I want to learn CadQuery to see if I can use it for my 3D printing projects, but at the moment I’m drawing a blank for a good project to climb the learning curve with. I looked over my teardown queue for potential inspiration and decided to take apart a Harbor Freight mini LED flashlight. This was one of the items they used to give away “Free with Purchase” to entice people like me to stop in, so it must have been made cheaply even by Harbor Freight standards. A good item to compare & contrast with an earlier giveaway keychain LED teardown.

The push button switch at the end unscrews to release a battery tray holding three AAA batteries. Given Harbor Freight price points, it’s not a surprise these batteries inevitably leak and destroy the device. I have several of these little flashlights and I picked this one as it had yet to corrode. The trio of batteries were wired in series, for a theoretical maximum of 3 * 1.5V = 4.5V.

I found no fasteners on the head of the device, nor any signs of glue, so it might be held together by friction alone. I pulled out my pliers and the thin metal construction could be peeled apart. It seems to be roughly the thickness of a food can, but peels much more easily, so probably aluminum instead of steel.

After unrolling that lip, I could remove the clear plastic cover, the shiny plastic reflector, and a circuit board with nine 5mm white LEDs soldered in parallel. There were no signs of a current-limiting resistor, so this design must have been dependent on cheap alkaline AAA battery internal resistance to keep this thing from burning itself up.

Connected to my bench power supply, I gradually dialed up the voltage delivered to this array of nine LEDs. At just under 3.3V, the power supply reported supplying 9 * 20mA = 180mA of current. I guess three cheap alkaline AAAs asked to supply 180mA would sag to 3.3V or less. Either that or the designer of this cost-optimized design decided they don’t care if these LEDs burn out.

While reviewing my notes about getting my Dell XPS 8950 fixed, I realized an item from my inkjet teardown slipped through the cracks: an aborted teardown of its 210XL and 211 ink cartridges. I wasn’t terribly interested in their internals. So when I encountered its robust construction I just shrugged and moved on. Still, there were a few interesting observations.

When I looked at the print carriage internals, I was not surprised to see the black cartridge had fewer electrical contact points than the color cartridge. However, I was surprised to see it’s not one-third the amount. It has to deal with just black instead of cyan, magenta, and yellow. Why does it have well over 1/3 the contact points?

I saw the answer when I flipped those two cartridges over and compared them side by side. The single-color black ink print head is double the height of the three-color ink print head. When printing purely in monochrome, it can print a band twice as high in a single pass so the paper can advance twice the distance basically doubling print speed. Assuming all else are equal, it would imply the black ink cartridge would need 2/3 the contact points of the color cartridge rather than 1/3. The actual numbers are 20 contacts for black and 36 contacts for color. Close enough for me to declare mystery solved.

Those contact points are on a thin flexible printed circuit and held down by four melted plastic rivets. I could cut them flush with a knife to free the thin sheet.

Once freed, I took a picture of the exterior side…

… and the interior side. The print head is to the left of these two pictures so it’s no surprise a bunch of copper traces lead that direction, but I was intrigued by the traces running off the edge to the right. What purpose did that serve?

I didn’t see any more rivets I could cut, and I couldn’t see anything else I could release. Trying to see what might be holding things in place, I gave the thin sheet a firm tug and it ripped off. The answer is, apparently, glue. Underneath the ripped-off sheet is the print head embedded inside cartridge enclosure. I saw no fasteners or clips. I think it is either glued in or molded in.

I tried prying against a corner and ended up braking off a piece. The whole thing is made of a very strong material. When it is over stressed, its brittle nature causes it to shatter instead of bend.

Looks like the enclosure is bonded strongly enough to the actual print head that they broke apart together. There’s no way to get further inside without being extremely destructive about it, which won’t teach me anything interesting, so I stopped here.

