Phoenix Bridge Analysis
Analysis of the Helmel Phoenix CMM Bridge Concept
At first glance, the mechanical bearing Helmel Phoenix CMM looks like any CMM, but the bridge underneath is unique. Bridge, legs and bearing ways make up a one-piece welded steel structure unbelievably strong and stiff, the only one-piece design we know of in the CMM industry. Leg connections are fully welded, stronger than bolted on legs. Way rails are welded using techniques developed over years to avoid overheating and warping, producing long-lasting strength and permanent integral accuracy of the stress-relieved bridge. This results in an utterly reliable, durable bridge at the heart of the Phoenix concept.
Through experience we avoid the bolt-on linear bearing ways found elsewhere. Every bolt creates structural stress, no friend of CMMs. Over time they invariably misalign. Even dowels between bolts add stress. Weld-in-place ways are more costly and difficult to produce, but are more reliable and extend the life of the bridge by multiple factors.
The bridge has two beams instead of one across the top. The X-axis bearing ways at the extreme front and rear of the bridge are finish-ground straight, flat and parallel in place using carefully developed techniques. Bearing span is maximized for optimal intrinsic carriage accuracy. Symmetrical in both XZ and YZ, this dual-beam design avoids asymmetric thermal distortion.
With the Z-axis centered between bridge beams, there is neutral center of gravity through the X-axis travel, a clear contrast to single-beam side-mounted Z-axis designs that produce imbalance and twist to the bridge. A side-mounted Z carriage moving across a single beam bridge with floating leg exerts increasing torque and twist to the structure causing mechanical errors that can only be corrected with software.
The steel X-axis scale is firmly adhered to a ground surface of our rigid, balanced and stress-relieved steel bridge for uniform thermal expansion, in contrast to designs that incorporate aluminum into the bridge beam, increasing thermal incompatibility, and reliance on temperature sensing and software correction. These designs must allow the X-axis scale to “float” to accommodate different expansion rates of aluminum and the steel or glass scale. This “bolt-on” is less integrated, like the separate bearing ways described above. Aluminum is an odd choice for scale surfaces in a metrology frame.
Countering Helmel, cantilever CMMs are even more mystifying, having an unsupported projecting axis with minimal bearing span across the top of the back plane with bearing ratios of 1:3, even 1:4 to the lengths of travel. The bearings are under opposed loads, one in compression – the other in lift, that change as the mass of the Z-axis carriage moves toward the outboard. Further, the Z-axis carriage is side-mounted on the projecting beam, inducing increased twist and downward deflection moving outward. This design is not an optimal metrology design, nor a long-life prospect.
The moving bridge CMM will remain at the core of precision coordinate metrology. The rigid, durable one-piece Phoenix bridge shines as a universal, enduring example of the type, and is emblematic of the intelligence and integrity of Helmel CMMs.
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