CNCOLETECH ZnSe Focus Lens

Basic optics gives a rough guide. With the same beam going in, spot size grows in step with focal length, and depth of focus grows with its square. So moving from 50.8mm to 63.5mm makes the spot about a quarter larger and the usable focus zone roughly half again as deep. Real numbers depend on your tube's beam quality and alignment.

A deeper focus means the beam stays narrow through the whole thickness of a sheet, so cut edges come out closer to square and less tapered. The cost is energy density at the surface. Fine raster engraving looks a touch softer, and on a 40W tube you may need a slower speed to cut through the same sheet.

• Choose 63.5mm if most of your work is cutting sheet thicker than about 3mm.
• Stay with 50.8mm if you do an even mix of engraving and cutting.
• Go shorter if detail engraving is your main job.

A 40W K40 has real limits on thickness whatever lens you fit. The materials and thickness guide sets expectations.

Look at your bed first. The stock K40 clamp bed often sits at a fixed height, and many owners remove it for an adjustable platform or a powered Z table.

1. Measure from the bottom of the lens tube to your current work surface at best focus.
2. Add 12.7mm. That is roughly where the new surface must sit.
3. Check there is room under the bed for that drop, including thicker sheets on top.
4. Fit the lens convex face up, toward the incoming beam.
5. Run a ramp test: lay a strip of scrap at a slope, fire a line along it, and find where the mark is thinnest.
6. Set your focus gauge or bed height to that point and note it down.

A nozzle for air assist also changes the picture. With a longer focal length, the gap between nozzle tip and work grows, which spreads the air stream. Some owners fit a longer nozzle to keep the air close to the cut.

The alignment and focus guide covers ramp tests step by step, and Lightobject's power Z table is one way to gain the bed travel.

Measure your old lens with calipers, then pick from the range.

• 15mm, 18mm, 19mm or 20mm at 50.8mm: stock focal length in whichever seat size your head has.
• 18mm or 20mm at 63.5mm: the deeper-focus option for common K40 seat sizes.
• 20mm at 76.2mm: longer still, which needs a bed drop of 25.4mm from stock; possible only on machines with plenty of travel.
• 25mm at 63.5mm or 76.2mm: rated to 400W and 500W, for bigger machines with 25mm heads; not a K40 part unless you have changed the head.
• 18mm PVD at 50.8mm, 30W to 100W: a cheaper PVD grade for occasional use.

CNCOLETECH describes its CVD blanks as II-VI material, and the model code IIVI-20-635 reflects that. II-VI is an established US maker of infrared optics. CVD zinc selenide absorbs less at 10.6 µm than PVD grades, so it runs cooler and resists thermal damage better, which counts for more as focal length and cut times grow.

Thickness ranges from 2.0 to 2.2mm on this lens. If your seat was cut for a thinner lens, the ring may sit a little higher; if it was cut for a thicker one, add the correct O-ring rather than overtightening. Point pressure on ZnSe cracks it.

Zinc selenide is soft and toxic, so handle it only with powder-free nitrile gloves or finger cots, and only by the rim.

1. Blow dust off with a hand air blower before any wipe.
2. Use lens tissue or a cotton swab wet with reagent-grade isopropyl alcohol or acetone.
3. Drag once, gently, across the face; use a fresh tissue for each pass.
4. Inspect against a bright light for haze, pits or brown spots.

Watch the cut, too. If a sheet that cut cleanly last week now needs a second pass, check the lens before touching the power setting. A film of resin on a 63.5mm lens costs power across the whole deeper focus zone, and turning up the current only cooks the film harder.

Against other lenses, CNCOLETECH's 20mm at 50.8mm is the direct stock-focus choice, and Cloudray's 20mm meniscus lens is its closest rival at that length. For short-focus detail work on 18mm heads, the Mssoomm 38.1mm lens sits at the other end of the scale. Air assist keeps all of them cleaner, and the upgrade order guide ranks it early for that reason.

• Keep a spare lens if you cut every day.
• Store spares in their case, never loose in a drawer.