K40 Chinese Laser Upgrade Plans

Safety Upgrades

First and foremost is safety. We are dealing with a high powered and invisible laser. It cuts, it burns, it slices and dices skin, eyes, and anything else in it’s path. Should I have the machine open to replace materials or debug and accidentally place my hand on the control panel, I could accidentally turn on the unit and fire the laser. A software glitch or a pet might also accidentally start the laser unexpectedly. We need to fix this.

To solve the issue of safety, a few additions/replacements need to be made:

System ESTOP Button

An ESTOP button is a big red mushroom shaped button that latches when you push it in. They are used in industrial machines as a way to stop and de-energize a system. For my purpose, I will use it in one of two ways.

1) If the mains power consumption is low enough and I can find a contact that can support the current needed, I will wire the mains power through the button. That is, if I press the button, the 120VAC being fed to the power supply will be cut off. I will also wire the laser ON/OFF circuit through a second contact so that the laser cuts out before the power supply has a chance to drain any stored power.

2) If the mains power consumption is significantly high, I will wire just the laser ON/OFF circuit through the button so that it acts as a safer laser ON/OFF switch. A circuit through the second contact would also be wired to the new control board to stop motor movement.

If I can, I will shoot for option 1. Given the tube fires at a max of 40W, that will give me around 0.5A at 120VAC with the power losses in the power supply (I’ll measure and report this later). I don’t trust the engineers who designed the laser power supply to have a fail safe system that won’t continue firing the laser after the signal was removed or that won’t fire on its own. There is too much at stake if they screwed up either the engineering or the wiring. It also allows me to stop the machine quickly and easily should something go massively wrong like a fire.

Laser Enable Key Switch

In addition to the ESTOP button, I will be replacing the laser ON/OFF toggle switch with a key switch. In a nutshell, you need a key to turn the laser on. Once you turn it off and remove the key, the laser will not should not fire. This is great for a couple reasons. 1) Since there isn’t a button you can accidentally push, you can’t accidentally push it. 2) If I have company over that gets a little drunk and curious, I can remove the key and hide it so they can’t burn a hole in their fingernails or set fire to my apartment.

Covered Test Fire Button

Alright, alright, part of this is me wanting a goofy movie-style “FIRE!!!” button, but there is a practical use. I don’t want to accidentally push it. If the button has a cover that keeps you from pushing it, again, you can’t accidentally push it. So, double whammy – I get to live my action movie dream every time I use it and it increases the safety of the machine.

Current Settings and Measurement

Potentiometer Replacement

Looking through schematics others have posted online, the current control knob on the machine is just a basic voltage divider circuit. Turning it one way or the other will sweep the voltage read by the power supply between 0V and 5V. Potentiometers are great for inexact things like volume control, but not for laser power control. They have two issues for this application. 1) They can wear out over time or oxidize. This means that there is no guarantee of a smooth transition of power as you rotate it. A bare spot internally might cause an intermittent short and therefore a massive jump in the laser power or else a short that takes out the power supply.  I don’t like either.  2) They are not precise. When it comes to engraving, I want to have the ability to dial in the power for any given material. Once I find a setting that works, I want to be able to return exactly back to that setting.

To fix the potentiometer issue, I will be adding a small Arduino board to control a DAC (digital to analog converter) alongside either an LCD or 7 segment display. The DAC will allow me to send a precise and repeatable value from 0-5V and therefore a precise and repeatable power level. The power level would be displayed on the display. The actual input device will be a rotary encoder read by the Arduino.

Current Sensor Replacement

Analog meters are cool, but they kinda suck. They aren’t usually calibrated well (if at all), and can fail quickly due to their mechanical nature. I’ve used them in novelty clock projects and they just don’t last that long. To replace it, I have two main options.

The first is to place a 1Ohm resistor where the analog ammeter is and measure the voltage across it (it would have to be amplified to read well). I’m not a huge fan of this option. Should the laser tube fail or short for some reason, I will be left with around 40,000V being sent to a control board which will result in a fun explosion and an unsafe machine.

The second option is to utilize a hall-effect current measurement board. These come in two flavors. The first flavor is a non-contact sensor that wraps around the current-carrying wire and sense the magnetic fields there. The second flavor is placed in series with the circuit. It has a thick, half-circle shaped loop with the hall effect sensor underneath. As current passes through the half circle loop, it creates a magnetic field that is measured. Both are safer options, but they are susceptible to electromagnetic noise which the laser and its power supply will make a lot of. They can also lack in accuracy for lower currents which we will be using for the laser.

