Mold makers strive to avoid difficult gaging situations by enforcing strict quality control processes that begin with ensuring their gages are properly calibrated to international standards of measurement. For best gaging practices, manufacturers start with applying primary standards for measurement and inspection that will ultimately lead to consistent, reliable gaging results.  Further, they use proper care and maintenance procedures to maintain accuracy and provide long tool life.

Measurement Defined

The definition of measurement units provides an interesting background on the origins of standards. Two units of measurement are common in the United States:  British yard and the meter, as defined by the Weights and Measures Act of 1878. These practical measurements have evolved to now be defined as the international inch in terms of light waves.

The meter is the basis of all measuring systems with the international inch defined as 25.4mm exactly. Over the years the meter has been defined in different ways. Currently the meter equals the distance light travels in a vacuum during 30.663318 cycles of a Cesium atom. Of course this cannot be used for regular measurement, so the physical relationship is translated by the National Institute of Standards and Technology (NIST) using lasers and atomic clocks and transferred to today’s most basic and accurate precision standard - gage blocks. Gage blocks are the standards that bring this technology to the shop floor where different sizes of blocks may be combined to give any required dimension.

Gage Blocks Are Key      Precision gage blocks are the primary standards vital to dimensional quality control in the manufacture of interchangeable parts. These blocks are used for calibrating precision measuring tools and for setting numerous comparative type gages used in incoming, production and final inspection areas. Gage blocks provide the most economical, accurate method of setting dial test indicators and other gages used in conjunction with surface plates for inspecting parts with exacting tolerances. Essentially, gage blocks consist of a hard stable material with a flat, parallel gaging surface on each end. The measuring surfaces are ground and lapped to an overall dimension with a tolerance of plus or minus a few millionths of an inch. Gage blocks may be stacked or “wrung” together to form accurate standards of any practical length, limited by assembly and handling of the wrung together blocks. Gage blocks are made in several grades or degrees of accuracy. Grade 0 is the most popular grade of gage blocks, as this grade is usually suitable for most applications and is a grade that offers the best combination of accuracy and cost. Higher accuracy grades of blocks, such as Grade 00 are primarily used as masters to check other gage blocks and for applications that require extreme accuracy. Grade B (±50 microinches) blocks are relatively inexpensive but are limited to workshop use where exacting accuracy is not required.

Another material option is ceramic gage blocks which are long-wearing and corrosion resistant. Webber ceramic gage blocks fill in the gap between steel and croblox® and are available in Grades 00 and 0. These blocks have favorable mechanical and thermal properties that compare the closest to steel of any alternative gage block material.

Steel is the most economical gage block material available, and steel’s thermal and mechanical properties are adequate in typical workshop environments. The main disadvantage of steel is its susceptibility to corrosion.

Various styles of gage blocks are available including rectangular, square and Starrett-Webber Heavy Duty. The use of gage blocks can also be extended by means of accessories that can be used with height gages, snap gages, scribers and dividers.

Even Work Surfaces  Every linear measurement depends on an accurate reference surface from which final dimensions are taken. Precision Granite Surface Plates provide the best reference plane for work inspection and layout prior to machining. They are also ideal bases for making height measurements and gaging surfaces, parallelism etc. A high degree of flatness, stability, overall quality and workmanship also make them ideal bases for mounting sophisticated mechanical, electronic and optical gaging systems. The most important element in the performance and life of granite surface plates is the percentage of quartz that is present in the stone. Quartz is more than twice as resistant to wear as the other minerals in granite. It provides bearing points that are of a hard, highly polished, smooth character which protect the accuracy and finish of both the surface plate and the tools and instruments used on it. For example, Starrett offers Crystal Pink® Granite that has the highest percentage of quartz of any granite. Starrett Superior Black Granite also contains quartz, though a slightly lower content than Crystal Pink. Higher quartz content means greater wear resistance. And the longer a surface plate holds its accuracy, the less often it will require resurfacing. Surface plates without work clamping ledges are recommended for sustained accuracy and reliability. Ledges are for work clamping purposes only. If excessive torque is used when applying clamps to ledges, it can adversely effect measurements taken near the plate edges. If clamping is important, T-slots and threaded metal inserts may also be installed in the surface. Some applications may warrant customized, special plates. For example, a custom-made plate might be needed for inspecting oversize parts and assemblies such as diesel engine blocks and crankshafts, vehicle frames, missile components, ground support equipment, and depending on size ‒ complex molds.  Or perhaps a special plate or a modification to a standard plate might be required for work-holding attachments of many types including threaded inserts, studs, adaptor holes, T-slots and dovetails etc.

The Starrett Tru-Stone Technologies Division specializes in custom granite applications that are ideal for various metrology equipment and other unique precision solutions. For instance, multi-plane bases with precise insert accuracy.

Granite Surface Plates are manufactured in three grades of accuracy including (1) AA – Laboratory Grade that is typically specified for precision operations in constant temperature gaging rooms and metrology departments, (2) A – Inspection Grade that is typically specified for general work in quality control and (3) B- Toolroom Grade that is typically specified for production checking work throughout the shop.

