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Probing for high-precision machining.

Two Precisely bored and contour milled holes, produced at 0 deg and 180 deg of the indexer, must look at each other within less than 0.0005".

For machining precision components, probing can yield faster setup on almost any part and, when combined with automatic tool setting and self-regulating machine tools, will yield higher precision and efficiency.

That's the experience at Toledo Scale's Spartanburg, SC, plant where the combination is increasing accuracy and output of milled and bored "counterforces," which are critical mechanical parts used on its electronic weighing systems. The basic counterforce can be described as a beam, having an extremely precise cross section to which electronic strain gages are attached.

Major differences in weighing capacity and the environment in which the counterforce must work result in hundreds of different configurations. To avoid corrosion and permanent set from overloads, the parts are usually made from hard-to-machine materials such as stainless steel and pre-hardened steel alloys.

The demand for ever-increasing accuracy in these precision parts led Jeff Pickens, manufacturing manager, Tom Johnson, senior manufacturing engineer, and Steve Jennings, manufacturing engineering supervisor, to examine new approaches to the most common problems in machining the counterforces. The problems included:

* machine positioning accuracy and alignment,

* tool wear and variation in tool size with change of inserts,

* temperature drift of the part and of the machine,

* registration errors in the fixture,

* distortions during clamping,

* variations in material hardness,

* human errors in tool setting,

* in-process inspection errors which lead to incorrect adjustments.

Though each error might be small in itself, their combined influence can cause excessive scrap, rework, and reduced output. The solution decided upon was combining all machining operations on the counterforce in a single setup on

a Monarch VMC-45B with a fourth-axis rotary table.

Monarch Cortland, Cortland, NY, was charged with the responsibility to design and build fixturing and provide all processing and software development as part of a complete turnkey package. While the machine and fixturing were being built, an in-depth analysis of advances in perishable tooling was made to benefit from any major improvements in cutting tools in terms of cutter life and replacement accuracy.

A major departure from conventional practice was the decision to use probing for automating the setup process and as the method for in-process inspection.

A three-pin driving fixture was proposed that would prevent distortion of the workpiece due to clamping. Programming and processing were to be arranged so that all setup activities including fixtures registration, tool length offsets, and location of part centerline would be done automatically by a Renishaw spindle probe and a Monarch Tool Length Indicator (TLI) to eliminate human error.

In-process inspection by the probe would monitor tool wear and automatically enter revised offsets, if required.

The effectiveness of probes as setup and inspection tools is frequently debated; detractors describe probes as delicate, costly, and time-consuming to use. When combined with an accurate CNC machine and driven by proper software, however, probes are capable of results that cannot be achieved otherwise. Here's how the probe is used in a setup procedure:

As with any measuring instrument, the probe must be calibrated regularly. X and Y calibration is done by measuring a master bore diameter of known size. Numerical adjustments, known as ball center offsets, are included in the CNC subroutines which control the probe and are automatically modified during calibration to account for the diameter of the probe tip, any runout of the probe tip relative to the spindle centerline, and any non-linearity of the probe itself.

Z calibration of the probe establishes the length of the probe relative to the tool length indicator by touching the probe and a master tool to the same reference Z surface. The master tool is then measured by the tool length indicator that defines the Z dimension between the probe and the tool length indicator. All of the measurements established during calibration are stored in the CNC and can be called up as required for inclusion in various setup or measurement subroutines.

Setup of a typical counterforce begins with tool measurement. A pre-programmed cycle automatically loads the tool in the spindle which traverses in Z to the TLI.

Each tool length is automatically measured and stored in the CNC in about 15 seconds. The 14 tools required are set in less than four minutes. The spindle-mounted probe requires five touches of the part blank to locate the part in X, Y, and Z, as well as finding the center of rotation of the indexer in Y and Z.

Probe cycles for finding centerlines are similar to manual procedures using dial indicators. Figure A illustrates the method of finding the indexer centerline.

Following completion of the above steps, all information necessary to commit the tools to cut has been stored in the CNC as follows:

1) The length of all tooling is known relative to the probe.

2) The probe location in Z relative to the workpiece surface is known.

3) The distance from the workpiece surface to the centerline of rotation of the part indexer is known, thus defining the Z location of the centerline.

4) The part program defines the Z motions of the tool in relation to the centerline of rotation.

5) The centerline of rotation is known in Y.

Tool length offsets for Z and workpiece offsets for X and Y are automatically set in the CNC and cutting begins. The dimensional outcome of the part will be dependent upon automatic measurements and independent of operator intervention. Often some "attitude adjustment" is required by new operators to avoid panic stops as they gain confidence in the hands-off procedures of automatic setup.

Figure B shows a cross section of a typical counterforce. The four "A" dimensions must be held to the same size within + or - 0.0005". The surfaces associated with dimension B must be flat within 0.0005" and dimension B must be the required dimension within + or - O.001". Finishes required are 32 microinches.

Stated in a different way, two precisely bored and contour milled holes, produced at 0 deg and 180 deg of the indexer, must "look at each other" within less than 0.0005". The end mill, which finishes surface B, must hold 0.0005" tolerance in Z.

After the sides of the part have been milled to finish size, the probe is reinserted in the spindle. The sides are measured for correct width and centrality with the indexer prior to beginning the opposing bores.

Four probe touches are required for this in-process inspection. If required, offsets in Y are automatically fine-tuned and the part is completed. Following completion of the part, machine performance is confirmed on a coordinate measuring machine.

Runoffs during the recent machine installation training at Toledo Scale resulted in two new operators running off five different parts in lot size of ten. Fifty good parts were produced using full automatic setup and in-process gaging.

For information, circle 192.


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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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Publication:Tooling & Production
Date:Jun 1, 1992
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