I’ve taken apart my retired Canon Pixma MX340 multi-function inkjet. Its task-specific plastic components are heading to landfill and its electronics core twist-tied to a sheet of cardboard for potential future reuse. I found a lot of interesting details as I went though this teardown and learned lessons that I hope to apply to future projects. I wrote down a lot of my observations here, so much that it has become pretty unwieldy to find specific information. Text search helps, but I also found myself frequently clicking “Next Post” and “Previous Post” to find a specific piece of information.

This post will be my first effort to help streamline finding references: all my MX340 posts listed in chronological order with as few words as practical (sometimes just a title excerpt) to remind my future self of their relative context. There are probably other ways to organize this information, but I am ignorant of the library science involved so this first effort is merely chronological.

Introduction

Tearing down inkjet printers as a learning exercise. General thoughts followed by an overview for this Canon MX340.

Phase 1: Functionality-Preserving Disassembly

• Examine power supply and removal of back and side panels.
• Automatic document feeder (ADF).
  • Lid is home to rollers but lacks its own motor. A preview of Canon’s part reduction engineering.
  • Control panel freed.
  • Motors and sensors.
  • Hinge was designed for easy removal, but I didn’t figure it out until after I had already ruined it.
• Scanner layer.
  • Closing damper mechanism.
  • Image sensor and flatbed glass.
• Inkjet printing engine.
  • Starting with the print head parking area.
  • Followed by a look at its paper feed path.
  • Then the paper feed motor and gears (including first look at encoder disc).
  • Paper output tray door has its own opening mechanism.
  • Print carriage motor (including first look at encoder strip) is adjacent to a WiFi module and phone line audio speaker.
• Declare phase 1 complete.

Phase 2: Probe certain electronic subsystems as system runs

• Start simple with cover switch and power supply.
• ADF
  • Photo-interrupter sensors pinout.
  • Stepper motor under oscilloscope.
• Scanner startup homing sequence and spoofing that sequence.
• Scanner stepper motor pinout and oscilloscope.
• Scanner contact imaging sensor (CIS) was challenging fun to decipher!
  • Illumination RGB LED under oscilloscope.
  • Research into Canon CIS products.
  • Finding the easily explained subset of pins (mostly RGB LED) on CIS connector.
  • CIS candidate image data line under oscilloscope.
  • Guessing the remaining pins on CIS connector.
  • Properly reusing this CIS will be hard, but I have some partial reuse ideas.
• Print carriage and paper feed DC motors
  • Motor control under oscilloscope.
  • Paper feed encoder disc sensor under oscilloscope.
• Control panel got a thorough examination.
  • To guide my exploration of how it works, I first reviewed of what it needs to do.
  • Then find the power and ground pins on its connector to main board.
  • Repeat exercise for connector to LCD screen.
  • Use that LCD pinout to inform probing the control panel IC.
  • Tracing circuitry required a big photo stitch of the control panel.
  • Power button and LED are wired directly to main board.
  • WiFi LED has its own power source, different from rest of control panel LEDs.
  • Pinout of control panel connector to main board.
  • Pinout of control panel IC including button matrix.
  • Oscilloscope examination of control panel communication.
    • Reporting button presses.
    • Updating LCD screen.
    • Control IC “Chip Enable“.
    • Oscilloscope trace appears to be asynchronous serial.
  • Logic probe examination of control panel communication.
    • Custom probe wires helped confirm: asynchronous serial 250000 baud 8E1.
    • Design list of six scenarios for logic analysis.
    • Decoding idle and four example button presses.
    • Picked out pattern for (but not yet decoding) LCD update data.
    • Logic analysis of control panel activity for machine states:
      • Plugging in.
      • Powering up.
      • Standing by.
      • LCD blanking and recovery.
    • How to decode LCD update data?
      • Custom Saleae HLA can’t do it.
      • Don’t overthink it, Calculator and Excel can do a lot.
      • Decoding LCD update data with Excel.
  • Now that I understand my captured scenarios, what’s next?
    • Define goals for desired data filter and go searching.
      • Wireshark is not the right tool.
      • Neither is Bus Pirate.
      • SerialTool comes closer but not quite.
    • I guess I’m creating my own tool!
      • Using USB serial adapter and pySerial library.
      • Get the full set of button matrix scan codes.
      • Decode LED control bit flags.
      • Project delayed by faulty USB serial adapter. Once I figured out it was faulty, I ordered a replacement which showed some mild evolution.
      • Challenges in simultaneously listening to two serial ports.
      • Process the commands, ignore bulk transfers.
      • Track known command sequences with a lookup table.
      • Quick-and-dirty data filter tool did its job.
  • Logic probe examination of LCD screen communication.
    • Power and ground already established, leaving five unknown wires.
    • Start in analog mode to probe pin assignment.
    • LCD screen pin assignments.
  • Summary of control panel findings.
• Paper feed motor examination goals.
  • Start by setting data recording objectives, then start hunting for quadrature decoders.
    • Logic analyzer candidates. Saleae no. Sigrok maybe?
    • Arduino support for sigrok is incomplete.
    • ESP32 Pulse Counter (PCNT) peripheral.
    • Go with known quantity of Arduino quadrature decoder library.
  • First pass: periodically output decoded quadrature counts.
    • Powering up.
    • Standing by.
    • Print a page.
  • Second pass: decode raw count into motion.
    • Decode based on sampling rate was unreliable.
    • Switch decode based on microsecond timestamp.
    • Timestamp can be improved, but probably not worth the effort.
    • Stay with integer math by using microseconds per count.
    • Graphing along with position.
    • Simple debounce eliminated majority of noise.
    • Observations on motor acceleration and deceleration.
    • Use dwell time to delineate motions.
    • Decoded output looks promising.
    • Summary of decoded motions.
  • Encoder disc delivers 8640 counts per revolution.
  • Paper feed motor examination summary.

