History of Metrology

Metrology is defined as “The science of weights and measures.”  Ever since Eli Whitney invented the concept of interchangeable parts to enable mass production we have needed some form of metrology.  Henry Ford used the concept to revolutionize automobile manufacturing.  This led to a powerful need to measure component parts of manufactured assemblies to guarantee that they are appropriately sized for the application.  Simply put, the stuff has to fit, and there is no time for correcting out of specification parts.  A lot of parts are standard, screws for instance.  Believe it or not, even screws have to be measured.  The person or company using them knows what the tolerance of the screw is, and depends on the screw to be within tolerance.  Screw manufacturers have to guarantee this, or they fail.

At first parts were measured with hand tools like vernier calipers, plug gauges and height gauges.  These tools are still used today for lots of measurements. 

Imagine a part one inch square with a ½ inch hole in the center.  Determining the diameter of the hole is relatively easy, but determining where the center is, is a lot more difficult especially if a fine degree of accuracy is needed.

One may wonder why the part even has to be measured at all.  After all, using a ½ inch drill will make a hole ½ inch in diameter, and machine tools are perfectly capable of placing the hole at the proper location.  This is mostly true, but consider if your company accepted a contract the make 100,000 of these parts, and the hole had to be ½ inch within 0.002 inches.  The drill is going to get gradually smaller due to wear and tear and will eventually be drilling holes that are too small.  This means that the part may not fit when it is picked up to be used in its intended application.  In order to avoid this situation and not have to defend yourself or your company in front of the customer, and possibly loose this contract and any future contracts with this customer, you need to guarantee that all you parts are inside the required tolerance..

This is a simple situation but it illustrates why companies institute so called Statistical Process Control.   In order to guarantee that all their parts are acceptable, typically the pieces being manufactured are sampled and measured.  Doing this allows them to determine when to change drills so that they never make a part with a hole too small.  By the way, the drills are also measured before they are used.

As manufacturing evolves, less expensive and better parts are being made, to everyone’s benefit. 

As you can imagine, metrology has become a necessity in order to reduce costs, control processes and compete effectively. 

Now back to the history.  Optical comparators, or as the British call them Shadow Graphs (a more descriptive name) were invented about 1910.  These measuring machines projected a shadow of the part onto a ground glass screen at a very accurate optical gain.  Commonly 10X, 20X, 50X and 100X are used.  In the beginning, a chart with an enlarged outline of the part and its acceptable variation would be clipped to the ground glass screen in order to quickly determine if the part was within the acceptable range by comparing the projected shadow of the part with the drawing on the chart.  The part would be moved via an X/Y stage to align itself with the chart.  If more accuracy was needed then a higher magnification would be used and a bigger chart provided.  This works but the system has obvious weaknesses.  More optical gain means less light and more size, which may not fit the screen any more.  This system gave way to stages with position encoders attached and digital indicators or readouts to indicate position.  In the late seventies, computer technology entered and units that could perform mathematics came on the scene. 

Our example of the part with the ½ inch hole can now be inspected with a comparator by picking up X/Y coordinates around the circumference of the hole and the readout with math capability can from these points determine both the diameter and the location of the center at the same time.

This whole scenario got expanded into 3 dimensions around 1960 with the invention of the Co-Ordinate Measuring Machine, CMM for short.  These machines originally came with 3 axes of digital readout with no geometric capability.  The lack of geometry meant that a company would need a very high powered inspector to get any use out of the CMM.  In the late 70’s and early 80’s the cost of computers diminished to the point that it was feasible to use a computer for a 3 axis digital readout and provide geometric calculations.  By the early 90’s the installed base of CMM’s was quite large but still there were no third party computer packages that could be retrofitted to existing CMMs. 

Enter Caliper Designs, Inc.  Our first product was a DOS based three dimensional package called PC-DRO that had the geometric capability needed.  There were all kinds of 3D software packages from the machine manufacturers, but a sizeable calibration industry, not tied to a manufacturer, needed an upgrade path if they were to keep their calibration customer when his computer finally stopped working.  Remember the pre-hard drive days? Some of the computers we replaced were tape cassette based.  The customer was glad to get going again with our software at $6,000 retail rather that a new CMM at $50,000 to 250,000.

Our company history section has more details about where we went from here.