Choose CMM scanning when a defined contact path is needed to assess form or profile, and consider touch-trigger inspection when selected points can support the drawing requirement. Neither method guarantees acceptance by itself. Agree on feature access, sampling locations, evaluation settings, measurement uncertainty, and the evidence required before approving a CNC part.
By SUUXIANG Engineering Team
What the two CMM methods measure
A coordinate measuring machine (CMM) uses a probe to establish measured locations on a part. In touch-trigger inspection, the probe contacts the surface at discrete, planned locations. Those points can be used to evaluate geometric features when their distribution is appropriate for the characteristic being checked. Renishaw: Touch-trigger probes
In contact scanning, a stylus stays in contact with the feature while moving along a path and collecting surface data. A scanning probe can also acquire discrete points, so the probe type alone does not tell a buyer how a particular feature was measured. Renishaw: Scanning probes
Here, scanning means tactile contact scanning on a CMM. Optical scanning and probing performed on a machining tool are different inspection arrangements; do not assume their results or plans are interchangeable with the CMM approach described here.
The relevant purchasing question is therefore: What evidence does this characteristic need, and will the proposed measurement reach and evaluate the right surface? A report that simply says “CMM inspected” leaves that question open. For the wider inspection context, see CMM Inspection.
Choose the method by characteristic
Start with the drawing callout, the part’s function, and the consequence of an incorrect decision. Then consider the accessible surface, stylus arrangement, coverage, evaluation, and uncertainty. Different characteristics on the same part can justify different collection strategies.
Use discrete contacts for a defined feature question
Touch-trigger points may be a practical choice when the inspection plan identifies suitable locations and the resulting measurement addresses the drawing requirement. Examples include checking selected faces, locations, or a bore size where the agreed method and sampling adequately support that decision.
Ask the supplier to show where points will be taken, including their distribution around and along the feature. A point plan concentrated in an easy-to-reach area may leave a critical region unexamined. Conversely, adding contacts without a reason does not necessarily improve the approval decision.
Consider scanning when sampled form matters
A scan path can provide a more detailed view of the sampled surface where shape variation is relevant. Renishaw identifies form and complex surfaces as applications for scanning data. Renishaw: Scanning probes
For a mating profile or a sealing surface, ask whether the proposed path crosses the regions that matter to function. The path, its extent, the data processing, and the evaluation must be specified together. A dense scan along the wrong line can miss the region that prompted the inspection in the first place.
Scanning describes how data is collected; it does not establish that the result is automatically accurate, free of uncertainty, or representative of every point on the surface. Review the coverage and the measurement process before using the result to approve the feature.
A bore can require both decisions
Consider a hypothetical bore with a diameter requirement and a separate form concern. A planned set of discrete contacts might support the diameter evaluation if the chosen locations and calculation are suitable. That reported diameter alone may not answer the separate question about form along the bore.
For the form question, request a path or other suitable strategy that covers the relevant axial and circumferential regions. A scan at one section of the bore does not automatically describe another section. Have the inspection plan state which characteristic each dataset supports and which surfaces remain outside its coverage.
The same distinction applies to a flat face: a reported feature result and an examination of local form are different approval questions. Ask for the evaluation required by the drawing rather than treating a visually detailed plot as a substitute for the specified result.
Plan sampling around the actual surface
A sampling plan should identify both which parts are inspected and where each inspected part is measured. A detailed scan of one selected part does not, by itself, establish conformity for the rest of a batch. Likewise, inspecting many parts at the same poorly chosen locations can leave the same feature risk unresolved.
Select parts deliberately
Agree on the inspection stage and the parts to be presented for approval. Depending on the project, a buyer may request a first article, a defined batch sample, or specified checks on each part. Record the selection method and whether the parts represent relevant setups, cavities, or manufacturing conditions.
If the selection does not cover a condition that matters to the approval decision, identify the gap. Do not imply that a sample report proves every unmeasured part conforms. The appropriate sample and any escalation after a failed result should be agreed for the project, not borrowed as a universal rule.
Map contact locations and scan paths
For each critical feature, request a sketch, annotated model, or program description showing the intended contact locations or scan path. Include the usable start and end regions, any excluded areas, and the part orientation required to reach them.
Consider a bore interrupted by a slot. A path confined to an uninterrupted portion may be useful for that portion, but it does not answer a question about the surface near the interruption. The buyer and inspection team should agree whether the drawing requires evidence there, whether contact is feasible, and how any interruption will be handled. If the required region cannot be measured with the proposed path, leave that characteristic unresolved until another suitable method or an approved change is established.
For repeated features, identify whether the plan covers each relevant instance or a defined selection. A report should distinguish measurements taken on actual features from nominal locations or inferred results.
Resolve inaccessible features before release
A deep recess, narrow slot, or obstructed internal surface may limit stylus access. Ask for the proposed stylus configuration and approach, then request evidence that the intended contact area can be reached without interference. More data from an accessible outside face cannot validate a separate inaccessible feature.
If access is inadequate, possible buyer choices include an alternative measurement method, a different setup, a drawing clarification, or a documented concession through the authorized approval process. None should be assumed acceptable without review. Keep the affected characteristic blocked while the necessary evidence or authorization is missing.
Agree on evaluation and uncertainty
A point set or scan is an input to an evaluation, not the approval decision by itself. The supplier and receiving organization should agree on the reported characteristic, the applicable drawing revision, the evaluation settings, and the acceptance rule. Record enough detail to understand how the result was obtained without turning the report into a software manual.
