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Cermet solves geometry problem.

Cermet solves geometry problem

Sometimes engineering specifications make the manufacturing engineer's job seemingly impossible. A case in point involves the machining of valve components at OK Tool Corp, Plainfield, NJ.

The valve shown in the drawing presents a trio of problems: (1) the workpiece contains four critical corners, one internal and three external, which must be generated with 0.005" radii; (2) four-digit tolerances must be held on three diameters; and (3) concentricity must be maintained within [+ or -] 0.0004" total. Machining is performed on an Okuma LC40 CNC turning machine.

OK Tool solved all three problems by switching from molded coated-carbide inserts to standard off-the-shelf cermet inserts from NTK Cutting Tools, Farmington Hills, MI. The new inserts, although standard, have precision-ground chipbreakers and 0.004" nose radii.

The workpiece is 1018 steel (BHN 160). This is a free-cutting material that presents no machining problems. However, to generate the critical 0.005" internal and external radii, OK Tool had to grind the nose radius on each insert.

During finish machining with these modified inserts, it was impossible to generate the radii consistently. Worse yet, the four-digit-diameter dimensions could not be held, and the finished workpiece failed to meet the specified [+ or -] 0.0004" tolerance for concentricity.

The initial solution to these problems involved sending the rough-machined parts out for finish grinding, but this usually took two weeks, and loss of in-house control was a constant concern.

In an effort to get the operation back in-house, manufacturing engineers first attacked the radii-generation problem. On NTK's recommendation, they substituted Grade XT3 cermet material for the coated carbide undergoing nose-radii grinding. They selected three geometries: a CPGH21.504AM5 positive-rake 80-degree diamond shape for the internal radius; a CNGG-4304R-N1 negative-rake 80-degree diamond for one critical radius at the end of the workpiece; and a VNGG-3304R-N1 35-degree diamond for the other two 0.005" radii on the OD.

The new inserts were chosen for two reasons. First, they are available from NTK as "off-the-shelf" standards with precision-ground chipbreakers. Second, and most important, the 0.004" precision-ground nose radii are also standard and available off-the-shelf.

The two negative-rake cermet inserts have 15 degrees of positive-rake chip control. The 0.004" nose consistently generates the critical radii, so that problem was solved.

Better parts at lower cost

After initial finish machining with the new inserts, engineers were impressed with the chip control and improved surface finish achieved. After a number of successful runs with consistent results, they decided to try cermet for machining the entire workpiece.

Finish machining with the cermet inserts has solved all problems, according to OK Tool. The 0.005" radii are generated accurately, and the four-digit dimensions are held consistently. Finally, concentricity is maintained within [+ or -] 0.0004". As a result, the firm no longer sends parts out for finish grinding. Overall machining time has been reduced, and total manufacturing costs have been lowered.

For finish boring and turning, machining speed is 800 sfm; feed rate ranges from 0.001 to 0.002 ipr; and depth of cut is 0.002".

For more information on inserts, contact NTK Cutting Tools, 27260 Haggerty Rd, Farmington Hills, MI 1002 48330 or circle 417.

PHOTO : CNGG 80-degree diamond (left) and VNGG 35-degree diamond (right) are cermet Grade XT3 negative-rake inserts. They feature standard precision-ground chip control with 15 degrees of positive rake and 0.004" nose radius.
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Title Annotation:machining of valves
Publication:Tooling & Production
Date:Jan 1, 1990
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