As manufactured from above drawings. Cut from 1/4" amber
transparent acrylic instead of ABS.
Initial thoughts: the inner ribs weren't necessary; the bracket+spacer works
pretty well. The cutouts for it can be removed and the top deck can also be
cut off at the front of the servos. The decision to put 4-40 bolts through
the springs was an afterthought; I had been planning on gluing a 1/8" shaft
into place. It works well because the torque keeps the backet from scraping
on the threads.
On modifying the HS-425BB. This is an easy one to mod because it is dual ball bearing. A servo consists of a small dc motor, a gear train, and a potentiometer for feedback. On some cheap servos the potentiometer is a mechanically critical component. On BB servos it cannot be. So, there are two things you need to do. Both require opening the servo case. First, open the back. There is a small screw securing the pot. Remove it, then give a good yank on the cables to pull the pot out. If you're trying to be super elegant you can replace the pot with two matched resistors, thus making it forever at exactly 50%. More easily, you just move the existing pot outside the case. I did that by cutting a notch under where the actual cable exits to allow room for the 3 feedback wires. Second, the output gear has a small tab on it that physically prevents 360 deg rotation. Remove it very carefully lest you slice off a finger using a hobby knife. There are probably other better ways, though, and modifying rc servos is better documented elsewhere on the internet than here.
There is still too much slop in the suspension because I didn't fix it. That was because I had this cut out by a different company and didn't want to assume the kerf would be the same and have it not fit. Since the interdeck space is only 3/4" inches on this and the bracket is about 3/8", there is only about 3/8" of suspension throw. I think this can be solved by a cleverly designed cutout that lets the bracket penetrate the top deck, but that remains to be seen.
The only tricky step remaining in construction is coring out the drive cog. I did it with a mill, which is not supposed to be required equipment. Fortunately the drilling requires no precision.
It took about an hour to assemble this vehicle.
Revision 02 has much less tread problems on hard surfaces or flat rugs. One rug, the "evil" rug, still causes problems, but I'm hoping that if I remove the rotational play in the suspension it will help somewhat.
 Tamiya Treads |
 Laser-cut parts |
 Unwrapped |
 Drilling drive cog |
 HS-425BB |
 Opened |
 Output gear tab |
 Finished servos |
 Side |
 Front |
 Back |
 Top |
On modifying the HS-425BB. This is an easy one to mod because it is dual ball bearing. A servo consists of a small dc motor, a gear train, and a potentiometer for feedback. On some cheap servos the potentiometer is a mechanically critical component. On BB servos it cannot be. So, there are two things you need to do. Both require opening the servo case. First, open the back. There is a small screw securing the pot. Remove it, then give a good yank on the cables to pull the pot out. If you're trying to be super elegant you can replace the pot with two matched resistors, thus making it forever at exactly 50%. More easily, you just move the existing pot outside the case. I did that by cutting a notch under where the actual cable exits to allow room for the 3 feedback wires. Second, the output gear has a small tab on it that physically prevents 360 deg rotation. Remove it very carefully lest you slice off a finger using a hobby knife. There are probably other better ways, though, and modifying rc servos is better documented elsewhere on the internet than here.
There is still too much slop in the suspension because I didn't fix it. That was because I had this cut out by a different company and didn't want to assume the kerf would be the same and have it not fit. Since the interdeck space is only 3/4" inches on this and the bracket is about 3/8", there is only about 3/8" of suspension throw. I think this can be solved by a cleverly designed cutout that lets the bracket penetrate the top deck, but that remains to be seen.
The only tricky step remaining in construction is coring out the drive cog. I did it with a mill, which is not supposed to be required equipment. Fortunately the drilling requires no precision.
It took about an hour to assemble this vehicle.
Revision 02 has much less tread problems on hard surfaces or flat rugs. One rug, the "evil" rug, still causes problems, but I'm hoping that if I remove the rotational play in the suspension it will help somewhat.