Counter rotation maximizes deep hole concentricity
Drill and workpiece rotation has a major influence on a hole's concentricity – a key measure of drilling accuracy
By UNISIG
In a routine drilling operation on a milling machine or drill press, a drill’s cutting edges rotate against a stationary workpiece. The opposite is true in holemaking on a turning machine where a stationary drill advances into a rotating workpiece. Either of these drilling methods produce sufficient reliability and hole quality for a wide range of applications. However, other tactics are necessary to produce more exacting tolerances and larger depth-to-diameter ratios.
Drill and workpiece rotation has a major influence on a hole’s concentricity – a key measure of drilling accuracy. When the drill alone rotates in a horizontal setup common for deep hole drilling, accuracy will vary as gravity acts on the drilling tool. A rotating drill can produce sufficient concentricity in relatively shallow holes, but performance will suffer as holes become deeper and less forgiving tolerance wise.
On the other hand, because the direction of gravitational forces relative to the workpiece constantly changes when the drill is stationary and the workpiece rotates, that arrangement can produce holes approximately twice as concentric as the rotating drill approach. While shops can perform rotating-workpiece deep hole drilling on a turning machine, a dedicated deep hole drilling machine using what’s known as counter-rotation will net much better results.
Benefits of counter-rotation
A drilling setup that involves both the drill and workpiece rotating in opposite directions will balance out drilling forces, which are never static in a constant net direction. The balanced forces keep the drill from drifting for a much more concentric hole. With the right equipment and setup, counter-rotation is possible for smaller gundrilled holes as well as larger holes drilled with BTA tooling.
In counter-rotation testing, UNISIG drilled a ¼”-dia. hole in a 30″-long, ¾”-dia. OD, 4140HT steel workpiece. This 120:1 depth-to-diameter application is one typically found in the production of power transmission shafts or aerospace linkages.
Drill drift at the 30″ hole depth was measured via ultrasound. With a rotating drill and stationary workpiece, drill drift was 0.026″; a stationary drill and rotating workpiece exhibited 0.015″ drift; and when both the drill and the workpiece were rotating, drill drift was only 0.009″. It should be noted that results will vary due to many factors, including material, depth-to-diameter ratio and the specific tooling involved.
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