Tool Presetting in the Drill Room: Length, Runout and Records

Before a drill bit reaches a spindle it is set into a collet to a defined length. That operation, tool presetting, fixes how far the drill protrudes from the holder and how well it is centred, and those two quantities decide the depth of the hole, the runout at the cutting edge and how the tool wears. It is a short operation repeated thousands of times, which is exactly why it drifts.

The presetting area is often treated as a store room with a gauge in it. Treated instead as a process, with defined settings, verified equipment and records per tool, it removes a class of defects that are otherwise attributed to the machine or to the tool supplier.

What Presetting Controls

The primary setting is the drill length, the distance from the face of the holder to the point of the tool. That length determines the depth the machine reaches for a given program and the depth of the backup that is consumed. Where lengths vary across a batch of tools, the machine compensates on average and drills some holes deeper than the program asks.

The second setting is concentricity, the alignment of the tool axis with the spindle axis. It is a property of the collet, the shank and the way the two are brought together. A tool that is set with a particle between the shank and the collet, or with the collet nut tightened unevenly, runs out at the tip, and runout produces an oversized hole and accelerated wear on one lip.

Drill Length and Hole Depth

Hole depth matters when a hole must break through a defined layer or when a controlled depth is required, and it matters more subtly for the backup material, which is consumed by the tool at each hit. A set of tools that is 0.2 mm different in length consumes the backup unevenly and changes the drilling conditions from one spindle to another.

The length is measured with a setting gauge or a presetter, and the tolerance is set by the depth requirement rather than by what the equipment can achieve. For a through hole the tolerance is loose, and for a controlled depth or a back-drilled feature it is tight. The tolerance belongs in the presetting instruction, and it should be stated in the same units the machine uses.

Runout and Its Sources

Runout is the total indicated movement of the cutting edge as the tool rotates. It comes from three places: the spindle, the collet and the tool. The spindle contribution is a machine property that is measured on a regular schedule, the collet contribution is a wear item, and the tool contribution is a manufacturing tolerance on the shank and the point.

Presetting can only influence the part that belongs to the assembly. A clean collet bore, a shank that is free of burrs and a nut that is tightened to a consistent torque keep the assembly contribution small. The measurement of the assembled tool in a presetter is the only way to confirm that it has done so, and it takes seconds per tool.

Collet Condition

The collet is a spring element that is compressed by the nut, and it loses its accuracy as it wears. A worn collet grips the shank at a reduced area, which allows the tool to move and to run out, and it also leaves a mark on the shank that changes the surface the next collet will grip. Wear is not visible at a glance and is measured rather than judged.

The service life of a collet should be defined in hits or in assembly cycles rather than in calendar time, because the load is what wears it. Where a drill breaks in the spindle, the collet involved is inspected before it is used again, since a fragment can be embedded in the bore. The interaction between the collet and the drill is described further in the notes on collet maintenance.

Tool Life and the Setting

A tool that is set correctly wears predictably on both lips and reaches its hit limit with a consistent hole quality. A tool that runs out wears one lip first, and the wear appears as a change in the hole diameter and in the burr before the tool reaches its counted limit. The result is a tool life that is shorter than the specification and a defect that is blamed on the tool.

Tracking the life of tools from the same supplier against the setting record separates the two causes. Where the life falls after a change of collet batch or after a change in the presetting procedure, the setting is the cause; where it falls across all settings, the tool or the material is. The counting method itself is described in the notes on hit count control.

Presetting Equipment

A presetter measures length and, in the more capable versions, runout on a rotating spindle. The equipment needs its own calibration, with a master tool or a gauge block, and the calibration is recorded on the same schedule as any other measuring instrument. A presetter that is out of calibration sets every tool in the room to the same wrong length.

The fixtures and the torque driver are part of the equipment. A torque driver that has drifted changes the clamping force, which changes both the runout and the seating of the shank. The setting station should be kept clean and covered, because a single particle of carbide in the collet bore is enough to create the defect the station exists to prevent.

Records and Traceability

The record for a tool should carry its identification, its diameter, its regrind count, the measured length, the runout, the collet used and the operator who set it. With those fields, a hole quality change can be traced to a specific assembly rather than to a machine or a lot.

Traceability is what makes the record useful. The tools loaded into a spindle for a given job are known from the machine log, and the presetting record for those tools is retrieved when a defect appears. Without the link between the two, the presetting data is a pile of measurements with no diagnostic value.

Verifying the Setting in the Machine

Once a tool is in the spindle, the assembly behaves as a system. The runout at the cutting edge can be measured with a dial indicator on a test spindle, and the length can be verified by drilling a test hole in a coupon and measuring the depth. Both checks are used after a spindle repair or a change of holder.

The machine check also detects a spindle problem that presetting cannot fix. Where a tool that measures correctly on the presetter runs out in the machine, the spindle or the holder is the cause, and the measurement that separates the two is described in the notes on spindle runout and in the notes on drill bit runout.

Process Control and Records

The controls in the presetting area are the calibration of the presetter, the condition and the life of the collets, the torque of the clamping nut, the cleanliness of the station and the tolerance applied to the length. Each is checked on a schedule, and each check is recorded against the equipment identification.

The output control is a sample verification: a small number of tools from each shift are measured again after setting, and the results are compared with the setting records. That audit catches a drifting gauge or a hurried operator before the defect reaches the drilling line. Its cost is a few minutes per shift, and its value is the class of hole defects that never appear. The relation between the tool and the hole is described in the notes on drill regrind and bit life.

FAQ

Does presetting affect hole position? It affects it indirectly. A tool that runs out drills an oversized hole and can wander more on entry, which shows as a position error in the finished board.

How often should collets be replaced? On a defined count of assemblies or hits rather than on a calendar interval, because the wear follows the load rather than the time.

Is a presetter necessary? A setting gauge is enough for length. Runout is the parameter that requires a rotating measurement, and it is the one that correlates with tool life.