Why Is Tolerance Selection the Most Consequential Decision in CNC Part Design?
A tolerance that is tighter than required will be paid for many times over — in machining time, in tool wear, in inspection time, in scrap rate, and in delivery delay. A tolerance that is looser than required will be paid for once — in a returned assembly that does not fit. The asymmetry between the two costs means that engineering judgment matters more than blanket "tight is safer" thinking.
GD&T (ASME Y14.5 / ISO 1101) provides a vocabulary for expressing tolerances that is more expressive than the older plus/minus system. It allows engineers to specify what matters (form, orientation, location, runout) and to relax what does not matter (irrelevant features). When used correctly, GD&T reduces cost without sacrificing function. When used incorrectly, GD&T creates parts that fail inspection despite being functionally acceptable, or parts that pass inspection but fail in service.
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CNC machine tool processing capability at Yuqing is documented per machine and per process. Tolerance selection is the bridge between the design's functional requirement and the shop's manufacturing capability.
What Are the Standard Tolerance Grades?
ISO 286 defines tolerance grades IT01 through IT18 for linear dimensions. Higher grade numbers mean looser tolerances; lower grade numbers mean tighter tolerances.
|
Grade |
Typical Application |
Typical Tolerance (at 100 mm) |
|
IT5–IT6 |
Gauges, master references |
16–24 µm |
|
IT7 |
Precision bearings, fits |
30 µm |
|
IT8 |
General precision machining |
46 µm |
|
IT9 |
Standard CNC machining |
74 µm |
|
IT10–IT11 |
Loose commercial machining |
120–220 µm |
|
IT12–IT13 |
Sheet metal, sand casting |
300–540 µm |
For a custom CNC machined part, IT8 to IT10 is typical. IT7 requires grinding or specialized processes; IT11 and above is achievable on lighter equipment.
The tolerance grade chosen must be matched to the machine's capability. A machine that holds IT8 under controlled conditions cannot reliably hold IT6 without additional setup, slower cutting, or secondary finishing.
What Is the Cost Multiplier of Tightening Tolerances?
Tightening a tolerance from IT10 to IT7 typically increases the machining cost by 3× to 10×, depending on the feature and the material. The cost comes from:
Slower cutting speeds to reduce tool deflection and thermal error.
Additional setups and repositioning.
Higher-grade tooling with shorter life.
More inspection time per part.
Higher scrap rate when parts fail first-article inspection.
The non-linear cost curve means that a single over-tight tolerance on an unimportant feature can dominate the part's total cost. The engineering response is to challenge every tight tolerance, ask what function it serves, and relax it where the function allows.
How Should Tolerances Be Specified?
Tolerances should be specified in three layers:
General tolerance (per ISO 2768 or ASME Y14.5): applied to any feature without an explicit tolerance. Defines the baseline capability expected from the manufacturer.
Feature tolerance (called out on the print): applied to specific features where the general tolerance is insufficient. Defines what matters for that feature.
GD&T tolerance (geometric tolerance): applied where form, orientation, location, or runout matters independently of size.
The general tolerance should be loose enough that the shop can hold it on every feature without slowing down. The feature tolerances should be as tight as the function requires, no tighter. The GD&T tolerances should be applied to features where the geometric relationship matters more than the absolute size.
A typical general tolerance for custom CNC machining is ISO 2768-mK (medium class) or ±0.1 mm on dimensions, ±0.2 mm on radii. Anything tighter must be called out explicitly.
What GD&T Symbols Apply to CNC Machining?
The most commonly used GD&T symbols and their applications:
|
Grade |
Typical Application |
Typical Tolerance (at 100 mm) |
|
IT5–IT6 |
Gauges, master references |
16–24 µm |
|
IT7 |
Precision bearings, fits |
30 µm |
|
IT8 |
General precision machining |
46 µm |
|
IT9 |
Standard CNC machining |
74 µm |
|
IT10–IT11 |
Loose commercial machining |
120–220 µm |
|
IT12–IT13 |
Sheet metal, sand casting |
300–540 µm |
Each symbol carries a tolerance value (in mm or inches) and is referenced to one or more datums. The datum scheme must be defined on the drawing and must reflect the functional reference of the part in the assembly.
How Are Datums Selected?
