4. Computer controlled cutting¶
The machine :
Chinese Laser Cutter CO2 model painted and rebranded under MlLaser name. It’s a 150 Watt CO2 Tube.
Before going into the details of what the machine is made up of, I would like to come back to a few important points.
Laser cutting is a manufacturing process in which material is cut using a large amount of energy generated by a laser and concentrated on a very small area. By focusing a laser beam, the temperature of a small area of material can be raised until it vaporises. The power of a laser varies according to the material to be cut and its thickness. This process allows a precise, clean and fast cutting of many materials.
The machine emits a large amount of energy, which generates a significant heat source. The machine may catch fire, so it is mandatory to stay in front of the machine when it is in use. The smoke emitted from the machine may be toxic, so it is important to check the properties of its materials before using it in the machine.
What the laser machine consists of :
- A CO2 laser tube, when an electric current passes through it, emits a specific light.
- A cooling system, to lower the temperature of the CO2 laser tube.
- A aspiration system, to evacuate the fumes created during the cutting process to the outside.
- A propelled air system that blows in front of the laser lens to prevent smoke, dust or other contaminants from obstructing the laser’s path.
- Mirror sets to reflect the light sent to the back of the machine.
How the laser cutter works at AgriLab :¶
As described above, the laser cutter at AgriLab is a Chinese Laser Cutter CO2 model painted and rebranded under MlLaser name. It’s a 150 Watt CO2 Tube.
The tube at the rear of the machine will create light under the influence of electricity. This light will be reflected by mirrors at the ends of the machine to the laser lens. The purpose of the lens is to concentrate all the light energy into a single point. This phenomenon makes it possible to have a large amount of energy on one and the same point.
The mirrors are surrounded in red
A water cooling system regulates the temperature of the CO2 tube. The reactions that take place in the CO2 tube give off a large amount of heat. In order for the system to work properly, it is necessary to cool the whole system to avoid any risk of overheating and deterioration of the material. The ideal temperature is 22°C
A fume extraction system is present on the machine. To cut, the machine heats up extremely strongly at one point, this burns and thus generates smoke. It is therefore very important to extract these fumes to the outside to protect the health of the user and those around him.
A small compressor is placed near the machine to send pulsed air to the laser lens. This expels any smoke or dust that may be in front of the lens and alter the laser beam.
Here are the different elements connected to the machine.
The machine has a safety device, a position sensor is placed on the machine frame. This sensor makes it possible not to start cutting if the protective cover is opened.
Basic safety rules:
- Keep a power/speed ratio according to the material.
- Use safe materials.
- Work under supervision of the instructors.
- Never open the lid while the machine is working.
- Never put you or something between the path of the mirrors of the machine.
- Identify the location of the extinguisher.
- Localize the emergency stop button.
- If possible use a wet fabric to extinguish small fires before use the extinguisher.
- Never look directly to the laser.
- Check the air extractor and liquid cooling systems, both should be working properly.
- Stay watching the process carefully, don’t turn back, because sometimes fires spread quickly.
- Keep a safe distance area.
- Be aware of the fumes.
How to use the machine :¶
|1||Turn on the machine and the computer. Simply turn the safety key and press the Start button on the computer.|
|2||Place your object cut out and focus. For our part, simply place the laser over the material with the arrows. Then press the “Z” touch button and lower the plate with the arrows. Place a metal plate on your material and below the nozzle. Press the “Datum” button for the machine to focus automatically.|
|3||Open LaserCut 6.1 on the computer and import your file.|
|4||Set up your layers by adding colours to your vectors|
|5||Set up your vector. Double click on your layer and enter the values for the desired speed and power.|
|6||Save your design and send it to the machine|
|7||Switch on the fume extractor and close the cover.|
Power and Speed¶
Power refers to the output energy of the laser, and speed to the speed (and acceleration) of the cart where the laser is mounted. There’s an important relationship between the laser power output, the velocity and the material to cut (and surface of it).
For cutting a piece of wood and a paper sheet, we shouldn’t use the same settings, the reason is Combustion.
There are multiple things than can change the effects of the laser in the material, for example humidity and the molecular structure of it.
For that is very difficult to have a standardized table for every kind of material, for this reason it’s a very important task for any Fab Lab to test every type of available material approved and safe for use in the laser cut.
We made individually multiple trials for the settings to understand the functions and effects of power and speed in the process of cutting and engraving.
This is a proposed compact test template.
One of the group asignement was to caracterize the laser cutter and more specially the kerf. To do the kerf we had the idea to cut a square in the woodand then do multiple slices in it. I means that the laser will do many rectangles in the wood many times and we can have an average of the length of the laser. First we cut a piece of wood and mesure it.
Then we cut it in different part with straight lines. We set the laser cutter with a power at 85 and a speed at 40. We got these values in the previous test.
And finally we mesure the total size of the remainers slices. We mesure wuth the caliper.
We did again with the picometer tool but with a smaller piece of wood to fit it in the tool. We mesure it also with the caliper.
Here is the table of our different results
|Try||Size before the slices||Size after the slices||Number of slices||Average of the kerf|
|1 caliper||98.0 mm||95.1 mm||10||0.29 mm|
|2 mircometer tool||21.12 mm||19.88 mm||5||0.248 mm|
|3 caliper||20.8 mm||19.8 mm||5||0.2 mm|
So when we do an avergage of these values, we can say that the kerf is 0.246 mm.
For the group assignment we carried out a few joint tests. We decided to make the same joints on different materials to see the possible differences. So Theo made the plans on Fusion360. Then we decided to do it on 3.9mm thick cardboard, 2.5mm thick Plexiglas and 3mm thick mdf wood.
Here are our conclusions:
On the cardboard, the simple press-fit and chamfer works very well and is very accurate. For nesting such as flexure or snap, the cardboard is too sensitive to deformation. Our tests were not conclusive for these nests.
On the plexiglass our main problem was the kerf.We found that our plexiglass parts were more openwork than expected. This is due to the laser melting the material too much. In addition, we can see that the flexure nesting has been subjected to too much heat. The cutting process has deformed the whole assembly due to excess heat. Concerning the snap nesting, it is not at all adapted to plexiglass. In both cases, our parts broke, which is explained by the fact that Plexiglas does not accept a large deformation.
MDF is the material that best fits all the joints we have tested. It fits perfectly.
In a group we carried out tests to find out the ideal focus for the machine. Our machine has the option of autofocus, which automatically adjusts itself according to the thickness of the material.
What is the interest of the focus?¶
The laser tube located at the rear of the machine emits a wide stream of light that must be channeled to a single point. This is achieved by using a convex lens. A convex lens refocuses the light at a single point (see photo opposite). In order to concentrate as much energy as possible on a small area, it is therefore necessary to focus. Focus is the action of adjusting the height of the material so that it is at the point of convergence of all the light beams.
Our machine make autofocus, but for the exercise we decided to focus manually. To do this we removed the honeycomb and positioned a sheet of paper on wood. We did the autofocus and measured the distance between the paper and the top of the lens. We made a laser dot to visualize the point of impact and its diameter. Then we moved around and lowered the plate. Then we made several laser points by moving the plate up.
Here are our results:
You can see that the machine’s autofocus is set correctly. Because it is at this distance that the laser achieves the smallest point.