As we saw on the previous page, the Yummy robot uses a pair of side-by-side right-angle Faulhaber gearmotors. Each motor needs to be attached to their respective half of the candy tin. The narrow wheels provide little tolerance, and the curved tin and lack of usable mounting holes in the gearbox add complications.
So, I went overboard by machining a beautiful snug mounting block out of Delrin plastic (acetal). I wouldn’t expect most builders to go to so much trouble. For me, it was the most enjoyable part of the project.
The motor simply slides into the block and is held in place with a couple of fasteners. Think of it like a sleeve or a cover.
Views of motor mounting bracket
The mounting block itself has screw holes to attach it to the candy-tin body.
Mounting block mates with interior of candy tin
Compare the numbered items on the left side of the above photo to the right side.
I prefer colorful plastic for my custom robot parts. Even if the parts are rarely seen, they are kind of like chromed engine parts that custom car fanatics love to show off by popping the hood.
Unfortunately, most blocks of plastic are only available in dreary colors such as off-white or black. Occasionally you’ll find green or red, but these usually denote a special properly like "glass filled". Color sheets are readily available in acrylic and expanded PVC, but rarely in other types of plastic and rarely in machinable blocks.
So, I often comb eBay for leftovers. In this case, someone sells cut-off medical-grade Delrin in various colors. The caveat is that the scraps are rough-cut with rounded edges and a notch. Substantial machining is required if you need a smooth square block, and that further reduces the size of the scrap.
Squaring scrap and then machining into precision blocks
Here was the approach I took:
Trimming side squaring face and squaring side of scrap plate
This is a really complicated mounting block, and it is non-symmetrical for the left and right motors. It requires a carefully laid out design.
Mounting block layout
One of the more difficult machining operations is trying to mill out a square pocket when the end mill is not permitted to cut all the way across. Milling in one direction leaves rounded corners (illustrated in yellow in middle of the photograph below). Milling in the other direction also leaves rounded corners (illustrated in yellow on the right side of the photograph below).
Steps to mill square interior of motor mounting block
The answer is to mill once in both directions, leaving only the intersection of the two corners rounded. That can be further reduced (but not entirely eliminated) with very small end mills.
Before any machining began, I needed to figure out the base dimension of the motor mounting block. I wanted to reuse the existing wheel rivet hole so that the wheels would be located exactly as they were in the original candy tin. The motor was placed into the candy tin and propped up with coins until the proper height was found. The coins were then removed and measured for height.
Using coins to find proper height for motor
I got lucky, in that the position was perfect using coins. Otherwise, I could have switched to paper or pieces of plastic.
After creating the design, but again before machining, the printed design was placed into the candy tin to see if it fit.
Template and wood block to center punch motor mounting holes in sheet metal
Upon determining that is was good, a wood block was placed underneath the tin and mounting holes punched with a metal centering punch.
Dimples in candy tin for drilling mounting screw holes
Drilling by hand is inaccurate. Drilling into flexible sheet metal by hand is really inaccurate. The dimples keep the drill centered and help prevent walking.
The holes need to be slightly oversize to allow the motor mounting block to be marginally repositionable. Making the block position adjustable ensures the wheel spins parallel to the body and doesn’t rub against it. Any slight mistakes in punching or drilling can be compensated for when enlarging the holes with a Dremel grinder.
Finally, we reach the motor coupler on the next page.