Drill-Geometry

A gun drill specification sheet can look intimidating the first time you see one. There are diameters, angles, lengths, and tolerances — and each number affects how the drill performs. I have ordered hundreds of gun drills. Here is how I read a spec sheet. Tip Diameter: The Number That Determines Hole Size The tip diameter is the most important number on the sheet. It determines the finished hole size. But the tip diameter is not exactly the hole size — there is an oversize allowance built in. ...

Problem Description A bad regrind ruins a gun drill faster than any cutting parameter mistake. I’ve sent drills out for resharpening and got back tools that couldn’t hold size for more than 10 parts. The tip geometry was wrong — the clearance angle was 2 degrees off and the drill rubbed instead of cutting. Gun drills have complex geometry: tip angle, outer clearance angle, inner clearance angle, chip breaker height, and coolant hole positioning. A regrind service that treats them like standard twist drills will destroy the tool geometry in one pass. ...

Regrinding a gun drill requires precision. The angles on the tip determine how the drill cuts, how chips form, and how the tool wears. Getting them wrong means poor hole quality and short tool life. Here is the procedure I use. When to Regrind I regrind a gun drill when any of these conditions are met: Surface finish degrades beyond spec Cutting forces increase noticeably (spindle load rises 20%+) Flank wear exceeds 0.2mm on the primary clearance Corner wear exceeds 0.1mm Chips change from short broken segments to long stringy ribbons For production work, I track holes per regrind and schedule regrinds at a fixed interval. For most steel applications, this is 150-250 holes per regrind depending on diameter and material. ...