Accurate alignment is the foundation of every Portable CMM measurement. Even the most advanced Portable CMM can produce incorrect results if the coordinate system is established improperly.
In this guide, we utilize PMT INSPECT, PMT’s PTB-certified 3D metrology software, alongside the PMT Portable CMM to detail key software features, common coordinate system alignment methods, when to use them, and how PMT INSPECT software simplifies the process.
PMT INSPECT 3D Metrology Software Interface Overview
1. Top Function Area
Integrates core functions such as CAD import/export, device connection, feature creation, and coordinate system setup. Users can also customize the device list so that only selected device appears during connection, helping streamline the workflow.
2. Project Management Area
Displays all project data in a clear tree view, including CAD models, point clouds, alignments, measured features, inspection reports, and color maps, facilitating process organization and traceability.
3. Central Main View Area
Displays CAD models, measurement data, and inspection results. Users can switch seamlessly between 3D View and Report View for efficient visualization, dimensional inspection, GD&T analysis, and data comparison.
4. Right-Side Toolbar
Provides quick access to frequently used tools, including CAD model/point cloud import, GD&T and color map display controls, annotations, and display options for efficient measurement and analysis.
5. Bottom Status Bar
Delivers real-time status feedback regarding device connection, probe diameter, measurement mode, units, and other key information.
Understanding Coordinate Systems in Portable CMM Measurement
To achieve accurate and stable measurement results, you must first know how to properly establish a coordinate system using the Coordinate Systems interface in PMT INSPECT 3D metrology software. The software features three modules: Create Coordinate Systems, CAD Alignments, and Point Cloud Alignments – each engineered to meet various workpiece structures, data types, and inspection goals.
1. Create Coordinate Systems
This module features Cartesian Coordinate System and Cylindrical Coordinate System. The Cartesian system is the most widely used method for defining X, Y, and Z directions, while the Cylindrical system is ideal for rotational parts such as flanges and gears.
2. CAD Alignments
Primarily used for contact measurement, this module includes the most common Portable CMM coordinate alignment methods: Plane, Axis, Center Point, Three-Plane Alignment, Best-Fit Measurement Objects. Other methods suit specific scenarios, such as Surface Points Alignment for irregular workpieces like sheet metal parts.
3. Point Cloud Alignments
Mainly for scanning scenarios, this method strictly requires a CAD model. By aligning scanned point cloud data with a CAD model, users can quickly evaluate overall deviation and analyze complex surfaces or freeform geometries.
Common Portable CMM Coordinate Alignment Methods
Based on alignment principles and application scenarios, we detail the three most common methods: Plane, Axis, Center Point, Three-Plane Alignment, Best-Fit Measurement Objects.
The workflow is explained from principles to operational steps and typical applications, allowing even beginners to get started quickly.
Method 1: Plane-Axis-Center Point Alignment
Principle: Define the origin, axis orientation, and reference plane via a center point, an axis, and a plane.
This fast, intuitive, and frequently used method follows the classic 3-2-1 rule (order: plane first, then line, then point). It is perfectly suited for regular workpieces requiring alignment without a CAD model.
Demonstration with a PMT Portable CMM to establish a coordinate system:
Step 1.1. Connect and Activate the Device
Open the PMT INSPECT software, navigate to Device on the Top Function Area, and select the device from the drop-down list. Next, click Device Connection. Once connected, move each joint according to the on-screen instructions to active the device.
Step 1.2. Probe Features
Probe Plane: Go to Measure > Plane. Use PMT Portable Measuring Arm to evenly probe 3 or 4 points on the top surface of the part. The points should be distributed as widely as possible and must not be collinear.
Probe Line: Go to Measure > Line.Probe at least 2 points on the front surface of the part. The previously measured plane must be constrained first.
Probe Circle/Point: Go to Measure > Circle. Probe point on the right side surface of the part, preferably near a corner or edge.
Step 1.3. Define Nominal Value
In the Model View on the left side, select the measured feature plane 1. Right-click and choose Define Nominal. In the drop-down list of Creation method , select Numerically. Enter the tolerance required by the drawing in the Parameters. Click Create.Repeat the same procedure for the line 1 and circle 1.
Step 1.4. Alignment
Open the Coordinate Systems menu and select Plane, Axis, Center Point. Under Parameters > Plane > Source, select the measured value (plane 1 -meas-). For Destination, map it to the nominal value (plane 1 -nom-). Repeat the same procedure for the Axis and Center point, then click OK to complete the coordinate system alignment.
Method 2: Three-Plane Alignment
Principle: The coordinate system origin is at the intersection of three mutually perpendicular planes; each axis is perpendicular to its corresponding reference plane.
Also a form of the 3-2-1 rule, three planes replace “plane-line-point”. Ideal for box-shaped or prismatic parts with three clearly defined perpendicular datum planes – making alignment more intuitive without the need to deliberately project lines or points.
Demonstration with a PMT Portable CMM for alignment with a CAD model:
Step 2.1. Connect Device and Import CAD Model
Connect and activate the device following Step 1.1. Navigate to File > Import > Import CAD Model. Select Fix surface orientation and import the CAD model in .step, .stp, .iges, or .igs format.
