Regrind

Not all regrinds are equal. I have learned the hard way that a bad regrind will cost you a broken drill and a scrapped part. A gun drill costs hundreds of dollars to replace, but a scrapped aerospace part can cost thousands. I inspect every reground drill before it goes into the spindle, and I reject about one in five regrinds from even my best sharpening vendors. Here is my inspection routine. It takes ten minutes and has saved me from countless spindle crashes, broken drills, and rejected parts. ...

About five years into my deep hole drilling career, I developed a habit that seemed paranoid at the time. Every time a reground gun drill came back from the tool grinding service, I measured it before it went into the machine. Tip diameter, shank diameter, overall length, tip geometry, coolant hole clearance — the full treatment. It added about two minutes per tool. I got the idea from a quality engineer who visited our shop for an audit. He watched me grab a reground drill from the crib and load it into the machine without checking it first. “Do you measure your regrinds?” he asked. “No,” I said. “They’ve been doing our regrinds for ten years. They’re always right.” He just nodded and wrote something in his notebook. I don’t know what he wrote, but I started measuring the next week. ...

Why Post-Regrind Inspection Matters I measure every reground gun drill before it goes back into production. Incorrect regrind geometry is one of the most common causes of poor hole quality that I see. The regrind operator might be having a bad day, the grinding wheel might have dulled mid-cycle, or the fixture might have moved. The measurement catches these errors before the tool goes into a part. A gun drill with incorrect geometry cuts poorly from the first revolution. It will produce rough surface finish, oversized bores, or excessive burrs. In some cases, it will break within the first few millimeters of cut. The inspection takes 2-3 minutes and saves hours of troubleshooting time. ...

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. ...

How to Read a Worn Gun Drill: Visual Failure Analysis from the Shop Floor Every gun drill that comes back to the regrind station tells a story. The wear pattern is the drill’s way of telling you what went wrong down in the hole — coolant pressure, chip packing, wrong feed, you name it. I have looked at thousands of pulled drills over the years, and almost every failure mode leaves a signature you can learn to read in about thirty seconds under magnification. ...

The Regrind Disaster That Cost Us a Weekend and a Customer We had been sending our carbide drills to Great Lakes Tool Regrind for almost six years. They were one of those shops that just worked — decent pricing, reasonable turnaround, never gave us trouble. A few hundred dollars a month, routine stuff, nobody thought twice about it. That is probably why when it went bad, it went so bad so quietly. ...