Drill Bit Point Angle and Web Thickness: What the Hole Sees
The point of a printed circuit drill bit is a cutting tool with a geometry that is chosen for the laminate, the hole size and the machine. The point angle, the web thickness, the relief and the surface treatment all influence the forces at the cutting edge, the way the chips leave the hole and the heat that is generated. They also change every time the tool is reground, which is why geometry control belongs in the tool room rather than only in the drill specification.
Hole quality is the sum of those effects. A drill with the wrong point geometry for the material produces more heat, more smear on the hole wall and more variation between the entry and the exit. None of those shows up as a dimensional error, so the problem is usually discovered later, at plating or at the microsection, when the tool that caused it has already been reground twice.
What the Point Geometry Controls
The point is the part of the tool that first contacts the laminate, and its shape decides how the material is removed. A sharper point concentrates the cutting force and enters with less pressure, which reduces the chance of the drill wandering before it bites. A blunter point distributes the load, which is more stable for a large hole but generates more heat and requires more thrust.
Geometry also controls the rigidity of the tip. A long, thin point is easier to break and deflects more under load, while a short, stout point is stronger but cuts less efficiently. Every tool is therefore a compromise between entry behaviour, strength and chip evacuation, and the compromise is selected per hole size and per machine.
Point Angle and Cutting Forces
The point angle is the included angle between the two cutting lips. A general purpose metalworking drill uses 118 degrees, while circuit board drills more often use 125 to 135 degrees. A wider angle produces a stronger point, a stiffer tool and better performance at high spindle speeds, and it also reduces the tendency of the tip to walk on a smooth laminate surface.
The angle interacts with the entry material and the backup. A hard entry sheet spreads the load and reduces burrs, and a point that is too sharp punches through it rather than cutting, which produces a larger entry burr. Where a stack is drilled with a soft entry material, a wider point angle improves the entry quality without any change to the spindle.
Web Thickness and Chip Evacuation
The web is the solid core between the flutes, and its thickness is measured across the point. A thin web leaves more room for chips and produces less heat, but it weakens the tool. A thick web is stronger and provides a better heat path away from the cutting edge, at the cost of chip clearance and of a larger thrust force.
Circuit board drills are often specified with a web thickness of 20 to 30 percent of the diameter, and the web grows as the tool is reground back from the point. The increase is small per regrind and significant over the life of a tool, and it is one of the reasons a drill behaves differently on its fifth regrind than on its first. Measuring the web as well as the diameter at each regrind is what keeps the geometry inside its window.
Relief, Land and Margin
Behind the cutting lip the tool has relief, the clearance that stops the drill from rubbing on the hole wall. Too little relief produces friction, heat and a polished, smeared wall, while too much relief weakens the edge and lets it chip. The land is the narrow band that supports the tool against the wall, and its width controls the stability of the drill in the hole.
On a small drill the geometry is difficult to see and easy to damage. The relief is measured in tens of micrometres, and a tool that has been handled roughly can have a chipped lip that is invisible without magnification but that produces a rough hole wall and a raised burr. Inspection at the tool room is therefore visual and dimensional rather than only statistical.
Coatings and Wear
Coatings reduce friction and wear, and their effect on hole quality is largest in abrasive laminates. A coating that has worn away on the cutting lip leaves the tool cutting with a different effective geometry from the one it was measured with, which is why wear is monitored on the lip rather than on the shank. Wear appears first at the outer corner, where the cutting speed and the temperature are highest.
Tool life is usually expressed as a number of hits, and the number should be tied to a measured hole quality limit rather than to a supplier recommendation. The method for setting that limit is described in the notes on drill regrind and bit life, and the effect of the machine on the same geometry in the notes on spindle runout.
Regrinding and Geometry Drift
Regrinding restores the point but removes material from the flute length and changes the web. The number of regrinds a tool can take is limited by the flute length and by the geometry that can be reproduced, and a tool that is reground beyond its limit will drill a hole that is dimensionally acceptable and metallurgically poor. Recording the regrind count with the tool number is the only way to enforce the limit.
The grinder itself has to be controlled. The wheel wears, the indexing drifts and the coolant condition changes, and the result is a batch of tools with a point angle that varies from the nominal. A periodic check of the ground geometry on a sample of tools, with the results recorded, is a small cost that prevents a large one.
Connecting Geometry to Hole Quality
The hole wall records what the tool did. A rough wall with a helical pattern indicates a worn or chipped lip, a smear indicates heat and insufficient relief, and a nailhead or a barrel shape indicates excessive thrust or a stack that moved. Each of those signatures has a geometry cause, and each can be confirmed by examining the tool that produced it.
The verification is a drilled test coupon examined in section, together with the tool measurements. The drills that produced the coupon are identified by number, and the geometry is measured after the run rather than before it. That comparison is described in the notes on drill stack and hit counting and in the notes on hit count control.
Verifying Geometry in the Tool Room
The measurements that matter are the diameter, the point angle, the web thickness, the lip height and the surface condition of the cutting edges. A tool room equipped with a tool maker microscope and a documented measurement routine can check all five in a few minutes per tool. The routine should include a sample from every regrind batch rather than only from the tools that look unusual.
The records should be kept per tool and per regrind, because a geometry problem is usually a batch effect. When a drilling defect appears, the tool records identify whether the affected tools came from one regrind run, one grinder or one coating lot, and that is enough to contain the problem before it reaches the plating line.
Process Control and Records
In production the geometry is controlled indirectly through the chip load, the feed and the spindle speed, which together determine the heat and the forces at the edge. A chip load that is too high overloads the lip and produces a rough wall; one that is too low rubs instead of cutting and produces heat. The window is narrower for small holes and for high aspect ratios, and it is documented per tool size.
The records that close the loop are the tool number, its regrind count, its measured geometry, the machine parameters and the hole quality result from the coupon. That set of fields turns a drilling defect into a tool issue rather than a machine issue, and it is described in more detail in the notes on chip load control.
FAQ
Does a wider point angle always give a better hole? It gives a stronger tool and better entry behaviour on smooth laminate. The best angle depends on the material, the hole size and the entry sheet.
Why does hole quality change after a regrind? The regrind changes the web thickness and the lip geometry even when the diameter is correct, so the cutting behaviour changes with it.
How often should tool geometry be measured? A sample from every regrind batch, plus any tool that produces a change in the coupon result. The measurement takes minutes and the information lasts until the next regrind.
