When and How to Sharpen Gundrills

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by Ryan Funk (left) & Eric Krueger (right), UNISIG

Gundrills are excellent candidates for re-sharpening

The modern gundrill is an engineering marvel, but like all tools, gundrills wear out, typically after drilling between 500 and 1,000 in. (12,700 to 24,500 mm). When re-sharpening is done correctly, the same gundrill can perform as well as a new drill as many as eight to ten times for all but the most demanding hole tolerances.

Before re-sharpening, worn gundrills offer valuable information manufacturers can use to optimize their gundrilling applications. With experience, it becomes possible to tie a wear condition back to the process. For example, if there’s a visible buildup along the cutting edge, it’s often because the rotational speed is too slow. Conversely, if the edge has worn faster than the tooling supplier’s data suggests, the tool is likely rotating too fast. On the other hand, a chipped cutting edge suggests that feedrate was too high.

The trick is knowing when it’s time to sharpen a gundrill. As they wear, they require more thrust and torque while producing more runout and experiencing greater drift. A dull cutting edge will produce irregular chips, which in turn cause spikes in coolant pressure, signs that failure is imminent. With camera inspection systems, it’s easy to look for the chips, premature wear or built-up edges that indicate it’s time for re-sharpening. Luckily, the re-sharpening process usually takes longer to describe than it does to perform on today’s sharpening systems, which should come equipped with a grinder, the appropriate gundrill fixture and equipment for calibrating and inspecting drill tips. After clamping a gundrill in to these systems, operators can use geometry data from the tooling supplier to calibrate the on-board camera and begin grinding the drill. 

The grinding wheel, turning in the direction toward the drill edge, makes contact with the drill tip after the operator confirms the correct rotational and X and Z axis orientations. A standard starting point grind will begin with the tip angled at +30º horizontally and +15º vertically with the rotation at +5º. The Y axis is used to hold the tip to the grinder while the drill feeds in along the Z axis at a rate of about 0.002 in. per pass.

Some gundrills include an outer secondary angle parallel to the front cutting edge where the primary and secondary angles meet. It’s critical that this primary facet is relatively narrow, as too much width will increase heat production and, consequently, reduce tool life.

The operator next moves to grinding the inner relief facet by moving the fixture -20º horizontally in the opposite direction from the primary angle. This movement results in the formation of a point position with a length that is exactly one quarter of the drill’s diameter, the so-called “D/4” position, but other lengths may be necessary depending on the drill material.

The operator then moves to the front clearance, a facet with a point close to–but not touching–the front cutting edge. With standard gundrill tip geometry, a 0º horizontal angle and rotation as well as a +26º vertical angle will provide the correct position. While cutting performance improves the closer this point gets to the cutting edge, optimal edge strength requires placing the point slightly behind the edge. If a tip’s geometry requires an outer secondary angle, the front clearance facet’s point should meet it. Otherwise, the point of the facet is placed between 0.020 in. (0.508 mm) and 0.030 in. (.762 mm) behind the front cutting edge.

The final step on the grinder provides the oil dub-off, a facet with an edge tangential to the flute of the gun drill. Operators position the grind fixture at -30º horizontally, +25º vertically and +65º rotationally. The gundrill tip then feeds into the grinder at a rate that prevents cutting into the front cutting edge. The optimal angle meets the inner relief angle at the corner opposite the gundrill’s outside diameter. 

After grinding is complete, the operator can use a hand chamfer to create additional clearance for optimal performance. The finished gundrill is now perfectly re-sharpened and ready for use, a process that takes fewer than ten minutes. SMT 

Ryan Funk and Eric Krueger are part of UNISIG‘s engineering team.

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