A first-article inspection report for a welded robotics frame can contain many dimensions and still fail to answer the question that matters: will the released modules assemble and align in the machine? The difficulty is not only measurement accuracy. A frame may sit differently on an inspection table, on a temporary fixture, and on its final leveling points. If the drawing does not define that inspection state, two competent inspectors can measure the same weldment and produce different conclusions.
For a robotics OEM evaluating a robotics frame fabrication company, free-state and restrained results should not be treated as competing versions of the truth. They describe different mechanical conditions. The engineering task is to define which condition predicts assembly, how the functional datums are recovered after welding, and what evidence must appear in the FAI package.
Why the inspection state changes the result
A welded frame contains residual stress, local shrinkage and geometric variation from cutting, forming, fit-up and welding. Its weight and support locations also influence the measured shape. When a long frame rests on an arbitrary table surface, it may twist toward the table. When clamps force it against a fixture, some variation disappears while new restraint loads are introduced.
The correct state depends on the released design intent. A self-supporting mobile robot chassis may need meaningful free-state requirements because wheel, battery and sensor interfaces must coexist without a production fixture. A machine base designed to be leveled and anchored may require inspection on the same defined support points used during installation. A thin enclosure frame might be accepted in a controlled assembly fixture if the mating panels establish its installed geometry.
Terms such as “flat,” “square” and “check on fixture” are not enough. The drawing or inspection plan should identify support points, allowed clamps, clamp sequence, orientation, stabilization time when relevant, and the features evaluated in each state. Otherwise the FAI documents an inspector’s setup rather than the product requirement.
Establish the functional datum structure first
Inspection planning should begin before the first frame is welded. The OEM should identify the surfaces and axes that control robot installation, drive alignment, module location, guarding, covers and site mounting. Those functional relationships form the datum structure.
A practical hierarchy often separates three groups:
- Primary support features that establish the frame in operation or installation.
- Precision interfaces for robot bases, rails, wheel modules, fixtures or payload equipment.
- Secondary cover, cable, guarding and cosmetic interfaces that should not drive the complete structure.
The datum scheme must also state whether the primary references exist as-welded or are created later by machining. Asking a robot welding frame factory to locate every feature from rough tube edges is rarely stable. Purpose-designed datum pads, tooling holes or machined surfaces give fabrication, machining and inspection a shared reference.
The OEM or system integrator owns the functional datum selection and tolerance analysis. DRAmetal can manufacture and inspect the released mechanical scope, but it does not infer robot accuracy, equipment safety, foundation behavior or complete-system acceptance from an incomplete drawing.
Use free-state inspection for unforced geometry
Free-state inspection evaluates the frame without clamps forcing it into nominal shape. It is useful for understanding whether the weldment can reach assembly in a natural condition and whether installation will require unreasonable force. Free-state does not mean “placed anywhere.” The inspection plan still needs a repeatable support arrangement.
Specify the intended orientation and support points. For a chassis, those may correspond to wheel-module or leveling interfaces. For a base frame, they may be defined pads. The surface plate itself should not become an undeclared datum. Shims used to stabilize the part should be documented, and inspectors should avoid support locations that artificially remove twist.
Typical free-state checks include overall twist, diagonal relationship, relative height of module pads, door or cover opening geometry, and the location of features that must mate before anchoring. The report should distinguish directly measured values from calculated relationships. Photographs or a simple setup sketch can make the result reproducible for later lots.
Use restrained inspection only with a released setup
Restrained inspection evaluates the frame in a defined installed or assembly condition. It can be appropriate when the product is designed to be bolted to a rigid foundation, located in a production fixture, or joined to another structure that establishes its final shape. The restraint must represent the real interface rather than conceal a fabrication problem.
The plan should define which locators are fixed, which clamps provide seating, the order of engagement and whether fasteners are tightened to a project-defined condition. Clamp force should be sufficient to seat the part, not to reshape a nonconforming frame. If excessive force is needed, record it as an assembly concern instead of reporting only the restrained dimensions.
