An aluminum housing can have an acceptable outer shape while a mounting hole fails its position requirement. It can also meet selected finished dimensions while the buyer still lacks evidence about the casting surface between them. A useful inspection plan starts with the feature and manufacturing stage that need a decision.
The practical difference between 3D scanning and CMM inspection
Optical surface scanning captures many points across visible surfaces for comparison with a CAD model. It is useful when the question concerns the shape of a casting across an area. Tactile coordinate measurement uses a contacting probe to evaluate selected features and their relationships. It is useful when a drawing requires a defined dimensional or geometric result.
The terms overlap: a coordinate measuring machine can carry optical sensors as well as tactile probes. Hexagon describes a CMM configuration combining laser scanning and tactile measurement. In this guide, the comparison is between optical surface scanning and tactile CMM inspection, rather than two mutually exclusive machine categories. [1]
Choose the method by the tolerance, surface, access, part size and measurement task. A scanner specification or CMM brand name alone does not establish that a particular feature can be accepted.
Before machining: examine the casting shape and available stock
A surface scan can help compare a raw casting with the intended geometry, investigate shape variation and review material available for later cutting. Hexagon describes color mapping for checking whether sufficient material is present; ZEISS also describes using a scanned cast blank to assess machining allowance and plan its machining alignment. Those examples explain the method, rather than establish a guaranteed result for every casting. [1, 2]
For a housing review, identify the faces and bosses that will be machined. Ask which CAD model represents the raw casting and which represents the finished component. Then agree on the alignment and the areas where stock needs evaluation.
Specify the part state: as-cast, trimmed, heat-treated or partly machined. Keep results from different states identifiable. If a region was not measured, record the gap and decide whether another view, setup or inspection method is required before accepting that region.
After machining: evaluate the features that control assembly
For a pump housing, the inspection discussion may focus on a machined mounting face, bearing bore, seal interface and bolt-hole pattern. Ask the supplier to connect each reported result to its drawing requirement and feature identifier. An overall surface comparison and a drawing-based dimensional layout answer different questions.
Agree on the datum setup, fixture condition and measurement strategy. A CMM result depends on how the feature is sampled and evaluated. The National Physical Laboratory's CMM measurement guidance covers point selection, part loading, alignment, temperature, surface finish and the relationship between software evaluation and drawing requirements. [3]
Neither method should receive a universal accuracy claim. NPL's task-specific uncertainty guidance provides a basis for considering the uncertainty of the actual measurement task. Where acceptance is sensitive to measurement uncertainty, agree on suitable evidence and the decision rule before approving the inspection plan. [4]
Read the alignment and color scale before the color map
When reviewing a scan report, first identify the CAD revision, units, part state, alignment and displayed deviation range. Ask how the alignment relates to the drawing's datum requirements and to the manufacturing question being investigated.
An overall best-fit comparison can be useful for studying shape. It should not silently replace a required datum-based evaluation of assembly features. If the supplier provides both views, have each view labeled with its purpose and alignment settings. Treat a change in alignment as a change in the evaluation that needs explanation.
A useful report identifies the evaluated areas, missing data and applicable limits alongside the image. Ask for explicit feature results wherever the drawing requires them. A green region on a chosen display scale is only one part of the acceptance evidence.
A practical example: reviewing an A356 pump housing
The following is an illustrative inspection plan, not a report of a completed Bohua customer project. Consider an A356 housing with an external cast profile, machined mounting face, bearing bore and bolt holes. The drawing and agreed control plan remain the basis for acceptance.
At the casting stage, the review could use a surface comparison to investigate the outer profile and stock around selected machining areas. The report would identify the casting, model revision, part state, alignment and areas evaluated.
After machining, a dimensional report could evaluate the specified face, bore and hole requirements using the approved datum setup and suitable measurement methods. Link these results to the same part or batch identifier used in the earlier review.
If the housing also has pressure-tightness requirements, retain a separate, project-defined test record. Dimensional conformity alone does not demonstrate pressure-tightness. Internal integrity, surface texture and other specified characteristics likewise need their own appropriate acceptance evidence. [5]
Five questions to ask when comparing inspection proposals
1. What is being accepted? Identify the drawing features, geometry and manufacturing stage. Separate a development investigation from a report used for shipment release.
