A three-mirror refit for the whole beam path, sold in two metals: hard, polishable molybdenum or higher-reflecting gold-coated silicon. Both drop into the 20mm mounts most K40s leave the factory with.
OMTech sells this three-pack in two materials under one name, and the material decides more than the brand does. The molybdenum set is bare, polished metal. The silicon set carries a gold reflective coating. Same 20mm diameter, same 3mm thickness, so either drops into a stock K40 mirror mount.
Molybdenum is the workshop brute. It is hard enough that OMTech calls it safe to polish, and it shrugs off the smoke film a K40 without good exhaust lays on everything. The price you pay is reflectivity: over 98% per bounce, where the gold silicon mirror is rated over 99.2%.
| Figure | Molybdenum set | Gold silicon set |
|---|---|---|
| Reflectivity at 10,600nm | Over 98% | Over 99.2% |
| Flatness | λ/2 | λ/4 |
| Beam left after 3 mirrors (on paper) | About 94% | About 97.6% |
| Tolerates hard cleaning | Yes | Gently only |
Those last-row percentages are plain multiplication of the rated figures across three reflections, not a measured result. Real losses also depend on dirt, alignment and how clean the lens is.
Flatness matters too. The Si set is specified to λ/4, the Mo set to λ/2, and a flatter face keeps the beam round rather than smearing it. On a K40, dirt and alignment usually cost more than that difference.
| Spec | Value |
|---|---|
| Diameter | 20mm |
| Thickness | 3mm |
| Mirrors per set | 3 |
| Mo version material | Molybdenum |
| Mo version reflectivity | Over 98% at 10,600nm |
| Mo version flatness | λ/2 |
| Mo version clear aperture | Better than 95% |
| Mo version centration | Under 3 arc minutes |
| Si version material | Gold-coated silicon |
| Si version reflectivity | Over 99.2% at 10,600nm |
| Si version flatness | λ/4 |
| Surface care | Scratch resistant, safe to polish (maker's note) |
| Compatibility | K40 boxes up to 200W CO2 machines |
| Package size | 3.11 × 2.8 × 0.87 in |
| Weight | 1.76 oz |
The point of a three-pack is a clean slate. Old mirrors wear unevenly, and chasing a weak cut through one fresh mirror and two tired ones wastes an evening. Pull the mains plug, let the tube and supply sit, and work through the path in order.
Expect to realign even if you never touched the adjusting screws. A new mirror sits a fraction differently in its seat, and on a K40 a fraction becomes millimetres by the far corner of the bed. The K40 alignment guide walks the tape-target routine corner by corner.
Keep the old mirrors in a labelled bag until the new path cuts cleanly. If something goes wrong mid-swap, you can put a known part back and isolate the fault.
Aftermarket mounts often use slotted retaining rings.
The two metals share a size but not a cleaning routine. Treat them the same and you will ruin the silicon set within a few cleanings. Know which one is in which mount before you reach for a swab.
For the molybdenum set, OMTech's own claim is scratch resistance and safe polishing. That makes it forgiving: a swab with isopropyl alcohol and moderate pressure lifts smoke film without harm. Gold coating is a thin layer on silicon, and scrubbing wears it through to grey substrate.
Never touch either face with a dry tissue, paper towel or shop rag. Dry wiping drags grit across the surface, and even molybdenum picks up fine scratches that way.
How often depends on your exhaust and material. A machine that cuts plywood daily with a stock fan may need a mirror check every few sessions. One that engraves acrylic with good extraction can go much longer.
Burned mirrors are usually a symptom. If mirror three keeps fouling, the head is taking smoke, and an air-assist head is the cure. Fit air assist early; the upgrade order explains why.
Plenty of 20mm gold silicon three-packs compete with the Si version of this set, and several makers sell molybdenum only in larger sizes. The OMTech set is unusual in offering both metals at the size the stock K40 takes.
The MCWlaser 20mm set is the closest match to the Si version: gold silicon, 20mm, three pieces, with a higher stated reflectivity. Cloudray's gold silicon mirrors come in 25mm, which suits upgraded mounts rather than stock ones. TEN-HIGH's molybdenum mirrors start at 25mm, so a K40 owner who wants Mo in 20mm lands back here.
Do this at the bench first:
A 25mm mirror catches a beam that has drifted off-centre with more margin, which is the main reason owners convert. It also means new mounts, new alignment and more parts. For a machine that aligns well today, a fresh set of 20mm mirrors restores lost power with less work. Pair it with a new lens if the old one is cloudy, since the lens usually wears first; see how to use a K40 for the full pre-cut checklist.
Most stock K40 mirror mounts take 20mm mirrors, which is what this set is. Measure the old mirror before ordering, because some K40 variants ship with 25mm mounts.
Gold silicon reflects more per bounce, so more beam reaches the lens. Molybdenum tolerates smoke and firm cleaning far better, which suits machines with weak exhaust.
The polished or gold reflective face points at the incoming beam. Handle the mirror by its edge with gloves so you do not leave skin oil on that face.
Yes. A new mirror seats slightly differently, and the error grows across the bed. Realign from mirror one outward with tape targets.
OMTech describes the Mo mirrors as scratch resistant and safe to polish. Do not polish the gold silicon version, because the coating wears through.
