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Positioning Components

One of the issues with designing with OpenSCAD is getting things positioned correctly. In a traditional CAD program, you just drag things around and things pop into place. Not so with OpenSCAD. You need to think through exactly where things need to go, and that involves knowing exactly where the parts are located in the coordinate system.

Coordinate Systems

Every individual part and assembly is created in its own coordinate system. When moving parts around, the part or assembly is translated and/or rotated into the proper position within this coordinate system.

You are in charge of the system used to design any part or assembly! All of this is a mental exercise. The important part of this process is being able to mentally visualize the gadget you are about to design, then identify points within that coordinate system that will be important in your design work. As much as possible, I try to think about each component the way a builder thinks and works. Most models are built on some kind of flat surface, so I make that surface the X-Y plane, and place the part the origin of the coordinate system. When assembling the model, I switch to a “pilot’s” view of the world. I place the origin of this system at some point on the front of the model and run the X axis down the fuselage to the tail. The wings are aligned along the Y axis, and the Z axis ends up aligned vertically .

For example, when designing an individual rib for an indoor model, I set up the coordinate system with the origin located at bottom front of the leading edge of the rib. The rib is constructed flat on the X-Y plane, and the lower point on the rib sits flat on the X-Z plane.

When we use this rib in the wing assembly, the coordinate system is different. For the wing, the leading and trailing edge spars should end up sitting on the X-Y plane, and the wing is centered relative to the X-Z plane.

That means we need to position the rib within the wing’s coordinate system. That will involve rotating it so it is oriented correctly, then translating that rib into its proper position within the wing structure.

Note

Of course, there is the issue of properly trimming the rib sit it fits nicely against a wing spar. There are a lot of details modelers never think about while building. I am working on ideas to streamline a lot of these details as I work on this project!

Visualizing Components

OpenSCAD make it easy to see the results of your work. You tweak the code, then generate a display that shows the results. (Of course, you never make typos, but if you do, you can fix then quickly!).

There is an interesting way to use OpenSCAD that I find handy.

I write a lot on my computer. Some of my writings are just normal text, some is code. Long ago I settled on a nice editing tool called vim that I have used for ll kinds of textual work. I really do not like working on some editing tool that is not as smart as my favorite editor. Fortunately, OpenSCAD lets me write code in my favorite editor, then save that work on my system. OpenSCAD can monitor a file you are editing, and then it sees a saved change, it will automatically regenerate your design so you can see it easily. That save a lot of time!

Using this scheme, I open up my favorite editor and OpenSCAD at the same time and position the screen windows as i like. I then set up OpenSCAD to use this feature, and turn off the OpenSCAD editor.

To do this, The critical point we need to remember is the location of the origin of the rib’s

coordinate system (bottom of the leading edge, right in the middle of the rib). That point must me moved into the proper position in the wing’s coordinate system. The translations need to slide the rib to the right spot, and any rotations needed to properly align the rib need ot happen before moving.

Align Function

To make this a bit easier, I wrote an align function that takes a list of six values:

\[[x_y, y_t, z_t, x_r, y_r, z_r]\]

Using this function involves calculating exactly what translations and rotations are needed to properly position that part. Since individual parts do not need to be aligned (unless you need to do that to properly create the part, I set up a file named partname_pos.scad within each assembly directory. This file lists the alignment values need to build the assembly out of the required components.

Using this concept, constructing an assembly is as simple as listing the components and aligning each one:

//###################################################
// wing.scad
// (c) 2021 - Roie R. Black
//***************************************************
include <./wing_data.scad>
include <./wing_pos.scad>

use <MMlib/position.scad>

use <./center/center.scad>
use <./right_tip/right_tip.scad>
use <./left_tip/left_tip.scad>
use <./wing_mount/wing_mount.scad>


module wing() {
    align(right_tip_pos) right_tip();
    align(center_pos) center();
    align(left_tip_pos) left_tip();
    align(wing_le_mount_pos) wing_mount();
    align(wing_te_mount_pos) wing_mount();
}

wing();

Calculating Alignment Values

In setting up the alignment values, you need to access design values. The dimensions of each component are important in figuring this out. There is no easy to avoid needing to do some annoying calculations. For instance, when positioning the wing posts on the side of the motor stick, we need to know how thick the motor-stick is and how thick the wing posts are. The offset needed is one half the sum of these two values:

post_x_offset = (motor_stick_thickness + wing_post_size)/2;

I have tried to name these dimensions to make it clear what the calculations involve. Unfortunately, some calculations need access to dimensional values defined in other directories. To make this a bit easier to manage, I am placing a data file in each directory where these values are either defined, of calculated as needed to define the component in the associated directory.