Installing an Amp meter
Some K40s, especially the more popular model, has a digital panel. Although it seems a favorite, it's working is actually disappointing. Between two k40s, one will output the same strength at 10% while the other indicates 30%. This makes cutting guess work when you use the panel. The solution to this, is to drop in a DC 30 mA Amp meter which can be found at any electronics store or on eBay for as little as USD 4.00
The return lead ends at the front of the Laser PSU screw terminals. It's the first screw terminal. Remove the return lead and connect it to the amp meter (+) terminal. The other amp meter terminal, marked (-) now connects to this first screw terminal at the Laser Power supply. Now the DC amp meter sits in series with the Tube's low voltage return lead.
While you're at it, why not laser cut a nice control panel for your K40 to mount the new control. You can remove the digital control panel all together and replace it with a mini Gerbil and or potmeter (5kOhm). Design inspirations can be found at thingiverse.com ! Also check the yellow/green earth connection at the K40 case to the mains plug (earth prong). Some K40s have a plastic washer (defect from factory), replace that for a metal one that conducts current.