Delta Star Arm Carriages

The arm carriages for linear motion of the Delta Star visited many different branches in its evolution.

Legacy Kossel Rollers

The first approach was the system of the legacy Kossel design of Johann C. Rocholl. The rollers were two printed halves each with three 623 bearings. The halves were connected with long M3 screws which could be tightened to firmly close the halves around the aluminum extrusion. The only contact with the extrusion were the bearings which acted as wheels to roll them up and down the extrusion. I liked this design as it used very easily sourced parts and let the aluminum extrusion act as both the frame and rail for linear motion. The system is driven by spools attached to the stepper motors and wound with fishing line attached to the rollers. For this design, I took the additional step of making shrink wrap "tires" for the 623 bearings using the largest wire shrink wrap I had. This helped reduce the wear of the steel bearings on the aluminum extrusion.

V Bearing Rollers With Encased Line

While the kossel rollers worked adequately, I wanted to see if I could improve on the stability of the rollers and do something about the positioning of the filament lines which had the potential to entangle the effector if it got too close to the perimeter of the print area. I designed some carriages that would use 623 VV bearings to capture the ridge of the aluminum extrusion with the V-groove of the bearing. A center plate acted as a filament guide to keep the filament in the center channel of the OpenBeam. The two carriage halves were connected through the plate with longer M3 screws. These could be adjusted to control tension of the bearings on the top outer channels of the OpenBeam.

I was pleased with the performance of these carriage. The amount of plastic and hardware was reduced a bit from the previous Kossel rollers. The tricky part of working was these carriages was adjusting the tension of the two connecting screws to make sure it was tight enough to prevent wobble when in use and loose enough that they do not overcome the torque of the steppers motors driving them.

Printed One Piece Sliders

The next approach was printing sliders which would directly travel on the OpenBeam with no bearings. This would remove a lot of bearings from the design. The first slider was a single piece I created in OpenSCAD that was simply a hole for the OpenBeam and a mount for the Delta arms. I threaded the drive filament through a hole in the center of the front face. The filament is pulled through and tied off. To tension the line, I simple slid an M3 screw through the filament loop and twisted it. I was very pleased with the performance of the printed sliders. Philosophically, it removes a lot of hardware and makes the printer that much more replicatable. These were also very quiet. The drawback of this design was the precision necessary to get them to perform well. Too tight and the stepper motors cannot get them moving on the OpenBeam. Too loose and they can start to bind and stall on the beam when the lateral motion of the arms tilts them and creates too much friction. Additionally, this single piece design made it very difficult to replace them. One end of the OpenBeam needs to be completely cleared to get the slider off. In my cycles of constantly trying new ideas, this was a big drawback.

Function Plates on Printed Slider

I first went about addressing the last drawback of the previous design by making modular function plates for the front and back of the slider. The design is similar to the once piece slider with the addition of mounting holes through the sides. This allowed adding an arm mount plate. Additionally, I began experimenting with a ratchet mechanism for dealing with line tension a little more elegantly. This first ratchet took each end of the drive filament in and they were tied into a loop. An M3 screw, again, twisted the loop and the ratchet catches the screw to maintain tension. This design still had the problem of the performance of the slider was dependent on getting a perfect print of the slider block.

Multipart Printed Slider

Next up was addressing the difficulty in tensioning and mounting the printed slider. The center block was broken up into four pieces. These all came together with the same four M3 screws that mounted the function plates in the previous design. This slider could be mounted to the OpenBeam without clearing one of the ends. Additionally, tension could be adjusted at least in the front to back axis by tightening or loosening the mounting screws. The four piece slider was also an easier print in that they could all be printed with the broad flat part of the piece laying on the print bed. This led to far better results when printing the tiny rails that go into the groove on the OpenBeam. This new design used the same function plates from the previous design. Addressing the easy mounting and removal of the sliders from the rail has been very helpful in reducing the amount of time and frustration involved with the iterative experimentation that accompanies designing these parts. This was also important in that the affect of wear on the printed sliders is still unknown with this design. These may need to be replaced occasionally so ease of access to these parts will also be helpful in this regard.

Conclusion

The new design is working well and is available on thingiverse. I will do a write up on the ratchet mechanism for line tensioning when I cover the other changes in the drive system.