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Redirecting your attention from the front of the robot to the back, let’s see how the rear wheel and speakers mounts are machined.
Robot wheel that pivots and rolls.
The rear wheel on the StreamHawk robot is a Lego wheel made of thin hard plastic. It was selected to match the color of the body and because the shape and material are fairly low friction.
The custom-designed wheel assembly contains several pins and setscrews to allow the wheel to both roll and turn/pivot. I’ve seen a number of pivoting wheels that get stuck in a perpendicular orientation (causing drag on forward motion), so an optional setscrew can lock the assembly to prevent the wheel assembly from pivoting.
Indeed, the pivot orientation issue appeared in StreamHawk and I have not yet figured out how to overcome this problem. A servo may be added to the wheel assembly to allow the robot to programmatically control the wheel angle. Or, two loyal readers (Joey and Dustin) have suggested moving the wheel away from the center of the pivot axle. That way, the offset wheel will have an uneven force applied against it, which may cause it to rotate and self correct.
Until I have time to modify the assembly, the pivot is locked in a forward direction.
The rear assembly bolt can hold steel washers as ballast to counterbalance the heavier front-end. Unfortunately, the Infineon TLE5206 motor driver chips don’t support a coast mode. This causes the robot to come to a sudden stop that tips the robot forward. The rear ballast reduces this condition.
A wheel axle pin of Delrin plastic machined on a lathe.
The rear assembly pins are made from a 3/8-inch rod of black Delrin (acetal) plastic. Delrin is strong and slippery. Many lightweight commercial bearings are made of Delrin.
A portion of the rod is turned down on a lathe to a diameter of 3/16-inch. The Lego wheel is slid on to test for a smooth rolling fit.
A length of narrow slippery plastic is difficult to machine because it bends away from the cutting tool. The proper solution is to apply the tip of a live or dead center to hold it in place. Alternatively, you may be able to squeak by with an ultra sharp tool, such as a polycrystalline diamond (PCD) lathe tool bit (McMaster-Carr #3316A34) shown in the middle of the above photograph.
After the axle diameter is correct, use a cut-off blade to cut the rod with about 5/32-inch (4 mm) of the original diameter remaining on the end. This provides a cap to the peg so that the wheel won’t fall off.
For a professional touch, chamfer the end of the peg cap. That is, flip the rod around so that the axle is held in the lathe chuck. Then, shave the edge slightly with a cornering, threading, or concave shape tool. (The concave radius tool bit pictured above is #3359A38 at McMaster-Carr)
Machining a wheel assembly from a block of black Delrin.
The wheel assembly is also made of black Delrin plastic. It requires a pretty thick chunk of plastic. I used some scrap pieces purchased in a lot of colored blocks on eBay. But, you can get a sample plate of 6″×6″×1/2″ (#8575K1) for $11 from McMaster-Carr.
The scraps are milled square, then milled to the drawing specifications, and then drilled.
The Devantech speech-synthesis board receives text wirelessly from the robot’s operator. Unfortunately, the Devantech module is not very loud with the attached speaker buried inside of the robot’s body.
Robot with a pair of rear-mounted speakers.
To make the speech audible, a pair of external speakers is mounted on the sides toward the back of the robot’s tube body. The speakers are common 8-ohm 1.5-inch diameter cheapos that can be salvaged from discarded electronics, toys, and PCs.
Since the speakers are highly visible, they need custom mounts with a style and coloring that matches the rest of the robot’s exterior.
A nasty scrap of ABS plastic with gouges and scratches.
Believe it or not, the fancy curved mounts come from an ugly uneven leftover piece of black ABS plastic (pictured above). I’m sure I considered discarding this scrap piece at some point, but wisely decided it was thick enough to be viable.
File off any raised edges on scrap that otherwise has a fairly flat side.
While it is possible to mill a clean square block with no prep work, you can preserve more of the material if you file away protrusions or other edge damage. This allows the scrap workpiece to sit more evenly in the vise from the start.
Machine only the portions of the scrap that will be used in the finished part.
Normally I’m in favor of cleaning up the entire stock workpiece before starting to machine the actual dimensions of the final piece. This ensures that all edges are square regardless of how the stock is held in the vise.
