What Is Milligram Mirror Treatment?

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  What Milligram Mirror Treatment Actually Removes
• 2
  Which Metals Take a Mirror Finish
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  Part Shapes That Suit Milligram Mirror Treatment
• 4
  How a Mirror Finish Is Measured and Verified
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  Where the Process Stops Being the Right Answer
• 6
  Milligram Mirror Treatment vs Other Finishing Routes

Every machined surface carries tool marks. A face mill leaves overlapping arcs, an end mill leaves parallel scallops, and a turning insert leaves a fine helix. Those marks are the surface. Milligram mirror treatment is the last stage that flattens them.

The name comes from the scale of removal. Instead of cutting a pass of 0.2 mm, the process takes away material measured in milligrams per square centimeter, or a few micrometres of depth. A typical mirror sequence removes 2–10 μm in total across all finishing steps.

That small depth matters. A part already milled to ±0.005 mm will not lose its tolerance when only a few micrometres come off. A part left 0.05 mm oversize for hand polishing will. This is the main reason the process is planned at the CAM stage, not after the part is made.

Removal happens in stages, and each stage has a job. Coarse abrasive cuts the peaks left by the cutter. Medium abrasive removes the scratches the coarse step created. Fine abrasive removes the medium scratches. The final pass removes almost nothing and only refines the surface.

Skip a stage and the surface shows it. A scratch from 400 grit will still be visible after 1500 grit unless the intermediate steps did their work. Mirror finishing is a chain, and the weakest link sets the result.

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  Depth per stageCoarse 3–5 μm, medium 1–3 μm, fine under 1 μm
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  Total removalUsually 2–10 μm from first abrasive to final polish
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  Why it holds toleranceRemoval stays far below the machining tolerance band

Aluminum finishes fast and takes a deep, bright mirror. Alloys in the 6061 and 7075 families respond well, though 7075 shows more grain structure when polished. Soft grades like 5052 tend to smear instead of cut, so they need lighter pressure and sharper abrasive.

Stainless steel is the classic mirror material. Grades 304, 316 and 316L all polish to a clean reflective surface. The 400-series martensitic grades such as 420 and 440C reach a higher gloss, but they are harder and take longer. Grade 303 polishes poorly because of its sulfur content, which leaves small pits.

Copper and brass are the easiest of all. C110 copper and C36000 brass cut quickly and hold a mirror with little effort. They also scratch just as quickly, so handling after finishing matters as much as the finishing itself.

Titanium and Inconel sit at the other end. Both are gummy and heat-sensitive. Titanium can reach a good finish with slow speeds and fresh abrasive, but Inconel usually stops at a satin look because the material work-hardens under the polishing pad.

Plastics are a separate story. Acrylic and polycarbonate can be polished to optical clarity, but they are not metals and behave differently under heat. PEEK and POM take a duller sheen rather than a true mirror.

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  Fast and bright6061, 7075, C110, C36000
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  Classic mirror304, 316, 316L stainless
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  Hard and glossy420, 440C, 17-4PH
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  Difficult303 stainless, Inconel, magnesium

Flat faces are the simplest case. A mold plate, a manifold face or a sealing surface with no deep pockets can be finished on a lapping plate or a wide polishing wheel. The whole face reaches the same finish at the same time.

Curved surfaces need a different approach. A shaft, a roller or a spherical seat is usually finished on a lathe with abrasive tape or a rotating mop. The tool follows the profile, so the finish stays even as long as the feed is constant.

Internal features are where the process gets expensive. A bore of Ø10 mm or smaller, a deep slot, or a cavity with a 2 mm corner radius cannot be reached by a polishing wheel. These areas need abrasive flow, a mounted point, or a shaped felt bob, and each adds handling time.

Sharp edges are a risk. Polishing rounds them. If a drawing calls for a sharp edge on a mirror face, the edge must be masked or the part must be finished before the edge is cut. Otherwise the mirror finish and the sharp edge cannot both survive.

Blind holes and cross-drilled passages are the hardest of all. Abrasive cannot reach the bottom of a blind hole evenly, and a cross hole breaks the surface with a burr. In most cases we finish the accessible faces and leave the internal passages as machined.

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  EasyOpen flat faces, outer diameters, shallow curves
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  ModerateBores over Ø20 mm, wide slots, spherical seats
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  HardBores under Ø10 mm, deep cavities, blind holes
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  ConflictSharp edges and mirror faces on the same part

Ra is the number most drawings use. It is the arithmetic average of surface deviations from the mean line, measured in micrometres. A mirror surface usually lands between Ra 0.2 and Ra 0.8 μm. Standard machining sits at Ra 1.6–3.2 μm.

Ra alone does not describe a mirror. Two surfaces can share the same Ra and look completely different. A surface with fine, evenly spaced scratches scatters light and looks hazy. A surface with the same average roughness but random, shallow texture can look bright and reflective.

That is why gloss is measured separately. A gloss meter reads reflected light at a fixed angle, usually 20°, 60° or 85°. For a mirror face, 60° readings above 90 gloss units are common on stainless and aluminum.

Inspection method matters too. A profilometer stylus with a 2 μm tip cannot resolve texture finer than its own radius, so it reports a flattering number. Optical profilometry or white-light interferometry gives a truer picture of a fine polish.

For production parts we agree the measurement method before the run starts. The same surface can pass or fail depending on whether the customer measures with a stylus, a gloss meter or a visual standard under a defined light source.

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  RaArithmetic mean roughness, the usual drawing callout
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  Gloss unitsReflected light at 20°, 60° or 85°
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  Visual standardAgreed sample under fixed lighting
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  Agree firstPick the method before the first part is finished

If the part is a hidden bracket, a mirror finish buys nothing. It adds cost, adds handling risk and adds a surface that shows every fingerprint. Function should decide the finish, not the other way around.

If the finish is only there to hide a machining defect, polishing is the wrong fix. A chatter mark or a gouge will not disappear. It will spread. The right move is to correct the cut, then finish.

If the customer needs a mirror inside a Ø6 mm bore 40 mm deep, the cost climbs sharply and the result is uneven. In that case a different route, such as electropolishing or abrasive flow machining, usually gives a better result for the same money.

If the part is handled after finishing, the mirror will not last without protection. A clear anodize, a lacquer or a protective film keeps the surface intact through packing and assembly.

Finally, if the alloy is 303 stainless or a magnesium grade, we say so up front. A mirror finish on those materials is either impossible or unreliable, and a satin finish is the honest recommendation.

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  Not worth itHidden, non-visible, purely functional faces
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  Wrong fixPolishing over chatter marks or gouges
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  Use another routeSmall deep bores, complex internal channels
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  Plan protectionAnodize, lacquer or film for handled parts