Casting and machining FAI: proving a part made in two stages

What the first article has to prove

A casting that is later machined is two products in one. The foundry makes a near-net shape, and the machine shop turns that shape into a finished part. A first article inspection has to show that both stages are controlled and that the finished part still matches the drawing.

That sounds obvious, but many FAI packs cover only the machined features. The casting is treated as incoming material, and the report starts at the first cut. The problem appears later, when a wall is thin, a bore breaks into a void or a datum shifts because the casting moved after stress relief.

The FAI should therefore be built around the full route: pattern or die, casting, fettling, heat treatment if used, machining, finishing and inspection. Each step that can change a drawing characteristic needs a record. That route is also the basis for the control plan and the change triggers. A report that starts at the first cut has already lost the evidence for the features that the cut removed.

Separate the as-cast and as-machined calls

Start by sorting the drawing characteristics into three groups: features formed by the casting, features created by machining, and features that depend on both. The third group is where most arguments live. A bore position depends on the casting datum, the fixture and the machine program at the same time.

For each group, name the condition to be measured. An as-cast surface may be inspected before machining, while a machined face is inspected after the final cut. If a feature is only accessible before machining, record it at that stage and say so in the report.

Do not let the supplier replace an as-cast check with a finished-part check. A shrink defect or a cold shut that sits under a machined face can be invisible after the cut, and the finished dimension may still pass.

Balloon the drawing once, with the route in view

Ballooning is the discipline of numbering every characteristic and carrying that number into the report. For a cast and machined part, add a route column beside each balloon. The column says whether the feature is checked as-cast, after a rough cut, after the final cut or at final inspection.

This prevents two common failures. The first is measuring a feature at the wrong stage and declaring a false failure. The second is skipping a characteristic because it is buried in a note or because the inspector assumes the casting supplier has covered it.

Include notes as characteristics. A call for no cracks, a casting tolerance, a surface finish, a thread and a marking requirement all need a method and a result. If the drawing refers to a casting standard, the standard becomes part of the inspection plan.

The casting evidence

The casting side of the pack should identify the material, the heat or lot, the pattern or die, the machine or line and the date. A material certificate is necessary but not sufficient. It proves the grade, not the condition of the part that was actually measured.

Add the as-cast dimensions that set the machining stock, the wall thickness at critical points and the condition of any sealing or pressure boundary. If the drawing zones porosity, the report should say how the zone was checked and what was found. A statement that the casting is sound is not evidence.

Where heat treatment or stress relief is part of the route, record the cycle and the batch. These steps change dimensions and residual stress, and a first article taken before them does not represent the production part.

The machined feature record

The machining record should name the fixture, the program revision, the machine and the operator. For each critical feature, record nominal, tolerance, actual and instrument. A CMM result needs the datum alignment and the program, not just a table of numbers.

Pay particular attention to features that depend on the casting. A true position call on a bolt pattern is only meaningful if the datum is the same datum used in the fixture. If the supplier uses a different practical datum to make the part easier to hold, the report and the drawing no longer describe the same part.

Check the first part after each setup, not only at the end of the line. A roughing cut can reveal a casting defect, and a finishing cut can move a thin wall. The final report should show that the part was stable through the route.

The first article should also show the link between the casting datum and the machining datum. Ask for a layout of the casting as received, with the stock on each machined face. If the stock is uneven, the part may clean up on one side and break into a void on the other. A simple stock map is more useful than a single wall thickness number.

The evidence set at a glance

The table below is a practical structure for a cast and machined FAI pack. It is not a form to copy blindly, but it shows the evidence that a reviewer should be able to find without asking for a second submission.

Approval and change control

Approve the report against a specific drawing revision and a specific route. If the foundry changes, the pattern is reworked, the die is replaced or the machining fixture is modified, the old approval no longer covers the part. A delta FAI can be enough for a minor change, but the decision should be written down.

A first article is not a one-off ceremony. It is the baseline for production. Keep the approved report with the control plan and the inspection instructions, and use it when a non-conformance or a process change appears.

If a characteristic is out of tolerance, record it and raise a non-conformance before submission. A report that hides a deviation in a footnote is worse than a report that states the problem and proposes a disposition.

Keep a copy of the approved pack with the drawing, the control plan and the tooling records. When a new lot arrives, compare the material certificate and the first-off dimensions with the approved baseline. That comparison catches a slow drift before it becomes a rejected shipment, and it gives the next FAI a reference for what changed.