Ohio, along with his inventive employee Donald V. Porter, started using granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite, more accurately referred to as black diabase, with the more wear-resistant surface plates being made of quartz-bearing granite. The quartz content of these granite surface plates increases the wear resistance of the plate as quartz is a harder stone. Black granite is dominantly used in machine bases, granite accessories, and custom applications for its superior stiffness, excellent vibration damping, and improved machinability. Quartz-bearing granite (usually pink, white, or grey) is often made thicker than black granite to provide equal load-bearing capabilities of the types of material used for surface plates, as it is not as stiff as black granite.

Damage to a granite surface plate will usually result in a chip but does not affect the accuracy of the overall plane. Even though it is chipped, another flat surface can still make contact with the undamaged portion of a chipped surface plate, whereas damage to a cast-iron plate often raises the surrounding material above the working plane causing inspected objects to no longer sit parallel to the surface plate.

Granite is also inherently stable, is non-magnetic, has excellent vibration damping characteristics, and will not rust.

On 3 August , Federal Specification GGG-P-463B was issued to provide requirements in United States customary units for igneous rock (granite) surface plates for use in precision locating layout, and inspection work. It encompassed new certification, recertification in the field, and recertification after resurfacing. GGG-P-463B was later revised and reissued on 12 September as GGG-P-463C, which provided common language and terms of classification for surface plate manufacturing and commerce. On 15 June an amendment was issued to the federal specification in order to include requirements in metric units.

Although GGG-P-463C was used extensively in American industry since its publication, the government did not issue any new revisions to keep up with advancements within industry. The American Society of Mechanical Engineers (ASME) decided to form a committee to revise the federal specification in accordance with modern technologies. Most notably, a more complete glossary was added with currently accepted definitions, and a new format was used that should be more familiar to current users of the Standard. ASME also recognised the need for updates to incorporate modern concepts such as traceability and measurement uncertainty that have undergone considerable development since . In June , ASME replaced Fed Spec GGG-P-463C with the American National Standard (ANS) ASME B89.3.7 ' Granite Surface Plates.[7] Iso standard defines ISO-2 for granite surface plates, but it seems the current in use is still dating back .[8]

Prior to World War II, almost all surface plates were made from ribbed cast iron with the ribbing used to increase stiffness without incurring the weight of solid construction. The cast iron was aged to reduce stress in the metal in an effort to decrease the likelihood of the plate twisting or warping over time.

Cast-iron surface plates are now frequently used on production floors as a tool for lapping granite surface plates to achieve certain grades of accuracy. The metal allows itself to be impregnated with the lapping media over a large flat surface.

Despite a fall in popularity among machine shops, cast iron remains the most popular material for master surfaces (different use from a surface plates) among laboratory metrologists, machine builders, gauge makers, and other high-accuracy industries that have a requirement for gauging flatness. Cast iron that has been properly cast is more dimensionally and geometrically stable over time than granite or ceramics,[9] is more easily worked to a higher grade of flatness, and provides a better bearing surface to assist the creation of other master standards. These specialized surface plates are produced in sets of three, by the company that will be using them, so the plates may be regularly verified and refined, including by the Whitworth three plate method, without the need to send them out to be reconditioned. Despite its high stability, cast iron remains unsuitable for use as a normal surface plate in high-tolerance production applications because of thermal expansion. The nature and use of a master surface, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.

Cast iron, unlike granite, has very uniform optical properties and, unlike glass or ceramic, very small light penetration depth which makes it favorable for certain optical applications.[10]

Glass is an alternative material and was used during World War II when material and manufacturing capacity were in short supply. Glass can be suitably ground and has the benefit that it chips rather than raising a burr, which is a problem when using gray cast iron.

The surface plate is used in conjunction with accessories such as a square, straight edge, gauge blocks, sine bar, sine plate, dial indicator, parallels, angle plate, height gauge, etc.

Granite surface plate calibration should be performed routinely to maintain proper flatness and ensure measurement accuracy over time. The intervals between calibrations depends on the environment where the surface plate is located. When needed, lapping or resurfacing is provided to bring measurements within grade guidelines. This procedure involves polishing the surface with an abrasive paste to remove all unwanted material. Granite surface plate calibration also includes cleaning and a light polishing.

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• Rolt, L.T.C. () []. Tools for the Job. HMSO. ISBN .

• Reid, David T. (). Fundamentals of Tool Design (3rd ed.). Dearborn, Michigan: Society of Manufacturing Engineers. ISBN 0--412-4.