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CMMs
are the most complex and fragile of metrology instruments around.
Preventative Maintenance, routine calibration and constant surveillance
are mandatory. The Monolith design attempts to simplify the design of a
CMM by reducing the number of materials, part count, points of adjustments
and the number of influential fasteners. The
last revolution in the metrology machine construction field occurred in
the late 1960s where friction was reduced by an order of magnitude
through the use of self-aligning air-bearings. Machines were cranked out
for the next 30+ years in all sorts of sizes and shapes. A great variety
of different structural materials, assembly techniques, computer mapping,
precise lapped metrology surfaces and air-bearing designs were employed.
This allowed for cost effective and flexible designs of many different
sizes. This
piecemeal, hodgepodge building block construction philosophy required the
design to accommodate variations in these components. Air-bearings were
designed with ball pivots and were constructed to self-align to the
machine frame. The typical machines today of this design have over 1000
key mechanical components, fasteners and mechanical interactive surfaces
that are critical to calibration of the CMM in the metrology frame. One
must appreciate that keeping this number of variables constant and
consistent over the duration of a machine calibration is a formidable and
almost impossible task. A survey of CMM installations indicated that 20%
of newly calibrated machines had improper and invalid calibrations. The
sheer number of mechanical variables and complex relationships
understandably overwhelms the service technicians. They
need to do their task in a finite and economically justifiable fixed time
period. If the do get the calibration correct, then the clock starts
ticking on the decay of the precision. For practical reasons, a one-year
period of time appears to be the magically number on most re-certification
requirements. This is the same one-year time frame for a much simple Gage
Block! Every
machine crash, acceleration jolt, temperature cycle, external vibration,
stress relief of change, mechanical failure or angry and frustrated
operator will cause some change in each of these 1000 mechanical
variables. Unless the machine is constantly monitored, there is no
insurance that the machine is accurate 10 minutes after the service tech
leaves the building. The
Monolith design philosophy has simplified the metrology frame from over
1000 mechanical components, adjustments points and fasteners to only 4
components, 0 fasteners and 0 points! This has been accomplished by the
engineering of Large Area Air Surfaces into these 4 simple solid
structural components. Some
interesting Comparative facts!
The
Monolith design, because of its drastically fewer components, provides for
a much greater and stable condition for Calibration & Certification.
And because of this extremely simple and rigid design, the cost for this
CMM is far less than a conventional CMM! Here
is an example of what makes up a typical CMM with Air Bearing Guides. Just
the primary bearing system has 3 axis times 8 bearings times 6 components
with 9 interacting surfaces. A total of 216 direct, highly critical,
mechanical interface surfaces at the primary support points of a CMM. 9
mechanical interface surfaces for each bearing!
In
the metrology field, there are many criteria for performance Speed,
capability, complexity, environmental degradation which all add to the
cost for the performance of inspection equipment where the CMMs are on
top of the food chain. So a CMM with a simpler design will have a much
greater cost savings.
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