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Probing into quality.

With the emphasis in quality shifting from post-process to in-process inspection, manufacturing engineers and quality control inspectors are making touch probes as commonplace in machine-tool magazines as extractors are.

Touch probes allow automatic pre-process, in-process, and post-process measurement and inspection of tools and parts on a variety of CNC machine tools, including lathes and machining centers. Integrating inspection routines into the machining cycle reduces the requirement to preset tooling and send parts to separate inspection stations--actually reducing overall batch cycle times and enhancing overall control of the process accuracy and repeatability.

When used for tool setup, probing systems can verify tool or cutter geometry and size, automatically entering offsets in the control's tooling register. Additionally, the same systems can be used to detect tool breakage and compensate for tool wear.

Probing systems in action

Higher accuracy, quicker delivery, lower cost, and improved documentation are just a few of the benefits touch-probe users are reporting. Probing systems, retrofitted to machine tools, enable machines to virtually set themselves up for complicated work. They reduce part handling and complex fixturing and ensure the first part is a good part.

Dave Morgan, manufacturing manager at Bley Engineering, an Elk Grove, IL, precision machining contractor, concurs. "Where setup is a significant percentage of the cycle time, probing can make the difference between profit and loss in our market," he says.

Bley Engineering and Jay-Em Aerospace Corp in Cuyahoga Falls, OH, exemplify how two aerospace contractors have sharpened their competitive edge with retrofit probe systems. At Bley, the probing system has eliminated time-consuming setups on a forged 7075 aluminum helicopter transmission housing. For Jay-Em Aerospace, the probe eliminates expensive fixturing and places bolt-hole patterns on aircraft wheels far more accurately.

For Bley, a one-year backlog of work on a Kuraki horizontal machining center made fast, foolproof setups a must. With a work envelope of 47"x 51"x 78" and repeatability of 0.0003", the machine is Bley's workhorse for helicopter transmission housings.

More than 60 hours of machining goes into the cone shaped part, with 9 1/2 hours the longest operation. Accuracy required on the ballistically qualified part is |+ or -~0.0005". "We remove all the draft and must be properly centered to split stock evenly between top and bottom, through the ribs," says Mr Morgan. However, small misalignments between upper and lower portions of the part, created during the forging process, complicate setup.

"Initially, we pre-qualified the part on a coordinate measurement machine (CMM), but couldn't do this for a production run," says Mr Morgan. "It takes too much time, increases part handling and paperwork. With probing on the machining center, there's nothing to lose, no serial numbers to confuse. We find our zeros on the machine for the part that's on the machine."

A Renishaw MP9 wireless probe is stored in the tool magazine and transferred to the spindle like a tool. Battery operated, it acts as an omni-directional sensor, effective in the |+ or -~X, |+ or -~Y, and |+ or -~Z directions. Each touch generates an infrared LED signal, which is read by an optical receiving unit, then transmitted to the Fanuc 11M control. The control relates the contact point to a known datum and automatically updates the tool offsets. Repeatability of the probe is 0.000 04" (2 sigma).

Bley uses a master leveling fixture to balance the helicopter part on its four legs. The probe cycle then begins. The probe touches several points around the bottom OD and several at the top ID. The control then computes a "best average" center. After the first reference cuts, the probe cycle is repeated to confirm the zeroes and update the offsets for the final operations.

Zero scrap

Setups are quicker, parts more accurate, and fixturing costs lower at Jay-Em Aerospace since it retrofitted a Renishaw probe to a LeBlond Makino vertical machining center. Jay-Em makes wheels and brake housings for a range of US military aircraft.

Expanding mandrel fixtures, each costing $3000 to $5000, were once essential for accurate setups to machine bolt holes in wheels, but not anymore. "With the probe, we can clamp a shell half to a $200 fixture, as long as the part is aligned within 1/4" of its center," says David Tenison, Jay-Em's operation manager. "The probe touches several features on the part, providing data for the Fanuc 11M control to calculate true position and tool offsets." The process usually takes less than a minute, including a 6 second magazine-to-spindle changeover.

Mr Tenison estimates the company saves one-half hour per setup over manual procedures. "It also cut our reject rate to zero," he adds.

Accuracy of bolt-hole patterns has improved dramatically with the probe. Jay-Em typically machines eight to ten holes to a maximum depth of 2" in each forged aluminum wheel half. Parts range in size up to 20" in diameter.