Free-state and restrained measurements can be paired deliberately. The difference helps the engineering team understand installation sensitivity. It can also show whether a datum pad is locally high, whether the frame rocks on its supports, or whether an assembly fixture is carrying a load the final installation cannot reproduce.
| Inspection question | Free-state evidence | Restrained evidence | Release decision owned by |
|---|---|---|---|
| Can the frame sit on its intended supports without forcing? | Rocking, support contact and natural twist | Not the primary evidence | OEM mechanical designer |
| Will precision interfaces align after installation? | Pre-installation relationship | Relationship on released locators and supports | OEM or integrator |
| Does a fixture merely locate or also reshape the frame? | Shape before clamping | Change after the defined clamp sequence | OEM and supplier jointly review |
| Can production inspection reproduce the result? | Support and orientation record | Locator, clamp and fastener record | Released quality plan |
Recover datums after welding with a planned process
Some robotics frames need machined mounting pads, rail seats, bearing locations or reference holes after welding. Post-weld machining should recover functional relationships, not chase every outside surface back to nominal. The model and drawing need machining allowance and accessible reference features that survive fabrication.
The process plan should identify how the weldment is located on the machine, which features establish the first setup, and which critical interfaces must be completed in a common setup. If the frame requires multiple setups, transfer datums should be deliberate and inspectable. Machining from a cosmetically convenient edge can move precision features away from the installed support system.
Datum recovery also needs a disposition rule for insufficient stock. Quietly moving a feature or enlarging a hole changes the released design. The supplier should stop and request engineering direction when the as-welded condition cannot support the specified machining result.
Put decision-ready fields in the FAI report
An FAI report should let a reviewer reconstruct the measurement condition and connect results to the correct design revision. A list of characteristic numbers and values is incomplete if the part state is unknown.
Useful report fields include:
| Report field | What it should identify |
|---|---|
| Part identity | Part number, frame serial or lot identity and quantity represented |
| Configuration | Drawing, model, BOM and approved-deviation revisions |
| Inspection state | Free-state or restrained, orientation, support points and clamp sequence |
| Datum realization | Physical features, targets, fixture locators or simulated datums used |
| Measurement method | Equipment, program or gauge identity and relevant setup notes |
| Characteristic result | Requirement, actual result, unit and acceptance status |
| Environmental or conditioning note | Only where the released plan makes it relevant |
| Nonconformance reference | Linked record rather than an unexplained handwritten change |
| Approval | Authorized supplier and customer review status where required |
Photographs should support setup traceability, not substitute for measurements. A fixture qualification record is also separate from the part FAI. If a checking fixture is used, its identity and current verification status belong in the report.
Confirm the measurement method predicts assembly
Coordinate measuring machines, laser trackers, height gauges, templates and dedicated fixtures answer different questions. Select the method from feature size, accessibility, uncertainty and functional risk. A high-resolution device does not correct an unstable support or a poor datum strategy.
Before production release, compare the inspection result with an assembly or mating check where practical. A master interface plate, released checking gauge or controlled trial assembly can reveal misunderstandings in datum simulation. The goal is not to replace dimensional inspection with fit-up; it is to confirm that the measured characteristics actually predict the intended mechanical interface.
For repeat production, lock the measurement program, setup instructions and fixture revision to the part configuration. Changes to support pads, weld sequence, machining stock or inspection software should trigger a documented review. Trend data can be useful, but it should not be used to accept a characteristic outside the released requirement.
Release a complete FAI and RFQ checklist
Before asking a supplier to quote or approve the first article, verify that the controlled package includes:
- Native model and released fabrication drawing.
- Defined primary, secondary and tertiary datum references.
- Free-state support orientation and measurement requirements.
- Restrained locators, clamps, sequence and acceptance purpose.
- Post-weld machining stock and datum-recovery plan.
- CTQ list tied to assembly and installation functions.
- Required report fields, setup evidence and record format.
- Mating gauge or trial-assembly expectations where applicable.
- Nonconformance, deviation and revision-control workflow.
- Production quantity, packaging supports and shipment condition.
The robotics industry hub provides the wider manufacturing context, while the robotics frame and chassis fabrication page describes the relevant build-to-print mechanical scope. Teams preparing an inspection package can use the editable robotics frame FAI and datum checklist alongside the existing robotics sheet metal quality plan.
DRAmetal manufactures and inspects frames against released drawings and project-defined inspection plans. The OEM or system integrator remains responsible for application loads, robot performance, foundations, safety functions, regulatory requirements, certification decisions and final system validation. When those inputs are controlled, send the frame RFQ and FAI requirements for a manufacturing review.
Need a Custom Sheet Metal Quote?
Send us your CAD drawings — DRAmetal's initial response target is within 24 working hours. Quotation, DFM and lead-time timing depend on file completeness and engineering scope.
GET A FREE QUOTE →