However, if the entire scrap piece isn’t going to be used, you can keep the piece thicker by not machining the entire surface flat. That is, if all the edges were machined perfectly flat then material in the middle would need to be shaved down to the height of the lowest scratch.
Drilling mounting holes in a workpiece.
Next, mounting holes are drilled so that the workpiece can be attached to a rotary table. Note that the scratched low-point is irrelevant and doesn’t affect the operation.
The speaker hole milled on a rotary table and test fit against the actual speaker.
The back of the speaker is 18 mm in diameter. That’s a pretty large hole to drill and I don’t own a drill in that size.
Instead, the workpiece is mounted to a rotary table and the hole is milled out using the same technique that created the robot’s collar. Rather than relying on a fancy measurement system to determine the actual size of the hole in the workpiece, it is started undersize and milled larger until the speaker fits.
Note the two 1/8-inch metal parallels under the both ends of the workpiece on the left side of the above picture. The parallels lift the workpiece slightly to allow the tip of the end mill to cut all the way through without making contact with the top of the rotary table.
Left: Machining the outside of the speaker mounts. Right: Removing the thin connecting plastic or flash.
The outer diameter of the speaker ring mount is milled using the same technique as the collar ring -- with an important exception. Because the speaker mount ring is not supported or held in place by internal screws, it will be difficult to mill all of the way through without potential damage or the ring flying out. Remember the golden rule of machining: Always secure the workpiece.
To avoid playing Frisbee, the outer diameter is milled until it is very thin, but not completely free. Then, the workpiece is removed from the rotary table and the ring is snapped out by hand. The thin strip of remaining plastic around the edges can be peeled off or removed with a razor blade, and then sanded smooth.
Finding the center of the plastic ring with a center square tool.
The ring is going to be cut in half to make two semi-circle (semi-ring?) speaker mounts. It isn’t critical to make the halves exactly even, so a line can be drawn by eye. Or, you can make it more precise by using a machinist’s center square.
Cutting a plastic ring in half with a hacksaw.
The ring can be split in half with a hacksaw, bandsaw, or table saw. If you choose a hacksaw, you might want to start the cut gently with the ring lying flat. This helps guide the final cut through the desired line. (In the picture above, notice that the bottom portion of the ring already has a shallow line of material removed.) Then, the ring can be placed in a firmer orientation in a vise for the final cut, as shown above.
Left: Milling the ends of a semi-circle flat. Right: Drilling the mounting screw hole using a metal ruler for positioning.
Place the freshly sawed semi-circle back into the milling machine to mill away the rough edges. A parallel placed across the top and supported by a pair of parallels on both ends can even the heights of the semicircle before machining.
After the ends are squared up, the semicircle can be flipped over to have a mounting screw hole added. A small metal ruler allows me to reasonably center the position of the drill by eye.
A No.43 drill with a 1/8-inch shank appropriate for mounting in a collet.
End mills are best held in a collet chuck because a collet grips the end mill firmly all the way up the shank. The collet is designed to function with vertical or lateral loads. Unfortunately, most hobbyist collet sets consist of only about 8-12 different diameters.
Since drills come in such a wide variety of diameters, an adjustable drill chuck is more suitable for most drilling operations. However, drill chucks are not designed for lateral forces, and as such should not be used for moderate or heavy milling operations.
Usually, when you need to change from a milling operation to drilling operation on the same machine, you must remove the collet chuck and swap in the drill chuck. This can be somewhat annoying and time consuming.
Since I use #4-40 screws for most of my robot parts, I often need to switch to a drill chuck to use a No.43 drill. But recently I found that Electronic Goldmine has these drills available (#G15724 for $1) with a standard 1/8-inch diameter shank that fits into a common collet. Other drill sizes are also available. So, I no longer need to swap out my milling collet just to drill a screw hole.
Semi-circle speaker mount epoxied and screwed in place on a robot.
Finally, the speaker mount is epoxied to the back of the speaker. It looks good for something that came out of a scratched and uneven scrap piece!
A single screw with some locknuts connects the robot body to the speaker mount. Black and yellow wires matching the robot’s color scheme enter the side of the robot.
Eagle-eyed readers may notice that the speaker mount is not screwed into the center hole that was drilled in an earlier picture. I changed my mind and drilled a hole towards the end of the semicircle to give it a more interesting look and to avoid covering the speaker wire solder points in case a future repair is required.
