Drill Runout in PCB Drilling: What the Collet Adds

Drill runout is the total indicated movement of the cutting edge as the drill turns, measured close to the tip rather than at the shank. It is the error that sits underneath every drilling parameter, because a bit that orbits cannot cut a straight hole whatever the feed rate.

The value that matters is the total, and it is built from three parts: the spindle bearing error, the collet and its seating, and the geometry of the bit itself. Measuring only one of the three explains a fraction of the hole quality that the machine actually produces.

What Runout Is and How It Is Measured

The measurement is taken with a dial indicator or a laser system on the cutting edge, with the drill mounted and clamped exactly as it is in production. The indicator is positioned five to ten millimetres from the tip, because that is where the cutting happens and where the amplification of any error is largest.

The spindle is turned by hand or at low speed during the check, and the reading is taken as total indicated runout over one revolution. Measuring at the shank gives a smaller and more flattering number, which is why the position of the indicator belongs in the record.

Spindle Error and Its Contribution

A new spindle in good condition typically holds runout below about 10 µm at the collet face, and a spindle that has reached 25 µm is usually scheduled for repair. Bearing wear develops gradually, and the first symptom is often a rise in hole position error on small drills rather than an obvious defect.

The spindle contribution also depends on speed. A spindle that measures well at low speed can show more error at the 100,000 rpm used for small holes, so the check should be repeated at, or near, production speed where the equipment allows it.

Collet Wear and Contamination

The collet is the cheapest component in the assembly and the most frequent cause of growing runout. It grips the shank on a taper, and any dust, resin or metal particle trapped between the collet and the shank tilts the bit instead of holding it straight.

Collets wear after a large number of tool changes, and the wear is not visible on the outside. A collet that has been in service for a few hundred thousand changes should be checked against a new one, and the check is a comparison of measured runout with the same drill, not a visual inspection. The same logic that governs bit life and regrind limits applies to the collet.

Runout of the Bit Itself

A new carbide bit is straight to within a very small figure, but the shank can be damaged by handling and the flutes can be bent by a jam. A bit that has been dropped should be treated as suspect even when the point appears intact.

The bit contribution is measured by rotating it in a precision holder that is known to be good, so that the collet and the spindle are removed from the result. Where the same bit measures well in the reference holder and badly in the machine, the fault is in the machine, and the reverse implicates the tool.

The Effect on Hole Position

Runout moves the cutting edge away from the spindle axis, and the hole follows the edge rather than the axis. The positional error that results scales with the depth of the hole, so a runout of 15 µm at the tip can become a position error of tens of micrometres at the exit of a deep hole.

That is why runout matters most on high aspect ratio work and on boards with a tight registration budget. A machine that is adequate for a 1.6 mm board with a 6 to 1 aspect ratio can be marginal on a 3 mm board with the same hole size, and the registration budget has to include the runout term explicitly.

The Effect on Hole Size and Wall Quality

A drill that orbits cuts a hole that is larger than its diameter, because the edge sweeps a circle wider than the tool. The oversize is not uniform around the circumference, and the wall carries a spiral pattern where the edge has rubbed rather than cut.

The rubbed surface is the more serious effect. Resin smears across the glass bundles, and plating adhesion suffers because the copper is deposited onto a damaged surface rather than a cut one, which is the relationship described in the notes on hole wall roughness and adhesion.

High Aspect Ratio Work

As the aspect ratio rises, the drill has less room to deflect before the wall interferes with chip clearance, and the consequence of any runout grows. Small drills are also more flexible, so a runout that is tolerable on a 0.5 mm bit becomes a position and wall problem on a 0.2 mm bit.

High aspect ratio work therefore justifies tighter limits: a spindle and collet combination that is acceptable for general work may need to be reserved for it, and the runout measurement should be part of the setup record rather than an occasional investigation.

Establishing an Inspection Routine

The routine has three parts. The spindle is checked on a fixed interval with a reference pin, the collet is checked whenever runout grows or a new batch of tools is opened, and a suspect bit is checked in a reference holder before it is scrapped.

Each check takes a few minutes and produces a number that can be trended. A rising trend points at wear and gives warning before the hole quality moves, which is far cheaper than discovering the limit through a rejected lot. The reference pin should be kept separately, handled carefully and re-verified periodically, since it defines the measurement.

Limits, Records and Consequences

A workable set of limits is under 10 µm for fine work, under 25 µm for general work, and a repair trigger at 25 µm. Those figures should be applied at the tip, at production speed where possible, and with the collet that will be used for the job.

The record should carry the measurement position, the speed, the collet identification and the reference tool. With those fields, a drift in hole quality can be attributed to the machine, the collet or the tool within an hour rather than over a week of experiments, which is the same approach taken to spindle runout and hole quality in general.

FAQ

Where should runout be measured? On the cutting edge, five to ten millimetres from the tip, with the drill clamped as it is in production. Measuring at the shank gives a smaller number and hides the error that affects the hole.

How much runout is acceptable? Below about 10 µm for fine work and below 25 µm for general work, with a repair trigger at 25 µm. The limit should also be compared with the position tolerance of the product.

Does a worn collet matter if the bit is new? Yes. The collet holds the bit, so a worn or contaminated collet tilts a perfect tool and produces exactly the same hole defects as a bent bit.