The version 2 prototype of 2 DoF arm mechanicals is complete! There are a few changes in the design that can be summed up as fit and finish issues
- switching the direction of Screws and moving the location of screws to make assembly easier
- increasing the length of the shoulder GT2 belt to make it easier to assemble
- moving to higher precision washers.
Items that are still on the to-do list are as follows:
- Finalize the stepper driver and controller (@dhylands is working on this)
- Develop end effector
- Get software running
This design is still in prototype phase and this is a short guide that will walk you through cutting and assembling the parts.
- Gather all of the parts
The purchased parts list can be found here, and the files for the parts that need to be cut or printed are here in Github. The parts that need to be laser cut (Or cut with a CNC) are the following files: arms.dxf and base.dxf. The arms.dxf file includes two parts. When setting up the laser keep in mind that there are parts of the file that should be engraved rather than cut. These are the areas around the holes that make space for washers and the stepper shaft. This can be seen in the photo below
While I used 6mm baltic birch, MDF or Acrylic could also be used
For the 3D Printed parts, shoulder.stl and elbow.stl, I used HIPS for the material, however PLA or ABS would also work, and I used a Lulzbot TAZ5 to print with a .5mm nozzle. This nozzle is a bit big for printing GT2 pulleys, however it did work. Any nozzle larger than .5mm should be avoided. When printing use supports for both parts. I used a 15% fill support in Cura. I had to do a bit of cleanup on the parts when removing the support.
2. Upper Arm Assembly
Before starting step 2 read all of this step. First put the skate bearings into the elbow and shoulder printed parts. There should be 2 bearings each. The bearings should be snug, but if they slip out that is okay. If they are really tight or really loose that is a problem and either your printer settings are off, or you may need to modify the files.
Then the shoulder is attached to the upper arm. This is a bit tricky as the M3x12mm screws need to be inserted as follows:
I used needle nose pliers to put the screw in place. This should be screwed into the upper arm and screwed through the upper arm.
Once this is completed add the stepper to the upper arm with 4 M3x12mm screws and washers. Leave the screws somewhat loose so that the stepper slides easily.
The Arm should now look like this, note that the stepper and shoulder.stl parts are on opposite sides.
3. Forarm Assembly
4. Connecting arms to base
Attach the forarm to the upper arm with a nut. For the shoulder joint, first put down the 280mm GT2 belt and use two large diameter washers, one below the base and one above and add a nut to the bottom side of the base to affix the shoulder. Add the stepper to the base with 4 M3x16mm screws. Add the 16 tooth GT2 pulleys to both steppers. The arm assembly should now look like this.
5. Adding and tightening belts.
To tighten the shoulder belt (the closed loop belt), just pull the stepper along the slots until the belt is just tight enough that most slack is gone, but not tight enough that it puts lateral pressure on the shoulder joint. If you tighten too much the shoulder joint will be tilted towards the stepper.
Next arrange the arm so the forearm is co-linear with the upper arm and ensure that the two ends of the GT2 belt overlap about halfway between the elbow joint and the stepper pulley. The picture below shows about the right spot.
Now, use two zipties to attach the two ends of the GT2 together as seen below. When you do this make sure the stepper is loose and slide it all the way towards the elbow joint.
Make sure that the zip tie doesnt catch on the shoulder joint, if it does just snake it out and flip it over (no need to remove the zip ties) Finally tighten the stepper to make the belt tight as shown.
Ta-Da the arm is now complete!