- Laser Cut Technology -

At FabLab Puebla, our facility is equipped with three advanced CAM-five brand-name laser cutting machines. These machines utilize cutting-edge CO2 tube technology, which generates a laser beam through the electrical stimulation of a gas mixture predominantly composed of carbon dioxide. A series of mirrors and a focusing lens then precisely guide and focus this beam onto the material's surface, enabling intricate cutting, engraving, or marking through the processes of melting, burning, or vaporizing the material.

Because of how the laser works, we need to calibrate the laser, which we'll explain using the following image:

1. Unfocused Laser Beam: This is the initial laser beam emitted from the laser source. It is typically wider and less intense than the focused beam.
2. Focusing Lens: The lens concentrates the laser beam into a smaller, more intense spot. The quality of the lens and its alignment with the laser source are critical for effective focusing.
3. Focal Length: This is the distance between the focusing lens and the focal spot. It is a crucial parameter that determines where the laser beam will be most concentrated. The focal length is predetermined by the lens' properties and must be calibrated to ensure the focal spot aligns with the surface of the target material.
4. Laser Nozzle: This part directs the laser beam onto the material. It is also used to expel any byproducts from the cutting or engraving process, such as smoke or debris.
5. Focused Laser Beam: After passing through the focusing lens, the laser beam is now concentrated into a smaller diameter, which increases its intensity.
6. Focal Spot: This is the point where the laser beam is most concentrated and, thus, where it will have the greatest cutting or engraving effect.
7. Depth of Focus: This parameter represents the range along the laser beam's axis over which the light remains within a small spot. The focal length and the diameter of the focusing lens both affect the depth of focus. A longer depth of focus allows for more tolerance in the material's surface height during the process.

The supplier recommends a distance of 5mm from the nozzle end for our machines to achieve the best focal spot against the material surface or near surface. However, in some cases, you may want to set the focus below the upper surface of the target to control the form of the kerf or cut zone, especially with thicker materials.

- Laser Machine Use -

Here are the three laser machine models we have in the fablab.

The American National Standard Institute (ANSI) considers Laser Cutters a class 1 laser based on ANSI Z136.1. These devices are safe when used as designed without manipulating the safety features. However this are some safety considerations to take into account.

Safety Considerations

• NEVER operate system unattended.
• Properly maintained fire extinguisher should always be in the area.
• Always keep the area around cutter free of debris, clutter, and flammable materials.
• Keep interior of the laser cutter clean and free of debris. Visually inspect the interior between uses and clean tray if debris or residue are present.
• DO NOT modify or disable any safety features of the laser system.
• DO NOT operate the laser unless all covers are in place and interlocks are working properly.
• DO NOT look directly into laser beam.
• DO NOT view directly with optical instruments.
• Use accepted materials only. Some materials engraved, marked, or cut with the laser can produce toxic and corrosive fumes.
• DO NOT operate if the view port is damaged, with any doors removed, or if any safety interlocks are defeated
• NEVER defeat laser interlocks built into the cutter. This could allow the beam to escape the laser cutter.

- Compatible Material List -

These machines can work with a wide array of materials, from soft fabrics like cotton and felt to hard substances like MDF, wood, and acrylic. The following tables provide a comprehensive guide for materials that the lasers can work with:

SOFT MATERIALS
Material	CUT	ENGRAVE
Paper	X	X
Cardboard	X	X
Foaming	X	X
Felt	X	X
Cotton and mixed fiber fabrics	X	X
Thick synthetic fiber fabrics	X	X
Synthetic fabric lycra and thin	X	X
Natural leather (leather)	X	X
Synthetic leather (leather)	X	X
Rubber (rubber or latex)	X	X
Cork	X	X

HARD MATERIALS
Material	CUT	ENGRAVE
MDF (pressed cardboard)	X	X
Wood	X	X
Plywood (pressed wood)	X	X
Plastic	X	X
Acrylic	X	X
Glass and crystals		X
Ceramic		X
Marble, onyx and other stones		X
Tile		X
Metal (apply special resin, laquer, etc)		X