2. What will actually be measured? Define the scan coverage or CMM feature list. Have the supplier identify inaccessible regions and explain how required characteristics in those regions will be addressed.
3. How will the part be located? State the required datum references, fixture condition and alignment approach. Record any approved alternative and its purpose.
4. How will the result support a decision? Request the nominal requirement, tolerance, result, unit and evaluation method for each required characteristic. Discuss task-specific measurement capability where necessary.
5. What will be supplied? Agree on the report format, part or batch identification, revision references, sampling scope and handling of nonconforming results. If point clouds or native project files are needed, include them in the agreed deliverables.
First samples and repeat production need different planning
A first-sample review is an opportunity to resolve model differences, clarify datums and establish the required inspection evidence before a program reaches repeat production. ZEISS describes optical metrology for initial samples and series production, alongside CMM inspection of finished parts. [6]
For repeat orders, agree on which characteristics require ongoing measurement, how parts are selected, and which changes trigger renewed verification. Examples to discuss include a drawing revision, tooling repair, fixture change or a change to the approved manufacturing route. The customer's requirements and control plan determine the response.
Keep the sampling decision separate from the choice of instrument. The presence of a scanner or CMM does not, by itself, mean every feature on every shipped part has been measured.
What Bohua can review with your drawing
Bohua's confirmed equipment includes Hexagon Global S and NANO Metrology CMMs, a German-brand 3D scanner and in-house CNC machining capability. Its quality-control scope includes dimensional inspection and project-defined inspection records. The appropriate method, part fit, feature access and deliverables are reviewed for the specific component. [5]
For a useful review, provide the controlled 2D drawing, matching 3D model, alloy and temper, casting process if specified, and the delivered machining or coating state. Identify the critical features, datum requirements, annual volume, sample quantity and required reports.
Include any disagreement you need resolved, such as a scan comparison that appears acceptable while an assembly feature fails its specified check. Supply the relevant report settings and revision information so the review can address the same measurement question.
Use the Start RFQ button below to request a casting and inspection review. Include your inspection requirements in the enquiry and follow the enquiry page's instructions for sharing drawings.
Technical references
[1] Hexagon — Optimising precision casting. Discussion of laser surface scanning, machining stock and combined optical/tactile measurement. Publisher: hexagon.com.
[2] ZEISS — Fast and safe milling of the tool on the basis of a digitized cast blank. An application example for scan comparison, machining allowance and alignment. Publisher: zeiss.com.
[3] National Physical Laboratory — CMM measurement strategies, Measurement Good Practice Guide 41, second edition. Publisher: npl.co.uk.
[4] National Physical Laboratory — Co-ordinate measuring machine task-specific measurement uncertainties, Measurement Good Practice Guide 130. Publisher: npl.co.uk.
[5] Bohua Casting — Quality Control & Inspection and the confirmed company capability records. Public capability page: www.bohua-casting.com/quality-control.
[6] ZEISS — Quality Assurance for Foundries. Inspection approaches across casting and machining stages. Publisher: zeiss.com.
Sources reviewed on 20 September 2026. Equipment-manufacturer examples explain measurement principles; they do not identify the configuration of Bohua's scanner or establish a project-specific acceptance limit.
FAQ
Can 3D scanning replace CMM inspection for an aluminum casting?
The answer depends on the feature, tolerance, access and validated measurement method. A surface comparison can be valuable for casting geometry, while selected drawing requirements may need tactile measurement or another method. Agree on the evidence required for each critical characteristic.
Should inspection take place before or after CNC machining?
Define the decision at each stage. Inspection before machining can help evaluate casting geometry and available stock. Inspection after machining verifies the required finished features. Record the part state and use the correct model and drawing revision for each evaluation.
What should accompany a scan color map?
Request the part or batch identifier, CAD revision, units, manufacturing state, alignment, deviation scale, evaluated areas and acceptance requirements. Required drawing characteristics should have explicit results rather than relying only on the image.