I received the remaining water-jet cut parts from Big Blue Saw a couple of weeks ago, and have slowly been working on taking them from rough water-jet cut pieces, to parts that will actually fit into the robot. The water-jet cuts are fairly rough when looking for precision parts, which is why a lot of work goes into the rough parts in order to get things to piece together nicely. To be honest, though.. I wish I had a CNC mill/router that I could have used to precisely cut the parts myself.

This isn’t to say that water-jet cutting is bad, but it is what it is. You have to account for the fact that water-jet cut parts aren’t 100% perfect to minute tolerances. Some of the issues I have with these parts are that the water-jet seems to have cut at a bit of an angle, which causes tolerance issues requiring some filing work to make sure everything fits together nicely. That said, I’m still happy overall with the water-jet cutting, and would recommend it to anyone trying to make more complicated shapes or numerous parts. I wouldn’t have been able to make all of the more complicated/curvy cuts by hand as precisely and quick as the water-jet service does. Plus, the edge finish has a pretty cool rough effect (Although I will be smoothing it out for this particular robot)

After a lot of work machining the parts, I finally put togethor the frame of the robot with all four legs for the first time (minus the robot’s feet, which I haven’t gotten to yet). There’s still a lot of tweaking and some machining I have to do on these parts before they’re finished, and then there are a lot of small parts that I still have to make. It’s been around 5 months since I started this project, and things are really starting to come togethor, but I’m thinking it’ll still be another few months at least before the robot even moves, let alone moves in a fashion resembling autonomous walking. At least it’s finally starting to look like an actual quadrupedal walking robot.

I also turned the axles on the lathe recently. Eight of the axles are simply straight 3mm shafts, however the four hip/base axles are 4mm in the center, and 3mm on either end with a center groove to hold an e-ring which is used to keep the flat-flex cable in place, and out of the gears. I still need to put the axles on the mill in order to flatten them out to keep set screws in place, and to drive the potentiometers I am using for axle position sensing. I’ll post more on those in the future when I get to that stage of the build

One of the other things I still need to complete are the motor controllers, and the actual robot’s brain (likely a Gumstix Overo, but I would have to make my own carrier baord). I have tweaked and fixed small bugs in the design of h-bridge prototype I built in the fall, however I want to move the same circuit into a shape that will fit into the actual upper-leg side piece. Luckily that gives me more space to work with than the current itteration of the board (which will be used for controlling the inner four motors), so it should be a relatively easy task once I find the time.

(Just a disclaimer, I wrote this late at night, so if may not make perfect sense. I’ll edit it for clarity when I get a chance)

Rewinding a few months back to the design phase of my robot, I had the issue that I wanted to use both radial ball bearings, as well as Teflon/bronze washers thrust bearings in the robot’s leg joints in order to provide as smooth motion as possible. Sounds easy right? I found some 8mm diameter bearings that I got for a really good price, and still but needed to find a large outside diameter Teflon washer with a small inside diameter to fit over the shaft. I couldn’t find such a washer at low volume and low cost. After some thought, I decided that I could get some standard 1/4″ ID washers, with a .750″ OD, which is slightly less than the diameter of the robot’s frame at the joints. The theory being that I could then bore out the inside diameter to 8mm, and have that slip over the bearing which I would then only seat part way into the frame. Sounds easy, right? To do it well, I needed to make a jig.

With the arrival of my new boring head for my Sherline mill today, I figured what better way to learn how to actually use it than by making a jig for boring my washers out. This was a better option than trying to make the bearing seats in the frame without having ever done any real boring before, and risking ruining some parts I’ve already put a lot of time into. I did a few quick tests on some scrap metal, and then got down to business. I cut two pieces of some thin bar stock aluminum, drilled, tapped and screwed them together, and then began to make the center hole, which was to be 8mm. I drilled a rough hole approximately 6mm in diameter into the aluminum, and then put it into the mill’s vice. Using the center finder, I (obviously) found the center of the hole to line it up, and proceeded to bore out the 8mm center hole. After that was done, simply unscrew the two pieces of metal that make up the jig, and in the bottom piece I made a flat bottomed hole 0.750″ diameter, and a bit under 1mm deep to seat the washer in place. The key, I discovered, is to do it slowly and patiently. I ended up cutting the recommended 20mil at a time, then smaller and smaller increments as I got closer to my desired final diameter for the hole.

