Sand Oil CNC Parts Guide

• 1
  What blasting actually does to a machined surface
• 2
  Media choice: sand is the wrong word
• 3
  Reading Ra on a blasted surface
• 4
  Where the oil film fits in
• 5
  Which metals take this finish well
• 6
  Blasting and oil film by material

A blasted surface is a deformed surface. Abrasive grains hit the metal at high velocity and each impact leaves a small crater with a raised lip. Stack a few million of those and you get a matte texture with no directional tool marks. The metal underneath is not removed evenly, and the top 5–20 μm becomes a cold-worked layer with residual compressive stress.

That stress is useful. It delays fatigue crack initiation on parts that see cyclic loads, such as brackets and housings. It also hides the feed lines left by a Ø50 mm face mill, which is often the real reason a drawing calls for a frosted finish.

The trade is dimensional. Blasting peens the surface outward in places and cuts it back in others. On a flat face, expect 2–8 μm of dimensional shift depending on grit size and pressure. On a sharp edge, expect more. A 90° corner loses its edge radius fast.

So the sequence matters. If a bore is held to ±0.005 mm, blast first, then finish-machine or lap that bore. Blasting after final machining on a tight bore is how parts come back out of tolerance.

Silica sand is rarely used in a modern shop. Free crystalline silica dust causes silicosis, and the spent media is hazardous waste. The word survives from old job tickets, but the actual grains are alumina, glass bead, silicon carbide, walnut shell or plastic media.

Each medium cuts differently. Alumina (typically 120–220 grit) is angular and aggressive; it produces a deep matte and strong compressive stress, but it can embed grit in soft aluminium. Glass bead is round and peens more than it cuts, giving a softer satin look with less material loss.

Silicon carbide is harder and faster-cutting, useful on titanium and hardened tool steel. Walnut shell and plastic media are soft, used for deburring and paint prep where you must not change dimensions.

For most aluminium housings we run glass bead at 0.3–0.5 MPa. For stainless covers that need a uniform grey, alumina at 0.4–0.6 MPa works better. The nozzle angle stays around 60–75° to the surface, because a shallow angle smears the peaks instead of cutting them.

Ra is an average roughness, and averages hide a lot. A blasted surface and a turned surface can share Ra 1.6 μm and feel completely different. Blasting produces a random, isotropic texture; turning produces a directional one.

That isotropy is the point for many cosmetic parts. Light scatters evenly in all directions, so the part looks uniform from any viewing angle. On a turned surface, a specular highlight runs along the feed direction and every scratch shows up.

The useful band for blasted cosmetic parts is Ra 0.8–1.6 μm. Below Ra 0.8 μm the texture starts to look polished rather than frosted. Above Ra 3.2 μm the surface holds dust and fingerprints, and cleaning becomes a problem in medical and food-contact equipment.

Measure with a stylus profilometer using a 2 μm tip and a 0.8 mm cutoff. Take three readings 120° apart on cylindrical parts, because blasting uniformity depends on how the part sat in the cabinet.

An oil film on a blasted surface is not a coating. It is a thin organic layer, usually 1–5 μm, that fills the crater valleys and leaves the peaks exposed. It darkens the metal slightly and gives a low-sheen, slightly waxy hand feel.

The film does two jobs. It displaces moisture from the valleys, which slows flash rust on carbon steel and cast iron. It also holds fine dust, so the surface looks cleaner in service. That is why blasted and oiled parts appear on machine exteriors and enclosures.

Cure conditions matter. A thin oil applied cold stays wet and collects debris. A heat-cured oil at 150–180 °C for 20–30 minutes crosslinks into a dry film that will not transfer to gloves or packaging.

Do not use oil on parts that will be anodized, welded, bonded or painted. The film contaminates the surface and causes adhesion failures that show up weeks later. Mask those areas or skip the oil step entirely.

Aluminium is the easy case. 6061 and 7075 blast cleanly, hold a uniform matte, and take an oil film without staining. Cast aluminium such as ADC12 has porosity, and blasting opens those pores; the oil then darkens them into visible speckles.

Stainless steel blasts well, but the surface passivates slowly. Blast media must be dedicated to stainless, or you transfer iron particles from a previous carbon steel job and get rust spots within days. 304 and 316 both behave this way; 17-4PH is harder and takes a longer cycle.

Carbon steel and cast iron are where the oil film earns its place. Blasted 1018 or 1045 will flash rust in humid air within hours. An oil film applied within the same shift prevents that, and it also covers transport damage during shipping.

Titanium and Inconel are harder to blast evenly because of their low thermal conductivity and high hardness. Use silicon carbide at higher pressure and expect longer cycle times. Copper and brass are soft; blasting embeds media easily, so use glass bead and keep pressure low.