A datum is the functional reference surface (or feature) from which all geometric tolerances are measured. Datum selection is the most consequential GD&T decision.
A datum scheme should reflect how the part is located in the assembly, not what is easy to measure on the shop floor. A part that mounts on three bosses should have the datum on those bosses; a part that mounts on a face and a hole should have the datums on the face and the hole.
A defensible datum scheme:
Identifies the primary, secondary, and tertiary datums in order of functional importance.
Uses manufacturing features (bosses, faces, holes) that the part-maker can reach.
Avoids redundant datums that complicate inspection.
Documents the datum reference frame in a note on the drawing.
A drawing without a clear datum scheme is a drawing that the shop will interpret arbitrarily, producing parts that are correct individually but fail in the assembly.
How Does Tolerance Stack Up in an Assembly?
In an assembly of multiple parts, the tolerances on each part stack up. The total clearance or interference at the assembly interface is the sum of the individual tolerances (worst case) or the root-sum-square (statistical).
A 5-part assembly where each part is held to ±0.1 mm produces a stack-up of ±0.5 mm worst case. The mating interface must be designed to accommodate this variation, or some of the parts must be held to tighter tolerances.
The engineering response is to identify the critical interface (often called the "key characteristic") and tighten only that interface, leaving the rest at general tolerance. A drawing that calls out ±0.05 mm on every dimension of every part is a drawing that has not considered tolerance stack-up.
What Are the Common Tolerance Pitfalls?
Common mistakes in tolerance specification:
Tight general tolerance: setting the baseline at IT8 or tighter when the part does not require it. Inflates every feature's cost.
No datum scheme: leaving GD&T symbols without a clear datum reference. Inspection becomes ambiguous.
Redundant tolerances: specifying both a size tolerance and a geometric tolerance that overlap. Creates inspection disputes.
Tight tolerances on cosmetic features: specifying tight flatness on a surface that is hidden in the assembly. Wastes cost on invisible features.
No bonus tolerance: using fixed-tolerance positional symbols when the size tolerance could provide bonus. Limits manufacturing flexibility.
Ignoring thermal effects: specifying tight tolerances at room temperature on parts that will be used at elevated temperature. The part will be out of tolerance in service.
Each pitfall is preventable by training, by drawing review, and by feedback from the shop floor.
How Is Machinability Considered?
Some features are inherently harder to hold to tight tolerances. The drawing should reflect this:
Thin walls: deflect under tool pressure; loose tolerance is the practical choice.
Long slender features: vibrate during machining; loose tolerance or multiple setups needed.
Deep cavities: tool deflection increases with length; loose tolerance or shorter tools needed.
Hardened materials (HRC > 45): tool wear is rapid; loose tolerance or grinding needed.
A drawing that calls for ±0.02 mm on a thin wall in hardened tool steel is a drawing that will fail in production. The shop should flag this at the quotation stage and propose a relaxation or an alternative process.
How Is First Article Inspection Used?
First Article Inspection (FAI) is the verification that the first part produced meets all drawing requirements. FAI is performed against the full drawing, not just the critical dimensions.
A defensible FAI:
Records all dimensions and GD&T characteristics with measured values.
Compares measured values to the drawing's tolerance band.
Identifies out-of-tolerance features and dispositions them (use, rework, scrap).
Documents the inspection equipment used and its calibration.
Is signed by the inspector and a quality representative.
FAI is the gate between prototype and production. A part that passes FAI can proceed to production; a part that fails FAI must be reworked or the process must be adjusted before more parts are produced.
How Is Process Capability Verified?
Process capability is the statistical measure of how well the process holds the tolerance. It is expressed as Cpk (process capability index), which compares the tolerance band to the process variation.
A Cpk of 1.33 is the typical minimum for a production process. A Cpk of 1.67 is preferred for tight tolerances; a Cpk below 1.0 indicates the process is not capable and will produce out-of-tolerance parts routinely.
Process capability is measured by:
Producing 25–50 consecutive parts under controlled conditions.
Measuring the critical feature on each part.
Calculating the standard deviation and the Cpk.
Acting on the result: re-derive tolerances if Cpk is low, tighten the process if Cpk is acceptable but the design requires more margin.
A shop that quotes tight tolerances without verifying process capability is a shop that will miss delivery dates or ship out-of-tolerance parts.
