Beyond Unboxing: Cordless Saw Extravaganza!

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First of all, buy my excess stuff! I’m still periodically adding goodies to the page. There are now MELONS.

As hinted in the Carly Rae Jepsen Wallbanger build report, I tore into a few different types of cordless saws to gauge at how adaptable to robot drivetrains they would be. Cordless drills have been a staple of small robot drives (in the 12-30lb range) for many years, but recently they’ve been getting a little… flimsy. The Sketchy Chinese Drill Co. Ltd. loves to cut corners. Not to say that these are intended as replacements for the drills, but expanding your robot part horizon is always a good thing.

During the week of robot mayhem leading up to Dragon*Con 2012, I binge-purchased 3 different types of cordless saws from the Harbor Freight store near my historical home base of Atlanta. I then proceeded to rip them apart and photograph their remains like the world’s most enterprising and aggressive medical examiner. The 3 tool-like devices I investigated were the 68242 18v cordless jigsaw, the 68240 18v cordless reciprocating saw, and the 67026 18v cordless saw.

Let’s start in the sequence of usefulness. First, we have the jigsaw:

From the same updated “Drill Master” line as the 18v drills I’ve come to love so much, and with a battery which even fits those drills, are these cute little cordless jigsaws. I didn’t check to see if they could, say, actually cut things, but that is not important here.

A few Phillips head screws later, and the case splits in half. Hmm, it’s not too exciting.

There are very few parts that go into making a jigsaw apparently. The crankshaft-like pin on the main gear engages with a stamped slot in the blade holder, which rides in sintered iron bronze guides.

That’s it.

The main gear rides on a roughly 6mm pin…. made of the same plastic the motor mount is made of. Hey, who thought this was a good idea? I was hoping it was at least black-oxide plated steel or something, but nope. Totally plastic.

Maybe this will be useful if you needed a gearset THAT BADLY, but there is also no easy way to couple that gear to anything. The motor is also not very worth pulling, since it’s the same type of generic 550 motor in the drills.

Verdict: Not very useful. Let’s move on!

Next is the reciprocating saw (“sawzall”) from the Chicago Electric line. These use batteries which are of course incompatible with everything else, so I couldn’t try to cut anything with it. Not that I was in the mood to anyway.

Cracking this one apart shows quite a surprising amount of metal. Well, shady cast aluminum, which may qualify as metal under certain tax brackets. I’m interested in what that right angle drive looks like.

The gearbox itself is modular, which is a big plus for this thing. The reciprocation mechanism is housed under the black stamped cover.

It’s a “scotch yoke” mechanism made of a stamping of steel….. welded?! to a precision ground rod. Given my adventures with welding things, neither of these components are likely heat treated. There is at least a real needle roller bearing that is doing the yoking!

A bottom stamping isolates the yoke from the crankshaft gear. I took apart the slider for kicks – the main guide bushing is pretty robust. It’s a solid iron-bronze bushing, which seems to be a Chinese tool favorite ingredient.

Here’s the crankshaft gear – it’s a machined spiral bevel gear sintering (as far as I can tell – the machining patterns don’t match up with any 3D process I know of). As spiral bevelly as it might be, it is not very useful because there’s no way to attach something to it that isn’t a crank pin. The assemble rides on a live shaft supported by ball bearings on one side.

The ball bearing and part of the spiral bevel gear is seen here. Unfortunately, I could not get that shaft out of the bearing at all, and ended up cracking the casting.

My curiosity satisfied for now, I elected to take off the motor.

…certainly not what I expected. So let’s see the thought process here.

“Hey 李小龙, what motor do you think we should put on this saw?”

“Not sure, 刘少奇… They want this to cost $0.45 less, but I’m kind of out of options from the motor factory. We’d need a custom shaft to couple to the gearbox and they will charge more for that”

“What if you just took the motor from the 18v cordless nose hair picker? It’s the same size as the motor we need for this.”

“They supply that with a gear already on it though.”

“So? Make a fucking adapter that goes to the gearbox we need that has a  cutout of the gear in it.”

