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Drilling with PDC.

Gundrills and twist drills tipped with PDC (polycrystalline diamond compact) are now cost-effective for cutting highly abrasive nonferrous materials such as silicon-aluminum alloys, filled rubbers, fiber-reinforced plastics (including carbon-reinforced plastics (including carbon-phenolic and other aerospace composites), and glass-fiber-reinforced plastics. Edge life of the tooling is conservatively 100 times longer than that of HSS drills, 10 to 15 times longer than carbide cutters. And, because of minimal edge wear, holes consistently show improved straightness, roundness, and surface finish.

The PDC tools tested are made by Crafts using Compax industrial-diamond tool Blanks manufactured by Specialty Materials Business Dept, General Electric Co, Worthington, OH. The blanks have a layer of polycrystalline diamond integrally bonded to a cemented-tungsten-carbide substrate by a high-pressure, high-temperature process. The substrate supports the diamond cutting edge. Tool blanks are available in a range of sizes and in various shapes including rounds, half-rounds, and rectangles.

In the manufacture of a PDC gundrill or twist drill, a PDC blank is precut to the approximate shape required for the tool. The rake face is polished to a mirror finish for smooth chip flow and a superior cutting edge. Then the tip is brazed into a pocket machined in a standard gundrill or twist-drill body. After brazing, the cutting edge is ground and lapped to produce final geometry and finish.

PDC tool geometries are generally the same as conventional tools, but it's possible that modified geometries can produce even better results. PDC tools are so new, however, that other geometries haven't been fully proved.

Starting conditions for drilling with PDC are the same as those used for drilling with carbide tools. The high wear resistance, though, may make it economical to operate at higher feeds and speeds without unduly shortening tool life. Optimum drilling conditions for a specific operation can be determined only by tests run at different feeds and speeds, keeping a record of tool life.

Use of cutting fluids is recommended. The same fluids used when drilling with HSS or carbide tools provide adequate cooling and lubrication in most cases.

PDC drills can be resharpened several times, thus extending tool life. Both the manufacture of the drill and subsequent resharpening require specialized equipment and know-how, and should be performed by experienced diamond-cutting-tool producers.

Production gundrilling

One of the most experienced users of PDC gundrills is the Industrial Measurement Group, Hersey Products Inc, Spartanburg, SC. The firm designs and manufactures meters and gages for precise measurement of liquids. Product components are made from a variety of highly abrasive materials, e.g., reinforced plastics, filled rubbers, hard bronzes, and other nonferrous metals.

The company converted several carbide gundrilling operations to PDC gundrilling over two years ago. The PDC tools are identical to the carbide drills previously used except for the tips. All operations are perfromed on an Eldorado gundrilling machine that has the necessary rigidity and accuracy needed for good cutting performance.

One operation consists of drilling a 0.184"-dia, 1"-long through-hole in the spherical hub of a rotor disc. The material is a hard, abrasive, glass-reinforced phenolic plastic.

Hersey industrial engineer Carroll R Shields, with strong encouragement by Manufacturing Engineering Manager J C Kolk, decided to give PDC gundrills a try. "We had no experience with PDC," says Shields, "but the rapid wear of carbide tools made it essential to find an alternative tool material.

"Carbide cutting edges dulled so fast that gundrills had to be replaced and resharpened after making about 100 holes. This meant tools were changed every hour. Tool changing took at least 10 min, reducing hourly machine output by 16 or 17 pieces.

"When we changed to PDC," he continues, "edge life skyrocketed to 1150 holes. Now we can complete a lot run before changing drills. And we're getting higher-quality holes with finishes that look almost ground."

Hersey also has switched from carbide to PDC gundrills for cutting 0.781"-dia, 1.5"-long holes in hard-rubber bushings, Figure 1. The rubber has a filler that makes it Extremely abrasive. At the same time, it's somewhat gummy, making it difficult to cleanly shear chips as the cutting edge dulls.

At one time, Hersey used HSS twist drills for this job. Edge life was only 15 holes. When the company changed to carbide gundrills, edge life increased to 60 holes--still unacceptably low. Changing to PDC increased edge life to around 1500 holes. Hole concentricity, previously difficult to maintain, is virtually perfect; hole finishes are mirror-like.

Throughout long lot runs, PDC edges shear chips cleanly. And, despite the work material's abrasiveness, PDC cutting edges show no evidence of chipping. Wear, according to Shields, is evidenced by a slight rounding of the cutting edge after hundreds of holes are drilled.

Hersey uses feeds of 0.005 ipr for all gundrilling with PDC--about 20 percent higher than when using carbide. The cutting fluid is the same--a water-soluble oil that has no adverse chemical reaction with plastics and rubbers.

The new drills have diameters corresponding to exact blueprint hole diameters. When carbide gundrills were used, they were ordered oversize to compensate for rapid wear. During the short edge life of these tools, hole diameters ranged from slightly oversize to somewhat undersize, taking full advantage of tolerances. With PDC, all holes are drilled close to exact size.

