Drill Bit Regrind And Hit Count Management

A carbide drill bit for a printed circuit board is a precision tool made to a tolerance of a few microns, and it wears out long before it breaks. The cutting edge rounds, the margin wears, the diameter changes and the surface of the hole deteriorates. Instead of discarding the bit, most shops regrind it several times, and the decision of how many times and when is one of the practical controls on hole quality.

This article explains what regrinding does to a bit, how the hit count is set, how a sharpened bit is inspected, and how the cost of the practice is balanced against the quality it produces.

Why Bits Are Sharpened

A worn bit is not simply a dull version of a new one. The cutting edge wears unevenly, the point angle changes, the margin that guides the bit in the hole wears away and the drill begins to wander. The result is a hole that is oversize, out of position and rough on the wall, and the roughness appears before the size does, which means the defect is present while the measurement still passes.

Regrinding restores the geometry by removing a small amount of material from the point. The bit becomes shorter, and if the operation is done well the cutting geometry is restored to near the original. If it is done poorly the point is asymmetric, and an asymmetric point cuts a hole that is oversize and lobed because one lip does all the work. The quality of the regrind therefore matters more than the number of times the bit has been sharpened.

What Regrinding Changes

Two things always change. The bit gets shorter, which reduces the available flute length and means that a stack that was drilled with a new bit may not be reachable with a reground one. And the diameter gets slightly smaller, because the grinding wheel removes material from the outside as well as from the point. That change is small, typically a few microns per regrind, but it accumulates, and it has to be accounted for where the hole diameter is critical, as it is for a press fit connector.

The geometry can also change in ways that are not intended. The point angle, the lip height and the web thickness are all affected by the setup of the grinder, and a machine that is out of adjustment will produce bits with a consistent bias. Since the bias is consistent, the holes it produces are consistent too, and the problem goes unnoticed until a customer measures the hole size or the position across a lot.

Hit Count And When To Change

The hit count is the number of holes a bit is allowed to drill before it is removed, and it is set from the laminate, the hole size, the stack height and the plating. A small bit in a hard, glass rich material reaches its limit quickly, while a large bit in a soft material can drill many more holes. The figure is established by drilling a test panel and measuring the hole quality at intervals rather than by taking a supplier’s recommendation unchanged.

The measurement that sets the count should be the one that matters for the product. Where the hole position is critical, the position is measured; where the wall quality matters, the wall is sectioned and examined. A count based on hole size alone will pass a bit that produces a rough wall, because the wall degrades first. Once the count is established, it is recorded per bit and enforced by the machine rather than left to the operator.

Inspecting A Reground Bit

A sharpened bit is inspected under a toolmaker microscope before it goes back into the crib. The check covers the point angle, the symmetry of the two lips, the condition of the cutting edge, the diameter at the margin and the presence of any chipping. A bit with chipped carbide is scrapped rather than sharpened again, because a chip produces a hole with a scratch and the chip grows with use.

An automatic optical inspection system is used where the volume justifies it, and it measures the same features from an image and compares them with a reference. Either way, the bit is measured after sharpening and not before it is used, because a bit that fails the check after it has drilled a panel has already produced the defect. The records show how many times each bit has been sharpened, and a bit that has reached its limit is removed from circulation.

Records And Their Consequences

The record that matters is a link between the bit, the holes it drilled and the panel that was produced. Where a machine records the bit identity against the lot, a hole quality problem can be traced to a specific bit and a specific sharpening, which turns a general complaint into a specific action. Without that link, the only available response is to change every bit and hope.

The record also supports the decision about the hit count. A shop that has data on the hole quality against the number of hits can raise the count and save money, or lower it and improve the quality, on evidence rather than on a habit. The fabrication sequence that the drilling belongs to is described under PCB design and fabrication.

Cost And Quality Balance

Regrinding is cheaper than buying a new bit, but it is not free. Each cycle costs the grinding time, the inspection time and the risk of a bit that is slightly worse than the original. A bit that has been sharpened five times is not the equal of a new one even if it passes the inspection, because the flute length and the margin are not what they were.

The balance depends on the hole. A generous hole in a thick panel with a relaxed tolerance can be drilled with a well used bit, while a small hole for a press fit pin needs a bit that is as close to new as possible. Many shops therefore separate their bits into pools for different products rather than treating all of them the same. The wider set of drilling controls is described under copper plating defects prevention and the production decisions that interact with it under layout decisions that affect production.

Additional Considerations for This Build

Practical attention to tool wear pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating tool wear explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, hole quality is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

FAQ

How many times can a bit be reground? Typically two to four times, depending on the original flute length and the hole depth. Beyond that the bit is too short to reach the bottom of the stack and the geometry is no longer reliable.

Does a reground bit drill a smaller hole? It does, by a few microns, because the grinding removes material from the outside diameter. Where the hole size is critical the bit diameter is measured and the drill program is adjusted.

Can a bit be sharpened in house? It can, and most shops do, provided the grinder is maintained and the bits are inspected afterwards. A grinder that is out of adjustment produces a whole batch of bits with the same defect.