"Between the probe and a switch to carbide tooling, we've seen great improvement in both the positioning of the bolt-hole patterns and a tighter tolerance of the holes," says Mr Tenison. "Our drawings generally call for geometric tolerance within 0.008" true position of a register diameter. With the probe, we usually place the bolt-hole pattern within 0.002" to 0.003" of true position--a 60% improvement," he says.

"Given the accuracy of the process, we've developed a quality procedure accepted by the government that certifies an NC program and process so that final inspection of the bolt-hole patterns has been eliminated," said Mr Tenison. "This shaved five to six minutes off inspecting first parts in the CMM room."

Still in the early stages of implementing statistical process control (SPC), Jay-Em is achieving impressive Cpk ratings. Explains Mr Tenison: "A Cpk of 1 means that total machine variance is right at the limit of the requirement. With the probe and carbide tooling, we're getting Cpks on our holes as high as 12, indicating we're machining within 1/12 of our allowed tolerance. Therefore, we'll never get a bad part."

Increased versatility

RCM is an Italian contract machine shop employing about 100 people. The shop specializes in the machining of a wide range of complex parts made from aluminum alloys, cast iron, and copper alloys. RCM's customers include automobile companies such as Ferrari and Fiat, and aircraft manufacturers such as Piaggio, and others.

RCM has a range of traditional machines and four completely automated machining cells to meet customer requirements on very short notice and with great flexibility. Four machining cells work 24 hours a day (three supervised shifts accounting for 20 hours and one unsupervised shift for the remaining four hours). The cells consist of two horizontal machining centers supplied by 25 pallets which are positioned by automated guided vehicles (AGVs).

RCM's typical machining processes are done in batch sizes ranging from a few parts to as many as 5,000 pieces. Many of the parts have more than 200 different details.

The tool store serves both machine tools and is controlled by a coordinate pick-and-place unit. Within the storage area are two Marposs Mida T60 touch probes with infrared transmission systems. The touch probes are used in a variety of different ways.

In one application, the probe is used to produce an aluminum gear-pump body. Seven body castings are machined using a fixture that holds eight parts. The probe has two functions. First, the probe checks certain dimensions, determining whether the part loaded on the fixture is consistent with the part program in the machine's CNC. Then the probe verifies the integrity of machining from the previous operation.

In another application, the probe is used in production of an aluminum clutch bell housing. One side of the casting is machined on a single fixture to make two different finished parts. The machining fixture is designed to locate two parts on opposite faces. Again, the probe is used two ways.

First, the probe has to determine that machining in the Z-axis is within 0.0006" (0.015 mm) on length measurements. A sensing cycle is performed on two reference faces that were machined previously on other machines. In this way, the theoretical position of such a reference to the machine spindle is obtained and can be referred to when other Z-axis machining is performed. This enables correction of the inaccuracies of the part/pallet positioning in the machine and inaccuracies from thermal drift.

Second, two bores are machined on opposing faces, the center distances of which have to be kept within the 0.0006" (0.015 mm) tolerance. This is done by detecting the center of the half-machined bore and comparing it with the center of the other unmachined half bore. If the two centers differ, the axes' origins are moved to compensate for inaccuracies in the clamping mechanism and table rotation.

In yet another application, a probe is used in the production of motorcycle gearbox cases on a standalone machining center. The fixture holds two castings mounted at 180 deg to each other.

The probe is used to determine the origins of the machining axes and to align the 360,000 angular position rotary table at a resolution of 0.0001 deg with the machine's X-axis. This check is done by probing two dowel pins located on the fixture. By measuring the pins' diameters, the centers can be determined and compared with the measurements stored in the CNC's memory. The rotary table can then be rotated accordingly.

Probes that control

Mec-Track is a medium-sized contract machine shop that uses two Monforts lathes each with two Marposs Mida touch probes.

A T36 probe with high-frequency transmission performs a regular inspection cycle on every fourth piece to check the inside diameter of the part. The diameter is measured by two touches, both at the top, after rotating the part through 180 deg. The machine's CNC then analyzes the results, compares them to tolerance limits, and automatically makes the necessary correction.

A Marposs Mida type A90 probe is used for automatic zero setting of the tool after replacement for wear or breakage. This use of the touch probes allows for flexible machining of a family of rollers. The benefits include automatic tool setting, measuring the part while it is in the machine, and increased productivity.

For more information on probes from Marposs, circle 290.

For more information on probes from Renishaw, circle 291.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:touch probes for automatic pre-, in- and post-process measurement and inspection of machine tools and parts
Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Oct 1, 1992
Previous Article:Optics improve CMM.
Next Article:Making sense of defense.

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