"Squish" and "Brontus"

Before this news group finally drowns in the noise, here are two links to robots. The first one, Squish, is a four legged mobile platform made
form 1/8" plywood. No sensors yet, but the controller has eight inputs waiting deperatly for data... .
http://www.robowerk.com/en/robots/squish.html http://www.robowerk.com/en/robots/squish/pictures.html
The second robot is Brontus, created by another Robowerker, Olaf. Brontus is made from 1mm laser-cut aluminum. Videos will follow next week.
http://www.robowerk.com/en/robots/brontus.html
Matthias
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That is exceedingly cool. I'm still trying to figure out exactly how the parts fit together to form those nifty 3D shapes. The lower-leg pic is enticing, but I think it doesn't show all the individual pieces involved... I'd love to see that.
How did you cut the plywood? It looks very smooth, like a laser cutter might do, except that I thought the edges of wood would be charred under such conditions. Did you have to treat the edges in any way (e.g. sanding)?
Why did you choose plywood for this project over, say, lexan?

Another great design. Where do you get aluminum laser-cut? I thought you needed to go to a water-jet machine to cut through that.
Thanks, - Joe
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Joe Strout wrote:

Thanks. Yes, there is a second 'side' part missing from the foot. It is the same as the left side without the mounting holes, but including the Servo case hole. I will take more pictures when I build a second one.

Laser cutting for wood would char the edges which can then not be glued anymore. This is done with a little CNC machine and a 1.2mm cylindrical cutter. Sanding is needed, but no big deal. This is alos nice because I can go only half the depth and create even better connections between the pieces.

Short answer: I had it laying around. Long answer: lexan and friends are more difficult to work with on a CNC machine because it melts at high cutting speeds. It's also somewhat harder to glue. But I am actually looking for a nice plastic that I can use for this kind of work.

Thanks. Olaf puts days and nights into his designs :-). I had it cut at a friends machine shop who usually builds huge racks and enclosures. Most shops do not like to laser cut aluminum at all, because it is so reflective and high energy is needed. Since my buddy cuts up to 30mm steel, 1mm aluminum was in range. In the long term, it supposedly ruins the laser though. I have not looked into water-jet. Do you have heard anything about it, goofd or bad?
Matthias
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So the whole foot is just 5 pieces? I'm having some trouble understanding how it fits together. Take the three long notches on each side of the side part. These mate with the roughly cross-shaped pieces you've laid out next to them. But why do the notches have an extra semicircular extension on each end? I was thinking that this was used somehow to make the pieces interlock -- but now I suspect that these are just an artifact of the CNC cutting process, and end up as (harmless) empty space inside the part when it's assembled. Can you clarify?
OK, maybe I do understand it now. Very clever design. I love the upper-leg servo bracket. How strong is it?
Also, if you don't mind my asking, what CAD program do you use to define these parts, and what process do you use to develop them? E.g., if doing some funky new part like that upper leg, do you prototype it first with foamboard, or go straight to the computer? In short, how should I go about developing the same skill? :)

Hmm, if you're going to sand the edges anyway, I wonder if a laser cutter would work just as well. Once you've sanded the "char" off, it should be just as good -- but then, I have no idea how deep the charring goes.

Yes, lexan works nicely on a laser cutter, but I agree it's harder to glue. ABS might actually be better in that regard -- it can be both glued, and (carefully) baked in order to fuse parts together (which I understand is how they set the models at Legoland).

No, I've only heard it recommended as an alternative for materials too tough for a laser. Apparently the process is similar. <http://www.dcwaterjet.com/ is one such service.
Best, - Joe
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Joe Strout wrote:

Correct, Matthias said he used a 1.2mm diameter cutter. You can't cut a square inside corner with that, so the cutter runs past.

The water jet has a fine abrasive mixture added. The machines I've seen use a 35KW pump that pushes a 1mm jet of water at 700m/s (Mach 2!), and can cut through 150mm granite, and also works well on glass. Solid carbide nozzles only last 20 minutes, but there are diamond composite ones now that last many hours. The advantage over lasers are obvious - no heat and no reflection problems.
Matthias, you might like to fix some unfortunate spelling errors on your pages, for example "bugger" where you meant "bigger" :-). Nice work anyhow!
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Clifford Heath wrote:

Thanks, Clifford.
Yeah, the German version comes first and then I write the English text. By that point, I am very impatient. Oh bugger, I'll spell check it. ;-)
Matthias
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Joe Strout wrote:

Yes, Clifford is right. These are the path of the cutter. If I wouldn't make that little dip, my software would create two dips instead to make sure that this corner is 100% milled out.