At the moment, I am leaning towards the second option – a hall effect sensor based unit. Should I have a failure somewhere, I’m much less likely to fry everything. I need to do more research to figure out what sensor units are available. Regardless of which I pick, they will be read by the same Arduino that is running the DAC and the value displayed on an LCD or 7-segment display.

Other Goodies

Water Flow, Control, and Temperature

I’ve seen a couple examples of people adding temperature and water flow sensors to their machines.  I like this quite a bit. It is extremely important to keep the laser tube cool. Should it overheat, it will go down the tubes. Knowing the temperature of the water reservoir and knowing that the water is in fact flowing is very helpful.  I will be adding temperature and flow indication to the unit. The temp will be found with a thermocouple and the flow with a simple flow switch. Both can be found online for cheap.

I will be adding to these as well. A liquid level sensor in the reservoir will tell me if the water is getting low. A DPDT (double pole, double throw) relay will be placed in the laser enable circuit as well as the control board ESTOP circuit to stop the laser and relay if the water isn’t flowing, the temp gets too high, or the water level too low. A button added to the panel will be added to the panel to turn the water on/off via a dedicated relay.

I want to use a relay instead of just wiring a bunch of sensor switches in series is for a couple reasons. First, wiring a bunch of sensor switches is begging for some horrible debug issues. Should any one go down, you have do dismantle the entire system to find the problem. Having everything go back to a master node that monitors things and either lights up indication LEDs or outputs on a screen makes things a lot easier. Second, it allows me to use analog sensors and tweak them as time goes by. If I don’t like an enable setpoint, I simply change the code. It also allows me to add warnings and alarms in the future should I want them.

I may also add a pressure sensor that would be used shut off the flow in the event a hose gets disconnected.

Fire Detection and Ventilation Control

Should the laser cutter catch fire, I would like the system to stop running and cut off the air ventilation system so it doesn’t make the fire burn hotter. Normal methods of fire detection become difficult in laser cutting applications. Smoke detectors will not work as the unit creates smoke during normal operation. IR (infrared) detectors don’t work because we are cutting with IR and burning the material we are lazing. UV (ultraviolet) detectors don’t work for reasons similar to IR. What is left is internal ambient temperature. A thermocouple placed in the machine will watch for temperatures to get too high which would indicate a fire.

If a fire does occur and the temp increases, a relay would remove power from the blower and a second relay would put the system into a soft ESTOP just like with the water monitoring system.

I don’t have the machine yet, so I don’t know the mechanical limitations of the machine. It may very well be that everything is pretty decent and doesn’t need a replacement. But, since I bought cheap Chinese crap, it could go completely opposite.

Air Assist Nozzle

This is a must. When you cut or engrave a material, you are doing one of two things: you are precisely burning the material or vaporizing it into a gas. An air assist nozzle blows air onto the cutting/engraving spot to help remove the fumes from the area. This allows the laser to focus more power directly onto the active surface as well as keep the smoke and fumes away from the expensive mirrors. In short, this means faster cutting/engraving, at lower powers which prolongs the life of the tube and gives you a better result.

I found this air assist and will be adding it to my build. http://amzn.to/2rnv8vu

Stepper Motors

Generally speaking, a stepper motor is a stepper motor. They are stupidly simple things and pretty hard to screw up. Unless there is a flat tooth or something in a motor, I expect to leave these pretty much alone.

A possible upgrade route would be to get stepper motors with either higher steps-per-revolution or higher torque. Both would allow me to cut more smoothly and the second would allow me to (possibly) run at higher speeds. Neither are actually necessary as I can simply slow down the machine, adjust the driver current, or adjust the acceleration in the firmware.

Another possible route would be to add a gear box to the steppers to add 2:1 or 3:1 gearing. Again, this might not be needed, but would increase both the motor resolution and torque of the system. The only downside to this comes from slack that will be found in the gearbox. Slack, in a nutshell, happens when a gear rotates the opposite direction and the small gaps in the gears allow the motor to spin a small amount without turning the drive gear. What this means is that I could get really smooth curves as long as the motors don’t have to reverse direction.

Belts and Timing Gears/Pulleys

Cheap timing belts have stretch and cheap timing pulleys don’t have good bite. The stretch will cause slack issues like I talked about with the gear box at higher speeds as well as possible oscillations as the carriages bounce like a ball. Good news is that both belts and pulleys are cheap and easy to find due to 3D printing, so this wouldn’t be hard.