I’m really betting it went down kind of like that. That’s what I’d do, anyway.

Anyways, the input pinion has a negative gear that fits onto the motor’s gear. I guess it’s a variant of a spline transmission, but it’s so Chinese.

The combination of nonremovable specialized output gear and nonremovable input…. thing has led me to give this thing a verdict of not very useful either save for maybe making a pokey-spike weapon for your robot or something.

Now, if the existence of CRJW is any indication, here’s the useful thing!

A cordless circular saw should consist at most of a motor, a gear, and a switch; two of those are interesting to me. This model, by the way, is from their third (out of like 5) line of battery-incompatible cordless tools!

Full disclosure: A little while back, I bought a Grizzly 18 volt cordless saw second (or third) hand for like $5, which led me to take it apart and discover what’s inside. So really I knew the conclusion coming into this teardown, but for the sake of informing everyone else, I’m doing the other two saws anyway. Additionally, the Grizzly saw seems to be a 2004 era vintage, so I wanted to check on the quality decline between then and now (the drills have gone downhill a whole bunch…).

Off the trimmings come! The metal nosecone of the gearbox poking out from the plastic is a good sign.

The cordless circular saw also uses a 700-class DC motor like the reciprocating saw.

And here is the assembly that was shown in CRJW’s build report!

The metal casting looks fairly stout, but it’s just an awkward shape. However, this gearbox is useful as-is. The shaft is supported in a ball bearing that is in the metal cone, so if you bolted it to a bulkhead or side plate in a robot it could be an immediately swappable part.

Inside the gearbox is this sintered assembly that consists of the spindle lock (for changing blades) and a solid ring gear/output shaft bushing assembly. I can tell that they are two different sinterings, but I wouldn’t be surprised if they were somehow the same material!

Here’s the gearbox split apart into components. As discussed in CRJW’s build report, the ratio is 5.2:1, single stage, using gears of (roughly) module 0.8 (about 32 pitch, but larger) that are 6mm in face width. All metal. Questionable metal, but better than that 1-stage-of-nylon, 1-stage-of-questionable-metal bullshit in the drills!

The output speed at 18 volts is about 3800 RPM.

So do I like these? Absolutely. They can be useful under certain circumstances – I think that they are far too fast for drive (CRJW may or may not disprove that), but in a situation where you have 2 motors and 4 wheels and can link them with chain or belt, a small amount of external reduction is reasonable. They are certainly more useful than the previous 2 saws, and I believe the gearbox is quite durable.

I’m satisfied with CRJW’s use of 2 plates to mount the plastic ring gear holder by itself, without the weird casting. The total weight of the gearbox assembly is about 18 ounces without the casting. I don’t anticipate using these on a robot quite yet, but I now have about 6 different saw motors and so have an option of it if I needed. Besides this HF version, I took apart the Grizzly and a “Speedway” brand saw (formerly retailed at homier.com which seems to have gone under) and they all have this same style of gearbox.

As a comparison, I also bought this Ryobi 18v saw secondhand. Ryobi is marginally more legitimate than a Chinese generic tool manufacturer, so I was expecting some custom hardware in this.

Ryobi is well known for making nicer 18 volt drills but whose chucks are patently impossible to remove – often needing to be cut off!

Long story short – no.

The gearing is spur instead of planetary, which, while it isn’t THAT terrible on its own, is integrated into the molded plastic case! The ball bearing in this case is just pressed into the plastic. As long as I’m not actually using the tool for the purpose it was intended, I’m gonna stick with the shady generic-brand import with their modular gearboxen. I’m wondering if the generics will move towards this design in the future too..

So here ends the lesson on cordless saws! I hope it spawns at least one retardedly fast robot besides the Carly Rae Jepsen Wallbanger!

The Triumphant Return of the Ragebridge: RageBridgeTwo

chorl

First, in accordance with the Charles Z. Guan Strategic Stuff Reduction Act of 2012, I’ve opened up a listing page of things I’m clearing out. Let me know if you want a thing. As I continue mining through my multiple mid-dens, more things may be added.

