Stating The Case For Spade Drills

Hole making is the most common of all metalworking operations. Virtually every machined part has bores and bolt holes, as well as other round and cylindrical features that must be cut into the workpiece. It's not surprising that hole making accounts for more machining time, and cost, than other operations.

It's also not surprising that reducing the cost of hole making is an important consideration in reducing the total cost of manufacturing. In that regard, spade drills offer manufacturers distinct cost and technical advantages over traditional twist, carbide and indexable insert drills.

Spade drills are two-fluted, end-cutting tools that incorporate a replaceable or indexable type of cutting insert. Internal passages in the toolholder channel coolant to the cutting edge of the insert. The external flutes on the holder provide efficient chip removal.

The Cost/Use Advantage

Spade drills can be used in virtually any application where a traditional drill can be used to cut holes generally ranging from 1/2 to 4 inches in diameter. A series of basic, rigid holders is used with several insert sizes to provide this wide range of diameters. The alternative is to stock drills for every diameter, a costly proposition and one which results in a sizable tool inventory.

The common brazed type of carbide drill can also be successfully replaced by "off the shelf" spade drill holders fitted with standard, replaceable carbide inserts. When the insert edge wears out, it easily can be replaced on the machine, or, with indexable spade drills, the second edge simply is indexed into place. No regrinding is necessary, further reducing cost. Special multidiameter brazed carbide drills can also be replaced economically with spade drill holders fitted with throw-away inserts. With spade drills, because only the insert cuts metal, the toolholder can be made from less expensive steel, providing another economic advantage to using them.

There is also a benefit to replacing indexable insert drills with spade drills. The primary advantage here is the ability of the standard spade drill to accept a wide range of different diameter inserts instead of the "one tool--one diameter" requirement of indexable insert drills.

Running at specified speeds and feed rates, carbide insert drills generally outperform other types of drills when metal removal rates are compared. The high metal removal rate can present a problem, however. High speeds and heavy feed rates consume large amounts of machine tool horsepower. If the horsepower is not available at the spindle to drive the drill, not only is the efficiency of the metal removal rate lost, but because the carbide insert drills cannot evacuate chips from the hole as efficiently at the slower speeds and feed rates, tool breakage also can result. Spade drills effectively solve this problem.

In one application, a manufacturer of machine tools replaced carbide insert drills with spade drills to overcome the tool breakage problem in a high speed, low horsepower hole-making operation. The company discovered that the spade drills could run at approximately 80 percent of the speed of the carbide insert drills, 80 to 130 sfm, and still produce holes within the required rough drilling tolerance of ±0.005 inch with surface finishes of 200 to 250 microinches. The reduction in machine tool downtime more than made up for the small reduction in throughput due to the reduced speed.

Herein lies an important, but subtle difference between traditional and spade drill technology. Beyond the advantages of lower tooling cost, spade drills also offer specific solutions to basic hole-making problems such as coolant delivery, chip removal and tool rigidity that have a direct effect on throughput and repeatability. These are important considerations for manufacturers who are trying to reduce the cost of hole making, because solutions to these problems reduce cycle time, an important, but often overlooked, factor in hole-making operations.

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