Runout Gauge — Measuring Wobble in the Faceting Lap
Runout Gauge — Measuring Wobble in the Faceting Lap
The dial indicator that tells the cutter whether the spindle is true
A runout gauge is a precision dial indicator used by lapidaries and faceters to measure the axial and radial deviation of a rotating component — a faceting machine spindle, a quill, a lap, or a dop — from true rotation. Excessive runout produces uneven facet meets, poor polish, and inconsistent angle indexing; competition-grade and optical-grade work is impossible on a machine whose spindle wanders by more than a few microns. The runout gauge is the diagnostic instrument by which the condition of the cutting platform is assessed and corrected.
What runout means
Runout is the deviation of a rotating part from a true circular path of rotation. Two components are commonly distinguished: radial runout, the lateral wobble of a shaft as it turns, and axial runout, the back-and-forth movement along the axis of rotation, sometimes called endplay. Both are read in thousandths of an inch or, more commonly in modern practice, in micrometres. A faceting spindle in good condition will hold radial runout below about ten microns and axial runout to a similar order; precision work is done on machines holding both well under five microns.
The instrument
A typical runout gauge is a dial indicator with a sensitive plunger or lever stylus mounted on a magnetic base, an articulated arm, or a purpose-built fixture. The stylus is brought to bear against the surface to be measured — the side of a quill for radial runout, the face of a lap or the end of a shaft for axial runout — and the indicator registers deviation as the part is rotated by hand or under power. Mechanical dial indicators with one-thousandth-of-an-inch graduations are still common; digital indicators reading in microns are now widely available and offer easier interpretation of small deviations.
Use in faceting
On a faceting machine, the runout gauge is used to verify the trueness of the master spindle, the lap surface, and any quill or dop that mounts the rough. A new lap should be checked for radial flatness across its working surface; a worn lap may have developed a high spot that prevents consistent facet meets. The cutter mounts the gauge so the stylus rides on the lap surface or quill body, rotates the spindle through a full revolution, and records the maximum deviation. Values within the manufacturer's tolerance are accepted; values above tolerance indicate a need for adjustment, regrinding of the lap, or service of the bearings.
Use in lapidary tooling more broadly
Beyond faceting, runout gauges are used in cabochon work, sphere cutting, and any operation where a rotating tool or workpiece must hold a precise relationship to its axis. A diamond saw blade with excessive radial runout cuts a wider kerf than the blade's nominal thickness, wasting expensive rough; a grinding wheel out of true vibrates and produces a poor surface. The same instrument used by jewellers to check turning and polishing equipment serves the lapidary equally well.
In the trade
For amateur and trade cutters, a basic dial indicator and magnetic base capable of one-thousandth-of-an-inch resolution is an inexpensive and worthwhile investment, costing a fraction of the faceting machine itself. Tighter-tolerance work justifies a digital indicator with submicron resolution. The instrument's value lies less in the day-to-day cutting routine than in diagnosing problems when facet meets stop coming clean — at that point, a runout reading is often the difference between productive troubleshooting and replacing parts at random.