Phase 3: Disassembly Without Concern for Preserving Functionality

• Print carriage as starting point.
  • Range of motion: 32.5cm.
  • Motor and mystery lever.
• Print head maintenance assembly.
  • Range of motion.
  • Topside components.
  • Underside components.
• Ink cartridge bracket.
• Print carriage removed.
• Mystery lever is a gear shifter!
• Distracted by ink graveyard.
  • Disposal flow path.
  • Peristaltic pump.
• Back to gear shifter.
  • Paper tray gears and cams.
  • Freewheel mechanism using capstan effect.
• Mass production priorities different from my project priorities.
  • But I do want easy serviceability.
• Paper feed motor+encoder removed.
• Print carriage disassembly to access encoder sensor.
  • Remove lower rail.
  • Open print carriage.
  • Pinout of encoder sensor.
  • Encoder strip delivers 600 counts per inch.
• ADF gearbox disassembly.
• Cursory review of main circuit board.
• Disembodied electronics nervous system still runs.
• Big group photo of all parts laid out flat.
• Salvage scanner flatbed glass.
• Disembodied electronics mounted on a sheet of cardboard.

And finally, the summary index. (You are here!)

Bonus item: aborted teardown of Canon 210 (black) and 211 (color) ink cartridges.

Whew, that was a lot to write down, but at least it wraps up documenting this lengthy project. Now I can document another lengthy saga that took place at the same time: debugging bug checks on my Dell XPS 8950.

I’m done taking apart my old retired Canon Pixma multi-function inkjet, with salvaging its scanner flatbed glass as my final act. While this marks the end of my teardown, this is not the end of my MX340 adventure: there are many components that have future project potential. I have several ideas that may or may not put certain parts to other use. But the end of my teardown is a good place to take a break, let those ideas stew for a while.

Since I want a change of pace, I need to clear MX340 components off my workbench. And since I want to reuse them in the future (or at least preserve the option) I can’t just sweep them into a box. There are some fairly fragile parts here, with my top concern being the X-axis optical encoder strip and its Y-axis counterpart encoder disc. Also, the contact image sensor bar would be more useful if it does not get scratched up.

