Cold Arbor Update 3: Deathrunner

Saturday, January 9th, 2010 @ 9:20 | Bots, Cold Arbor, Project Build Reports

Certainly the centerpiece of Cold Arbor is the 10″ milling saw, but circular saws are useless without motors. So I have commissioned the construction of a custom-wound brushless motor (hereafter termed Deathrunner) to drive the saw. While I certainly could use a commercial outrunner, high performance and solid motors cost some serious money. Factor in IAP’s excessive amounts of usable build time (for those of us who are enthralled with anything but academic work), and I’m better off pumping out a motor of my own design, while maybe learning a thing or two about said motor design.

The reference motor is the HobbyKing 70/55 outrunner. Deathrunner’s physical layout is practically identical, with key areas made more supple to take the impacts of battle. In this latest generation of  Inexpensive Chinese Brushless Motors (ICBMs), the distal end of the can is supported by its own bearing. This ensures that the can won’t begin resonanting at high speeds, or just… fall off.

I never trusted motors which hung a huge bell-can off a single set screw connection to a shaft. The original Snuffles motor suffered from can-wiggle to the point that the magnets started falling apart.

We begin with a Giant-Ass Steel Pipe (GASP – is someone keeping track of these random acronyms that I make up while writing 7AM build reports about the night before?)

Hey, speaking of the original Snuffles motor… here it is! I intend to harvest the stator, more or less the only functional part remaining in the motor, for Deathrunner. This motor has been completely trashed. The can is physically out of round, the shaft connection has been stripped and Loctited countless times, the bearings are crunchy, and the chrome plating is scratched, tarnished, or completely eroded away in places.

The steel pipe is 3.5″ OD and 2.875″ ID, 5/16″ wall thickness. I picked this pipe as a compromise between several motor designs that I want to build, all requiring slightly differing can diameters.

Unfortunately, this means that I had to turn down the pipe to make the 3.25″ OD can for Deathrunner. We’ve seen what happens before when I try to make a big steel pipe into a little steel pipe.

You know, the exact same thing.

The difference is that this time I had a real (sort of) boring tool. Optimally, a piece this large is finished off on a chunkier machine, but I was already at MITERS, where we’re limited on tool sizes, and didn’t really care to seek out another shop.

The consequence of hanging a tiny bit way the hell out beyond the holder is CHATTER. You can observe my amazing finish on the inside here.

I decided to just put up with the chatter because nobody should ever be seeing the inside of this motor anyway, and if nobody’s looking at it…

Plus, all those ridges ought to make the magnet epoxy stick better, right?

I cracked open the old Snuffles motor. Check out the scrape marks on those magnets – and the missing chunk.

Beginning the uninstallation of the windings. The motor is wound “DLRK” style, a symmetrical winding that results in three easy to terminate connections.

Easy to wind also means easy to unwind. Here’s the successfully recovered stator. It is integrated (epoxied, pressed, you name it) onto an aluminum mounting post.

The second custom part for Deathrunner is this… can end? Bearing holder?

It’s BOTH!

The Integrated Aluminum Post (IAP) is conveniently 40mm in diameter at its widest point, the bottom of the motor. A 6808 metric miniature deep groove bearing is 40mm on the bore and 52mm diameter.

This just screamed can bearing!!!! to me, made all the better when I discovered an Ebay bearing supplier that was even sketchier than VXB, who supplied me with…

Google Earth bearings? Kiss my leg? Kill my lizard? Lower Tanudan Kalinga?

Whatever. They’re steel, round, and slightly greasy. That counts as “bearing”.

Making the REAL can endcap. Why am I using a Bitch Chuck instead of the more convenient 3 jaw? Because the 3 jaw couldn’t hold the pressure of me cramming a boring bar full width into the face of the aluminum round. The piece jammed and rotated in the chuck, throwing off the established axis of rotation forever.

Well shit, there goes about 5 minutes of work, but it’s a big chunk of aluminum to just toss and start over. Using a 4-jaw chuck, I dial-indicated the piece back to 0.0005 runout in a moment of totally uncontrolled OCD. It’s a number that I have a feeling the Old Mercedes can’t actually hold any more, but hey.

But the other advantage of the Bitch Chuck is the ability to get the exact same centering after you take a part out and flip it around. Ordinary self centering chucks only self-center on one specific orientation of the part. If it moves, as events transpired, or you have to take things out… all bets are off.

The  Bitch Chuck requires the use of an indicator and is thus excessively bitchy and inconvenient, but I’ve become pretty adept at wiggling the jaws to the right position.

Above is a picture of the other side of the can endcap being finished.

Test fit. Because all the mechanical connections from can to hub will be by cross-drilled screws, nothing needs to be a press fit. This is great, because it means I can disassemble my test fits. It was tantamount to me discovering that I left the internal shoulder 2mm too long. A return trip to the machines took care of that.

Most of the protoforms of the motor are assembled here. Technically, this is the entire motor, but…

… it needs a mounting flange to adapt the 6 hole bolt circle of the IAP to the rest of the robot.

I’ve been anticipating Deathrunner for a while now – it was originally going to be an EV motor for the next wacky scooter. So I invested a chunk of cash in custom full-circle magnets from Super Magnet George. The thing I hate worst about motor construction besides winding 9000 turn stators is installing the magnets, because inevitably they want to snap together onto either eachother or your fingers. A full magnet circuit comes with a few electromagnetic penalties, but installs so much cleaner.

These custom magnets are N42 composition and I ordered a full circle of 14 plus two spares. They are perfect – as in, I’m sure they were made right to dimension, but I bored the can a hair too large, resulting in the ~2mm wide gap in the circle.

I suspect just the thickness of the epoxy alone will close the cap. Else, that’s what they invented index cards for.

What’s left to do on Deathrunner? All secondary features – holes need to be drilled, slots need to be cut, and threads need to be tapped. Those are all mill operations. I wanted to quickly pop off the protoforms of the parts before continuing.

Oh, and the thing needs winding. I’m settling for running the entire robot on 23 volts (7S A123s), so the motor will be wound rather hot. The original motor boasted a 176 Kv, which actually puts it in the right speed range to drive the saw to design speed. I’ll probably seek double that.

 

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