Finishing
Polishing is the operation that separates a fabricated part from a manufactured one. It is also the operation most often described with a word that means different things to different shops.
"Polished" on a drawing can mean a flame pass that takes seconds or a diamond sequence that takes an hour. The two look completely different and the price difference is large, so the word by itself is not a specification.
| Method | Result | Time | Where it belongs |
|---|---|---|---|
| Flame polishing | Glossy, slightly rounded, hazy through the thickness | Seconds per metre | High-volume parts where the edge is not examined closely |
| Machine buffing | Glossy face or edge, with some risk of burning | Minutes | Face polishing after scratch removal, simple edges |
| Diamond edge polishing | Water-clear, sharp, light-transmitting | Long, a skilled hand operation | Display, awards, vitrines, premium fixtures |
| Hand polishing (grit sequence) | Good on small areas, varies by operator | Slow | Local repair, unusual geometry |
| Vapour polishing | Near-optical across the whole part | Batch process, needs equipment | Optical and medical parts, not general fabrication |
A hydrogen-oxygen flame is passed along the edge. The surface melts briefly and flows, then cools glossy. It is fast and it produces a good-looking edge on a cheap part.
Its limits matter: the edge is rounded, so two flame-polished parts do not meet with a crisp line; the flame introduces stress, which matters if the part will be bonded or cleaned with solvent; and it does not produce a clear edge, only a glossy one. Light shone into a flame-polished edge dies within a few centimetres.
The edge is machined flat and square, then worked through a sequence of abrasive papers and diamond compounds on a felt or cotton wheel until it is optically clear. On a thick edge the result is a pane that glows along its length when lit, which is the effect most display work depends on.
It is slow, skilled work and it is priced per metre of edge. On a thin flat panel it can cost more than the material and the cutting combined, which is why the specification should ask for it only where it is seen.
Face polishing is usually repair work: removing a scratch, a machining mark or a haze over an area. The sequence is the same as edge polishing but the risk is different. A face is large and flat, and a machine polisher held in one place generates enough heat to dish the surface.
On a visible face, polish a small area by hand and check under raking light before widening. A patch polished to a different level shows as a lens.
A photograph of an accepted finish, kept with the purchase order, resolves most finish disputes. Adjectives do not survive a change of shift; a sample does.
Any hole, slot or engraving in a polished face leaves a matte area that must be re-polished if it is visible. This is why a design with many holes in a visible face costs more than one with few, and why we ask which faces are seen before quoting.
The sequence is machine, anneal, polish. Polishing before annealing means the oven may dull the finish and the part will need polishing again.
Substantially, and for many parts it is the right choice. It is not a substitute where the edge is viewed closely or where the part will be bonded, because it rounds the edge and introduces stress.
Yes, on the outer faces and edges. Polishing near a bond line risks opening the joint, so the assembly is usually polished before the final bond, or the bond is positioned away from the polished edges.
Marks are usually from skipping a grit in the sequence. Go back one step and work through again in the opposite direction, then polish. On a face, work the whole area rather than the spot, or the patch will show.
Only marginally, a few hundredths of a millimetre. On a part with a tight dimension to a polished edge, the polish is normally applied before the final measurement, or the dimension is taken from an unpolished reference.
Yes, on the edge, with a diamond polish. A diamond-polished cast acrylic edge is visually indistinguishable from glass at normal viewing distance and much lighter and safer.
Polishing is priced by the metre of edge, not by the part, which is why two brackets of the same size can differ threefold in finishing cost. A 200 × 150 mm bracket has 700 mm of edge; the same bracket with a 60 mm return has 820 mm, and if it is a tray with an internal cut-out it can easily carry 1.5 m. The finishing line on a quotation is therefore a measure of edge length, and it is worth counting before you compare suppliers.
| Finish | Relative cost | Time per metre | What it looks like | Best use |
|---|---|---|---|---|
| As-routed | 1× | <1 min | Matte tool marks, slight edge chipping | Hidden edges, parts that get bonded |
| Sand and scotch-brite | 1.6× | 1–2 min | Uniform satin, no tool marks | Where a matte edge is the design intent |
| Diamond edge polish | 3× | 3–5 min | Clear, mirror-bright, sharp reflection | Retail, cosmetic, visible edges |
| Flame polish | 2× | 1–2 min | Glossy but slightly wavy, rounded corner | High volume, forgiving geometry |
| Face polish | 8–15× | 15–30 min/m² | Removes shallow scratches across the face | Repair, or an optical surface |
The cost step between diamond-polished and flame-polished is where most projects should stop and ask what the edge is for. A flame-polished edge on 8 mm sheet reads as glossy from a metre away and shows a slight ripple from 200 mm; a diamond-polished edge reads the same at any distance. If the part is a shelf edge behind glass, flame is the value choice. If it is a jewellery riser that a customer picks up, diamond is the only one that survives close inspection.
Two cost traps. Polishing before bonding doubles the work, because the joint has to be polished again after assembly — polishes done in the flat are wasted at the seam. And polishing a part with drilled holes adds hand work per hole, so a panel with forty 4 mm holes can cost more to finish than a much larger plain panel. Both are visible in a quote as a finishing line that does not match the edge length, which is usually the first thing worth querying.
A photo of an accepted part plus which edges are seen is the specification. We will match it and quote the minimum polishing that gets there.