NEVER CUT THIS!!!!
Material	DANGER!	Cause/Consequence
PVC (Poly Vinyl Chloride)/vinyl/pleather/artificial leather	Emits chlorine gas when cut!	Don't ever cut this material as it will ruin the optics, causes the metal of the machine to corrode as chlorine is released and ruins the motion control system.
Thick ( > 1mm ) Polycarbonate/Lexan	Cuts very poorly, discolors, catches fire	Polycarbonate is often found as flat, sheet material. The window of the laser cutter is made of Polycarbonate because polycarbonate strongly absorbs infrared radiation! This is the frequency of light the laser cutter uses to cut materials, so it is very ineffective at cutting polycarbonate. Polycarbonate is a poor choice for laser cutting. It creates long stringy clouds of soot that float up, ruin the optics and mess up the machine.
ABS	Melts / Cyanide	ABS does not cut well in a laser cutter. It tends to melt rather than vaporize, and has a higher chance of catching on fire and leaving behind melted gooey deposits on the vector cutting grid. It also does not engrave well (again, tends to melt). Cutting ABS plastic emits hydrogen cyanide, which is unsafe at any concentration.
HDPE/milk bottle plastic	Catches fire and melts	It melts. It gets gooey. It catches fire. Don't use it.
PolyStyrene Foam	Catches fire	It catches fire quickly, burns rapidly, it melts, and only thin pieces cut. This is the #1 material that causes laser fires!!!
PolyPropylene Foam	Catches fire	Like PolyStyrene, it melts, catches fire, and the melted drops continue to burn and turn into rock-hard drips and pebbles.
Epoxy	Burn / Smoke	Epoxy is an aliphatic resin, strongly cross-linked carbon chains. A CO2 laser can't cut it, and the resulting burned mess creates toxic fumes ( like cyanide! ). Items coated in Epoxy, or cast Epoxy resins must not be used in the laser cutter. ( see Fiberglass )
Fiberglass	Emits fumes	It's a mix of two materials that can't be cut. Glass (etch, no cut) and epoxy resin (fumes)
Coated Carbon Fiber	Emits noxious fumes	A mix of two materials. Thin carbon fiber mat can be cut, with some fraying - but not when coated.
Any foodstuff ( such as meat, seaweed 'nori' sheets, cookie dough, bread, tortillas... )	The laser is could cut food, however people cut things that create poisonous/noxious substances such as wood smoke and acrylic smoke.	If you want to cut foodstuffs, consider sponsoring a food-only laser cutter for the space that is kept as clean as a commercial kitchen would require.

- Characterization of Laser -

As all three machines have similar capabilities, we created a series of tests to standardize and characterize their focus, power, speed, rate, kerf, joint clearence and types.

Power and Speed

To characterise the laser cutters power and speed we made a two test charts. That visualise the combined power and speed. For the first test the min power we used was 10% from a 100 Watt laser, and from there we did an incremental increase of 10% until reaching the maximum power of 95% as it's not recommended by the manufacturer to up to 100% power. Similarly, for speed, we started at 15 mm/s and gradually increased it by 5mm/s until reaching a speed of 40 mm/s. By doing so, we were able to determine the optimal power and speed settings for each material.

In this test we can observe on the back part of the cut that power percentages from 30% or lower werent able to cut the material, and 10% wasnt even able to engrave it. In 40% the laser barely cut through qith the slowest speed being the only correct cut. All other percentage were able to cut the material however the cleanest ones was 50% with a speed of 40 mm/s, we can also observe that the slower speeds become really burn which means a worst kerf.

The second test is more detailed making the matrix bigger, to test more speeds, still having an increase of 10% power but adding speeds every 10 mm/s up to 100 mm/s.

The results of the second one are a bit more extensive so we made a table to show the results, however it's worth noting that there was a problem with the number of combinations and layers so the first line where we had the combination of speed 10 mm/s in the range of power 10% till 50% was lost. However we can see a linear pattern between engraving and cutting were theres a correlation of lower lower power needed to cut whereas higher speed require more power and we can see a clear line where this happens.