"Another benefit," reports Shields, "is that the number of drills carried in inventory has been reduced greatly. Moreover, constant in-house regrinding of carbide drills is no longer required."

PDC proliferation

Having proved the benefits of PDC gundrilling, Hersey is tooling up to perform turning, boring, and cutoff operations with the material. Part designs almost invariably require interrupted cuts, which don't appear to affect the life of PDC.

Both old and new machines have been successfully tooled up. Hersey's newest machine, a Model SL4A Mori Seki CNC turning center, originally equipped with carbide, now is outfitted with single-point PDC tools. The machine produces intricate internal and external contours without using form tools. Standard toolholders are used.

When turning and boring parts made of bronze alloys, Figure 2, edge life of PDC tools is typically around 2000 pcs vis-a-vis 50 to 100 pcs for carbide. As with gundrilling, there is closer control of sizes and surface finishes as well.

"The tools help to fully use the inherent productivity of our machines," says Manufacturing-Engineering Manager Kolk. "Fewer tool changes are an obvious benefit. In addition, we can optimize machining conditions for higher output without being plagued by excessive cutting-edge wear. Quality benefits are even more important. We are holding consistently tighter tolerances."

New twist on drilling

Users of PDC twist drills report equally impressive results. Several airframe manufacturers are using such tools to produce precision holes in components made from advanced composites. On one carbon-phenolic, for example, life of carbide twist drills was 50 to 60 holes. With PDC-tipped drills, edge life increased to at least 3000 holes. Added benefits are more consistent hole sizes and smoother finishes.

Automotive engine and transmission manufacturers are already planning to replace conventional drills with PDC tools, particularly for producing holes in valve and pump bodies. Most automakers already use PDC for turning, boring, reaming, and other operations on high-silicon aluminum alloy parts.

Conservatively, PDC tool life is 20 times that of carbide. On the basis of early tests, it appears PDC twist drills will confer the same benefits as other PDC tools when drilling high-silicon aluminums.

Work also is being done to establish optimum drilling conditions. The results will help develop advanced drilling machines for use in manufacturing silicon-aluminum alloy parts for the next generation of engines and transmissions.

Because of their long life, PDC tools are of special interest to users of CNC machines. One machine-tool builder, Hardinge Brothers Inc, Elmira, NY, has tooled up a CNC universal turning center with a twist drill, triangular boring inserts, and a brand-new PDC tool--a ball-nose end mill used for form boring, Figure 3.

The workpiece used for testing tool performance is a 2"-long, 1-1/2"-dia aluminum cup, which is machined from a length of hexagonal bar stock. The cup is machined from the solid in two chuckings. In the first, one end is faced, the sharp corner is radiused, and the OD is profiled using two plunge cuts and a finish cut to check down the diameter.

In the second, the other end of the cup is faced to length, the remainder of the OD is rough and finish profiled with the same tool, and the cup's interior is drilled. The hole is 4-5/64" dia.

Next, the interior is tapered using a boring bar that takes two rough cuts and a finish cut. The bottom of the tapered bore is rounded with the end mill.

Cycle time for all operations is 1 min, 50 sec. Although the rough and finish cuts are made with the same tools, low-microinch finishes are attained. This is possible because PDC maintains high edge sharpness. It takes sharp cutting tools to produce sharp-looking parts.

Future of PDC drills

On the basis of tests, and production experience with PDC gundrills and twist drills, it's apparent that the tools are capable of substantial productivity and quality improvements when drilling highly abrasive nonferrous materials. One reservation voiced by many tool engineers--that PDC drills might be damaged by impacts with drill bushings--is unfounded. When proper clearances are provided, and the cutters and bushings are aligned correctly, the possibility of chipping by bushing contact is no greater than that of a carbide tool being damaged.

A benefit of PDC drills that hasn't been mentioned yet is elimination of subsequent finishing operations. PDC gundrills, for instance, often produce holes to such close size and finish tolerances that reaming is obviated. If located in the right place--directly opposite the cutting edge--a PDC burnishing wear land will impart a smooth surface finish to a freshly drilled hole.

When exceptionally smooth surface finishes are required, drills can be made with fine-grain Compax blanks. Because of the small size of the crystals in the PDC layer, keener cutting edges can be produced than when larger-grained blanks are used. Fine-grain PDC also is more easily brought to a high polish.

Along another line, PDC step drills and reamers are being developed by the auto industry. These tools enable multiple-diameter holes to be cut in one pass. Tool geometries are the same as for conventional drills.

To sum up: PDC drills are being put to work on a variety of equipment--from venerable gundrilling machines to CNC machining centers. Productivity benefits resulting from longer tool life make the tools cost-effective when drilling difficult-to-cut nonferrous materials. And improved control over hole sizes, geometries, and finishes results in substantial quality improvement.

for more information, circle E10.
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Title Annotation:polycrystalline diamond compact
Author:Sauer, Robert H.
Publication:Tooling & Production
Date:Dec 1, 1984
Previous Article:Service centers: strength in steel.
Next Article:Planting ions: seeds for precision-tool life extension.

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