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Designing hole-making tools is a game of compromises, says Goran Melin, drilling product manager at Seco Carboloy (Warren, Michigan). "Where you gain in one area, you must pay somewhere else," he says. The trade-offs usually include tool life, accuracy and stability. Tool designers are constantly juggling these variables to find the best solution.
Seeing how these variables are managed in the case of the company's new PerfoMax line of indexable drills reveals the kind of insights required for successful hole-making operations. These new drills have been engineered to make indexable drills a viable choice in a wider range of applications.
Here's the issue: Of the tooling choices that have been available for drilling holes, indexable insert drills are usually the low-cost solution, according to Mr. Melin. Indexable insert drills provide up to four edges per insert, and the drill bodies can typically be used for 40 to 50 insert changes, depending upon the part material. Thus, the cost per cutting edge can be considerably lower with indexable insert drills compared to brazed or solid carbide drills, or drills with replaceable carbide crowns.
Normally, however, you need a wider tolerance when drilling with indexable drills, Mr. Melin says, because the tool effectively has an asymmetrical cutting edge. While indexable drills use two inserts, only one insert covers from the center, and the other covers from the inboard insert out to the full diameter. Uneven cutting forces occur because there is only feed force and virtually no cutting speed on the center insert, while the peripheral or outer insert removes material more efficiently because of the higher speed. The result is that deflection of the drill body occurs as the tool enters the cut.
"Before you are into the full cut, you have to be aware that you have an unstable situation," Mr. Melin says. "The longer the drill, the more deflection you will have." So for longer drills, users have been instructed to reduce the feed to 30 to 50 percent of recommended feed during the first millimeter of depth, and then increase to the normal feed rate. This suggestion is unpopular with many users because they would prefer to use the standard drilling values given in programming software and not have to manually re-program for an entrance feed. Also, in cases where a long drill may be required for only a few operations, the operator may prefer to use the same drill for general use in place of a shorter drill. Doing so, however, would degrade accuracy and productivity.
By comparison, twist drills and solid carbide drills have two cutting edges to equally divide the chips that are produced during each revolution, promoting easier chip evacuation and permitting higher drilling feed rates. This fact contrasts with indexable drills, which effectively have only one cutting edge covering the entire revolution—the cutting edge of the outer insert that removes most of the material. This tends to restrict chip evacuation and limits drilling feed rates. However, indexable insert drills do not create as much friction because they lack the land margin that is inherent with twist drills. Thus, indexable drills can run at higher rotational speeds (rpm). Unfortunately, drill deflection becomes a concern when users also seek to maximize feed rates, especially in drill lengths of four times the drill diameter and longer. For these reasons, indexable drills have mostly been used for pre-finishing holes, or for finishing holes where there is at least a 0.008-inch (0.2-mm) tolerance. With indexable drills, you can get the economy you want, but not always the accuracy you need.
To move beyond these restrictions, Seco Carboloy engineers did five things when they designed the new line of indexable drills.
- They left more material at the periphery of drill body to resist bending under cutting forces.
- They incorporated an insert pocket that is shaped to hold square inserts, because 90-degree corners are inherently strong.
- They created two coolant holes and large chip flutes with a flute angle and relieved front to get coolant and chips out more effectively (the idea is to reduce heat buildup and promote longer tool life and consistent surface finishes).
- They added a wear-resistant coating that helps to prevent chips from adhering to the drill body and shortening its life.
- They had a universal cutting grade developed to combine a hard, fine-grained substrate with a coating that permits higher cutting speeds.
Mr. Melin says that the new drills will enable users to significantly increase the productivity of many drilling operations, including those with long overhang, workpieces with poor fixturing and thin-walled parts. By improving drill stability and chip evacuation, more accurate holes are possible; thus the cost savings associated with indexable drills is attainable in applications where they were once ruled out by tolerance requirements.
The drills cover a range from 0.594 inch to 2.375 inch in diameter and have drilling depth capabilities from two to five times the drill diameter. The drills are available with a choice of several shank styles, including a universal shank with four flats that can be applied to many lathe applications.
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