Second Y Pulley

The machines I’ve seen typically have one Y (think “North” and “South) drive system on the left side. If the carriage assembly is well done and tight, there may not be any issue with this. If it is sloppy, I need to add a second Y pulley on the right hand side. In this way, both sides get pulled at the same time which can stop skewing as the cut/engraving moves to the right side of the machine.

Z Axis Bed

A flaw of this machine is that there is no easy way to adjust the focus height of the laser. Adjusting the focus means raising/lowering the material bed (if the machine supports it) or screwing with the optics. Neither of which are simple. So, if you want to engrave an 1/8 inch thick piece of plastic and then a 1 inch thick piece of wood, you have to change the settings. A Z axis bed would move up or down as needed to make sure the engraving/cutting surface is where it needs to be with ease.

Honeycomb Bed

Nearly all machines I’ve seen have a solid, flat, sheet metal bed that you place the cutting/engraving materials on. This works for engraving, but terrifies me for cutting. If the laser is cutting all the way through the material, it has a chance of bouncing off of the metal behind it and becoming a danger to nearby people should the reflected beam find its way out of the machine. It will also trap smoke and fumes underneath the material causing burn and scorch marks. Replacing the sheet metal bed with a honeycomb allows the laser and fumes a place to go.

Placing ceramic tiles upside down underneath the honeycomb bed (with the backs facing up) would absorb any stray lasers and safely scatter anything it can’t absorb evenly in all directions. The idea here would be to stop reflections entirely.  Another option is to get graphite plates specifically made to absorb IR lasers. These can be a tad pricey though…

The aluminum bed I will likely use: http://amzn.to/2pR65Ri

I don’t expect the laser and optics to be up to par. Given this is a laser cutter, these are all fairly important.

Laser Tube

Nearly all of these machines ship with poorly made, poor quality, reject tubes. Actually, this is pretty common for a lot of electronics – factories will make components for a customer, test them, give “quality” parts to the customer, and then sell the rejects to budget Chinese companies. Chances are, the tube in the machine will either not be marked with a manufacturer, have the manufacturer mark scrubbed off, or have the sticker hastily removed with all the residue left behind. What I’m getting at is that the tube should be replaced at some point.  I will run this tube into the ground, but sooner than later, I need to replace it with a professionally made tube.

A new tube ranges from $150 to $250 for a decent one, so this will wait a bit.

Optics

Mirrors and lenses are a pretty huge deal for a laser system and it is important to have good ones. Why? Because every time a laser bounces off a mirror, it looses a small amount of energy. For example, a  cheap mirror might have a reflectance of 95%. That sounds okay, but when you consider that three of them are used in the system, it adds up. The laser after the first mirror will be at 95%, after the second will be 90.25%, and the end laser output will be 85.7%. In edition to losing power, this also means that the mirrors will heat up and degrade faster. Replacing the mirrors with better ones helps to fix this.

Lens upgrade if needed: http://amzn.to/2pRy1V5

Laser Combiner

IR is invisible to the human eye. This means that the only way to line things up without changes to the system is to move the laser, do a test fire, and see where you get a burn mark. This means that to line up your laser, you have to burn something. With a laser combiner, you can add a low powered, red laser into the same path as the IR laser beam. The end result is that you can align mirrors and center your materials without risk of injury or damage to the material or machine.

There are a few other bits and bobs that I want to update or improve. These are things that don’t really impact the functionality of the machine, but may help either its aesthetics or general usability.

Replacement Window

I’m not a huge fan of the orange viewing window. Given that I paid less than $400 for the machine, I very highly doubt there is anything special about this window except the color. From the sources I’ve found, acrylic in and of itself is opaque to the wavelengths put out by the laser. What this means is that I should be able to drop in a 3-5mm clear acrylic window without fear of eye damage. Of course, I need to do a bit more research/testing to verify this, but I’d really like a clear window.

Internal LED Lighting

Lighting is crucial and a pretty cheap upgrade. I will buy some LED light strips and mount them inside the cutting chamber so I can align and calibrate the laser easier.

Mirror Alignment Covers

To align the laser mirrors, you typically place a piece of tape or paper in front of a mirror and fire the laser on low power. You can see where the laser is by the burn mark on the tape/paper. Unfortunately, the fumes and smoke from this process can contaminate mirrors are lenses. An easy fix for this is to 3D print covers that fit over the mirrors to keep smoke off.

Auto Laser Focusing

There is a great little sensor board from Sparkfun that has a time-of-flight (TOF) distance sensor on it. Simply place it on the laser head carriage, and viola! With some simple math, you have the distance between the material and the laser lens.