OMTech Laser vs. Ortur Rotary: 3 Inch Lens

The manual accompanying my OMTech 60 W CO₂ laser clearly states it has a 1.5 inch focus lens:

Which I had always assumed was the case, even though a short lens like that would typically be used for fine engraving due to its smaller spot size. One could argue the carton should have included a 1.5 inch lens in addition to whatever was in “its optics”, but it didn’t.

It has a 2 inch lens, as I confirmed while switching to a 3 inch lens to get more clearance over the Ortur rotary than the stock lens allows:

The bottom of the lens (its planar surface) sits inside the nozzle at (about) the same level as the joint just above the assist air fitting:

That’s the proper focus distance for the 3 inch lens, with the lens 3 inch = 3 × 25.4 = 76.2 mm above the platform. There’s obviously some room for quibbling about the optical center of the lens vs. the lower surface and so forth and so on, but a ramp test shows it’s Close Enough™:

Which adds an inch of clearance, enough to prevent obvious collisions:

Changing the lens requires removing the air fitting, during which operation I also moved the clamp holding the focus pen. Because that changed where the switch trips, the Focus Distance also changed:

• 2 inch lens = 12.7 mm
• 3 inch lens = 12.7 + 25.4 = 38.1 mm

The clearance under the nozzle depends only on the lens:

• 2 inch lens = 18.5 mm
• 3 inch lens = 18.5 + 25.4 = 43.9

I’ve been using step gauges for manual focusing with the 2 inch lens:

I figured a rod would be more appropriate for the 3 inch lens and, hey, now that I have a rotary, I can engrave it:

Through no fault of mine at the lathe, that stick is exactly 43.9 mm long, but “44 mm” fit better.