Thermally stable construction materials. Make sure that the primary members of the machine construction consist of materials that are less susceptible to temperature variations. Consider the bridge (the machine X-axis), the bridge supports, the guide rail (the machine Y-axis), the bearings and the machine’s Z-axis bar. These parts directly affect the machine’s measurements and motions accuracy, and constitute the CMM’s backbone components.
Many companies make these components out of aluminum because of its light weight, machinability and relatively low cost. However, materials such as granite or ceramic are much better for CMMs because of their thermal stabilities. In addition to the fact that aluminum expands nearly four times more than granite, granite has superior vibration dampening qualities and can provide an excellent surface finish on which the bearings can travel. Granite has, in fact, been the widely accepted standard for measurement for years.
For CMMs, however, granite has one drawback-it’s heavy. The dilemma is to be able, either by hand or by servo, to move a granite CMM around on its axes to take measurements. One organization, The L.S. Starrett Co., has found an interesting solution to this problem: Hollow Granite Technology.
This technology uses solid granite plates and beams that are manufactured and assembled to form hollow structural members. These hollow structures weigh like aluminum while retaining granite’s favorable thermal characteristics. Starrett uses this technology for both the bridge and bridge support members. In a similar fashion, they use hollow ceramic for the bridge on the largest CMMs when hollow granite is impractical.
Bearings. Nearly all CMM manufacturers have left the old roller-bearing systems behind, opting for the far-superior air-bearing systems. These systems require no contact between the bearing and the bearing surface during use, resulting in zero wear. Additionally, air bearings have no moving parts and, therefore, no noise or vibrations.
However, air bearings also have their inherent differences. Ideally, look for a system that uses porous graphite as the bearing material instead of aluminum. The graphite in these bearings allows the compressed air to pass directly through the natural porosity inherent in the graphite, resulting in a very evenly dispersed layer of air across the bearing surface. Also, the layer of air that this bearing produces is extremely thin-about 0.0002″. Conventional ported aluminum bearings, on the other hand, usually have an air gap between 0.0010″ and 0.0030″. A small air gap is preferable because it reduces the machine’s tendency to bounce on the air cushion and results in a much more rigid, accurate and repeatable machine.
Manual vs. DCC. Determining whether to purchase a manual CMM or an automated one is quite straightforward. If your primary manufacturing environment is production-oriented, then usually a direct computer controlled machine is your best option in the long run, although the initial cost will be higher. Manual CMMs are ideal if they are to be used primarily for first-article inspection work or for reverse engineering. If you do quite a bit of both and don’t want to purchase two machines, consider a DCC CMM with disengagable servo drives, allowing manual use when needed.
Drive system. When selecting a DCC CMM, look for a machine with no hysteresis (backlash) in the drive system. Hysteresis adversely affects the machine’s positioning accuracy and repeatability. Friction drives use a direct drive shaft with a precision drive band, resulting in zero hysteresis and minimum vibration
Post time: Jan-19-2022