Does Blasting to Sa 2.5 Change Bracket Dimensions?

Yes, Sa 2.5 blasting removes metal from a 45 steel bracket, so dimensions do move. The move sits at the surface, inside a band tighter than the ±0.2 mm pitch tolerance. Sequence decides whether that band matters. Blast first, cut the mounting face last, and the tight numbers are produced after the grit has stopped.

What Sa 2.5 Actually Specifies

Sa 2.5 is a preparation grade in ISO 8501-1, one step below white metal. The grade names what has to be gone: oil, grease, dirt, mill scale, rust, paint, oxide. Light staining may stay. What the grade does not name is depth. Two brackets can pass Sa 2.5 at different profiles, because cleanliness and texture are separate callouts.

Drawings go wrong right here. Someone reads the 2.5 as an Ra value and hands the blast shop a number the standard never set. The grade carries no depth value. Where a surface needs texture, say so in a second callout and check it with a profile gauge. A bracket heading for paint needs cleanliness only, and Sa 2.5 says nothing about steel loss.

Where Blasting Removes Metal

Grit strikes the casting, flakes the scale off, and takes steel with it. On a flat field the loss follows the profile, peaks down to the valley floor. At an edge the same stream hits two faces at once and rounds the corner. Thread crests, reamed lips and thin wall sections show the change first.

None of that shifts a hole center. A hole drilled to ±0.2 mm keeps its center through a blast, while its edge loses sharpness and its mouth picks up a radius. The face around the hole is what suffers, because peaks come off and valleys stay. Blast a machined pad and the plane the machine left is gone. Re-cut it and the same face is paid for twice.

Why Machining Comes After Blasting

The process table runs sand cast, gate removal, quench and temper, cast-on test bar, blast to Sa 2.5, machine the mounting face, passivate as the drawing confirms. Blasting sits at step five, the mounting face at six. That order is a dimensional call. The 0.1 mm flatness and the hole pitch at ±0.2 mm are drawing targets, and both are cut after the last abrasive touches the part.

Reverse the two and the shop either masks finished faces or pays for a re-cut. Inspection gives a second reason. Blasting before machining strips the cast skin off the mounting area, so magnetic particle or dye penetrant can find sand holes and shrinkage before a cut is spent on them. The order costs one extra setup and a second fixturing.

Masking the Faces You Keep

Any feature at final size when the nozzle arrives needs shielding: threads, reamed holes, machined pads, the datum face itself. Rubber plugs and bolt-on plates do that work. The rule is about fit. A blasted mating face touches its partner on peaks, the peaks crush under bolt preload, and the joint settles after torque. A bracket flush at assembly sits loose a week later.

Masking has a limit. A face that has not been machined yet cannot be masked, and a face about to be blasted should not be machined. Two routes work: shield the finished features and blast the rest, or blast the whole casting and cut afterwards. The process table takes the second, so masking here is mostly about holes rather than pads.

Acceptance: Comparator, Not a Reading

The inspection table lists a Sa comparator plate for this callout, not a surface tester. An inspector holds the blasted area against the reference photographs under agreed light and judges the grade. That tool fits the question, because ISO 8501-1 defines the grade by appearance. A profile gauge reports a depth the standard leaves open. Fix the lamp and the viewing angle in the note, or two inspectors split on a borderline part.

Measure the mounting face on its own, with a CMM or a surface plate, against the 0.1 mm flatness. One tool answers the surface grade, the other answers the dimension. Put it on the PO that way: Sa 2.5 to ISO 8501-1, masked features listed by name, blast scheduled before the mounting face is cut, and post-cast machining signed off on a CMM report.