Making productivity gains in drilling holes may not seem to be the most challenging machining problem, but there are three factors above all others that can affect drilling speed, tolerance and tool life:
- Coolant
- Spindle speed and feed rate
- Rigid toolholding
Coolant keeps the tip of the drill at a relatively low operating temperature and helps to extend tool life. The build up of heat in a hole can cause the tip of a drill to curl, dulling its cutting edge and causing it to plow through a workpiece. That could cause the drill to break in the hole. "Some people think that cooling is the only function of coolant," but it plays other roles, said Gregory S. Antoun, president of ChipBlaster (www.chipblaster.com).
The coolant keeps the tip of the drill at a favorable operating temperature and also keeps the workpiece lubricated. Lubrication helps the cutting tip to keep its edge, extending the drill's sharpness and its life.
The concentration of lubricant in a coolant depends on the job and the tool, Antoun says. "With a single-point turning tool with a relatively small contact area, a 5-percent (concentration of lubricant in a coolant) is probably okay. As you increase the contact area of the drill, the concentrate level must go up to at least 8 percent," Antoun said. Tapping holes after drilling requires the highest concentration of lubricant in the coolant – 12 percent, he notes, adding that lowering the concentration for drilling also could reduce tool life by as much as 90 percent. Coolant also flushes chips from the hole. "If you don't get your chips out of the hole, you'll break the drill," said Frederik-Sundstrom, drilling-product manager for Seco-Carboloy (www.carboloy.com). It is as simple as that. Chips can bind, lock, seize and freeze a cutting tip, causing the tool to snap.
European machinists use techniques that combine minimal amounts of coolant with shop air to reduce the large volumes of coolant typically needed for drilling, Herman notes. ChipBlaster, Seco-Carboloy and Komet of America Inc. (www.komet.com) all recommend that coolant feed pressures be kept high, and Herman and Sundstrom say very high pressures only add to coolant's effectiveness. Also, all three companies recommend that machinists use through-thetool coolant technology for holemaking, but that is not always possible.
Through-the-tool coolant technology has a limit. As holes approach sizes of 0.1969 in. (5 mm) and smaller, coolant cannot always be supplied through the bit, and external coolant techniques must be used. While some square-type and g-type drills provide through-the-tool coolant technology for drills smaller than 0.1969 in., pecking techniques usually are required to drill holes that small.
In pecking at a hole, the machinist advances the drill to one or two times its diameter, clears the hole and flushes it with external coolant to remove chips. While pecking may be time consuming, it is better than the alternative, says Mike Herman, director of distributor sales for Komet of America. "If you break a drill that small in a hole, you might need a wire EDM to get the drill out, or you might be facing scrapping a workpiece worth thousands of dollars," he said.
Finding the correct spindle speed and feed rate for the drill is the second critical item for increasing productivity in drilling, Seco-Carboloy's Sundstrom says.
Tool suppliers provide detailed lists of cutting tool data in their product catalogs, and those specifications have to be followed exactly. Producer specifications also help machinists to determine which tool — an insert drill, a carbide or high-speed steel drill, a tip drill, a slow spiral (12-degree helix) drill, or the more standard 35-degree helix drill — is the right tool for the job. For example, slow spiral drills often are recommended for materials that are extremely hard or that could become "gummy" when they are machined, such as titanium.
Finally, rigid tool holding is vital for making gains in productivity in drilling. A poorly held workpiece can lead to poor hole surfaces, chatter, and a host of problems and a scrapped part, while reducing the working life of the drill.
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