A partial arc measurement on a CNC part is not automatically a measurement of the drawing radius. When only a short circular segment is available, the buyer and supplier must agree on the section plane, included segment, point treatment, fitting method, and acceptance characteristic before inspection begins. That agreement prevents a plausible-looking fitted radius from becoming an unapproved substitute for design intent.
A partial arc is a visible or accessible portion of a circular feature rather than a complete circle. It may be a fillet in a pocket, a relieved profile, a blended transition, or a curved surface interrupted by adjacent geometry. The issue is common on CNC parts because the functional curve is often bounded by tangencies, tool access, intersecting walls, or a design blend.
The essential distinction is simple: a radius reported by software is the result of a defined calculation on selected data. Drawing compliance is an approval decision against the actual requirement. Those are related, but they are not interchangeable.
Why short arcs create approval risk
A complete circular feature supplies broad geometric evidence for its center and radius. A partial arc supplies less of that evidence. Small changes in the chosen endpoints, the section plane, the point distribution, or the fit algorithm can move the reported center or radius. That does not mean the part is acceptable or unacceptable by itself; it means the measurement definition needs to be controlled.
This is especially important when the drawing calls out a nominal radius but does not say whether that radius is a directly accepted size, a profile-controlled contour, a tangent blend, or part of a modeled surface. A nominal radius may communicate design intent without defining an independent inspection characteristic. Conversely, a radius dimension with its own tolerance may require a method that establishes that characteristic as intended.
The buyer should first ask what the radius is meant to protect:
- A mating component’s clearance or contact path.
- A controlled blend between two surfaces.
- A tool-generated relief that must avoid a sharp edge.
- A contour that belongs under profile control.
- A radius whose own size is functionally important.
The answer determines the inspection plan. It should not be inferred after a report produces a surprising value.
Define the measurement before choosing the instrument or software
“Measure the radius” is incomplete for a partial circular segment. A usable instruction says what data represents the arc and how the result will be interpreted.
Start with the section plane. If the intended arc exists in a specified two-dimensional section, the inspected points must be projected or acquired in that same defined plane. A scan taken on a neighboring surface, at a different height, or across a changing blend can describe another curve entirely.
Then identify the start and end of the included arc. Those endpoints should be defined from drawing geometry, CAD references, nominal tangent boundaries, or mutually approved physical references. They should not be selected after looking for the most favorable result.
A practical inspection definition records:
- Drawing and CAD revision.
- Feature name and functional purpose.
- Datum alignment and section-plane definition.
- Arc start and end references.
- Whether the reported value is radius or diameter.
- Units.
- Probe compensation or surface-compensation treatment.
- Acquisition approach and the raw measured points.
- Included and excluded regions, including any transition region.
- Circle-fit method and any fit constraint.
- Software name and version.
- The acceptance characteristic and disposition authority.
The record is modest compared with the cost of debating an ambiguous report after parts are complete.
What circle fitting does—and does not—tell you
Circle fitting estimates a circle from measured points. Different fitting approaches answer different questions. In PC-DMIS documentation, least-squares fitting minimizes average squared radial distance, while minimum-separation fitting is framed around minimizing the maximum deviation between concentric boundary circles containing the data. Maximum-inscribed and minimum-circumscribed constructions serve mating or containment interpretations rather than being universal radius answers. Hexagon: Best Fit Type (for Circle)
For a buyer, the conclusion is not that one algorithm is always correct. It is that the algorithm must match the characteristic being accepted.
Keep the focus on the limited segment and the approved characteristic. A fit suitable for process trending does not automatically establish functional acceptance.
Fixed-radius fitting needs special care. Hexagon describes fixed-radius construction as using a pre-given diameter while solving the circle position to minimize maximum radial distance; the radius is not allowed to vary. Hexagon: Best Fit Type (for Circle) Therefore, it cannot independently establish an actual measured radius. It can support an analysis of how selected data departs from a nominal circle, but reporting the fixed nominal value as the measured radius is misleading.
A decision framework for the drawing requirement
Use the drawing requirement, rather than the available software menu, to select the inspection result.
When the radius is independently toleranced
If the drawing assigns a radius and a direct tolerance, agree on a free-radius method that calculates radius from the included data. Preserve the fit type and raw data in the report. If the segment is short or includes a transition, require engineering review of the method before treating one computed value as approval evidence.
When the curve is controlled by profile
If a profile tolerance controls the surface, the primary acceptance characteristic is profile under the drawing requirements, including any specified datum references, not a separately fitted radius. A radius fit may still help diagnose a machining condition, but it should be labeled as supporting information. Do not convert a profile requirement into a radius-only pass/fail rule without design authority approval.
When the drawing uses a basic or nominal radius for a blend
A nominal radius may describe intended CAD geometry while continuity, adjacent surfaces, or profile control carry the acceptance burden. Confirm whether the nominal value is informational, derived from the model, or intended as an independently verified size. This clarification belongs in the inspection plan, not in an operator’s personal convention.
When a fixed nominal circle is functionally relevant
Use a fixed-radius evaluation only when the approved question is deviation from that nominal circle and the position solution is understood. Label it as a nominal-circle or fixed-radius assessment. Do not call it “actual radius.”
