Gun-Drill

I used to think chip breaking was entirely about feed rate. If the chips were too long, I increased the feed until they broke. That works up to a point, but the drill’s chip breaker geometry sets the baseline for what feed rate will work. Understanding chip breaker geometry helps you choose the right drill for the material and know whether a regrind is maintaining the correct geometry. What a Chip Breaker Does A chip breaker is a groove or step ground into the rake face of the gun drill tip, just behind the cutting edge. As the chip slides across the rake face, it hits the chip breaker and curls. The curl creates bending stress in the chip, and when that stress exceeds the material’s fracture limit, the chip breaks. ...

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. ...

Cross-training operators to run both gun drilling and BTA machines increases shop flexibility. When one operator is out, another can step in without losing production time. In my shop, we run both processes daily, and before I implemented the cross-training program, we had three dedicated gun drill operators and two dedicated BTA operators who could not cover each other. One sick call meant a machine sat idle. Training Timeline and Structure The training takes about 2-3 weeks for an operator who already knows one machine type. The trainee learns the differences in tooling, parameters, and troubleshooting between the two processes. I break the training into three phases: observation, supervised operation, and independent operation. ...

The guide bushing supports the gun drill at the entry point of the workpiece. It prevents the drill from whipping or deflecting during the first few millimeters of engagement, which are the most critical for hole straightness. I have spent years selecting and maintaining guide bushings for different applications, and the right bushing choice is one of the most important tooling decisions in deep hole drilling. Guide Bushing Material Selection The most common bushing material is hardened tool steel, typically A2 or D2 tool steel hardened to 58 to 62 HRC. It provides good wear resistance at moderate cost. For high production or abrasive materials, I use carbide bushings. For soft materials where marking is a concern, I use bronze bushings. ...

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. ...

I have a process I use for every new gun drilling job. It starts with the bore print and works through diameter, shank, length, tip geometry, and coating in that order. Skip any of these steps and you risk a tool that wanders, breaks, or wears out before it reaches depth. Here is my full sizing and selection process, along with the reference tables I keep pinned above my bench. ...

Tool change time on a deep hole drilling machine gets overlooked because the drilling cycle itself is long. A typical BTA drilling cycle might run five to fifteen minutes, so a three-minute tool change seems insignificant. But add up thirty tool changes per shift across three shifts and suddenly you are losing 4.5 hours of spindle time per day. Quick-change tooling addresses exactly this hidden productivity drain. The Cost of Conventional Tool Changes On a standard machine, changing a gun drill or BTA drill head involves stopping the spindle, retracting the tool past the guide bushing, loosening set screws or a drawbar mechanism, pulling the tool, inserting the new one, tightening, resetting coolant pressure, and restarting. I have timed this at 4.5 to 8 minutes for a competent operator on a typical machine. ...

Spindle torque limits protect the drill from overload. When the torque exceeds the limit, the spindle stops. This prevents the drill from twisting apart under excessive load. I have seen gun drills snap at torque loads just 200 percent above the normal cutting level. A torque limit catches the overload long before that point. Here is how I set up and manage torque limits on my machines for deep hole drilling operations. ...

Tool load monitoring tracks the spindle load during drilling and alerts the operator when the load exceeds a threshold. It catches problems before the drill breaks. In my experience, load monitoring saves more tools than any other preventive measure I have implemented in my shop. I set up the monitoring on the CNC control. The control reads the spindle drive current and compares it to the baseline measured during the first cycle with a fresh tool. I set the alarm threshold at 120% of baseline and the stop threshold at 150%. These numbers are starting points — I adjust them for each material and application. ...

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. ...