Sandblasted CNC Machining Services: 6 Buying Checks

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  Key takeaways
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  Which surface finish fits your part
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  What sandblasted CNC machining services actually change
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  Parts that suit blasting and parts that do not
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  Media choice, pressure and the numbers to put on the drawing
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  How to judge a sandblasting shop before you place the order
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  Six steps from drawing to blasted parts

Blasting fires abrasive media at a machined surface under compressed air. Each particle chips a small amount of material and leaves a shallow crater. Thousands of craters together read as a matte gray surface with no directional tool marks. That is the whole effect. It is mechanical, not chemical, so the base alloy does not change.

The depth of those craters sets the roughness. Coarse media at high pressure cuts deeper and gives a rougher finish. Fine media at lower pressure gives a softer look. On aluminum we normally land between Ra 0.8 and 1.6 μm with glass bead at 2–4 bar. On 304 stainless, aluminum oxide at 4–6 bar gets a comparable visual result.

Blasting also rounds sharp edges. A 0.2 mm burr on a machined corner disappears after a few passes. For a cover plate that is fine. For a sealing lip or a press-fit bore, that same rounding can cost you the fit. This is the trade-off engineers miss most often.

Blasting is not a repair process. A deep gouge, a scratch from handling or a warped face will still be there after blasting. The surface just looks more uniform, which can hide the defect from a casual visual check. If the part is out of tolerance, blast first and inspect after, or the fault gets buried.

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  It removes tool marksMilling and turning lines are gone. The surface reads uniform from any angle.
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  It sets the base for coatingPaint, powder coat and adhesive bond better to a blasted profile than to a polished one.
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  It does not change dimensions muchMaterial removal is in the micron range. A 0.02 mm shift is normal on a light pass.
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  It will not hide defectsGouges, porosity and warping survive the process.

Blasting earns its place on cosmetic and functional-prep surfaces. Aluminum front panels, EV battery brackets, robot arm covers, camera housings and enclosure lids are all common. Anywhere the customer sees the part and you want a low-glare, fingerprint-resistant surface, blasting does the job at a lower cost than anodizing alone.

It is also the standard prep before powder coating and before many structural adhesives. A blasted steel surface at Ra 1.6–3.2 μm gives the coating a mechanical key. Skip that step and the paint flakes at the edges.

Do not blast sealing faces, o-ring grooves, bearing bores, thread flanks or any surface with a fit callout tighter than ±0.02 mm. Mask them or blast before the finishing cut. If a bore is blasted, the effective diameter grows and the seal leaks.

Thin-wall parts are another risk. A 0.8 mm aluminum wall can deform under high blast pressure. Keep pressure low and hold the nozzle further away, or skip blasting on that face. Tell the shop which faces matter. A marked-up drawing saves a rework cycle.

Media selection follows the workpiece. Glass bead is the default for aluminum, brass and copper because it cuts gently and leaves a bright matte. Aluminum oxide is sharper and cuts faster, which suits steel, stainless and titanium. Walnut shell and plastic media are for soft plastics and thin sheet where you cannot risk erosion.

Pressure and standoff distance control the cut rate. On glass bead, 2–4 bar at 100–150 mm standoff is a normal range for a uniform cosmetic finish. Push above 6 bar and you start peening the surface, which can work-harden a thin aluminum skin and make it brittle.

Grit size drives roughness. Fine glass bead at 100–170 mesh gives Ra 0.8–1.6 μm on aluminum. Coarser media at 60–80 mesh moves toward Ra 1.6–3.2 μm. If you need Ra 0.2–0.8 μm, plan on a fine bead pass plus a tumbling or polishing step; blasting alone rarely gets there.

Write the target on the drawing. A note such as blast all over, Ra 0.8–1.6 μm, mask faces A and B is enough for a shop to quote and inspect against. One word like matte is not a specification. Two shops will read it two different ways and you will get two different surfaces.

Ask what media they stock and how they track it. A shop that reuses contaminated media will drag steel grit into an aluminum batch and leave dark specks. Media should be segregated by alloy family and replaced on a schedule, not topped up forever.

Ask how they mask. Silicone plugs, vinyl tape and machined fixtures all work, but the choice matters for a tight bore. Tape leaves adhesive residue in blind holes. Plugs cost more upfront and are reusable. For a 10,000 part run the plug pays for itself.

Check whether blasting happens before or after the final machining pass. A shop that blasts first and machines after will produce a clean sealing face with a matte finish everywhere else. A shop that machines first and then blasts everything will round your fit surfaces. Both are valid; only one matches your drawing.

Finally, ask for the inspection record. Surface roughness should be measured on a sample from the batch, and visual checks should note color and uniformity. A supplier who cannot show a roughness trace is guessing. GreatLight runs 100% inspection before shipment and supplies reports on request, including roughness values where the drawing calls for them.