Four common disputes and how to prevent them
A blended fillet is treated as a pure circle. A scan may cross a tangency transition or a noncircular blend near the ends. Including that region can change the fit, yet silently deleting it is not an answer. Define the usable circular region from CAD or approved geometry, state why any transition region is excluded, and retain the original data.
A fixed nominal radius is reported as measured. The software output may show the nominal radius because that value was imposed. The report should state that radius was constrained and should separately report the agreed deviation, center location, or other calculated characteristic.
More points are mistaken for more geometric coverage. Denser points within the same small angular span can improve description of that local surface, but they do not turn the segment into a full circle. Review span, point placement, and endpoint definition separately from point quantity.
Customer and supplier evaluate different segments. One party measures from visible edge to visible edge; the other avoids transition material. Both may obtain defensible calculations for different data sets. The cure is an approved start/end definition, not an argument over which result looks better.
Approval workflow for partial arcs on CNC parts
- Identify the governing requirement. Mark whether the feature is an independently toleranced radius, a profile-controlled surface, a tangent blend, or another functional geometry.
- Set the alignment and section plane. Reference the applicable datums and define the plane or surface path on which the arc will be evaluated.
- Freeze the included segment. Specify arc start and arc end using CAD coordinates, nominal intersections, tangency boundaries, or other clear references. State any approved transition region excluded from the fit.
- Choose the acquisition and compensation treatment. Define contact or noncontact data capture as applicable, plus probe compensation, filtering controls if any, and treatment of invalid readings. Keep raw points available for review.
- Choose the fitting rule. Name the fit algorithm, constraint status, radius-versus-diameter convention, units, and software version. If fixed radius is selected, state that it is a constrained nominal-circle evaluation.
- Run a method comparison when risk is material. Apply the same agreed segment to the relevant CAD or representative measurement data and compare the proposed reporting approaches. The purpose is to confirm interpretation, not to search for a passing algorithm.
- Approve the acceptance statement. Design authority—not the inspection operator—approves whether the result is judged against radius, profile, location, or another characteristic. Any change to tolerance or acceptance logic requires a documented drawing clarification or authorized revision.
- Release a controlled report format. Include the setup, selected data, excluded region, algorithm, constraints, result, and disposition. Preserve the definition for later lots and revision changes.
A compact buyer checklist for the RFQ and FAI plan
- Send the current 2D drawing and, where available, the 3D model.
- Identify the partial arc as a CTQ feature when it affects assembly or tool function.
- State whether the radius itself, profile, tangency, location, or a combination is the acceptance requirement.
- Define the datum alignment and section plane.
- Mark the intended arc start and end on the drawing or CAD view.
- State whether an endpoint transition is included, excluded, or awaiting design clarification.
- Require the report to show radius versus diameter, units, fit method, and constraint status.
- Ask for raw-point retention and documented exceptions rather than silent endpoint removal.
- Require design authority approval for any interpretation that changes the drawing’s apparent acceptance logic.
For CNC procurement, SUUXIANG should receive the drawing revision and the agreed inspection definition together. Its quality assurance discussion is most useful when the report characteristic is defined before machining rather than reconstructed after inspection.
Hypothetical approval decisions
Consider a pocket fillet that blends into a vertical wall. The drawing shows a radius, while a profile control governs the contoured surface. The buyer may decide that profile is the acceptance result, with a free-radius fit shown only as diagnostic information. The approved segment excludes the modeled transition zone, and the report identifies that exclusion. No tolerance has changed; the parties have clarified what the existing drawing controls.
Now consider a curved relief intended to mate with a cylindrical component. The radius has its own tolerance and location is also important. The buyer may approve a free-radius fit for actual radius, plus a separate center-location evaluation relative to the datum system. A fixed-radius fit may be included to visualize departure from nominal, but it cannot replace the actual-radius result.
Finally, imagine a supplier receives a drawing with a nominal radius but no direct tolerance, no profile callout, and no defined blend boundaries. The correct outcome is not an assumed pass/fail method. The feature remains an open clarification item. Review any title-block or general tolerances as well. If the governing requirement remains unclear, keep the inspection plan pending until design authority provides a documented clarification.
FAQ
What is partial arc measurement on a CNC part?
It is evaluation of a circular segment that does not cover a complete circle. The inspection definition must identify the measured section, endpoints, fitting method, and acceptance characteristic.
Can a CMM report prove the drawing radius from a short arc?
A CMM can calculate a result from acquired data, but whether that result proves drawing compliance depends on the approved feature definition, selected segment, fitting rule, and governing tolerance.
Is least-squares fitting always the best choice?
No. Least squares answers an average-deviation question. A boundary, mating, profile, or fixed-nominal-circle requirement may need another explicitly approved evaluation.
Can we discard points near the arc endpoints?
Only under an agreed, documented procedure. Scan-segment controls can define start/end selection and endpoint discarding; changing those settings changes the data used to construct the arc. Hexagon: Constructing an Arc from Part of a Scan
Does adding more points solve a short-arc disagreement?
Not by itself. More points may describe the selected segment more densely, but they do not resolve an undefined section plane, different endpoints, a transition blend, or an unsuitable acceptance rule.
If your drawing includes a short radius segment, discuss the inspection brief with the drawing revision, CAD data, and proposed acceptance definition before release.