Today a Coordinate Measuring Machine is not always installed in the perfect operating environment of 20 degrees C +/- 0.5 degrees C. and as a consequent the material used in the CMM structure is critical to ensure the maximum CMM accuracy is retained. Quoted CMM accuracy is today many times better than that offered by the early generation of Coordinate Measuring machine and CMM material science less critical.
Granite was used as the build material for the very early CMM’s more than 40 years ago. The granite surface plate was already established in manufacturing because of its wear resistance, slow thermal response and relative low cost; it was a natural progression to build CMM’s from granite.
A granite surface plate is only used as a base reference and any thermal growth has no impact on its function since as long as the plate remains flat its size is not relevant. This is not the case when considering a high accuracy CMM frame structure.
Coefficient of Thermal Expansion
All materials have a coefficient of thermal expansion and some materials expand more than others. The popular materials used in CMM frame construction over the past 4 decades are shown tabulated.
The thermal expansion of aluminum is 3.5 x more than granite yet today over 80% of CMM’s supplied annually have an aluminum frame.
Rate of Thermal Diffusion
The rate at which the CMM frame responds to a temperature change is very important to ensure the retention of maximum CMM accuracy. All materials have a different rate of thermal diffusion.
The thermal diffusion of aluminum is 60 x faster than granite. This actually results is less thermal growth than granite since the aluminum acts as a heat pump and stabilizes quicker to the changed temperature. Granite takes 8 hours to fully dissipate thermal changes.
This is one principle reasons that modern CMM frames are produced in aluminum. But not the only one…
.Today ALL CMM’s are error compensated since the demanded CMM volumetric accuracy is impossible to achieve with inbuilt mechanical accuracy only. Even granite CMM’s are today error mapped.
CMM’s have 21 degrees error compensation which updates the CMM metrology software with compensated CMM XYZ position real-time. The measuring scales used by the CMM need to be also mapped against a laser reference standard and their errors included in the map.
Most CMM suppliers offer thermal compensation on their CMM’s whereby the machine scale temperatures are monitored and the error map is adjusted according for thermal growth.
Granite, unlike aluminum, does not expand at a uniform linear rate and therefore the thermal compensation of a granite CMM is less precise than that of an aluminum frame CMM due to the linear expansion of aluminum.
Aluminum reflects about 90 percent of radiated heat and emits only a small percentage of the heat that is taken on. This low transmission makes aluminum the ideal CMM material since absorption of thermal changes is critical to maintaining CMM accuracy.
It also well documented that dark colors (black granite) will heat up more than light colors (silver aluminum). This is another huge negative for use of granite in CMM frames.
Granite is a porous material acting like a sponge and absorbs moisture when the atmosphere is humid. This moisture absorption by granite results in structural deformation of the CMM frame that cannot be compensated inducing CMM measuring errors.
The use of a granite CMM’s in a high humidly countries is not recommended.
The ALL-Granite CMM Myth – IT IS NOT ALL GRANITE
The all- granite CMM is a myth since its air bearings and central carriage are manufactured in aluminum. The vertical bridge beam supports are also typically produced in aluminum or steel and thus only the Bridge beam and Z ram are produced in granite.
This means the all-granite CMM does not expand uniformly when subjected to a thermal change.
Z Axis Stiffness
The stiffness of the CMM Z ram is critical to ensuring accuracy. The greater the CMM Z measuring range the more critical becomes the Z axis stiffness.
As shown in the adjacent chart the Modulus of Elasticity of Aluminum is 33% stiffer than solid granite; granite Z rams are core drilled to reduce mass and allow cable management making them even weaker still.
Some suppliers switch to ceramic from granite for their Z ram material on taller measuring range CMM’s due to the low stiffness of granite.
CMM Accuracy comes from the Measuring Scales
The measuring scales should be fixed to the CMM structure only at its reference end to allow for a free-floating scale thus allowing the CMM structure to move independent of the measuring scales. This is achieved by mounting the scale in a track as shown below. Most granite CMM suppliers fix (glue) the measuring scales directly to the granite inducing additional measuring errors as the structure expands.
Heavy Granite CMM Internally Generated Heat Sources
Heavy granite CMM’s require large heavy duty motors to drive the heavy CMM structure. These motors create local heat which can directly dissipate through the CMM frame causing measuring errors. Typically granite CMM suppliers install cooling fans inside their machine covers in an attempt to minimize the impact of these local heat sources on the CMM frame.
What is The Perfect CMM Structure?
- Lightweight CMM structure offering good dynamics without use of large heat producing motors.
- Free floating measuring scales
- Frame produced from heat reflecting material
- Frame produced from fast reacting material to thermal changes
- Frame has linear and predictable expansion
- Frame material not affected by humidity
- Frame components produced from common materials
- Stiff Z axis column