Wednesday, January 22, 2014

Delta Bot Build Part #3: Effector Rods & Hot-End platform

Finally, more progress on the 3D printer! With the frame designed, printed, and bolted together, it was time to move on to the moving parts. All part files are up on Thingiverse.

I ended up going through several revisions of both the hot-end platform, and the carriage mounts for the rod-ends. The versions shown in the pictures are not necessarily the latest version. I've kept the Thingiverse file list updated with the latest files.

Hot-End Platform on the Left, Rod Sleeves on the right.
I chose to use 8.5mm OD x 7.25mm ID carbon rods for the effector rods. I sourced them from Hub Hobby. The smaller diameter rods just didn't seem to be stiff enough. Another nice thing about this particular diameter is the Traxxas 5525 rod ends slip right into the ID of the tube. One thing I did notice was the rods seemed to split easily at the ends. I thought a simple solution was to print some thin-walled sleeves to epoxy over the ends to reinforce the rod-ends. 

My original, wimpy carriage rod ends
My first go at the design for rod-end carriage mounts looked fine on Solidworks, but after I printed them I thought they looked weak and flexy. You'll see later, beefier versions printed in yellow in the photographs below.

Fixture for making the effector rods
One key dimension to get right is the rod ends- the exact length isn't extremely important, but the consistency of length is. If possible each rod's eyelet to eyelet distance should be as close as possible to the same length as the rest. I accomplished this by making a simple fixture. My intended rod length was 250mm, so I drilled some holes in some plywood scrap 250mm apart, and pressed in a pair of 3mm SHCSs.

To make the effector rods, I first cut the rods to length (230mm) using an X-acto razor saw. Then, I epoxied the printed rod sleeves onto each end of the six rods.  When I epoxied the Traxxas 5525 rod ends into the ends of the carbon rod, I did it one rod at a time, and after applying the epoxy I slid the tie-rod ends over the screws in the fixture. So, when the epoxy finished curing, each rod was almost exactly the same length.

Finished Effector Tie-Rods, lightweight and strong as heck
The next thing to do was to bolt everything together and see how much backlash / slop there was in the system. Everything fit well- after revising the design several times to fix some perceived strength concerns, and to adjust fits, types of screws used, and method of clamping the rod-ends into place. One disturbing thing I found out was that the parts in a bag of Traxxas 5525 rod ends not all ends were the same- some had slightly different fittings on the end. I ended up having to shim the parts using 3mm washers to maintain a consistent spacing.

Note the yellow parts in the pictures, different than the original black shown above.

Assembled frame & platform. I really need a better photo backdrop!

Close-up of the frame and effector rods

Again, the carriage mounts and platform shown in the photos are not the latest and greatest, I did more revisions after they were taken.
Unfortunately the assembly isn't nearly as slop / backlash free as I'd like. I can grab the platform and wiggle it back and forth about 1mm. Much too much slop to produce a quality print. The slop all seems to be coming from some side-to-side play within the Traxxas tie rod-ends. A friend suggested going to magnetic arms- and I ordered some spherical 10mm magnets off of e-bay to give it a try. The Cherry Pi Delta 3D printer on thingiverse has a really cool implementation that I'll probably modify and try out.

Another option would be to use a pair of springs (or rubber bands) on the ends of each arm to pre-load the arms and eliminate the slop. I might try that short term as it's quick and easy.

Next up was to design and print endstop switch and adjustment screw mounts. At least initially I'll use the trusty RadioShack sub-miniature lever switch.

Mounts seen in yellow. The carriage is all the way up, with the M5 screw triggering the endstop switch
 I think the next step will be to build the platform for the heated bed, and start wiring it up!

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