I decided the minimum effort way to store these components is to revisit an idea I had earlier with prototype circuits: mount components on a sheet of cardboard. My priority here is to ensure parts don’t damage each other and that wires are not jerked around. I cut apart a cardboard shipping box and started punching holes for me to secure components with twist ties.

I can even power up the system in this state and watch it go through its power-on self test, probing any circuitry if needed. If I want to run the test, though, I need to make sure the print carriage is dangling over the edge of my table. Given how it extends slightly below cardboard level.

I wouldn’t call this safely packaged — I wouldn’t ship it in this condition, for one thing — but it should be good enough for me to keep everything together without causing damage. Having all of these literal loose ends tied up on a sheet of cardboard means I have the option to stand it vertical leaning against a wall. This only consumes only a few square inches of desktop space, and far less than the full volume of an intact MX340. It’s a good way to clear my workbench so I can think about other things.

That takes care of physical organization, next up is my first stab at information organization.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

My old Canon Pixma MX340 multi-function inkjet is about as taken apart as I want it to be right now. After taking a big group photo of all the components, I started gathering up the plastic that I don’t plan to keep. That’s when I noticed a loose end: two glass panels in the flatbed scanner assembly.

Early in this teardown, I discovered those panes of glass were much thicker than those used in LCD screens and thus far more robust than I had given them credit for. They also had nicely beveled edges so I’m much less likely to cut myself while handling them. Those two traits made it interesting to salvage those panes for my own use, but I forgot about them until now.

These two panes of glass were held with double-sided tape. It was not a surprise to discover the tape had yellowed and hardened, and the adhesive had dried up. It was pretty easy to peel both pieces of glass off their plastic frame.

Some residue was left behind as I peeled, but majority were easy to clean up. Some small streaks will need to be either scraped off with a razor or cleaned off with a solvent.

Underneath the glass is the image sensor homing marker. I thought it was a thin piece of paper, but it was actually a more substantial sheet of plastic and I’m curious why it had to be this thick. It’s almost as thick as the #11 knife blade I used to get started peeling off its adhesive.

I really doubt I’d reuse this homing marking for anything else, but it won’t take much space to hang on to a thin strip of plastic for now. To preserve context I will keep it alongside its matching contact image sensor bar so at least they’re available if I think of something to do with that sensor. Fortunately, I have a convenient piece of cardboard I can use to keep it with the sensor bar.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

I’ve torn down my retired Canon Pixma MX340 multi-function inkjet almost to its individual components. I was a bit surprised the remaining electronics still ran through its power-on self-test sequence. It failed the test, of course, given how almost all mechanical components have been disassembled. But the fact it ran at all was enough to motivate keeping all the electronics together until I pick off individual pieces for future repurposing.

The same could not be said of its mechanical components. Most of the plastic pieces are very specific to a MX340’s mission of handling paper and that hasn’t been my area of interest. Now that they’ve been taken apart, I no motivation to put them back together again. Another part of this lack of interest is the fact that, thanks to 3D printing, it’s easy for me to create tailored plastic pieces for future projects. I think I will keep the gears because, even though they can be challenging to repurpose, they are difficult to 3D print well. The remaining plastic are landfill bound.

I don’t have any metalworking capability, though, so all these miscellaneous metal bits will be added to my jar of salvaged parts. Joining my existing collection of screws, springs, and shafts.

Before these parts go their separate ways, I laid them all out together one last time. I had originally thought it would be neat to lay them out in a way that maintained their relative position to each other like an exploded-view engineering drawing. But due to how a MX340 is built, it quickly became an impossible task to maintain 3D space relationship of multiple layers on a flat 2D layout.

Ignoring my failure to maintain spacial relationship, this “group picture” showed the large number of parts that go into a multi-function inkjet. I believe an inkjet is the most mechanically complex consumer electronic equipment still on the market. Especially now that VHS decks, audio cassette players, and CD changers have disappeared. Given their complexity it’s amazing inkjets are still sold for well under a hundred bucks. Now that I’ve taken one thoroughly apart I find it more believable they might be sold at low to no profit (or even a loss) for the intent selling profitable ink cartridges.

