The Gauge Shop

Introduction

Machine Frame Construction

Bearing Systems

Scales

Did you know that granite weighs about the same as aluminum?

It has been our long held view at Starrett that if you start off with a stable, well designed machine frame, that you provide the basis that will allow you to attain the maximum performance from the other peripherals. After all, you can only deliver accuracy with a machine frame that is stable.

The best CMM frame will provide many years of excellent service but as any of us who have bought personal computers know, the hardware and software (and to a lesser degree) the probing system will become outdated during this period. The one constant is the MACHINE FRAME.

Enter any gage lab or inspection room and look at the base standard for precision measurement surfaces, and you will see a GRANITE surface plate. Granite surface plates are used because they are thermally stable, exhibit a very low coefficient of expansion and have excellent vibration dampening capabilities. Also, the working surface of a granite surface plate is precision lapped, which provides an ideal air bearing guide surface. However, the primary disadvantage of a standard granite member designed into a moving structure is weight, which would produce too much mass to accelerate and decelerate quickly.

This, along with the perceived cost benefits have made aluminum the easy choice for most CMM manufacturers, even though it expands at 0.000012 per inch per degree fahrenheit compared with granite at 0.000003 per inch per degree fahrenheit. This is a ratio of nearly 4 to 1.

The solution to the weight problem is not easy, but after serious consideration, Starrett patented a manufacturing process which combines the lighter weight benefit of aluminum with the thermal stability of granite. Thus, Hollow Granite Technology was born!

Hollow Granite
Hollow Granite is a patented Starrett design that features granite members that are manufactured to form a hollow structure that is lapped and finished as a solid member. Starrett Hollow Granite Technology is employed on all moving bridge components, including the main support, outboard leg and bridge. The machine table and the guide rail, which are static components, are produced as solid members. Interestingly enough, aluminum has the same density as granite. A cubic inch of granite weighs almost the same as a cubic inch of aluminum.

Ceramic
Incorporating ceramic in key components of the machine design is also advantageous. Ceramic has similar thermal characteristics to granite, with a high strength-to-mass ratio, making it an extremely stiff material. This makes it the ideal material for the Z ram, which when fully extended can exhibit distortion problems. For example, Starrett eliminated this problem by designing a Ceramic Z ram as standard on every machine offered. Ceramic is also utilized as the BRIDGE cross beam on larger machine designs, where it is not practical to use granite members.

It is important to note that when ALL of the major components of a machine exhibit the same low thermal characteristics, like Starrett machines, they become extremely stable overall, making them able to ensure a high degree of consistency and accuracy, even under less than ideal working environments.

Starrett on Coordinate Measuring