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Modern Machine Shop 73 Deep-Hole Machining Modular fixturing reduced setups for this mold component during machining on a USC-M platform. Unisig's use of this fixturing in its own machine-building process is a lesson taken from moldmaking customers. Mr. Fettig says it works like "stacking Lego blocks" to provide both repeat- able, rigid mounting and single-setup access to six sides of a workpiece. The most important consideration in drilling isn't necessarily the depth or the diameter of the hole: It's more about the ratio between the two. the necessary clearance for chips and coolant. Typically, wear pads on the periphery help keep the tool in place and provide a burnishing action that smooths the workpiece surface. Point angle and location influence cutting thrust and radial force on the tool at a given set of parameters. The most precise hole making is achieved when tool and workpiece rotate together, he adds, and rotating the workpiece by itself typically produces better results than rotating the tool by itself. Solid-carbide gundrills are particularly rigid. These tools offer the advantage of resharpening, but they are effective only at diameters smaller than about 300 mm. After that point, they tend to become expensive and/or and fragile, Mr. Fettig says. For larger diameters, tools are available with steel shanks and brazed carbide tips, with "V" notches rolled into the steel for chip evac- uation. Alternatively, indexable-insert models eliminate the need to discard the entire tool when worn. That said, these models might not be as precise as brazed-tip tools because each insert- tool connection is subject to variance. BTA Drills Whereas a gundrill is designed for internal cool- ant delivery and external chip exhaust, BTA drill- ing routines involve evacuating chips and coolant through the center of the tool itself, in the oppo- site direction of the feed. As a result, holes should be at least a certain size—about ½ inch—before considering this tooling, Mr. Fettig says. As is the case with gundrills, BTA tools are available with