Carbide Drill Bits - High-speed Drill Bits

Increased productivity in drilling requires that drill bits function at a higher speed, a faster penetration rate or both.

Defining high-speed drilling can be somewhat arbitrary. Some drill bit manufacturers refer to high-speed drilling as drilling at spindle speeds fast enough for penetration rates that are three to ten times greater than those considered conventional. Other drill bit makers simply define high-speed as drilling faster than usual.

Factors that affect drill bits at slower speeds become more pronounced and more critical as drilling speed or feedrates go up. Major considerations are heat, chip removal as the tool feeds deeper into the hole, and runout.

As drilling feeds and speeds increase, the problem of providing sufficient coolant to remove the chips becomes more critical. The volume of chips produced, if not removed, can cause chip jams, recutting and high heat that shortens tool life.

When reducing cycle times by 90 percent, there is simply not much time to remove chips and a high-pressure coolant flow is required. Many older machines do not have sufficient coolant to remove the chips, but in some cases, they can be retrofitted to increase coolant flow.

At lower speeds, high-speed steel drills often are effective, and provide a tool with relatively high bending strength and toughness. At higher speeds, however, carbide or even ceramic tools are required. These tool materials trade some of the toughness of high-speed steel for greater wear and heat resistance.

Drill and toolholder balance also becomes critical at drill speeds that exceed 10,000 rpm. Shrink-to-fit and hydraulic toolholders usually are picked so that the drill and toolholder remain within acceptable out-of-balance limits.

Older machine tools often can not provide the concentricity necessary to take full advantage of the precision drills that are available today.

Total runout should be held to at least 0.001 in. In lathe drilling, with both the turret and chuck contributing to hole concentricity, runout at the tool should be no greater than 0.0005 in.

Toolholders provide the link between the machine tool spindle and the cutting tool, and they must have the same rigidity and concentricity qualities as the spindle. For example, consider a coolant-fed, indexable drill with a 4-in. projection in a toolholding system that allows a maximum of 0.0002 in. within one inch of the toolholder face. The tip or cutting edges could runout more than 0.001 in. Toolholders exist that can true the tip or cutting edge to within 0.0001 in. Such a toolholder can improve tool life significantly and can increase workpiece accuracy at high drilling speeds.

For high-speed drilling, toolmakers design products that prevent chips from coming into contact with the tool's cutting edges and flutes.

Hot chips can soften and smear against the tool, filling in microscopic crevices in its edge surface. This built-up edge becomes the new cutting surface and may force the tool off-center, leading to a failure. High-pressure coolant, delivered through the tool, can effectively flush hot chips away from the tool and out of the hole.

Choosing the correct coolant for high speed drilling is critical, and the choice of coolant must match the material being drilled. For example, when drilling steel, the cooling effect is paramount, and when cutting aluminum, lubricity is key.

Most common high-speed drills are made from a high-temperature grade of solid tungsten carbide. Where chatter may be a problem, a good choice is a tool made from a finer grade of carbide. Cutting-tool manufacturers offer carbide grades with grain sizes of 0.0000197 in. (0.5 microns) or smaller, as compared to a more standard 0.0000984 in. (2.5-micron) grade. Carbide tools with smaller grains resist wear with less of a sacrifice in toughness.

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