Nov 1, 2015

How to Make 3D Printed Building Sticks

Early on in my 3D Printing experience, I immediately became interested in designing connecting parts. After some fails, some lessons and minor success, I took a break. I came back to this project and now created some simple connecting parts I call "Building Sticks".

The goal for the Building Sticks was two-fold. First, I wanted something that would be fun for kids (and me) to build stuff - like a construction toy. Second, I wanted something I could use to build or prototype simple functional things - like phone stands or business card holders or even just ideas for larger custom objects.

Design Size

One project I had in mind for the Building Sticks was quite large - a printer enclosure - but I've learned that starting small is smart until the design is right. I also knew that the size of the model I created would have an influence on the connector design - particularly the clearances between parts (more on this below). I decided to start quite small - with sticks that were 5mm square around the waist (width and height) and around 30mm - 60mm long (although that measurement would not influence the connector at all).

Connector Design

Before I even finalized the shapes and angles of the objects to be connecting, I needed to get the connectors right. The main goal was to make sure they snapped together easily but also provided a firm hold. Not an easy balance to reach. From prior experiments, I decided on a mortise and tenon design. Here's the basic steps I took to get this done:

  • Model the tenon on a small block that was the same length as the width of the Building Sticks. This would allow the tenon to be inserted both straight on and from the side for 90 degree connections.
  • Add a "bump" on both sides of the tenon - perfectly in the middle of the tenon - as the method to provide a more secure "snapped in" connection on the mortise (which would have an opposite indent to receive the bump). I did this by overlapping a sphere onto the tenon, perfectly centered, then sizing it so it stuck out approx 0.50mm from each side.
  • Create the mortise on a separate block by subtracting a copy of the tenon from an equally sized block (5mm wide and high). This created the exact negative shapes on the inside of the mortise, including the bump, which is now an indent.
  • Create clearance between the Mortise and the Tenon to allow for smooth fit. Open up the mortise - and the indent - by just a small amount to give clearance so that the tenon is not too tight. THIS IS THE CRUCIAL STEP. I do this using the "Push" command in 123D Design on the inside walls of the mortise and the indent - and/or on the outer walls of the tenon (in the opposite direction). 

Clearances on the Connector

As mentioned above, the clearances given between the connecting parts is the most important part of this whole design. I found that at this size, clearances between touching parts should be somewhere in the 0.25mm to 0.30mm range for each side touching. That means for the mortise and tenon, the sum of those clearances became 0.60mm - which was to allow for 0.30mm for each side of the connection. The diagram below shows all the clearance values I ended up with.

Your Printer May Vary!

Once I "perfected" my connector design, I started printing on two different printers, and quickly found that the clearnace values I used for the Polar3D did not work well on the TAZ4. The TAZ4 has a flatter first layer and therefore pushes the filament wider than the Polar3D - so the parts were too tight. Make sure you test on the same printer you intend to use for printing final parts. That also means that if you end up using my models, you may need to adjust and customize the mortise and tenon sizes to fit better for your printer.

Shapes and Angles

The fun part of this design came after I had the connector working well. I could now experiment with all sorts of Building Stick shapes and angles. The only limitation was really "printability" - that is, making sure the shapes I dreamed up would actually print well on my print bed (reducing overhangs, etc). I found that printing both the mortise and tenon along the bed sideways worked best. Building upwards a tenon or mortise was ok, but not as clean, and not as strong (given the grain of the print layers were now perpendicular to the weakest part of the print (vertical layering).

I started with some basic shapes and will expand that as I get experience with what I need in my building. That's the best part - that I can customize these shapes for precisely what I might decide to build. I ended up with straight sticks in two sizes (30mm and 60mm), single height right angles, double-height right-angles (these are magical ;) and the "H with arms" - which are basically 4-way connectors that allow straight and right angle connections in any 4 directions.

Polishing the Design

Sharp corners on 3D Printed parts often come out sharp enough to cut skin. I learned this the hard way when my Pi Bracelet gave me a significant cut on my wrist when it got caught between me and my backpack. Now I almost always try to round the edges on designs which have high-touch. I do this simply using the "Filet" tool in 123D Design, which makes this super simple. A filet radius of 0.5mm worked well on the Building Sticks in the size I was printing.

That's really the only finishing touch I added to this design, but you can add all sorts of other touches if you want to spruce these up - even personalization embossed into the sides of the sticks.

Now What?

My main goal here was to get a basic design for connection which was reliable at one size. I'll now try the following things:

  • scale the design larger to use it as building blocks for a printer enclosure.
  • Create fun toy designs - perhaps hands, heads, feet, tails and other parts to let kids create their own characters.

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