Taking this apart was a lot of fun! But when putting together this picture, I realized I missed an item on the to-do list: salvage the scanner flatbed glass.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

I unplugged everything from the main board of my Canon Pixma MX340 multi-function inkjet so I could take a look at components on that circuit board. This examination was the originally planned end point for my teardown. Now I’m looking over what I have on hand to see where to go from here.

The first thing I did was to plug everything back into the main board. This turned out to be a lot easier than I had originally expected, because (aside from two exceptions) every connector was distinct from another so it was easy to make sure I didn’t plug something into the wrong place. I believe this was another sign of intentional design for serviceability.

Once everything was plugged in, I had the disembodied nervous system of a Canon Pixma MX340 separated from almost every mechanical component. I plugged in a power cord and it turned on and started running its power-on self-test sequence. Nice!

I kept the two encoder-equipped DC motor assemblies intact enough to run through their respective self tests, so the paper feed motor could run through its sequence and the print carriage motor could do the same.

There were a total of four optical interrupter sensors. Two in the automatic document feeder and two in the paper feed mechanism. They were all designed so the “normal” position blocks the beam and the power-on self-test sequence doesn’t do anything requiring those beams to be unblocked. So even though these sensors have been separated from their respective mechanisms, all I had to do was to stick something opaque into these sensors to fool the computer.

Beyond that, though, fooling the computer got more complex. The scanner tries to run through its homing sequence. It would fail because the sensor bar has been separated from the homing marker and I don’t see an easy way to fool the system. I guess I could set up a short rail so its motor can move the marker into view and out of view, but that’s more effort than I care to expend on such a project.

And finally, the computer complains it can’t talk to the absent ink cartridges. I expect that communication to be protected in multiple different ways in the interest of protecting their revenue stream, so I’m not even going to try to figure out how to spoof it.

Since this disembodied MX340 nervous system still runs, I’m inclined to keep the electronics together as I dispose of the rest. But before that, they get to hang out together one last time for a big group photo.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

I’ve learned a lot from taking apart my retired Canon Pixma MX340 multi-function inkjet, and there are many more potential lessons that I’ve chosen not to pursue. The paper-handling details of its ADF is one, its main circuit board is another. There are a lot of electronics design lessons on this intricate board, but they’re beyond my current skill level to understand. (In comparison, the control panel was a much simpler single-sided circuit board and I enjoyed tracing through it.) But I’ll still take a cursory look at the main board.

So far, I’ve kept everything plugged in so I could keep it running and probe component interactions. Now I will unplug everything so I can take a look at the board itself. I took many pictures as reference as I went, to increase the odds I can put it back together, but I would later learn it was unnecessary.

Unplugging everything left a large plastic shield.

Removing the shield uncovered the fact that landline phone jacks (for its fax functionality) are on a separate circuit board. I’ve reused salvaged jacks before so these may yet find another use.

Finally I have the main board by itself. As already stated it’s much more complex than my skill level can reverse-engineer, and I have no motivation to do so anyway.

The circuit board as a minimum of two layers, possibly more but I don’t know enough to tell.

Apparently production volume of mainstream Canon inkjets are high enough to amortize up-front cost of custom electronic components. I picked a few large pieces and tried searching for them based on their markings, coming up empty handed across the board. An example is IC702 here marked with a Texas Instruments logo. It should have been a slam dunk but all I got were chip vendors promising to sell me a TI OACC3TTC 81024 without having any idea what it is. The same story repeated for three other chips before I threw my hands up and quit trying.

There were many unpopulated footprints on the main board, presumably to support features of other models in the product line. I can speculate on two of them. This looks like an Ethernet port, something I would have appreciated as its WiFi module is now out of date due to its WPS dependency.