The end result turned out perfect. The teflon washers are simply seated into the jig, and held in place with the top plate. Since the material is soft, I can use just an exact-o knife to cut away the excess material on the inside, to a perfect 8mm diameter using the jig. For the bronze washers, I chose to re-align the jig in the mill and use the boring head to to bore out the center, a few washers at a time. The reason I’m using both Teflon and bronze is due to the fact that the Teflon washers are approximately 0.63″ thick, and the bronze washers are thin. I use the Teflon as the main thrust bearing washer, and then have the bronze washers to act as shims to make sure the joint is nice and snug, despite any tolerance issues in machining the frame. (I am far from an experienced machinist, so several parts of the robot are designed such that I can make mistakes and can recover from them instead of having to rebuild complicated parts)

[caption id="" align="alignright" width="150" caption="Perfect Fit of Bearing assembly"][/caption]After seeing how well the boring head worked out on the jig, I decided to try it out to bearing seat for the leg joints. After carefully measuring with the center finder, I began to make the 8mm diameter x 3mm deep cut to seat the bearing into the frame. It came out surprisingly well, and aligned concentrically with the 3mm reamed shaft/pilot hole I had to start with. I can run a shaft through both bearing and 3mm hole without binding. Now I only have another 23 bearing seats to go… On that note, the rest of the parts have been water-jet cut, and should arrive later this week. A lot of work goes from turning the water-jet cut roughs into final parts, but progress is definitely being made. I have also gotten some work done on the motor controllers in the past little while, but still need to finish up the design revision before sending out to get more PCBs made. When those get back, I have to build them and finish programming them. If all goes well, I’m hoping to have the robot workign with a very basic walking gate in a few months. Then the fun begins with experimenting with more complicated control algorithms, although I have a feeling progress will slow down as the weather turns nicer and the diving season kicks back into high gear.

Over the past week, I had a couple of nights free to get some machining done on the Quadruped. Initially I had to clean the edges of the waterjet cut parts, and eliminate the connecting bits. After that, filing some corners into sharp edges where parts fit together. Plenty of drilling and tapping was next in order, as the design is held together mainly with screws. For the body and one leg, so far I’ve had to drill and tap a little over 50 holes to keep it all together, all without breaking a tap. (This is my first project that involved tapping screw holes)

For those who are just venturing into machining, tapping small holes such as 2-56 is nowhere near as hard as it sounds, and horror stories of broken taps ruining parts can be easily avoided if you take your time and do everything carefully.

Here are a few hints for easily tapping small holes that I found useful:
(experienced machinists can skip on ahead)

• Make sure your pilot hole is drilled perpendicular to the surface, and free from metal chips before tapping
• Use lubrication. I used WD-40, although there are specialty tapping fluids available
• Use a tapping block to ensure the tap is perpendicular to the part’s surface, and cocentric with the pilot hole. You can easily make one by drilling a hole the size of your tap’s shank into any firm material large enough to keep the tap straight. I don’t recommend using wood, metal is the best choice.  Unfortunately, wood was all I had available and I found it absorbed the WD-40, and the tap did bring some sawdust off.
• Make sure your part is firmly held in place so that it’s easy to keep the tapping block firmly on the surface. For thin parts, using a vise is a very good idea.
• Make sure that you back off a quarter of a turn often while tapping, when you feel increased resistance. This will break the forming chips and keep them from clogging your tap. I usually did three half twists forward, then one half twist back and it seemed to flow nicely
• Keep in mind how deep you have to tap. Typically twice the width of the screw you are using should be enough to properly secure your part. Just me mindful of the type of tap you are using, since a plug tap will have to go a bit deeper than this, and a taper tap will have to go much deeper.
• Clean the tap often. The chips will build up and stick, especially with the use of lubricants.
• Wikipedia has more information on tapping, and if you look around there is a lot of wisdom floating around on the net, but hopefully I have brought some of the best points togethor here

After all the finishing, drilling and tapping, I finally put the parts togethor to form a leg. Although there is still a lot of work to go in boring the holes to seat shaft bearings, making the shafts, boring holes to fit gear hubs, making more holes to mount various sensors, pins, cable guides, etc, it’s starting to finally come togethor. So far it looks really good, and I don’t think I will need any major changes to the design before I send out to get the parts for the other three legs waterjet cut.