Drill Regrind: Extending Bit Life Without Losing Quality
Drill regrind is the process of restoring the cutting geometry of a used carbide bit so that it can be returned to production. Done properly, it extends the life of an expensive tool and keeps hole quality stable; done casually, it produces a bit that looks sharp but drills off centre and generates heat.
The distinction matters because a reground bit is a new cutting tool in everything but its shank. Its geometry has to be measured, its runout checked in the holder, and its performance proven on a sample of holes before it goes back on the machine.
Why Regrind Is a Process, Not a Repair
Every regrind removes material from the point, and each cycle changes the relationship between the flute, the web and the cutting edge. After several cycles the bit can be shorter, more flexible and less able to clear chips, even though its diameter is unchanged.
The number of regrinds a bit can take is therefore finite, and the limit is set by the flute length and by how much web thinning the grinding operation can restore. A shop that tracks cycles per bit knows when the tool is no longer economical, whereas one that regrinds until the bit breaks usually discovers the limit on a customer panel.
Geometry That Must Be Restored
A correct regrind restores the point angle, the relief angle behind the cutting edge, the chisel edge and the symmetry of the two cutting lips. Symmetry is the least visible and the most important, because a bit with one lip longer than the other drills a hole larger than its diameter and wanders as it cuts.
The measured parameters should be recorded per bit rather than per batch. Lip height difference and point angle are both quick to check under a toolmaker microscope, and a bit that fails either is not returned to a machine that drills fine pitch or high aspect ratio holes.
Point Angle, Web and Chisel Edge
The point angle of a PCB drill is typically 130 degrees, and the chisel edge at the centre of the point is the part that does no cutting but pushes material aside. As the web thickens through successive regrinds, the chisel edge lengthens and the thrust required to start the hole rises.
Web thinning is therefore part of a competent regrind rather than an optional extra. A thinned web reduces the thrust at the point, which reduces heat and improves hole position, and it is the parameter that most clearly separates a controlled regrind from a simple sharpening.
Runout and Coating After Regrind
Runout is measured with the bit mounted in its collet, not in a separate fixture. A bit that measures zero runout on the bench can show several micrometres when it is clamped in a worn collet, and the drilling result follows the assembly rather than the bit alone.
Coated bits add a complication, because grinding removes the coating from the point while the flutes remain coated. The exposed carbide wears faster than the coated surface, and the transition between the two can become a stress riser. Coated tooling is usually reground less often and replaced earlier than uncoated tooling for that reason.
Setting Hit Count Limits
The number of hits a bit can take depends on the laminate, the stack height, the hole size and the backup material, so a universal number is meaningless. A useful starting point comes from the tool supplier, and the real limit comes from measured hole quality on the product.
Once the limit is established, it becomes an input to the machine rather than a matter of operator judgement. A machine that counts hits and flags the tool at the limit produces a stable process, while one that relies on the operator to notice rising position error produces defects in the interval between noticing and acting. The counter should also distinguish between hits through laminate and hits in entry material, because the load on the cutting edge is not the same in both.
Measuring Hole Quality After Regrind
The change that a poor regrind produces is easiest to see at high magnification on the hole wall. A rough wall with smeared resin, a hole that is larger than the drill diameter, or a position that has moved all point to a geometry problem, and the chip load setting should be confirmed before the bit is blamed.
Position error is best separated from wander by measuring entry and exit separately, as described in the notes on drill wander. If the entry has moved but the exit has not, the bit is deflecting; if both have moved by the same amount, the machine or the fixture is responsible.
Handling, Storage and Contamination
Carbide is hard and brittle, and the cutting edges are damaged by contact more easily than by cutting. Bits stored loose in a box chip each other, and a bit that has been dropped should be treated as suspect regardless of how it looks. Plastic carriers that hold each bit separately, with the point clear of the container wall, are a small cost against the price of a scrapped panel.
Contamination matters in the same way. Oils from hands, residue from a cleaning bath or debris left in the flutes all change the cutting conditions, and the flutes should be inspected for packed material before the bit is returned to the machine rather than after a defect appears.
When to Scrap a Bit Instead of Regrinding
A bit is scrapped when the flute length is too short for the deepest hole it must drill, when the web is too thick to thin effectively, when a chipped corner cannot be removed within the diameter tolerance, or when the shank itself has been damaged.
The diameter tolerance is the constraint that decides most cases. Regrinding a bit below its nominal diameter produces holes that are undersized and an aspect ratio that is worse than the one the design assumed, and the consequence shows up later on the plating line as well as in the drill. A hole that is 5 um undersized on a 0.25 mm diameter reduces the cross sectional area available for copper by about four percent, and it also makes the barrel harder to plate to the minimum thickness at the centre. The consequence shows up later as aspect ratio limits being exceeded on a job that was previously comfortable.
Records and Supplier Requirements
A regrind record should carry the bit identification, the number of cycles, the measured point angle, the lip height difference, the measured runout and the hit count at which it was removed from service. Those five fields explain most of the variation seen on the drilling floor.
Where regrinding is done by an outside supplier, the acceptance criteria should be written into the purchase specification rather than left to the supplier method. A defined point angle, a maximum lip height difference and a packaged return condition turn regrind from an unknown into a controlled input to the process.
FAQ
How many times can a PCB drill bit be reground? It depends on the flute length and the amount removed per cycle, and most carbide bits take several cycles before the geometry can no longer be restored. Each cycle should be recorded and the limit set by measurement.
What is the most important parameter after a drill regrind? Lip symmetry. A difference in lip height between the two cutting edges makes the bit drill oversize and push off centre, and it is the defect that most often escapes a visual check of the point.
Should coated drill bits be reground? They can be, but grinding removes the coating from the point and exposes uncoated carbide at the cutting edge. Coated bits are usually reground less often and replaced sooner than uncoated bits as a result.