This unpopulated footprint for connector CN502 is labeled “Card” and has nine pins, matching nine contacts on a SD card. MX340 have a feature for scanning a document directly to PDF file on a USB memory stick. Looks like a sibling model could write out to a SD card.

There were a few other unpopulated footprints but I had no speculation on what they might be. Bringing to a conclusion all I had expected to get out of looking over this circuit board. I plugged everything back in and was mildly surprised it still ran.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

I’m finishing up the teardown of my old Canon Pixma MX340 multi-function inkjet. After completing my exploration of the print carriage encoder, I looked over my workbench and saw: Hey, the automatic document feeder (ADF) gearbox is still intact! I’ll take it apart now.

Since I don’t expect to need to know how to mass-produce a paper feeding mechanism, I’m not going to spend too much time to understand exactly how all the mechanisms work together.

Mechanically, a stepper motor drives at least three shafts, each turning a set of soft rubber rollers. I wonder if these gears are all specifically designed for this device or if they might be standard parts from a catalog. If the latter, I would love to browse through that catalog.

As with the rest of the device, everything came apart nicely now that I recognize the system of clips Canon engineers use to ease disassembly and repair.

The ADF lid is likewise an assembly of injection-molded parts held together with easily disassembled mechanisms. Within this assembly I noticed several freewheel mechanisms implemented with a coil of metal, now that I understand what I’m looking at.

I appreciate precision ground metal shafts, much more satisfying in the hand than an injection-molded plastic shaft. I keep thinking it would be cool to reuse them in another project, but due to their precision nature they’re typically tailored to a specific purpose and not easily reused elsewhere. They usually end up just as toys for me, roughly analogous to fidget toys but with no moving parts. When my projects need metal shafts, I end up having to cut new ones tailored for my project. My shafts are not as precise as these mass-produced units, but good enough for their own respective purposes.

Next up: the original finish line of my teardown, the main circuit board itself.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

After taking apart the print carriage of my old Canon Pixma MX340 multi-function inkjet, I figured out the pinout for its quadrature encoder. Sadly, it didn’t look very easy to tap into those signals on the printer mainboard.

So I soldered wires to the print carriage instead.

I routed these wires through the front with nice long flexible wires, to my Arduino Nano running a quadrature decoding library. Providing 3.3V power and manually moving the carriage back and forth across its entire range, I saw ~7637 counts. I divided it by centimeters but didn’t get a very nice number. I tried Imperial measurements, and it worked out to 600 counts per inch.

Earlier I decided the paper feed encoder delivered 8640 counts per revolution. Trying to correlate the two measurements, I went back to measure the diameter of the paper feed shaft at 9.75mm. That works out to a circumference of 30.615mm or roughly 1.2 inch. 8640/1.2 = 7200 counts per inch. That’s 12 times the horizontal axis resolution!

Such a huge discrepancy in resolution between horizontal and vertical axes can be explained by how this print engine moves. The paper feed motor needs to advance paper with high accuracy to make sure one print head pass lines up exactly against the next pass with no gaps or overlaps in between. The print carriage motor then moves the print head across the page at a controlled rate, which is the key here: the steady rate of motion means the printer control system can interpolate between those 600 counts per inch to synthesize virtual steps in between real hardware steps. Doubling to 1200 or quadrupling to 2400 (or more) are valid options when print carriage motor moves at a known controlled speed.

Now that I have my answer, I no longer need these wire taps. At first I was ready to disassemble the print carriage again so I could unsolder these wires, and I wasn’t trilled about the risk of damage and losing springs when I take it apart again. Then I decided to not take that risk, save myself the time of disassembly, and just cut off those wires today. I’ll unsolder the remaining stump later, if ever. Right now I would rather spend my time disassembling the ADF gearbox instead.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

There’s one final sensor I’m trying to access as I tear down my old Canon Pixma MX340 multi-function inkjet: its print carriage linear quadrature encoder. Removing the lower rail allowed the print carriage to slide free, but I first tried to see if I can keep it on the rail. I was afraid sliding it off the rail would unpredictably release the spring-loaded tension mechanisms I can partially see. (White plastic below the belt and behind sheet metal in picture below.)

