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Workholding for flexible inspection.

The CMM provides an incredible amount of dimensional information much more quickly than traditional surface-plate inspection and almost as accurately as dedicated gages without their attendant inflexibility. Part change-overs are accomplished by merely installing a new program and fixture. Even when the CMM is tooled for high production inspection, other areas of the machine's measuring volume are available for random inspections of other parts in between regular measurements. Factory-hardened CMMs can even be placed on the shop floor.

Flexibility like this is making CMMs the fastest-growing segment of the inspection market. As CMMs expand from lab uses to production tasks using direct cOmPuter control (DCC), a new type of tooling is required. This tooling must rigidly support the part in a known location, speed the part-orientation process, and foolproof part loading to avoid crashes of expensive probe heads.

Volume variations

For the job shop, fixturing the CMM usually consists of mounting the part onto the table with tape, clay, or strap clamps with little part-to-part repeatability. With small lots, manual positioning-corrected by the CMM's axis-skewing software-works fine. Flexible clamping systems are an excellent solution for parts in jobshop inspection. These systems, Costing $10,000 and up, provide a variety of fixturing solutions, but setting them up can be quite time-consuming. This makes them more suitable for small to medium-lot production situations.

In high-volume applications, a dedicated fixture can be mounted to the machine and calibrated for position within seconds. Such a tool should locate the part, secure it in a stable manner, and permit the probe to access all of the features required for inspection.

CMM fixturing is quite different from metal-removal fixturing. It more closely resembles that used by EDM or other noncontact machining processes. If fixture design is approached as it would be for machining, the resulting tool will be less than optimal. Part orientation The first decision is part orientation. For bridge or gantry machines, a majority of the inspected features should be on the top or sides of the part. Horizontal-arm machines place inspected features toward the arm, or on the top or sides. Rotary tables or dual-arm machines facilitate inspection on opposite sides of the part and simplify tooling design. Also, an articulating probe head, such as the Renishaw PH9, makes inspection much easier. Access to datum surfaces for part alignment, and the position of locating surfaces that support the part, will also influence part orientation.

Close cooperation between programmer and tool designer will insure that all measurements can be made without interference from the tooling, that the part will be easy to load, and that the part cannot be positioned improperly in the fixture.


Casting or machining datums are not always used for locating the part on a CMM fixture. Our method is to leave the datums exposed and accessible for probing wherever possible. Part geometry can be constructed using CMM software features. We locate the part as square as possible to a machine axis, constructing a plane through which the three point locators will pass. In the remaining axis, we need only position the part within a small range to permit the CMM to find a locating feature. This way, the machine automatically orients the part, providing for part skewing and keeping tooling costs down.

Often, the locating elements can rest on part features adjacent to the datums that remain constant throughout the manufacturing cycle. If no further machining takes place there, the fixture can be used for subsequent measurements.

We regularly produce tooling that will hold parts from the as-cast state right through to finish inspection without any adjustments to fixture or part. Castings sometimes present problems using this method because their surfaces are rough. CMM measurements to locate datums are subject to a small surface-finish error of about 0.005" to 0.015" for sand castings or 0.001 " to 0.005" for investment castings. If this repeatability error is objectionable, we sometimes use oversize locators at the datum positions that permit the probe to touch on the locator, adjacent to the datum surface.

A nest can be used to position a part within range, providing a rough orientation and fool-proofing the fixture against incorrect loading. Point locators-spherical or conical rest buttons, for example-are used to support flat surfaces. To locate surfaces that are not flat, commercial flat rest buttons or pads can be used, or the sides of dowel pins exposed by mounting in a partial mounting hole. Cylindrical shapes can be held in a Vee. Commercial fixturing components should be applied wherever possible to reduce cost-standard handles, clamps, locators, and other stock devices are readily available.


Clamping is required on light or unstable parts, on fixtures that are transported, and on machines with vibration-damping systems (to offset the effect of machine dynamics; i.e., the CMM jumps and the part doesn't). As with conventional tooling, clamping forces should be applied directly over a support or the part will deflect. (It takes very little force to deform a part when you are looking for errors as small as 0.000 05".)

Commercial toggle clamps are quite useful, but don't overlook other solutions. Light clamping is sufficient for stable parts held against stylus forces or machine movement. Unstable parts must be clamped more securely. Commercial clamps are often modified with longer clamp arms to provide probe clearance. Spring-loaded or eccentrically actuated clamping arms are used when a range of part diameters must be held or when machining operations change part diameter.

FMS inspections

In flexible manufacturing systems, fixturing requirements remain the same, but the part is often attached to a pallet that must be loaded onto the CMM table. In such cases, a pallet receiver is installed to accept and clamp the pallet in position, and sensors must be added to verify pallet position. The cell integrator will provide a receiver matching that used on the metalcutting machines, or in some cases, we have designed and built special pneumatic pallet receivers because most CMMs do not have a hydraulic system. These perform the same function, but at a reduced cost and complexity.

Palleted parts and shuttle loading of parts provide safer loading of heavy workpieces, protecting the operator from injury and the machine from accidental damage. Such systems will be increasingly used as CMMs move into high-volume inspection. Complicated solutions are not necessarily required. Simple part-handling devices can permit safe loading by an operator or by standard pallet-handling equipment found in an FMS.

For more information, contact Vitullo & Associates, 25311 Dequindre, Madison Hts, MI 480714241 or circle 174.
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Title Annotation:fixtures for coordinate measuring machines
Author:Vitullo, Robert
Publication:Tooling & Production
Date:Apr 1, 1991
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