Thanks. Quite strong. I have yet to break one. The interlocking helps, but I was alos very generous with the glue ;-)

I am using QCad. It's shareware and works well. 2D only, but since the CNC process is 2 1/2 D only as well, I have no troubles.
I create the pieces first by sitting on the sofa for hours, thinking ;-). Then I start by drawing the six sides of a cigar box with overlapping corners. Next, I remove what I don't need for the shape, for example the round corners on the foot and the holes. When all that is done, I do the interlocking, creating a gap on one part and removeing everything that won't fit into that gap from the other part.
I did do some 3D modelling on the PC a while ago which really helps me to envision the finished part. Brain-Foam, I guess.

I'll give it a try if I come across someone who'd do if for free. The post-CNC sanding is very very minimal: just bput the part flat on the paper and go back and forth twice. This removes the splinters from the milling process. With the laser, I'd have to sand the shaped sides which sound like a lot more work. But I'd give it a try.
Matthias
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That makes sense. So we could leave those little dips out if we were targeting a laser cutter instead of a milling machine.

Fair enough. Wood glue bonds very strongly. Working with other materials might require some experimentation to find the best bonding method.

Great! I recently did a survey of CAD apps available on my platform, and settled on QCad as well. It does everything I need, and does it very efficiently once you learn how it works. Are you willing to post the QCad files?

Sounds like a good process. I may have to try my hand at it myself. I wish I could think of an easier way to prototype it with foamboard, though. 1/8" foamboard is cheap and reasonably strong, and can be cut with a knife, but such cutting is rather tedious -- I'm not sure I'd have the patience to cut all the edges in parts such as these. Hmm, maybe my first project should be a CNC machine good enough to cut foamboard!

Yeah, I was thinking about that. A custom modeling program which assumes all 2D parts, but lets you assemble them in 3D, might not be too hard. I don't have time to write it though, alas.

You're probably right: with a laser cutter, we'd be better off using some material which doesn't char.
Best, - Joe
P.S. I hope you don't mind all my questions. Until this thread, I was totally ignorant of your work, and as you can tell, I'm quite excited about it!
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Joe Strout wrote:

Yes, that would be the way.

Stick around, I probably will in a few weeks or so. Depends on my other projects.

Well, look at the edge of those yellow Lynxmotion parts: charred. Plastic will probably always char.

Thanks for the questions. I hadn't really published anything yet on the robots side. I always like to get good feedback. And if you are interested in having those parts laser cut in an exciting material, then it sure adds to the project.
Matthias
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One more question, if you don't mind. How exactly do you make the attachment of the bracket to the non-horn side of the servo?
Thanks, - Joe
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Joe Strout wrote:

I remove the bottom plate from the servo, then drill a hole and push a Nylon bushing into that. The axle itself is a brass screw with its thread removed form the top 1/8th inch.
Matthias
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Thanks for the explanation. That certainly gives me food for thought. I think I'll start a separate thread about this issue.
Best, - Joe
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I've been thinking a lot lately about the use of servos in robotics -- my previous experience has been mainly with standard motors. So I've spent a lot of time looking at various servo-based robots (mostly walkers of some sort, often bipeds) to see how you take an output shaft that's on only one side of the servo, and turn this into a nice self-supporting motorized joint.
Here are the methods I've seen so far:
1. Don't Bother. Some people just bolt a plate onto the servo horn and let that be the sole connection to the next section of the arm/leg/whatever. This seems to work fine for lightweight models, but it must put a lot of stress on the servo axle.
2. Wrap-Around Mounting Bracket. LynxMotion's "Multi-Purpose" bracket provides mounting holes on the top of the servo, and a hole for a pivot on the bottom side, as seen here: <http://www.lynxmotion.com/images/html/build083.htm> This holds an unmodified servo nice and snug and provides a bottom pivot by means of the bracket.
3. Modified Bottom Plate. In another thread, Matthias Melcher