Without sliding it off the rail, I could access the two fasteners I couldn’t access before. Removing them allowed two separate pieces of black plastic to move apart slightly, but they were both within the lower rail. The carriage has to slide off before I could pull those pieces apart.

As I slid the carriage off the lower rail, my fear came to pass: I heard a “pling” announcing a spring departing to seek new adventures. Its former home highlighted in red on the left, and its companion still in place on the right. Thankfully I managed to find this spring later, because every spring I lose decreases the odds I can repurpose the entire print carriage assembly intact for a future project.

Carefully setting down the rear cover with its many springs, I could see inside the carriage. Front and center is the encoder sensor, but its pins go through the circuit board to the other side. The flexible ribbon cables are also connected on the other side. I will have to remove four more screws before I can see how they are wired.

The flex cable connectors were expected, as are the ink cartridge contacts. The surprise on the front is a pair of electrolytic capacitors. I guess inkjet cartridges need small bursts of buffered power to do their thing.

I wasn’t interested in reverse-engineering the ink cartridge interface, but I was interested in how the electrical contacts are implemented. Each contact is a thin spring-loaded metal blade. (My thumb nail is pushing on one.) Unfortunately, it appears if I want to see more I would have to remove the contact assembly from the circuit board. I haven’t had a great success rate unsoldering components with this many pins, so I will save my unsoldering practice session for later. Right now I’m staying focused on the optical encoder sensor.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

I’m close to the end of tearing down my Canon Pixma MX340 multi-function inkjet. After the paper feed motor was freed from the plastic base, there’s not much more I could do to that assembly because I wanted to keep the motor and encoder together. So I turned my attention to the print carriage assembly, with its own encoder I have yet to access.

Earlier disassembly saw fasteners I couldn’t access from the front. I will now take apart the print carriage rail in order to free the carriage itself and allow access to those fasteners.

Looking at the assembly I saw the backbone sheet metal was folded up top to form the upper rail, and there were folds left and right to keep the carriage constrained. The only thing that could move is the lower rail, which is a separate piece of sheet metal. (In above picture, the lower rail is visibly covered with darkened lubrication grease.)

At this point I noticed the lower rail is mounted slightly tilted from the bottom of the backbone. I measured it was raised by 2.07mm on the left but only 1.41mm on the right. This slight tilt was probably part of factory calibration to ensure the print head travels exactly parallel to the paper surface at all times.

Before releasing the lower rail, I unhooked the encoder strip’s tension spring on the left.

The drive belt was also unhooked from the right side tension pulley.

An vertical alignment reference marker is visible to the left of this lower rail screw.

Removing the screw made it clear there’s allowance for a few millimeters of lower rail adjustment.

By removing the lower rail, I have destroyed its precision factory alignment. But with the rest of the printer taken apart I doubt it matters anymore. I’ll ignore that and keep digging.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

The process of taking apart my old Canon Pixma MX340 multi-function inkjet has been remarkably easy, something that took deliberate effort by Canon. I recognize their effort and I am grateful. It’s been a fun learning experience and I realized I’m a little sad to be close to the end of mechanical disassembly. But that’s not going to stop me from finishing the job!

The paper feed motor and adjacent gears (including the rotation quadrature encoder) is the final metal assembly still attached to the base. It also hosts the plastic-and-foam assembly that sits under the paper as it is printed. A typewriter platen is responsible for both feeding paper and holding it against inking impact. Here, the two tasks are handled by two separate parts. The metal shaft with a gray friction coating is responsible for feeding paper, and the rectangular black plastic-and-foam assembly holds the paper directly under the print head. I’m not sure which part technically counts as a platen here, but I’m going to call the rectangular plastic-and-foam assembly the platen.

I see several Philips-head fasteners blocked by the friction-coated feed shaft.

