Why tolerances do not hold in CNC machining

The program is right, the machine is good, the operator is experienced — and the part still misses size. The machine is usually not to blame. Six things move a dimension, and most of them repeat because nobody noticed them the first time.

1. Temperature — the most underestimated cause

A dimension depends on the temperature of the part. The standard says so explicitly: under ISO 1 the reference temperature for dimensional properties is 20 °C. Every dimension on a drawing is therefore valid on the assumption that the part is at 20 °C, unless something else is stated.

A part that has just left the machine is not at 20 °C. Cutting heat has warmed it, and as it warms it grows. How much depends on the material:

In numbers: a 500 mm aluminium part warmed from 20 °C to 30 °C grows by 0.116 mm. On tight-tolerance work that alone puts the part out. It returns to size as it cools — so the part you measured correctly at the machine reads differently the next morning.

The practical consequence: do not measure a precision part while it is warm; let it stabilise in the inspection room. With aluminium and stainless this wait matters more than with steel.

2. Tool wear

As an edge wears it removes less material: holes shrink, shafts grow. The drift is slow and one-directional, which is why the first parts are right and part fifty sits on the limit. In series work tool changes are planned rather than waited for, and measurement is built into the batch instead of left to the end.

3. Deflection: tool, part, fixture

Cutting force pushes everything. A long tool walks away from the part and the bore comes out crooked. A thin wall deflects under the force and springs back once the tool has passed. A clamp deforms the part while holding it, and the part opens up when released. None of this is fixed by a better program; it takes a shorter tool, a different depth of cut and the right fixture.

4. Released residual stress

Plate and block stock carry internal stress from rolling and heat treatment. As material is removed that stress is released and the part bends on its own. This is why a part can be in tolerance on the machine and out of it once unclamped. Where a lot of material comes off, stress relief between roughing and finishing — or distributing the passes symmetrically — is the answer.

5. Workholding and reference errors

Every setup adds a positional error. A part finished in one setup on five axes is more consistent than the same part made in three. And it matters which face the measurement is referenced to: if the drawing names no datum, the manufacturer and the customer rest the part differently and get different numbers.

6. The measurement itself

Sometimes the part is right and the measurement is wrong. A caliper and a micrometer do not read the same; the same part measured at a different temperature gives a different number; an uncalibrated instrument produces a systematic error. As the tolerance narrows, measurement uncertainty takes up a serious share of it — you cannot police a 0.01 mm tolerance with an instrument whose uncertainty is 0.01 mm.

So what should you do?

• Separate the critical dimensionsNot every dimension has to be tight. Identify the ones that genuinely are and leave the rest to the general tolerance; both cost and risk fall.
• State the datumsA tolerance with no reference cannot be measured. The drawing has to say which face the part is set on.
• State the measuring conditionOn tight tolerances, say at what temperature the part will be measured. The default is 20 °C, but writing it ends the argument.
• Discuss the process togetherThin walls, deep pockets, parts where most of the block comes off — discussed before production these are manageable; discussed afterwards they are scrap.

At Q TECH inspection is in-house: a COORD3 ARES CMM with a Renishaw PH20 five-axis head. On precision work we let the part stabilise before measuring and we build measurement into the process, so drift is seen when it happens rather than at the end.

• tolerance
• ISO 1
• 20 °C
• thermal expansion
• tool wear
• stress relief
• CMM inspection
• Ankara Ostim