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.
Critical Measurements
Tip Angle
The tip angle is the most critical dimension on a gun drill. I measure it with an optical comparator or a magnifier with a protractor reticle. The tip angle should match the original tool specification within 1 degree.
A tip angle that is too steep (wider included angle, e.g., 35 degrees instead of 30 degrees) causes the drill to climb over the material rather than shear it. This produces chatter and a rough surface finish. I have seen this happen when a regrind operator sets the wrong angle on the fixture.
A tip angle that is too shallow (narrower included angle, e.g., 25 degrees instead of 30 degrees) increases the cutting forces. The drill has to push harder through the material. This reduces tool life and increases the risk of drill breakage in deep holes.
| Material | Recommended Tip Angle (included) |
|---|---|
| Steel up to 30 HRC | 30 degrees |
| Hardened steel 30-45 HRC | 28 degrees |
| Stainless steel | 25 degrees |
| Aluminum | 35 degrees |
| Brass / bronze | 40 degrees |
Clearance Angle
The clearance angle behind the cutting edge provides relief so the drill does not rub against the workpiece. I check the clearance angle with a protractor or a comparator.
The standard clearance angle is 8-12 degrees for most materials. Too little clearance (below 6 degrees) causes the flank of the drill to rub against the bore wall. The rubbing generates heat and creates a burnished or glazed surface finish. In extreme cases, the rubbing can weld material to the drill flank.
Too much clearance (above 14 degrees) weakens the cutting edge. The edge becomes fragile and chips easily, especially when entering the workpiece or encountering a hard spot in the material.
Drill Diameter at the Tip
I check the drill diameter at the tip using a micrometer. The diameter should match the specified drill size within 0.01 mm. Each regrind removes material from the tip and reduces the diameter slightly.
The drill body (behind the tip) is typically 0.02-0.05 mm smaller than the tip diameter. This relief prevents the body from rubbing in the bore. The tip diameter is the size that determines the finished bore diameter.
I track the diameter reduction with each regrind. A 10 mm drill typically loses 0.01-0.02 mm per regrind. After 5-8 regrinds, the drill is undersized by 0.10 mm and should be retired.
| Drill Diameter (mm) | Typical Max Regrinds | Retirement Diameter Loss |
|---|---|---|
| 3-6 | 3-5 | 0.05-0.08 mm |
| 6-12 | 5-8 | 0.08-0.15 mm |
| 12-20 | 8-12 | 0.15-0.25 mm |
| 20-30 | 12-15 | 0.25-0.40 mm |
Coolant Hole Clearance
The regrind process can leave grinding debris in the coolant hole. A blocked coolant hole means no coolant flow to the cutting edge, which means tool failure within seconds.
I blow compressed air through the coolant hole before and after regrind. The air should flow freely through the hole and out the tip. If the flow is restricted, I use a small wire (0.5 mm diameter) to clear the hole and then blow again.
For gun drills with two coolant holes, I check both holes individually. I have seen drills where one hole was completely blocked by swarf while the other remained open, resulting in uneven coolant distribution and a rough bore surface on one side.
Edge Condition
I check the cutting edge under 10x-20x magnification. The edge should be sharp with no visible burrs, chips, or grinding burns. A burred edge produces poor surface finish and increases cutting forces.
Grinding burns show as discoloration on the cutting edge. Blue or brown discoloration indicates overheating during the regrind. The overheating can create micro-cracks in the carbide that will propagate during cutting and cause edge chipping.
I reject any drill that shows grinding burns. The regrind parameters need adjustment to reduce heat generation at the grinding wheel.
Regrind Count Tracking
I stamp a number on each drill shank after every regrind. The number tracks the total regrind count. This is important because drills that have been reground too many times have a reduced diameter and thinner cross-section at the tip, making them more susceptible to breakage.
I retire a drill when it reaches the maximum regrind count for its diameter class. I have broken more tools from pushing them one regrind too far than from any other single cause.
Inspection Report
I record all measurement results on a regrind inspection form. The form includes the drill ID, regrind number, tip angle, clearance angle, tip diameter, coolant hole status, and edge condition. The form goes with the drill to the machine so the operator knows the drill has been verified.
Key Takeaways
- Measure tip angle within 1 degree of specification after every regrind.
- Maintain clearance angle at 8-12 degrees to prevent rubbing without weakening the edge.
- Check coolant hole clearance with compressed air before and after regrind.
- Track regrind count and retire drills at the maximum for their diameter.
- Reject any drill with grinding burns or edge damage from the regrind process.