For example, a supplier and receiver can collect data on the same bore but apply different feature constructions, filtering, exclusions, or evaluation settings. If their reported results differ, first compare the measurement scope and settings before concluding that either part or instrument has changed. The organizations should agree on the governing interpretation and resolve any remaining difference before release.
Discuss measurement uncertainty in relation to the decision being made. The buyer should request the uncertainty information or assessment appropriate to the characteristic and ask how a result near a tolerance boundary will be handled. A displayed pass result is insufficient when the parties have not agreed how uncertainty and the acceptance rule affect release.
A change in scan speed, path, stylus, fixture, software setting, or other relevant measurement condition may affect the approved method. Ask engineering to assess and validate the change for the feature and intended decision. There is no universal point count or speed that makes every inspection plan suitable.
Buyer guide: specify the inspection before ordering
Inspection requirements are easiest to resolve while the drawing and quotation are being reviewed. The goal is an inspection scope both parties can execute and interpret, with unresolved items visible before a shipment is presented for acceptance.
Decision criteria to include in the RFQ
Provide the drawing and model revision, identify critical characteristics, and explain the intended function where it affects inspection. Specify the required inspection stage, part selection, report format, traceability, and any customer-defined acceptance rule. Where a measurement method is mandated, state it; where it is not, ask the supplier to propose a method and explain its coverage.
Request answers to these questions:
- Which characteristics will use discrete contacts, contact scanning, or another agreed method, and why?
- Where will points or scan paths be placed, and which required regions cannot be reached?
- How will interrupted surfaces, edges, burrs, or other local conditions be treated during measurement?
- What feature construction, filtering, exclusions, and evaluation settings apply to each critical result?
- What information will identify the part, drawing revision, inspection method, and report revision?
- What uncertainty assessment and acceptance rule will be used for a result near a limit?
- Which changes to the approved method require review, and who can authorize them?
- What happens if a characteristic cannot be measured or the required evidence is unavailable?
For stylus-specific records, the related guide to CMM stylus qualification records addresses the qualification scope separately. Request the records relevant to the proposed setup rather than assuming a generic statement covers every configuration.
Numbered workflow for inspection and approval
- Define the requirement. Identify the drawing revision, characteristic, tolerance, function, and applicable acceptance rule. Mark any ambiguous callout for engineering clarification before inspection.
- Choose the part sample. Agree which parts and manufacturing conditions the evidence must represent. Record how inspected parts will be identified and what the sample does—and does not—support.
- Propose the measurement strategy. Assign discrete contacts, contact scanning, or another suitable method to each characteristic. Show intended locations, paths, feature access, and exclusions.
- Review the complete setup. Check the proposed stylus, fixture, part condition, and relevant qualification evidence. Confirm that the method reaches the required region and that the uncertainty assessment is suitable for the decision.
- Agree on evaluation and reporting. Define feature construction, processing, settings, acceptance rule, and report contents. Resolve differences between supplier and receiving methods before using their results interchangeably.
- Inspect and review the evidence. Match each reported value or plot to the correct part and drawing characteristic. Investigate missing coverage, unexpected results, or changes from the agreed plan.
- Disposition and retain the decision. Approve only the scope supported by evidence and authorized acceptance. Record deviations, open items, responsibilities, and the approved method for future orders; reassess relevant changes before relying on it again.
Compact buyer checklist
Before approving the inspection plan or the parts, confirm:
- The drawing revision, critical characteristics, and part identities match the report.
- The part sample and feature sampling locations answer the intended approval questions.
- Required surfaces are accessible, and gaps or interruptions are documented.
- Each size, position, form, or profile result uses the agreed evaluation.
- Supplier and receiver settings, uncertainty treatment, and acceptance rule are understood.
- Method changes have been reviewed, and missing evidence has an owner and disposition.
Buyer FAQ
Is CMM scanning always better than touch-trigger inspection?
No. Choose the collection method for the characteristic and the required coverage. A suitable discrete-point strategy may answer a defined feature question; a contact scan may be useful when the sampled shape needs closer examination. Both need an appropriate setup and evaluation.
How many points are enough to approve a feature?
There is no universal count. Ask the inspection team to justify point locations or scan coverage against the feature geometry, drawing requirement, access, and intended decision. A count without a location map does not show whether the relevant region was measured.
Can a scanned bore be approved from a single plot?
Only if the plot and supporting report provide the evidence required for the specific characteristic and agreed scope. Check which regions were scanned, how the data was evaluated, and whether the report addresses the actual drawing requirement. A plot of one region cannot establish coverage of an unmeasured region.
What if the supplier and receiving inspection disagree?
Hold the affected decision and compare part identity, revision, sampled locations, stylus and setup, evaluation settings, and acceptance rules. Agree on the governing method or investigate the difference with the responsible engineering and quality teams before dispositioning the parts.
Can a CMM report approve an entire batch?
A report supports the decision within its identified inspection scope. A report for selected parts does not establish that every unmeasured part conforms. The buyer and supplier should agree on batch sampling, traceability, and the disposition of any failed or missing evidence before release.
To discuss a drawing-based CNC part and its inspection requirements, contact SUUXIANG with the drawing, critical characteristics, and desired report scope. SUUXIANG is the public brand of Dongguan Xuxiang Precision Mold Co., Ltd.
Sources
- Renishaw: Scanning probes — Contact scanning, discrete-point capability, and form measurement.
- Renishaw: Touch-trigger probes — Discrete-point measurement with touch-trigger probes.