When deciding to get into the laser cutting game, one of the cheapest entry points is the K40 Laser. Floating around the $400 mark, the K40 is a great entry level laser for all of your CNC laser projects.
With the cheap price tag comes the cheapness of the machine. The K40 comes with a shoddy lens, cheap mirrors, no air assist and manual controls. Don't fret, the popularity and cost of these machines has created a huge community online and with that a ton of hacks that can be done to turn this into a great cutting or engraving machine.
I'm going to go through the various mods I have done to increase the precision, reliability and power of my K40.
Lens and Mirrors
If there is only one thing you change in your K40, let it be your lens and mirrors.
Lenses have set focal lengths. The laser tube creates a bean that measures somewhere between 3-5mm. That beam is directed through the lens which in turn converts the beam into the shape of an hour glass. The thinnest point of this hour glass is where you should put your material. In a stock K40, the focal point, or the thinnest part of the hour glass, it set at 50.8mm.
There are various focal length lenses out there, 50.8mm is a good starting point. Just make sure you replace the lens in your K40 with a good lens with proper coatings. The stock lens is of poor quality.
SI Mirror: Silicon Glass Gold Coated. High reflective index. Needs cleaning often. Good for up to 50w of power.
CU Mirror: Copper mirror, better than a MO lens, a bit below SI in reflectivity.
MO Mirror: Molybdenum mirror. Can withstand a lot of abuse. Has less reflectivity than SI mirrors. Good for up to 100w of power.
K9 Mirror: Bad reflective index and really easy to damage with dust or mishandling. This lens is standard on K40 lasers.
I personally have a set of SI mirrors in my machine and love them.
Laser cutters produce their beam in a tube. That beam then has to bounce off of three different mirrors before it finds its way through the lens. this is why it is absolutely critical to make sure your beam is perfectly aligned if you want a good clean cut.
The K40 is made in China. It's slow boat ride to your house will almost guarantee a few bumps along the way that will throw its beam out of alignment.
k40laser.se has a great guide on aligning your beam. It is my absolute go to when I align my beam.
Aligning the mirrors is easy, but if you have a poorly aligned tube to begin with no amount of mirror movement will help you. The K40's laser tube is mounted with metal straps that don't offer any form of adjustment.
Print yourself a set of these and replace the metal straps with them. It will make aligning your beam a lot easier.
Air Assist, Exhaust, and Bed Oh My
The bed that ships with the K40 is a waste of material. There really is no reason to keep it. Get yourself a honeycomb bed which allows for support and airflow under the material you are cutting.
Adding a stream of air that exits the nozzle along with the beam will offer cleaner cuts and prevent smoke and debris from heading up your nozzle and dirtying or damaging your lens. It also helps to prevent flareups from thicker material which requires more power to cut. Flareups can quickly turn into a fully fledged fire which would ruin your day.
I bought a cheap air brush compressor and ran some flexible line to the nozzle of the K40. Printing up some some wire track on my 3D printed allowed me to keep the flex line and the wiring for the laser crosshair out of the way.
Using a laser on any material will cause smoke, especially when one engraves plexiglass. The smoke that is produced will create a haze on the material which is nearly impossible to remove. One way to combat this problem is to create positive pressure inside the cutting chamber by adding a fan blowing air into the chamber.
I added a fan to the side of the unit which brings outside air into the cutting chamber. That air is then sucked out of the chamber by the exhaust system taking any of the lingering smoke with it.
I have never ruined a piece of plexiglass since.
Much like the other components that came with the K40, the exhaust fan sucks (see the pun I made there).
I 3D printed a mount for the back of the K40 which allowed me to connect some aluminum dryer hose. I also added an inline exhaust fan which sits inline of the aluminum hose. Plenty of CFM sucking power.
The K40 comes with a pot for controlling the power, an analog mA meter and a few buttons, not much monitoring or control.
I added a flow sensor to monitor the coolant flow, a submersible temperature sensor to monitor the coolant temperature and a temperature/humidity sensor to monitor the temperature of the laser tube itself. I wrote some Arduino code to monitor all of these sensors and output them to a 20x4 LCD display.
I also added power switches to control the exhaust fan, coolant pump, air assist compressor, laser crosshair and some lighting inside the cutting chamber. I also replaced the old analog mA meter with a new digital one and cut a new panel for all the buttons and display on the K40 itself.
All of these hacks has turned my K40 from a cheap mass produced ok laser cutter to a reliable great one.
Over the next few months I will be creating a lift table for my beloved K40. This will allow me to move the table up and down to set the focal point. Keep an eye out for a detailed how-to on DIY Hacker.