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.

Cooling The Cutting Edge

One of the most important concerns in any metalcutting operation is coolant delivery, and it is critical to economical hole making. Today, machining centers are capable of producing spindle speeds of 8,000 to 10,000 rpm for drilling operations, and in some cases up to 15,000 rpm can be used depending upon the material being drilled. Carbide inserts and coatings such as titanium carbonitride (TiCN) make it possible to use spade drills in high speed drilling applications. Higher spindle speeds are the key to reducing hole-making cycle times because they allow a significant increase in feed rates, up to a tenfold increase in some cases, which, in turn, increases throughput and reduces cost-per-hole.

Higher speeds and feed rates, however, generate high temperatures, which can significantly shorten the life of tooling, and higher feed rates create more chips, which must be evacuated from the hole. In order for a hole-making operation to benefit from higher speeds and feed rates, coolant must be delivered effectively to the cutting edge of the drill.

Every hole has a depth and a diameter, and the ratio of depth to diameter is particularly important when considering coolant delivery. If coolant is being flooded onto the workpiece, the depth of any drilled hole is limited to one diameter. Even at that depth, sufficient coolant may not reach the cutting edge of the drill because chips may block it. The heat generated by the drill in the workpiece often boils off what coolant eventually does reach the cutting edge.

Spade drills eliminate this problem because they are coolant fed; the coolant is introduced to the cutting edge of the drill through the body of the tool itself. Typically, there is a shaft down the center of the holder with veins branching from it. Coolant is sprayed through these veins directly to the bottom and sides of the hole where it is needed most and will do the most good.

The coolant-fed design assures that coolant gets to the cutting edge of the drill all the time, regardless of the depth of the hole. With a coolant-fed spade drill, depths of ten diameters are common, and depths of 40 diameters are possible with pilot holes and sufficient coolant pressure and volume to disperse chips adequately.

Recently, there has been considerable attention focused on high pressure coolant-delivery systems. These systems, some of which are designed to maintain pressures as high as 1,200 psi at the drill tip, help reduce drilling cycle times because the high pressure breaks chips and forces them out of the hole without the need for a pecking cycle.

Because spade drills, by their design, are efficient at moving chips away from the cutting edge of the tool, most spade drilling operations can be performed using coolant pressures of less than 100 psi. However, they also perform well in high pressure coolant applications. The stronger shank and the insert locking mechanism help prevent the tip from being driven off center by the higher coolant pressure.

Keeping Chips Moving

As chips build up in a hole, particularly in deep holes, the cut can be driven off center and out of round, roughing up the surface finish, and maybe even breaking the tool. Most conventional drilling operations require a peck cycle to remove chips between drilling cycles.

The secret to moving chips up and out of a hole is heat. Properly used spade drills effectively put the heat generated during the drilling process into the chip. Heat generated at the cutting edge of the drill exceeds the maximum range of the material that is being drilled. High temperature, with the coolant acting as a quenchant, hardens chips so that they crack and fracture easily. This embrittlement process breaks the chips into smaller pieces, making them more manageable and more easily removed from the hole.

The result is the production of smaller chips, which can be evacuated more efficiently from the hole. This chip removal action makes modern spade drills well-suited for drilling large diameter holes in one pass.

For example, one spade-drill user replaced standard 1-inch, high speed twist drills with spade drills in a hole-making operation on a large aluminum casting. The shop was able to drill the holes in one pass, eliminating the peck cycle, at twice the feed rate of the twist drills.

Tool Rigidity Means Higher Throughput

Efficient removal of smaller chips also means that the spade drill holder can be more rigid, because a massive chip gully isn't needed. Tool rigidity is a critical issue for improved drilling performance. Spade drills offer rigidity similar to other cutting tools, but more than traditional drills. Consequently, they can handle higher speeds and feed rates, and, they also can easily absorb the ill effects of misapplication.

Three-Way Winner

Spade drills offer manufacturers significant cost benefits. The design of spade drills helps solve the three key hole-making problems manufacturers face when trying to improve productivity through reduced cycle time. By overcoming difficulties in coolant delivery, chip removal and rigidity, spade drills can significantly reduce the cost of hole making.