...which works well, as you can see in his "Squish" robot: <
http://www.robowerk.com/img/squish/leg_up_hi.jpg
But of course this means modifying the servo bottom plate, as well as a brass screw. (Is there anywhere you can by pre-drilled servo bottom plates? Seems like this would be a common need...)
4. Specialized Servos. The new HITEC robot servos (introduced with Robonova-1) have optional "idler horns" opposite the regular horns, providing a slim built-in solution to this problem, as seen in this review: <http://www.botmag.com/articles/hitec_servo.shtml> They also lose the standard mounting flange and instead offer, on the side away from the horns, mounting holes (which seem to do double-duty in holding the bottom plate on), and extenders which can make the width on that side equal to the total width across the dual horns; this makes it very convenient to bolt one servo to another using just a couple of flat plates.
Also in the specialized servos category would be the Bioloid servos, as well as the new LEGO NXT servos I suppose.
Are there any other solutions to this problem that I may be missing? I really like the look of the Robonova servos, but I prefer the wider selection available with standard servos. I'm a newbie to robotics, and I can tell this is a problem that's been solved many times... if you have any insight, I'd love to hear it.
Thanks, - Joe
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Joe Strout wrote:

A through-shaft is probably the way to go. Anything that puts a heavy side load on the bearings from one end is going to give trouble. Hitec apparently puts an "idler" opposite the output shaft on their new servo, but it's just a bearing attachment point, not a true through-shaft.
These gearboxes
    http://www.servocity.com/html/spg785_gearbox.html     http://www.servocity.com/html/worm_gearbox.html
are through-shaft devices, but they're rather bulky.
We're getting closer.
                John Nagle
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Yes, I'm amazed that it's so hard to find servos that work this way; the Bioloid ones are the only ones I'm aware of.
In fact, in a lot of ways, standard servos seem ill-suited to robotics -- for example, the way the cable connects on the same end as the servo horn, pretty much guaranteeing that it's going to get in the way of the joint, and also preventing the servo from fitting through a full-surround mounting hole in a frame. I suspect that the form factor was designed for their applications in RC models, and now there's just too much pain in deviating from the standard (with a few exceptions).
But I digress.

Right, but at least the idler supports the load on both sides of the servo; I think this must help substantially. A through-shaft would certainly be better of course.

Hmm, yes, those are neat though. That second one (the Tamiya worm gearbox) actually doesn't look much bulkier than a servo, but of course it lacks position control. If you could cram some sort of position feedback into that thing, it might make a really interesting robotic joint.
As for the first one, that suggests another one to add to my list: replace the servo horn with a gear, which drives another gear connected to a through-shaft. This does widen the servo assembly by probably 50%, but on the plus side, you could choose your own tradeoff of servo range/speed vs. power just by changing the gears. Hmm.
Anybody know where to find gears that connect securely to a servo shaft?
Thanks, - Joe
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Joe Strout wrote:

True, but we're finally starting to see a few devices that aren't just variants of R/C airplane parts. The important innovation is that some of the newer servos will read back position and torque to the controlling computer. That advances control from blind positioning to force feedback.

http://www.servocity.com/html/mechanicals___accessories.html
Servo City has a collection of collars, shafts, and other small parts for connecting things to R/C servos.
                John Nagle
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Yeah, but no gears (that fit on the servo shaft) that I could find. They do have sprockets and chains, which could serve the same purpose, but in a bulkier and more Rube-Goldbergish way.
Best, - Joe
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Joe Strout wrote:

Hmmmm. They used to sell gears that incorporates splines for attaching to servos. Convenient though it's usually not hard to use a standard servo disc and just mount the exact gear you want over it.
-- Gordon
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Gordon McComb wrote:

If you don't have the catalogs of
    Small Parts, Inc.     Stock Drive Products     Berg
you should. They're full of gears, shafts, collars, and related mechanical parts. That's where you get such things. Prices are higher than in the R/C world, but the quality is higher.
                John Nagle
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John Nagle wrote:

But none of these have servo splines, so there's still that issues, and when we're talking about R/C servos, I wonder if it makes sense to spend more on the gear than it does on the servo. You can get a nice nylon or Delrin gear replacement Traxxas gear, for example, for $3-4 retail.
-- Gordon
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