The K40-III laser uses a 12mm-diameter zinc selenide
lens in its head.
The lens is in a holder, screwed on the head. The holder has no groove inside
to hold the lens centered; the lens just lays curved side down on a hole in a
round machined aluminium block. The block has to be removed when the lens is
cleaned, and it is difficult to keep the lens properly centered after reassembly.
 Lens holder assembly, without air assist |  Original holder with lens |  Original holder with lens |  Original holder with lens |
 Lens, diameter |
A groove was machined in the lens holder, in its inner side, matching the diameter of the lens. This keeps the lens securely in place, without sliding to the side during reassembly.
The machining was done on a lathe. It turned out that the tools on hand were not suitable for the task, as their faces weren't tapered aggressively enough; the result was a curved wide hole instead of a sharp-edged one.
A tool was improvised from a broken piece of a saw blade, acting as a tool bit. This approach has all sorts of issues; the tool is flexible and springy instead of rigid. But for soft aluminium it turned out to be sufficient, even if requiring careful and slow approach.
The saw blade was placed in the tool holder, with a tool as a support. It required a fairly long overhang, to get the assembly into the relatively small part.
The tool has specific behavior. The blade bends when loaded, then slowly springs back to straight position as the load goes down. The blade edge was carefully pressed against the spinning workpiece to bend about 10 degrees, then was left to return to zero deflection as it scraped off the aluminium from its path. This was repeated until the desired shape was reached. (Yes, this is all sorts of ugly. But it worked.)
 Machining of holder |  Machining of holder |  Machining, improvised tool |  Machining, improvised tool |
The lens was found to fit snugly into the hole, with only minimal wiggle.
 Modded holder with lens |  Modded holder with lens |  Modded holder with lens |  Modded holder with lens |
The attempts left the lens sunk fairly lower, about 2 millimeters. To get it back to original position, matching height was machined off the sleeve of the lens holder.
The lens was cleaned with acetone and fingerprints and other gunk removed. The head was then reassembled.
 holder, removing height |  holder, removing height |  holder, removing height |  holder, removing height |
The illuminating cross was also removed from the head. Its lens was smoke-damaged and the cross did not project properly anymore, and the power cable was later cut and partially removed because of being pulled through the air assist tube and connector, where it caused bad leaks. New laser centering adjustment is planned, maybe a pilot red beam of a coaxially mounted red laser.
 Removal of cross light |  Removal of cross light |