LVOUP ZnSe Focus Lens

The price is depth of focus. A short lens holds its tight spot over a shallower band, so the beam spreads quickly above and below the focal point. On thin material that hardly matters. On thick stock the cut tapers and the bottom edge chars, because the beam is already out of focus by the time it gets there.

• Good matches: photo engraving on wood, anodised aluminium marking, leather, card, veneer, thin acrylic.
• Workable: 3mm plywood and acrylic, with focus set carefully.
• Poor match: 6mm acrylic or thick plywood in one pass. A 2-inch or 2.5-inch lens does that better.

How much finer the spot gets depends on the beam arriving at the lens. A K40 tube with dirty mirrors or a skewed path loses most of the benefit, so clean and align before you judge the lens. The routine is in the K40 alignment guide.

Plenty of owners keep two lenses: this short one for engraving work and a 2-inch lens for cutting. Swapping takes a few minutes with a lens tube that unscrews, plus a new focus height. That is a cheaper route to good engraving than any controller upgrade.

LVOUP makes it from China-made PVD zinc selenide, AR-coated front and back, and quotes 99.5% transmittance. It is a reasonable grade for an engraving lens that sees moderate power.

Factory K40 beds and their spring clamps are set up for a 2-inch lens. You have three ways to close the gap:

1. Shim the work up with a flat spacer block cut to the right thickness.
2. Fit an adjustable bed, such as a manual lab-jack style table or a powered Z table.
3. Use a head with an adjustable lens tube, where the lens itself can move.

Then find true focus rather than trusting the arithmetic:

1. Lay a strip of scrap on a slope under the head.
2. Burn a straight line along it at low power.
3. Find the thinnest point of the line and measure lens-to-surface distance there.
4. Make a spacer from scrap at that height and keep it by the machine.

A powered Z table is the tidy answer if you plan to swap lenses often.

Write the new focus height on a label inside the lid. Months later, when you swap back to a cutting lens and forget the number, that label saves a ramp test.

The upgrade guide explains where an adjustable bed sits among the other K40 upgrades.

Once the seat is right, fitting takes minutes. Unplug the machine, let it sit, and remove the lens tube from the head. Keep the old lens in its case in case the new one needs a comparison.

• Orientation: curved face up, toward the incoming beam from mirror three.
• Gloves: clean nitrile, and hold the lens only at the edge.
• Ring pressure: firm enough that the lens cannot rattle, never tight enough to stress it.
• Cleaning: puff off grit, then one slow pass with solvent-wet lens tissue, isopropyl or acetone.

PVD ZnSe generally costs less than CVD grade and has a reputation for shorter life under heavy use. On an engraving lens that works at modest power, that trade is often fine. Keep it clean and it will outlast a cutting lens pushed at high current.

Air assist protects an engraving lens from smoke, and it matters even more at short focal length. The lens sits closer to the fumes rising from the work, so the nozzle's airflow is what keeps it clear.

The 101.6mm option needs about two inches of extra depth below the head, which a stock K40 cabinet rarely has without a lowered bed. Treat it as a special-purpose lens.

Other makers sell the same job in different sizes:

• Cloudray 20mm: CVD ZnSe at 50.8mm, the like-for-like cutting partner to this lens.
• CNCOLETECH 20mm: CVD ZnSe at 63.5mm for thicker stock.
• TEN-HIGH 18mm: PVD at 38.1mm, the same engraving focus for 18mm heads.

Owners who both cut and engrave often end up with two lenses, one short and one standard. That pair covers most K40 work without a single change to the tube or controller.

Settle three questions at the machine first: your head's lens diameter, how far your bed can move, and whether you mostly cut or mostly engrave. If you are new to the machine, how to use a K40 covers the basics that should come before any lens swap.