Now, onto the real content.

With the version 1 of the Ragebridge controllers having proven themselves at Robot Battles 2012 (after being the stuff of nightmares prior to that), and now that I understand what I’ve been doing wrong the whole time, it’s time for me to make an update which addresses the little shortcomings and fine details that were lacking in version 1. Recall the major hardware issues:

  1. Gate driver bypass caps were placed incorrectly, thus causing the gate drive power traces to have higher impedance to pulsed currents, with the resultant voltage spikes some times causing instability.
  2. Gate drive current return path was long and loopy: through the main power ground, and back in through the narrow logic regulator trace.
  3. Non-rotationally symmetric current sensor placement meant there was a major current bottleneck for one side
  4. Long power traces in general made for more bottlenecks
  5. The logic regulator inductor was (and apparently has been in many of my past designs) too small which caused the regulator to become unstable.

Luckily they were all solvable with component-level hacks (no more wire cutting and trace jumping!) so I can easily roll them up into an upgrade. Next, there were a few little gripes I wanted to resolve which didn’t impact the function of the board so much as was just annoying.

  1. None of the 3-pin digital input headers had the correct orientation for standard servo cables. I had intended to make 2 of them “potentiometer” style inputs, reading [5v] [Signal] [Ground] from one side to the other, and 2 of them R/C style inputs reading [Signal][5V][Ground], but messed up (and didn’t check), making the latter two [Signal][Ground][5V] instead. Basically this meant I had to hard-solder pigtails to the board so as to not plug in a normal servo cable and explode everything.
  2. The board is 2.2″ wide, which precluded it from fitting vertically (space-saving) in a 2″ frame, fairly standard for the 12 and 30lb classes. Only Clocker could fit it vertically in a custom little “rack”.
  3. It still uses that ATMEGA328 breakout board called the Arduino Nano. Not only is that kind of unnecesary, but the thru-hole package of the Nano actually makes routing pretty difficult since I have to fit signal traces between the thru-holes.
  4. I could only read 60A peak with the “single bypassed” ACS714 current sensors. With a little help from a fan (and maybe 4oz copper) I don’t doubt this board can flow more than that.

With these issues in mind, the goals for RB2 were clear:

  • Squish the design down to 2″ wide maximum
  • Rebuild the schematic from scratch to eliminate possible cross-generation schematic copying problems, which is what led to my incorrectly placed buscaps and ignorance of the logic regulator problem for so long.
  • Widen all the power traces and make sure all the gate drive traces have their own low-impedance path back to the driver chip and the power supply
  • Make the current sensors “double bypassable” to read peak currents up to 90A and make the layout as much of a 180 degree rotation as possible to preserve the bottleneck-free layout.
  • Board-mount the ATmega328! It’s still going to be Arduino, but I’ll forego the extra $12-20 breakout board.

It’s also important to note that RB has been printed on 1oz copper using the cheap prototyping service from MyroPCB. Even moving to 2oz copper would help alot with the current bottlenecks. Ideally I’d try to get it fabbed from 4oz copper.

Here’s the first shot of the layout progress. The master layout remains essentially the same. The board is now 4.5″ long and 2″ wide:

Beginning the layout is usually the hardest for me because that’s when the problem is totally unconstrained. I usually jiggle the major power components around in a rough grid snap (to ensure reproduceability) first. After that, I usually work from the “outside in” – gate drive components get arranged next, then the logic fills in the loose volume. I’ve gotten all the way to the “so, where to put the logic components?” stage here, so they remain in a formless blob in the center. It does show that I have space to play with however.

For the ATMEGA chip, I knocked a footprint and schematic symbol from the Sparkfun Electronics library. The one difference is that I added to the ATMEGA’s pin names in the schematic their equivalent analog and digital pins when the chip is brainwashed into Arduino mode. This just makes mapping from old to new schematic easier. I’ll update my EAGLE Library under the Useful Stuff / references page with this device.

There’s one more little detail about this board, but I’ll keep going for now. Let’s see if you can figure it out.

