Zero tolerance in CNC machining: how many points are too few?

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  Key takeaways
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  What zero tolerance in CNC machining actually means
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  Why a single point can pass a bad part
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  Factors that set the point count
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  How many points are too few: a working rule
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  Measurement method changes the answer
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  Step by step: set the point count for a new part
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  Point count by feature type

Zero tolerance in CNC machining is often read as "make it perfect." That reading creates trouble on the shop floor. No machine holds an infinite limit. A tolerance of zero means the allowed band is zero, so every part is scrap unless it lands on one exact number.

In practice, engineers use the phrase for a drawing where every called-out dimension carries a tight limit and is inspected. The band might be ±0.005 mm on a bearing bore and ±0.05 mm on a mounting hole. Both are zero-tolerance callouts if both are measured and both are enforced.

The real question is not whether zero tolerance is possible. It is how many measurement points you need before the inspection tells you something true about the part. Measure too few and a tapered bore passes. Measure too many and you pay for data nobody reads.

GreatLight machines to ±0.005 mm (±0.0002 in) and inspects 100% before shipment. That capability only helps if the drawing tells us where to put the probe.

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  CalloutA dimension plus a limit, such as Ø25.000/24.990 mm.
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  PointOne measured location on that feature, at a stated depth and angle.
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  PassEvery point sits inside the limit. One point outside is a fail.

A caliper reading at the mouth of a bore says nothing about the bottom. If the tool deflects, the hole tapers. If the boring bar chatters, the hole goes oval. Both parts pass a one-point check and both fail in assembly.

Roundness error is invisible to a two-point check. A three-lobed bore can measure 25.000 mm across every diameter and still be 0.02 mm out of round. That is why roundness needs at least three points at 120° apart, and four points at 90° when you suspect a lobed form.

Position needs its own logic. A hole pattern checked at only the first hole misses cumulative error across a 400 mm bolt circle. The last hole is where stack-up shows up.

Depth and step features behave the same way. One touch on a shoulder does not reveal a tapered floor or a burr left by a dull insert.

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  TaperTwo points at different depths, top and bottom of the bore.
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  OvalityThree or four points around the circumference at one depth.
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  PositionOne point per hole, plus the two holes farthest apart.

Start with the fit. A press-fit bearing seat at Ø40 H7 needs more points than a clearance hole for an M6 screw. The bearing has a defined interference; the screw hole has 0.3 mm of slop. Same part, very different inspection budgets.

Then look at the process. A 5-axis cycle that machines a feature in one setup holds form better than a three-setup route, where re-clamping shifts the datum. Fewer setups mean fewer points are needed to trust the result.

Material matters too. Aluminium 6061 cuts clean and holds size. Titanium Ti-6Al-4V and Inconel spring back and work-harden, so bores drift after the tool leaves. Those parts earn an extra point at each end of the bore.

Finally, consider what the customer does with the data. If the part feeds a CMM report for an aerospace or medical build, the drawing may already name the points. Follow it and add nothing.

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  Fit classInterference fits get the densest coverage.
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  Setup countMore re-clamping means more check points.
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  MaterialSpringback alloys need depth-wise points.

For a critical cylindrical feature, four points is the floor. Two at the top of the bore, two at the bottom, offset 90°. That catches taper and gross ovality. Six points adds a mid-depth ring and is enough for most sealing and bearing surfaces.

For a flat mating face, three points define a plane. Six points in a 2 × 3 grid show twist and bow. Twelve points rarely change the decision on a milled face that was cut in one pass.

For hole position, check every hole once, then add the two holes with the largest separation. On a 12-hole pattern that is 14 points. It costs minutes and catches the stack-up that a single-hole check misses.

The test is simple. Ask what defect each point would catch. If two points catch the same defect, drop one. If a point catches nothing, it is too few points turned into too many.

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  Bore, general4 points minimum: two depths × two angles.
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  Bore, sealing6 points: three depths × two angles.
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  Flat face3 points for a plane, 6 for twist.
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  Hole patternOne per hole plus the two farthest apart.

A caliper resolves about 0.02 mm on a good day and depends on operator feel. It cannot support a ±0.005 mm callout. Use it for stock checks, not for final release.

A bore gauge or an air gauge reads to 0.001 mm and is fast on the shop floor. It measures diameter well and roundness poorly. Pair it with a roundness check if form is critical.

A CMM gives position, form and orientation from one setup. It is the right tool for a first article and for any drawing with GD&T. It is slow, so production runs often use gauges plus a CMM audit.

Optical and laser scanners cover freeform surfaces fast but struggle on deep bores and blind features. Match the tool to the feature, then set the point count.

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  Caliper±0.02 mm. Stock checks only.
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  Bore gauge0.001 mm resolution. Diameter, weak on form.
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  CMMPosition and form. Best for first article.