Introduction to Ornament.SCAD

The SCAD file generates random ornament designs for your printer.  Of course your printer may need tuning before printing the ornament.  Save a back-up copy of the Ornament.SCAD file so you won’t have any regrets when changing parameters while adjusting your creations.

The first thing to do is enter your hot end extruder nozzle diameter into the SCAD file.  The ornament design is scaled vertically from this value to draw the string patterns without them touching.  Then enter the shape of the ornament by the “number_sides”: i.e. 3 for a triangle, 4 for a square, 5 for a pentagram, and 60 for a circle. The “maximum_angle_step” is how rapidly the strings are rotated (it will become a random number less than this entry). The “number_strings” sets the amount of strings in the pattern and the “number_sets” allows for multiple sets of string patterns.

There are two ways to generate the random numbers used in the designs.  The first approach uses a “seed” number.  And, since the “seed” number is the same each time you run the program (unless you enter a different seed value) the string art pattern is repeated. Enter a different “seed“ number to change the pattern.  Un-comment the line for the “random_vect” function that contains the “seed” variable to use this approach.

The second way to generate random numbers is to use your processor’s clock register contents. Forget the years, months, days, or hour values.  When you trigger “GO” to start the SCAD program it grabs the clock milliseconds as a seed value.   That’s pretty random.  Un-comment the line for the “random_vect” function that DOESN’T have the “seed” variable.  Now each time you click “preview” or F5 you get a new random design.  BUT, while they are nice to look at, you can’t save those designs.  You have to “render” or (F6) the design before you can generate an STL file.  And, when you “render” the design the program re-runs creating a new random design.  So, use the “render” (F6) button to run the program and when you get a good design you can generate the STL file.

The random number generator is creating numbers that fall between -1 and +1.  Lower in the program these random numbers are scaled to variables that move the strings around.  Feel free to try changing the scaling factors or optimizing things by adding or subtracting constants.

The strings are cylinders with four sides (rectangles).  The diagonal is the extruder diameter.  Then the cylinder is rotated into the X-Y plane and rotated 45 degrees along the cylinder axis.  Now the height is less than the extruder diameter (by 1-0.707=0.293).  This way the strings/cylinders can be stacked up at the extruder diameter pitch and they never touch.  The ends are then cropped to the size/shape of the ornament.  The strings/cylinders widths are stretched/scaled a small amount in the X-Y plane to trigger a touch more plastic during slicing and printing.  Note: in this case, the stacked cylinders/strings in the model-download STL files are 0.5*0.707=0.3535mm thick so a slicing thickness of 0.35mm will always intersect with a stacked string and only occasionally get two slices through the same string.  Note: the optimum slicing thickness will be just a touch smaller than the string thickness.

Optimization Hints:

The program generates random values for X, Y, Vx, and Vy i.e. the center of the first (bottom) string and the rate it moves in each direction as the strings are stacked up.  There’s a variable “ang” for the orientation angle of the bottom string and “ang_step” for the incremental rotation step applied to the stack of strings.

So, you’ve found a “seed” value that produces an interesting pattern but it’s not quite right.  Since you’re using the “seed” to generate the pattern you can rerun the program as many times as desired.  The ornament size, shape, number of strings, and the rotation rate can be varied using your initial data inputs before each program iteration.  There is an “ECHO” of the initial random numbers and the values of the X, Y, Vx, Vy, ang, and ang_step variables for each set of strings in the SCAD Console window each time you run the “Ornament.SCAD” program.  By going down to the lines of code where these variables are computed you can modify the values by adding or subtracting an additional value.  Note: there already is a “+0” in the code as a start. Remember, you’re just changing the position and orientation of the bottom string.  The stack is moved and rotated with respect to the bottom string.

Tuning Slic3r® version 1.3.0

These ornaments are essentially a “bridging test” for your printer.  The goal is to get as much filament in the crossing threads as possible for the best result.  The nozzle diameter for your hot end extruder that you set in the SCAD program is the upper limit for the size of a single thread of plastic.  The vertical spacing of the cross threads in the STL file are set to this nozzle pitch.  The thread line patterns in the STL file were scaled in width to fool Slic3r® into extruding as much “thread” plastic as possible without laying down multiple passes. A factor of 2 worked for a 0.5mm nozzle. For smaller nozzles you may need to reduce that scaling factor.

In Slic3r® (Print Settings) -> (Layers and Perimeters) -> (Use adaptive Slicing) unclick that box. Then under (Print Settings) -> (Layers and Perimeters) -> (Layer height) set the slicing height.  For a 0.5mm nozzle, I used 0.35mm.  When I tried 0.4mm and 0.45mm Slic3r® missed some of the cross threads because the slices fell either above or below the threads. The downloadable STL files in this model are generated for a 0.5mm nozzle so, when using a 0.35mm slicing thickness some slices will fall between the thread Z-heights but the next higher slice should capture that thread pattern.

Detecting the thin filament bridges in the ornaments.  (Print Settings) -> (Layers and Perimeters) -> (Extra perimeters if needed), (Detect thin Walls), and (Detect bridging perimeters) click those boxes. In that same section I have the (Avoid crossing perimeters) unchecked.  The (Detect bridging perimeters) box was the one that I found that was key to capturing the crossing threads for the print.

Turn off Support.  Slic3r® (Print Settings) -> (Support material) -> (Generate support material) unclick that box. 

I found that the Slic3r® default speed settings were generally OK.  The only glitch with my printer was at: (Printer Settings) -> (Extruder 1) -> (Speed).  This is the speed that print filament is pushed through the hot end.  The default was 40mm/second.  I’m using 3mm filament and the hot end stepper is geared down. As a result the stepper’s response to all retract commands was a loud “Burp” as it couldn’t move that fast.  Setting the speed down to 5mm/sec made for a happier response.  I also set (Printer Settings) -> (Extruder 1) -> (Extra length on restart) to 0.15mm since I saw a slight gap before plastic was deposited after a move and I wanted a bigger drop of plastic when the printer starts drawing a string.

Don’t forget to save your modified configuration file.  I.e. File -> Export Config … and give it a unique name for loading next time.

Setting the Printing Temperature

Higher temperatures cause the plastic to string out in fine hair-like threads.  You want the printed cross filaments to flow out of the extruder with enough tension to keep them straight until they cool, but not neck down to hair-size threads.  Each roll of plastic is likely to print differently.  Start walking the temperature down until you obtain satisfactory results.  Also at lower temperatures the hot end can’t melt the plastic as fast so you may need to slow down the print speed.  I found that slower print speed and lower hot end temperature were the next most important adjustments in printing the ornaments.

I hope you enjoy your ornament creations and the process of tuning your printer.