I’ve now added some of the major power planes and a logic ground plane. One of the things I’ve never done in the past is a logic ground plane, for some reason. It makes trying to tie all the ground nets together so easy! And a wide plane is lower impedance than narrow 10 mil wide traces hopping through 9 vias. I just had to be careful with the bottom-layer traces not cutting the plane in half or totally isolating a ground pad, which would defeat the purpose.

The big SMT 6 pin header has disappeared – it just took up too much floorspace.

A few hours later, the first round of iteration is complete. Really I should send the board out to manufacture right now…

…but I decided to wait a while this time. I didn’t do anything with the board for about 2 days, upon which I returned and started optimizing. The differences are very minor, mostly in component fine placement. There’s now plenty of room to attach standoffs to the board at its mounting holes. Furthermore, the mounting holes have been made into a slightly smaller square that is the same size as a 52mm DC fan’s mounting pattern. This means I should be able to get some active…. microcontroller? cooling going on with this board.

Even though I say microcontroller (the ATMega would be receiving the best airflow in the house), almost any airflow at all would make the FETs’ thermal resistance to ambient air lower and hence increase the current rating of the board. Maybe there’s enough space to fiddle with putting a temp sensor on it…

Alright, one more layer of optimization. I’ve organized the top row of parts, which are 15v logic supply regulator related, into a better arrangement. Previously, they were extremely close together (soldering would have been difficult and hell if an IR lamp can reflow the pads in the dark alleyways of my L/C skyscrapers). I’ve space them apart alot more, but that involved moving the regulator chip itself down. This in turn meant I needed to shift some passives around. Whatever, the component density has only increased as a result.

(Now, none of this would be a problem if I would only use 0603 or 0402 SMT parts, but I’m too heavily invested in 1206 and fuck people with good eyesight.)

That is pretty much the final version of the board as it stands. With each of these revisions, I was sending it off to Advanced Circuit’s FreeDFM service in order to double check that it can actually be printed. I also ran a full Design Rule Check every time I changed a bunch of things between iterations. This was mostly me clicking “Approve” to all my vias-in-pads and insufficient solder mask clearances, but it did catch a few too-close-spacings.

Speaking of vias-in-pads, apparently they’re generally discouraged in automated PCB assembly, so in the interest of possibly getting this board professionally stuffed, I went through all the difficult tunneling in the gate drive and tried to un-pad my vias, at least connecting them to the pad with a short neck trace if possible.

This is a picture showing the VSS (everything-power-return) highlighted. Each gate drive now has its own private access to VSS at the location of the FET. This makes sure that 8 switching current pulses aren’t trying to octo-penetrate the thin logic regulator supply trace at the right in order to get back to the regulator. While it “worked” in RB version 1, it’s definitely not optimal. This general idea of not having signals mix with power is called Kelvin connecting, after the dude who first used it to sense very small voltages.

Highlighted now is the 15 volt distribution bus, which I’ve made strictly tree shaped this time. I try to adhere to the rule that any time power is being dealt with, closed loops are a bad idea.

The 5 volt bus follows a similar tree structure.

The large ground plane, which is the logic return path, is actually not a closed loop either. On the lower left corner, it was purposefully broken by judicious gate drive trace routing. It’s topologically a big mirrored C shape with the ends of the C right at the regulator output.

I’m pulling out all the stops I know to make sure that this board works on the first try. Ideally I’d get this sent to manufacturing through Myro tomorrow and have it ship right before Maker Faire 2012 in New York. This will probably be my on and off diddling project for most of the semester as a result. If it works out, I am now much less opposed to letting a few out “in the wild” for testing and subsequently investigating how much it would cost for Myro to just assemble all the parts, minus cables and through-hole pin headers and capacitors, for me.

To make it easier for them, I’ve placedallthe parts on the top side. That was the surprise – go back and look at the first few pictures now. Notice how all the parts are red (top layer)?

Now, all top side parts doesn’t instantly mean manufacturable. Once I build a few I might find where problem areas are. Say, anyone know if you can tell pick-and-place machines to put on the short and flat parts first?