All surface plates must pass a critical final inspection to prove that their surfaces are within the specified tolerance. The final inspection is typically done with an autocollimator in a controlled atmosphere. This instrument is checked and certified against standards traceable to NIST.

Take Good Care

Since standards are so critical and form the basis for all precision measurement, extra care and regular inspection should be taken to ensure they are accurate.

Gage Blocks

To properly care for gage blocks, wringability is a key test for evaluating the integrity of the surface condition of a gage block. Wringability may be defined as the ability of two surfaces to adhere tightly to each other in the absence of external means and it is an important property of gage blocks. Blocks that don’t wring may give erratic and unreliable results and are recommended to be replaced.

To prepare gage blocks for wringing together, blocks must be free from nicks and burrs because imperfections on one block may damage the surface of the other block.  Blocks may be checked for defects with a gage block stone before wringing. A stone with serrated grooves is recommended because it gives a better “feel” for nicks and burrs that catch the edges of the serrations.

If stoning is required, this process will remove a small amount of raised material, improving repeatability of readings and providing block sizes that appear to be truer to their original tolerances. Ultimately, blocks that are free of defects will wring together better.

To wring gage blocks, all surfaces must be clean. An oiled Wring Pad can be used for wiping the surfaces of the blocks. Next, a dry pad can be used to remove as much oil as possible. The blocks should then slide together without any feel or bumps or scratching, and should adhere to each other strongly after being rotated into place.

Formal tests exist for testing wringability if a problem with a block is suspected. Tests may be done by the user of the blocks and does not require a laboratory to perform the test. Wringability is an important property of gage blocks, but is fortunately a quality that can be controlled and monitored readily by the user of the blocks. With proper use and care, quality gage blocks will provide long, reliable, accurate service.

Granite Surface Plates

The granite surface plate is a precision piece of equipment and as such, must be properly installed and maintained. Before use, the plate should always have a reliable support system furnished with the plate. Typically this is a hard rubber pad attached to the bottom of the plate forming a non-distortable 3-point support system. The pads are installed during manufacturing and the plate rests on them throughout lapping, inspection and shipping. They are a critical factor in surface plate accuracy and must not be removed. Plates up to and including 6’ wide x 12’ long should be furnished with this non-distortable 3-point support system. When mounting the plate on a stand, be sure only the pads are resting on the stand. Never support the plate by the ledges or under its four corners, as this will completely void guarantee of accuracy.

Plates larger than 6’ wide x 12’ long are supported on multiple support points (6 or more points) consisting of granite pedestals and leveling wedges. The number of support points and positioning is determined by the size of the plate. The height of the granite pedestals, or even if they are used, is determined by the thickness of the plate and overall working height desired.

Nylon slings are highly suggested when lifting the granite and if a forklift must be used to move the granite surface plate, a protective padding should be placed between the metal forks and the granite.

Once set-up, plates do not require extensive care and maintenance.  Primarily, keeping the surface clean and free from buildup of dust, dirt, grease, grime and other foreign particles will maintain accurate tool readings and extend the life of the plate. Environment and usage have much to do with the frequency of cleaning. However, as a rule, it is recommended to clean a plate daily if it is used each day. If the plate is not used for an extended period of time, it should be covered with a surface plate cover.

There are some do’s and don’ts for using granite surface plates. Do not use granite surface plates as workbenches or lunch tables!  Dropping wrenches or hammers on plates can chip and nick the surface, and spilling coffee or drinks on granite can cause permanent stains. Grease from food particles will do the same.

Work pieces to be measured must be set down gently on the plate, as a sudden jolt or blow to the plate with a heavy metal object can chip or nick the surface. Also, when inspecting work pieces, especially small parts, it is recommended to use different areas of the surface plate because using the same spot over and over, year after year, will wear that area of the plate.

Surface plates should be checked on a regular basis for wear using a repeat reading gage with a manual indicator. Generally, long before a surface plate has worn beyond specifications for overall flatness, it will show worn or wavy spots, which will produce measurement errors. The reading gage will readily detect these error-causing areas.

Simply set the gage on the plate and zero it at any point on the table. Move the gage over the plate and if there are hand movements in the indicator more than .000025” for a “AA” plate, .000050” for an “A” plate or .000100” for a “B” grade plate, then you have indication that the plate may have some high and low spots and be out of tolerance. Specifically, the tolerance must repeat from side to side within the specified tolerance range in order to be an accurate plate.

 An effective inspection program should include regular checks with an autocollimator, laser or electronic levels, providing actual calibration of overall flatness traceable to NIST If tolerance variations are excessive, the plate can be transferred to work involving less accuracy or it can be resurfaced to restore its original level of accuracy. Outside facilities or manufacturers are available to assist with the resurfacing process.

Conclusion

Mold makers that produce the highest quality molds use the best quality control practices. The quality of products is only as good as the accuracy and reliability of measurements.

This begins with relying on primary standards to ascertain that gages are properly calibrated and are being used to check parts on optimal work surface conditions. Due to a number of factors including shop contaminants, normal wear and tear and more, gages can regularly require adjusting. By checking gage readings and accuracies with gage blocks and ensuring a reliable reference by using granite surface plates, manufacturers have the essential tools and methods for maintaining primary standards that will ultimately lead to reliable gaging and better quality parts.