Computer Controlled Cutting

Group assignment:

Characterize your lasercutter’s focus, power, speed, rate, kerf, and joint clearance

Document your work to the group work page and reflect on your individual page what you learned

MSI Chicago Fab Lab

Epilog & Glowforge Laser Cutters

By Alan Han, Kyle Pierre NFR, Dan Meyer

Characteristic Epliog Mini 24 #1 Epliog Mini 24 #2 Epilog Legend 36EXT Glowforge
Bed Size 305 x 610 mm (12 x24”) 305 x 610 mm( 12 x24”) 914mm x 610mm (36” x24”) 279mm x 495mm (12” x 20.4”)
Focus Lens 50.8mm (2”) 50.8mm (2”) 50.8mm (2”) 12mm (0.5”)
Power 35 Watts 35 Watts 75 Watts 45 watts
Rate 1778 mm/sec (70 ips) max 1778 mm/sec (70 ips) max
Pulse 100-5000Hz 100-5000Hz 10-5000Hz
Kerf (w/ 2” ens) 0.10-0.18 mm (.004-.007”) 0.10-0.18 mm (.004-.007”) 0.10-0.18 mm (.004-.007”) 0.2-0.6 mm (0.008-0.025”)
Joint Clear 0.25 mm (.010”) 0.25 mm (.010”) 0.25 mm (.010”)

Epilog Legend 36EXT technical specifications

Epilog focus-lens options and data

Mini 24 material settings

Glowforge technical specifications

Frequency* number of times a laser pulses per second. lower frequency -> less heat applied to material so dots through material, less charring of wood; higher frequency -> more heat applied so lines through material, melting of material

Power* amount of energy delivered per pulse. lower power -> shallower engraving so use for harder mateirals; higher power -> deeper engraving or cutting.

Speed* speed of the carriage in raster mode. lower speed -> deeper engraving or cutting; higher speed -> shallower engraving; when in doubt, use a slower speed

”*” from Legend 36EXT manual

Laser-able Materials

The most popular materials to laser cut in the MSI Fab Lab are the following: 1. 305 x 610 mm (12 x24”) sheets of 4mm thick cardboard with the specifications of:
ULINE (24 x 12”) 90 kg (200 lb) Corrugated Pads S-13742 4 mm (5/32”) thick C Flute

  1. 3mm thick cast acrylic from Inventables

Laser Focus

To review how focus works on the Epilog Legend Mini laser cutters we reviewed the user manual by searching and reviewing all the mentions of focus. It seems the focal length is based on the focal mirror spec 2” focal length lens are standard. With some plastic materials a de-focus can be used to improve cutting quality.

“Once you have the correct speed and power settings you can improve your engraving results even more by taking the focus lens out of focus (lower the table) by about 1/16 (1.5 mm) of an inch. This technique enlarges the focus beam a little bit and provides more beam overlap on each pass of the laser. The greater overlap produces a smoother engraved surface on the plastic and eliminates the grooves that you sometimes see when engraving plastic.” Page 119

“(Acrylic) Cutting Note: Adjusting the standard focus distance so it is closer to the lens by about .030” (.762 mm) will produce better edge quality on 1/4” acrylic and thicker. Two passes may produce better results and allow for cutting through thicker materials. There are two types of acrylic: cast is better for engraving (creates a frosted look when engraved) and extruded acrylics are better for smooth-edged cutting.” Page 143

Epilog also has a good Article called Focus Lens 101, which we read through. Of note was that a standard 2 inch lens “Produces spot size of 0.004 to 0.007 inches in diameter.” This leads into the topic of kerf.


Dan remembered having offsets of around .010 inch in acrylic, however today we’ll be working with cardboard so results may differ. we setup a series of squares in Inkscape. Of note while drawing these squares we noted the size of the selected object was always the line thickness bigger than the drawn size. For example a square drawn to 1x1” with a line thickness of .003” ended up being 1.003” when selected. Considering the laser beam width of .007 max, this .003” could throw off press fit fits. However upon closer inspection using View, Outline in Inkscape We realized that the line thickness in Inkscape is centered around the center of the line thickness, so this was an unfounded worry. Because of this for our experiments we labeled our squares with the drawn size, which should be accurate.

Joint Clearance

Given the above notes a joint clearance of between .007 min and .012 inches max due to laser heat affected zone may be expected. This means for perfect fit parts the offset may be in the range of .007 to .012 inches. We wondered if it is possible for some materials to swell back out after being cut as well. 1. We setup a 8x8” workspace in Inkscape via Menu: File, Document properties
2. Pressing Shift+Ctrl+F opens fill and stroke menu to set:
-Fill tab: No paint
-Stroke paint tab: Flat Color, RGB 0,0,0 A: 255 Opacity 100%
-Stroke style tab: width: 0.003 in (0.08 mm) 3. We drew 5 boxes across with varying decrease in size.
1.000, 0.993, 0.990, 0.988, 0.985 (25.40, 25.22, 25.15, 25.10, 25.02 mm)

alt text Above: 1.000, .993, .990, .988, and .985 boxes svg file ready for laser cutting

Kerf Width ULINE S-13742

Our experiment was done cutting 305 x 610 mm (12 x24”) sheets of 4mm thick cardboard with the specifications of:
ULINE (24 x 12”) 90 kg (200 lb) Corrugated Pads S-13742 4 mm (5/32”) thick C Flute We used the MSI Fab Lab Epliog Mini 24 #1 laser cutter. Here is a chart of the results:

Square Size Inside Size Outside Size Difference inch Difference mm
1.000 0.997 1.004 0.007 0.18
0.993 0.997 0.990 0.007 0.18
0.990 0.988 0.995 0.007 0.18
0.988 0.986 0.990 0.004 0.10
0.985 0.980 0.988 0.008 0.20

It was very hard to measure the cardboard with a calipers. We wondered if the kerf angle would impact measurements. The over all kerf width seemed to be 0.007 in the cardboard being used. So we decided to go with a 0.007 offset for one side of our parts when drawing in 3D CAD. This was a bit different than the .010 inch kerf width in acrylic.

Power, frequency, and speed settings for material cutting

We would like to write a few words about optimizing lasers to reduce kerf. The goal is to cut material as quickly as possible without burning or excessive loss of material. It is also important to use settings that produce the cleanest, smoothest, and most square (vertical) edges. While manufactures provide guidance for these settings on theirs sites and in their manuals, one should verify that the recommended settings obtain the desired results.

Vertical and horizontal kerf Width for Legend 36EXT

ULINE S-13742 and 3mm acrylic