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.
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!