SPEED/POWER
Speed/Power	10	20	30	40	50	60	70	80	90	100
10	NOTHING	NOTHING	NOTHING	NOTHING	NOTHING	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT
20	NOTHING	ENGRAVE	ENGRAVE	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT
30	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT
40	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT
50	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	MARKED CUT	MARKED CUT	CLEAN CUT	CLEAN CUT	CLEAN CUT
60	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	CLEAN CUT	CLEAN CUT	CLEAN CUT
70	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	CLEAN CUT	CLEAN CUT
80	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	MARKED CUT
90	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE
100	NOTHING	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE	ENGRAVE

Kerf

When a laser cutter cuts a line, it burns away material. Because a laser beam has a certain width, you end up with a part that is slightly smaller than you want, unless you account for this. The kerf of a laser is the amount of material that gets burned away

To figure this out, we cut out a series of rectangles. I tested it using both 3mm MDF and 3mm acrylic. By obtaining the number of lines, the formula becomes easy as I have that number of inner lines that count for a full kerf. By sliding all pieces to one side we create a gap that we are able to measure using a caliper. This means that the kerf can be calculated by dividing the gap by the number of cuts.

MDF 3mm 10 cuts Speed 40 mm/s Power 50%

• 1.23 mm/10 = .123 mm kerf

Acrylic 3mm 21 cuts Speed 40 mm/s Power 50%

• 5.31 mm/21 = .252 mm kerf

Joints

One of the most common applications of laser cutting is the creation of joints for assembling parts without the need for additional hardware like screws or glue. These are known as laser cut joints.

In the following images we show the final drawings of some joints we laser cutted. This images include the equations and variables used to design them parametrically. The original files, as well as a DXF prepared to cut them on a small scale can be found HERE.

The following designs makes use of the deformation of the material to create a joint. When designing snap joints, the deflection of the material is a critical factor to consider, as it determines how much the material can flex to allow the interlocking parts to engage or disengage. Using the formulas from the chart below which comes from the following book, you can calculate the maximum deflection (y) allowed for a given material and cross-sectional shape before it yields or breaks. These calculations take into account the modulus of elasticity (E), the moment of inertia (I), the load (P), and the geometric constants for the cross-section (such as h, b, c, etc.). Which an also help you determine how to scale the dimensions if you change the size of the joint or the material thickness, which directly impacts the deflection and the stress distribution in the snap joint.

Therefore using this formulas we arrived to the following parametric deisgns using snaps. However it's worth noting that this is all a really simplifies approach. In practice the design for a final product should pass through an iterativ process of prototyping and testing to achieve the right balance of strength and usability.

Finally we also created combs for our material to test for the proper PressFit dimensions so the final product could be as precise as possible. This was done in Fusion 360 where each groove goes from the the material (thickness + Kerf) to (thickness - Kerf) with half a kerf with each step.

A living hinge is a thin, flexible hinge created from the same material as the two rigid pieces it connects, allowing them to pivot relative to each other. In laser cutting, a living hinge is made by making a series of closely-spaced cuts into a material, typically wood or acrylic, which allows it to bend along the line of cuts

Living hinges are widely used in product design for applications such as lids on packaging, flexible joints in toys, and movable parts in furniture, providing a combination of utility and elegant design simplicity.

So inspired by Deffered Procrastination who in their webpage calculated a way to get a parametric design of a living hinge. This is done by getting the minimum Torsional Links the thickness of the material the laser Kerf, the maximum torsional stress allowed and the minimum lenght of link. You can see the following image they provided for a visual clarification of the different part of the living hinge.

Nomenclature

• Θ = Total bend angle of the piece (Θ=θ×n)
• k_laser = Laser Kerf (m)
• l = Torsional link length (m)
• n = Number of columns of torsional links
• t = Material thickness (m)
• G = Torsional Modulus of the material (Pa)
• τ = Torsional Stress (Pa)

From this formulas we obtained the following design made in Solidworks which you can download HERE. The main parameters to change is the width of the material, the laser kerf and finally the bend angle that's needed.