Delta Star Diagonal Rods

I wanted to avoid getting some very specific joints, magnets, or carbon fiber rods so I explored printing rods and joints for the Delta Star. I have not tried any of the non-printed common approaches to diagonal rods, but I suspect they are awesome. Many are using the Traxxas universal joints or rare earth ball magnets with carbon fiber rods. They are probably very light, very rigid, and very precise. They are not, however, cheap and printable which are two of the goals of the Delta Star

Printed Joints and Dowel Rods

My first rods used the existing designs for printed universal joints with wooden dowels for the rod itself. I believe these are from some of original Rostock designs. I happened to have some appropriate sized dowels around (I like to make knitting needles out of them) so this was a good fit. I printed the rod ends and universal joints using the old cartesian printer and cut the dowels to the same length. Initially the fit was perfect and I just push fit the dowels into the rod ends and there was sufficient friction to keep them oriented and attached. 

I then used M3 screws in the universal joints. I used a drill through one axis of the universal joint to allow it to rotate around the M3 shaft and used a tap to put threads on the other axis to use two small M3 screws to create a two part axle. I later learned this is not the best approach, but it was good enough.

Printed Joints and Printed Rod Segments

I initially tried printing full rods and half rods but was having some warping and bowing problems. I decided to print the rods in smaller segments. My final attempt with this approach ended up with exploiting the segments to double as the forked rod end for connection to the joint.

Printed Joint Dependency on Threading

While the rod and joint designs worked, they had a number of shortcomings. First, it was difficult to get a very firm joint that did not wobble in use. Second, the tiny M3 screws holding the rod to the joint depend on the joint being threaded and the threads holding the screw in. This also leads to them often loosening themselves after extended use. I looked around on the internet and realized I was using the printed joints incorrectly. I looked closer at RichRap's Rostock build which used printed joints. This approach is actually using captured nuts to hold an M3 screw fixed with the joint able to float with no threading necessary. Another M3 can then go through the rod end and middle of the joint for the other axis. This obviously solves all the shortcomings of how I had implemented the printed joints. The problem now was that this would take a lot of work to switch to this because my use of the rods depended on the space efficiency of how I put together the joints. Rather than redesign my effector platform and carriages, I decided to redesign the joints and rods.

New Printed Rod and Joint Design

I created a wide fork for the rod end so that I could directly embed small M3 screws into the rod. This solved the problem of holding the screw fixed to create a split axle the joint could revolve around. The problem is this design would not allow inserting the screw with the joint in place and the screw is not reliably fixed in place. This could be solved by embedding a nut into the fork and screwing the M3 through the fork and nut, but this would add even more width to the fork. Instead, I designed the joint to split so that it could capture the embedded screws.
This image is of an older version of the joint, but the concept is the same. The screw goes through the two halves of the joint. This screw works as both the second axle and holds the two halves firmly together. I use a locknut to hold the entire joint together and the remaining part of the bolt can be used for mounting onto the carriage or effector.

I am very pleased with the performance of the new joint. There is no dependency on threading of the joint. The joint holds firmly together and the single piece fork keeps everything together with no wobble. The biggest shortcoming is depending on a very precise print of the fork. The recess for the M3 screw head should be very tight. Slight variations led to a crack forming in some of the screw head holders when I pressed the screw head in with pliers. They work fine, but it does not seem reliable in the long term. I will probably have to reprint some of the rods.

Future Rods

My next set of rods will have a little different design. I'd like to help with the rigidity of the rods and eventually trim away to get the rods lighter. Additionally I would like to be able to firmly connect the two rod ends with one screw using additional plastic to ensure no rotation about the screw.