Introducing the Concept of Unistraint

 

Unistraint, marker for scale.

 

We have solved the age-old problem of how to create very long artifacts for the purpose of machine calibration by inventing a new concept for supporting large calibration artifacts for measurement.

The Problem

When calibrating large articulating-arm CMM’s, laser trackers, optical scanners, and for very large coordinate measuring machines--large calibrated artifacts must be used. Large artifacts, however, come with an inherent limitation.

At least from the time of Lord Kelvin and Clark Maxwell, science has been obsessed with the concept of kinematics--the principle that a rigid body has six and only six degrees of freedom. According to their rules there is only proper constraint and over-constraint. But they take one hidden factor for granted.

 

Unistraint

 

Unistraint

 

In the real world there is no such thing as a rigid body, meaning there are in fact seven--not six--degrees of freedom. Elasticity is the seventh degree of freedom. And it is this last degree of freedom that has thus far hindered the adoption of very long artifacts for high-precision calibration.

By reasoning on this plane, we are introducing the concept of unistraint to cope with the problem of supporting really large artifacts in all seven degrees of freedom. In this approach, the artifact and the structural support become one monolithic entity.

The knock-out punch of this monolithic approach is the use of the fundamental concept of structural sheets to produce a device incapable of appreciable length variation due to elastic deformation. By combining several large, sheets of light-weight structural metal, enormous rigidity is achieved with an artifact that can be picked up with one hand.

The finishing touch is to combine the invar-artifact, with its nearly zero thermal coefficient of dimensional change, with the light-weight aluminum structural member. This combination is achieved using the unistraint concept by supporting the artifact not by six points to be kinematically correct, but by literally millions of points of contact within a cocoon of stiff structural foam, achieving the unistraint condition.

Our Solution: The Unistraint Calibration Artifact

Our Unistraint device is a unique concept in long calibration artifacts (patent #86325970).

This concept is based on simple principles of physics. All of the trivial fussing about the original camber or waviness of the artifact and all of the smoke screen of stiffness-to-weight ratios become gibberish.

The difference with Unistraint is that it is not based on constraint, or for that matter over-constraint, but on the simple principal that “if it doesn’t bend, it doesn’t matter.” We call this new doctrine: unistraint.

The basic design is based on the resistance of very thick sections of material to bending. In the case of the Unistraint it is the resistance of four, huge, sixteen-inch thick sections of aluminum to bend due to the minor load imparted by change in the force of gravity when the ball-bar-like structure is moved from vertical to horizontal, and of course any interim position.

 

Unistraint anchor and trivet

 

unistraint info 3

The fact that the sixteen-inch thick sections are only 0.020” thick has very little effect on the bending moment generated by their width. This brings us back to the original premise that “if it doesn’t bend, it doesn’t matter.”

An invar-alloy bar ball (or trihedral nest) forms the body of the device, and is encapsulated in high-stiffness foam: 250,000 PSI compressive modulus, connecting all the elements of the artifact together to form a pseudo-monolithic structure. This entirely eliminates the influence of the original geometry of the artifact

Furthermore, there has always been a serious problem with slippage in the rotary position of large calibration artifacts during use. This has been entirely eliminated through the application of two gear-tooth-like Hirth plates that form a preloaded coupling, both locating the artifact at a very accurate incline and locking it there without even microinches of movement.

 

Unistraint ball end

 

We are building these giant calibration artifacts in two off-the-shelf standard lengths. We have the really giant 90.5” (2.3 meter) long version--part number US-2.3-B for the ball-bar version, and US-2.3-TH for laser-tracker version. And there is a shorter 50” long version (1,270mm), part number US-1.27-B (for ball-bar version) and US-1.27-TH (for laser-tracker version).

One of the two standard designs is a simple ball-bar. The body of this ball-bar is invar-alloy with its almost zero rate of thermal expansion and contraction; and unlike carbon and glass fiber composites it has absolutely zero sensitivity to moisture. This ball-bar is equipped with one-inch diameter balls of either hard stainless steel or zirconium-oxide ceramic.

Our second standard configuration has the same general construction but with a magnetically preloaded trihedral nest on each end to hold a spherical retro-reflector (SMR) for use in calibrating laser-trackers. This standard configuration is designed for a 1.5” diameter SMR, but we can also supply a version that is compatible with 7/8” or 1/2” diameter SMR’s.

 

Unistraint

 

For calibrating articulating arm measuring machines, standard high-grade 1” diameter ball ends are used on the artifact. Our three ball probe end 3B-KP is highly recommended.

A bonus with the design of the Unistraint ball-bar is that the version used to calibrate laser-trackers can be converted to a standard ball-bar by just screwing on accessory ball-bar ends, part number ABB-1, into the trihedral mounts.

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