12.3 Castings, Weldments, Sheet Metal, and CMM Evaluation

12.3 Castings, Weldments, Sheet Metal, and CMM Evaluation

Profile of a surface is the workhorse control for castings, forgings, weldments, and sheet metal because those surfaces are not prismatic and cannot be controlled by size or position alone. The true profile is defined by the drawing's basic dimensions, and the tolerance zone is a band around that true profile. Senior-level work adds two complications: stock allowance on surfaces to be machined later, and the actual CMM evaluation logic that converts a cloud of measured points into a pass/fail verdict.

Stock Allowance and Unequal Disposition

A casting or forging drawing often leaves stock allowance — extra material on a face that will be machined away later. The true profile of the as-cast surface includes that stock allowance: the basic dimensions define the as-cast (pre-machined) surface, and the profile tolerance controls how much the as-cast surface can deviate from that nominal as-cast shape. The machined surface is then defined by a separate set of dimensions or by a different profile callout after machining.

When the designer needs to bias the tolerance zone toward or away from the material, unequal disposition is used. The FCF carries the unequally-disposed symbol (a circle with U inside, per Y14.5-2009) followed by a value that says how much of the total tolerance lies on the additive (material-add side) of the true profile. For a casting face that will be machined, the designer may want most of the zone to be additive (extra material) so the as-cast surface tends to have stock remaining to clean up; a small portion is subtractive so the casting is not excessively thick. Example: profile of a surface 1.0 U +0.7 means a 1.0 mm total zone with 0.7 mm on the material-add side and 0.3 mm on the material-remove side. The Senior technologist must read the U value and orient the zone correctly.

Weldments and Sheet Metal

On weldments, profile controls the as-welded geometry: the true profile is the nominal welded assembly, and the zone accounts for weld buildup, shrink, and distortion. On sheet metal, profile of a surface is used to control formed flanges and bends where the bend radius and angle are basic and the profile zone accommodates springback and tooling variation. In all cases the surface being controlled is not a simple cylinder or plane, which is why profile — not position or size — is the right tool.

CMM Evaluation Logic

A coordinate measuring machine (CMM) evaluates profile of a surface by:

1. Establishing the datum reference frame from the physical datum features on the part (this is the alignment step).
2. Sampling a set of points on the controlled surface with the probe.
3. For each measured point, computing the deviation from the true profile along the direction normal to the true profile at the nearest point.
4. Tracking the sign of each deviation — positive for the material-add side, negative for the material-remove side.
5. Reporting the profile result as the width of the smallest band containing all measured points; for an unequal-disposition zone, also checking plus and minus sides separately against their own allowances.

The zone is a bilateral band (two curves/surfaces equally disposed about the true profile, or unequally disposed if U is specified). The part passes if all measured points fall within the band. The inspector's reported value is the width of the smallest band that contains the actual measured points; this is compared to the specified tolerance.

CMM Evaluation Walkthrough

A cast boss has profile of a surface 0.8 (no U, no datums — a self-reference profile that controls form only). The CMM:

1. Aligns to the part. Because the FCF has no datums, the alignment uses the controlled surface itself as a best-fit reference, which is permitted for a no-datum profile that controls form only.
2. Takes 24 points around the boss.
3. Computes each point's normal deviation from the true (basic) profile.
4. Finds the maximum positive deviation = +0.31 mm and the maximum negative deviation = -0.22 mm.
5. Reports the profile as the total width of the band containing the actual surface: 0.31 - (-0.22) = 0.53 mm.
6. Compares 0.53 to the 0.8 tolerance: pass.

Now suppose a U +0.6 had been specified (total 0.8, +0.6 / -0.2). The inspector checks +0.31 ≤ +0.6 (pass) and -0.22 ≥ -0.2 (fail, since 0.22 exceeds the 0.2 subtractive allowance). The part fails on the subtractive side even though the total band width 0.53 < 0.8. This sign-aware evaluation is exactly where unequal disposition traps inspectors who only report total band width.

CMM Evaluation Table

Common Traps

Three traps recur on the Senior exam. (1) Sampling strategy: a sparse CMM scan can miss the worst point and report a passing width that a denser scan would reject. The Senior technologist should specify density on critical surfaces. (2) Datum alignment: establishing the DRF from the wrong features (or using a free-form best-fit when datums are specified) corrupts every downstream deviation. (3) Reporting sign: collapsing a signed deviation into a single magnitude loses the U-side comparison. With unequal disposition, the sign must be preserved and each side checked against its own allowance.