ADVANCES IN FIVE-AXIS CMM INSPECTION

Five-axis CMM inspection brings the same efficiency to part measurement as five-axis contouring has brought to metal cutting by dramatically improving throughput, opening up lean efficiencies, and documenting parts to ever-tighter tolerances.

Five-axis processing is moving into the mainstream as a powerful strategic solution to lean manufacturing. On the metal cutting side, contoured parts can be machined in-the-round from a single set-up and prismatic parts machined on four or five sides, as well as angles. The automated processing optimizes in-cut times, speeds throughput, and reduces operator intervention to save on labor burden.

One-stop processing eliminates the error-stacking that is unavoidable with part transfers and multiple set-ups, while five-axis positioning optimizes part access, allows use of shorter tools and enables more direct approaches for higher accuracies and reduced process variation.

But manufacturers have also been challenged to achieve similar gains in coordinate measuring machines (CMM) inspection or to effectively apply five-axis strategies. Traditionally, CMM part inspection can present a serious bottleneck to faster processing, as well as compliance with ISO and QS9000 requirements, and Six Sigma disciplines. Quality control departments are being pressured to provide faster inspection feedback to the manufacturing line.

Parts must be produced to ever-tighter tolerances. Inspection routines are more detailed and require higher data density and more documentation. However, 3-axis CMMs are frequently constrained by machine dynamics from achieving faster cycle times. Rapid acceleration/deceleration creates inertial errors and vibration that diminish accuracy.

Historically, few five-axis CMMs have been available because the technology has been mainly limited to “three + two” machines with indexing heads. Such heads historically provided incremental adjustment (2.5 deg, 5 deg and 7.5 deg, for example). The head must be unlocked, rotated to position, then re-locked. Next, the head and stylus tip must be calibrated for each new angle position before measuring can begin.

All of these steps slow the measurement process and complicate programming and calibration. Most importantly, they prevent CMMs from delivering rapid, automated throughput that can keep pace with the processing gains of five-axis machining. In practice, the overwhelming majority of parts today are inspected on conventional three-axis CMMs with access and approach issues being handled either by changing to a specialty probe (with calibration after each change) or re-orientation/refixturing of the part.

Innovative CMM head designs offer technology solutions to this QC dilemma. Created to bring automated five-axis measurement capability to most CMMs – both new and existing machines – the new heads have a low-mass, low-inertia, motorized design, with rotary drives in horizontal and vertical planes to provide a wrist-like action for infinite positioning.

Rather than incremental positioning, they can position to any angle within their sweep range, and unlike incremental heads, the new heads allow a stylus to be qualified only once using a calibration sphere. The head and stylus can then be used for all angular positions without further calibration, providing a level of automation not previously available. This capability can save hours per day on calibration, enabling unmatched CMM utilization.

Two head designs include one optimized for high-speed continuous scanning operations and the other for high-throughput touch probing. Each uses the same universal CMM controller, which adds true, continuous 5-axis motion control to the machine. This allows rapid repositioning of the probe and access to features. It also allows the lightweight, 2-axis head to give the probe “touch” motion, minimizing the need for the stop/start motion of the CMM’s full structure. The goal is to minimize CMM motion and machine dynamic errors that can degrade measuring accuracy.

A REVO scanning head is engineered for high-speed precision measurement of contoured surfaces and complex geometries requiring high-volume data-point collection to validate part fit and form with high accuracy. It gathers form data from blisks, turbine blades, cylinder bores, valve seats and valve guides, and provides most of the measurement motion in scanning routines, minimizing CMM motion and error forces.

It enables continuous measurement of complex geometries that would require probe changes, repositioning, calibrations or part refixturing when using a three-axis CMM and indexing head. Infinitely adapting position while measuring on the move, the head maintains stylus tip contact to changing surfaces at scanning speeds of up to 500 mm per sec, compared to typical rates of 10 to15 mm per sec for conventional CMMs equipped for scanning. The head can collect up to 4,000 points per sec vs. 200 to 300 data points for typical scanning CMMs.

Five-axis software drives the measuring head and synchronizes its motion with the linear moves of the CMM. The head rotates on the fly to reduce delays in moving between part features, shortening cycle time. Look-ahead algorithms drive the probe path in a single continuous motion. The software uses error-mapping routines that compensate for the effects of geometry, temperature and machine dynamics. Greater bandwidth than a conventional CMM enables accurate, high-speed path following.

The PH20 two-axis touch-trigger motorized head provides faster inspection speed and 5-axis capability for touch trigger CMM measurement. Its compact design enables installation on new CMMs, including smaller machines, as well as retrofit to the vast majority of existing touch-trigger CMMs. Low-inertia two-axis motion (in the head) allows extremely fast “head touch” point gathering. This measurement speed, combined with infinite positioning and avoidance of stylus changes and calibration, allows the head to typically deliver up to a three-fold increase in inspection throughput.

A proprietary kinematic joint of three rollers and six balls gives extremely repeatable stylus tip location for more than 60 percent improvement over CMM-driven touches on size error and 35 to 40 percent better positioning.

The PH20 two-axis touch-trigger head fits the widely-used TP20 touch-trigger probe system, allowing an economical upgrade to 5-axis inspection with existing probe modules, styli and accessories. Both heads are part of complete five-axis packages for installation on new or existing CMMs. The integrated system includes CMM controller with five-axis processor, machine cabling, and application and metrology software. Open interface design with I++DME command protocol enables use of most existing programs and application software, present and future. The Renishaw service organization handles comprehensive retrofits for existing CMMs.

Five-axis CMM inspection brings the same efficiency to part measurement as five-axis contouring has brought to metal cutting – and for many of the same reasons. It dramatically improves throughput, opens up lean efficiencies, and documents parts to ever-tighter tolerances. Retrofits of such systems allow manufacturers to realize greater value and productivity from their CMM investment.

Denis Zayia is the national sales and marketing manager for Renishaw Inc., 5277 Trillium Boulevard, Hoffman Estates, IL 60192, 847-286-9953, Fax: 847-286-9974, [email protected], www.renishaw.com.