What Is the Relationship Between Tolerance and Material Selection?
Some materials are more difficult to machine to tight tolerances than others. Aluminum alloys (6061, 7075) are machinable to ±0.025 mm routinely. Stainless steels (304, 316) are tougher and typically machined to ±0.05 mm. Tool steels and titanium alloys are more demanding and often require grinding for sub-±0.025 mm tolerances.
The drawing's tolerance and the material's machinability must be aligned. A ±0.01 mm tolerance on 17-4 PH stainless steel in the H900 condition requires grinding or EDM, not standard CNC milling.
The processing of mechanical parts service at Yuqing covers a range of materials and tolerance bands; the engineer should specify both on the drawing.
How Should Tolerances Be Communicated?
Tolerance communication is the bridge between the design and the shop. A defensible communication package:
The drawing (with general tolerance, feature tolerances, GD&T, datums, and notes).
A 3D model with PMI (Product Manufacturing Information) embedded, where applicable.
A specification of inspection requirements (CMM, gauge, hand tools).
A statement of any critical characteristics (KCs) and their statistical requirements.
A list of cosmetic requirements (surface finish, marking, packaging).
The shop needs all of this to quote accurately and to inspect correctly. A drawing that omits the datum scheme forces the shop to guess; a specification that omits the inspection method allows the shop to use whatever is convenient.
Conclusion
Tolerance selection is the engineering discipline that defines the geometric precision required for each feature, expresses it in GD&T terms that are both manufacturable and verifiable, and matches the design's functional requirement to the shop's process capability. The
CNC machine tool processing and mechanical parts processing capabilities at Yuqing support a range of tolerance grades and GD&T specifications; engineers should specify the tolerance that the function requires, no tighter. A well-engineered tolerance specification is the foundation of cost-effective, on-quality CNC production.
Frequently Asked Questions
What is the typical general tolerance for custom CNC machining?
ISO 2768 medium (mK) is typical, equivalent to ±0.1 mm on dimensions and ±0.2 mm on radii. Tighter general tolerances should be justified by functional requirement.
When should GD&T be used instead of plus/minus?
GD&T should be used when the geometric relationship between features (position, orientation, form, runout) matters more than the absolute size. For holes that must align in an assembly, position tolerance is the correct call. For a shaft that must be round to seal, circularity or cylindricity is the correct call.
How does bonus tolerance help manufacturing?
Bonus tolerance allows the manufacturer to use the size tolerance as additional positional or orientational tolerance. A hole with a size tolerance of ±0.1 mm and a position tolerance of 0.3 mm at MMC (Maximum Material Condition) can use up to 0.3 mm of bonus from the size tolerance, depending on the actual produced size.
What is the relationship between IT grade and machining cost?
Each step of IT grade (e.g., IT10 to IT9) typically increases machining cost by 30–80%, depending on the feature and material. Going from IT10 to IT7 can increase cost by 3× to 10×.
How is Cpk calculated and what is acceptable?
Cpk = min((USL - mean) / 3σ, (mean - LSL) / 3σ), where USL and LSL are the upper and lower specification limits and σ is the standard deviation. A Cpk of 1.33 is the typical minimum for production; 1.67 or higher is preferred for tight tolerances.
How are tolerances verified on large structural parts?
Large structural parts are typically verified with a coordinate measuring machine (CMM), a laser tracker, or a portable measuring arm. The choice depends on the part size, the tolerance band, and the number of features.
What is the difference between MMC and LMC?
MMC (Maximum Material Condition) is the condition where the feature contains the most material — for a hole, the smallest diameter; for a shaft, the largest diameter. LMC (Least Material Condition) is the opposite. Bonus tolerance is allowed at MMC, not at LMC.
Can tolerances be relaxed in production?
Tolerances can be relaxed only with engineering approval and drawing revision. A shop that loosens tolerances without authorization is shipping non-conforming parts, regardless of whether the parts work in the assembly.
How is thermal expansion handled in tolerance specification?
The drawing should specify the temperature at which the tolerance applies. Parts measured at 20 °C that will operate at 200 °C will be dimensionally different; the tolerance must be wide enough to accommodate the operating condition, or the inspection must be performed at the operating temperature.