Step 2.2. Select and Probe Features
Under Pick from CAD, choose Plane. Select three mutually perpendicular planes from the CAD model as plane 1, plane 2, and plane 3.
Select each plane in Model View, right-click and choose Probe Measured. Probe the corresponding planes on the part following Step 1.2.
Step 2.3. Alignment
Go to Coordinate Systems > Three-Plane Alignment. Under First plane > Source, select the measured value (plane 1 -meas-), and map the Destination to the nominal value (plane 1 -nom-). Repeat the same procedure for Second plane and Third plane. Click Align to complete the coordinate system setup.
Compared with workflows without a CAD model, inspecting with a CAD model makes operations simpler, more accurate, and enables advanced functions like GD&T automatic evaluation, batch inspection, and auto-report generation. This ensures higher consistency, automation, and data reusability, making it ideal for batch inspection.
Method 3: Best-Fit Alignment
Principle: Minimize the overall deviation between all measured points and the nominal CAD model to achieve a global best fit.
When a workpiece lacks regular datum features (planes, lines, perpendicular planes) – or when you focus on overall contour deviation rather than a specific datum – the Best-Fit Alignment is the ideal choice. This method strictly requires a CAD model.
Demonstration with a PMT Portable CMM to establish a coordinate system:
Step 3.1. Connect Device and Import CAD Model
Connect the device and import the CAD model by following Step 2.1.
Step 3.2. Select and Probe Features
Under Pick from CAD, select Circle. Choose three circles from the CAD model as circle 1, circle 2, and circle 3.
Select each circle in the Model View, right-click and choose Probe Measured. Measure the corresponding circles on the part to capture their actual positions. Ensure you constrain the reference plane before measuring circles.
Step 3.3. Alignment
Select circle 1, circle 2, circle 3 in the Model View. Navigate to Coordinate Systems > Best-Fit Measurement Objects > Start to complete the coordinate system setup.
Quick Comparison: Choosing Your Alignment Method
| Criteria | Plane, Axis, Center Point | Three-Plane Alignment | Best-Fit Measurement Objects |
| CAD Model | Optional | Optional | Required |
| Stability | Depends on probing method
and feature geometry |
High | High |
| Typical Applications | Simple, regular parts;
on-site inspection and repair |
Standard parts
such as housings and brackets |
Complex assemblies
and real-time adjustment |
| Key Advantages | Flexible and fast | Stable, standardized,
and highly repeatable |
Minimizes overall deviation
across the part |
There is no absolute “Best” alignment method. The selection depends on part geometry, available datum features, and inspection objectives. PMT recommends choosing the method that best fits the application requirements.
FAQs on Alignment in Portable CMM Measurement
The sections above clarify the three common alignment methods, but in real applications, technical difficulties may arise. This section provides answers to frequently asked questions related to coordinate system alignment in Portable CMM measurement.
Q1: When inspecting a gauge using Plane, Axis, Center Point alignment, my first measurement was perfect, but after re-aligning, the results showed massive deviations. Why?
There are two common causes for inconsistent measurement results:
Form error & sparse probing: If the physical workpiece has surface flatness errors, taking too few probing points or shifting where you probe will alter the calculation of the datum plane.
Altered alignment logic: If you change the order of your features or select different surfaces, your reference shifts. The measurement data hasn’t actually changed; your coordinate system has. Experienced engineers always follow this rule: The inspection logic must strictly obey the assembly logic.
Q2: In Portable CMM’s contact measurement, how do experienced engineers decide which alignment method to use?
To determine the best alignment method, experts follow a 3-step approach:
Analyze the drawing: Analyze the workpiece structure, drawing datums, and inspection purpose.
Prioritize stability: Choose large, stable features as the primary datum based on site conditions.
Simulate assembly logic: Imagine how the part is positioned in actual assembly; align the measurement coordinate system with assembly datums.
Q3: The same part passes with Three-Plane Alignment but exceed tolerance with Best-Fit. Which result is correct?
The answer depends on your assembly logic. If the part is actually located by three planes, trust the Three-Plane. If it relies on multiple features working together, trust the Best-Fit. Different coordinate systems naturally lead to different conclusions.
Conclusion: PMT Multi-Scenario Measurement Solutions
From Plane, Axis, Center Point, to Three-Plane Alignment and Best-Fit Measurement Objects, alignment methods continue to evolve to make digital inspection data perfectly reflect the physical reality of the part. For complex geometries, high-precision inspection, and assembly analysis, simply establishing a coordinate system is no longer sufficient; replicating complex assembly logic and delivering streamlined tolerance analysis is the new benchmark.
By combining the precision of the PMT Portable CMM with the versatile alignment methods of PMT INSPECT metrology software, manufacturers can quickly deploy accurate, repeatable, and automated quality control workflows right on the shop floor.
Interested in optimizing your quality control workflow? Contact PMT Technologies today for a customized 3D inspection demo!
