Granite - Is this the material of choice for CMM Manufacture?

Originally all Coordinate Measuring Machines were manufactured to a high very precision and were mechanically accurate.  It was not uncommon for manufacturers to offer 2 different grades of CMM accuracy for the same model; the higher accuracy being attained by dedicating more care and attention in the final assembly and calibration process.

The first error mapping techniques were introduced to aid the linear accuracy calibration of individual CMM axes. The magnitude of errors introduced into the map was very small, generally limited to scale error and squareness error, indicating the intrinsic accuracy in-built into the CMM. Later many CMM manufactures were building CMMs from Aluminium; an initiative that started a trend that has subsequently become the norm.   

The only issue they faced was how to make Aluminium CMMs accurate. Answer: throw the large errors prevalent in an Aluminium structure into the error map and force the map to make the CMM structure accurate.

Maps were designed to only meet the accuracy standard currently in vogue and assist the manufacturer in passing the "test". One by one manufacturers went to Aluminium structures attracted by much reduced manufacturing costs. Manufacturers became dependent upon error correction software; their confidence in this fundamental technique increased with time and subsequently relaxed manufacturing tolerances further as they improved with experience error collection and mapping procedures.

Aluminium CMMs are typically built by unskilled personnel and have no accuracy whatsoever until error mapped into specification. Aluminium CMMs offer no benefit, other than price, to the end users. Manufacturer of Aluminium CMMs claim Aluminium is the perfect material for CMM build, but how so since its coefficient of expansion is almost four times that of granite and yet its specific weight is only 1% less than granite, therefore Granite is the choice of material for CMM Manufacture not Aluminium.

Material	Specific Weight (Kg/dm3)	Expansion Coeffecient(1/K)	Temperature Diffusion Rate (W/mK)	Elasticity Module (103N/mm2)	Material Metrological Ranking
Steel	7.25	10.4x10-6	42-63	90-180	+++++++
Aluminium	2.7	23.8x10-6	210	72	++
Ceramic	3.85	8.0x10-6	28	370	+++++++
Granite	2.8	6.5x10-6	3.5	NIL	++++++++++

Wenzel has retained the original manufacturing techniques for CMMs Wenzel and has grown dramatically by bucking the Aluminium and error map trend and has subsequently become the 4th largest CMM manufacture in the world as a consequence. Wenzel is delivering CMM structures that are manufactured to exacting tolerances, built by skilled tradesmen and achieve their competitive accuracies without the use of error mapping techniques. The key to the Wenzel success is a vertically integrated manufacturing facility where the investment has been in people, manufacturing processes and thoroughbred engineering with no compromises.

A CMM built mechanically accurate over 20 years ago still remain in regualr use today, what length of life can you expect from an Aluminium CMM today?
Wenzel are proud to be able to quote CMM accuracies of its products with and without error maps. The difference is an indication that Wenzel structures are intrinsically accurate. Maybe the CMM industry should be asked what the accuracy of its product is with the error map switched off?

The Wenzel CMM accuracy is inbuilt, not added on and so we ask the CMM industry a sincere question; 

Why map it when you can lap it?