Turning a plastic handle allowed me to lift that shaft up and away, exposing those fasteners.

The fastener left of center is spring-loaded though it’s not clear to me what forces that spring is intended to absorb. The center of this platen is a soft porous foam discolored by a few high-traffic areas of ink absorption. I used this printer for many border-less photo prints. The printer apparently accomplishes this feat by shooting ink beyond the borders to make sure everything is covered, and that ink ends up in this foam. There is a distinct over spray pattern corresponding to 4″ x 6″ photo paper in addition to a less distinct pattern for less frequent full width 8.5″ x 11″ photo paper prints.

Removing the platen and flipping it over, I see several holes where oversaturated ink can drip down to the ink absorbent pads below. It looks like I never needed that provision as the lower pad is still pristine white.

Removing the platen also allowed me access to the remaining fasteners. I had hoped there was only a single piece of stamped sheet metal as they would make it easier to keep the paper feed motor and shaft assembly in one piece. Unfortunately they are two separate pieces. So after lifting them out of the base, things are a little loose and floppy.

Once I made sure none of the rotating pieces are in contact with my table surface, I pressed the power button. The power-up self-test sequence still runs! Things sounded weird because the print carriage was not designed to run facing upwards, and the paper feed motor is no longer driving a bunch of gears. Plus the printer complained that no ink cartridges were installed. But still, it ran! I’m glad it’s still in running condition as I still want to probe the print carriage encoder. I will start that process by disassembling its rail assembly.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

Looking at the inner workings of my retired Canon Pixma MX340 multi-function inkjet, I understood many details to be results of making very domain-specific optimizations for mass production. Such details wouldn’t be useful doing anything else, and time and effort required for such work wouldn’t make sense for hobbyist-level projects.

On the other hand, I also found the design reflected a priority for easy serviceability. Something I do aspire to in my own projects. I know serviceability hasn’t always been a design priority in my own projects and it shows. Some people who had build their own Sawppy rover got confused or encounter problems in assembly or repair. By not putting any effort into serviceability, I have implicitly assumed I’ll always have my workbench and that wasn’t always true. Fixing my Sawppy rover in the middle of Maker Faire Bay Area was a huge pain.

With that experience I now recognize effort went into making the MX340 easy to service. Every fastener is a Philips-head screw, the vast majority of which can be turned by a #2 Philips driver. Nothing is glued down, welded, or hydraulic pressed. Everything can be taken apart and reassembled.

This is the result of deliberate decision by Canon and they had allocated the engineering time to improve serviceability. Some of which is quite elaborate: look at this mechanism securing the shaft that hosted the rotary quadrature encoder. This assembly could have been press fitted into the stamped sheet metal chassis. Simple, reliable, cheap. But that’s not what Canon did.

They designed this plastic bearing carrier so I could turn its arm…

… and lift the entire assembly out of its stamped sheet metal chassis. No tools required.

Why did Canon decide to invest this engineering effort? I assume there must be a monetary payoff. Perhaps this reduces their costs to service devices under warranty? Whatever their reason, my teardown experience indicate it is a very rare thing among consumer electronics manufacturers. It makes teardown projects like this one so much easier and more rewarding. I chose not to reassemble the paper tray sheet feed gearbox to see exactly how it worked, but because it was easy to non-destructively take it apart, I could have done so and that’s is a luxury I don’t take for granted. Up to this point, and as I proceed forward.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.

The paper sheet feeding mechanism in my retired Canon Pixma MX340 multi-function inkjet is a mechanical marvel. There are details I didn’t understand until a second or third look, like the spring that was installed on top of the big black gear driving the output cam. When I first saw it I thought it wasn’t a big deal. It looks like a spring, probably there to absorb some kind of shock to the system. No big deal, except I was wrong. The real story was actually much more interesting: it’s a freewheel mechanism.

My lesson came from a different part of the gearbox. This pair of gears were mounted on a static shaft that did not turn, so this pair exists to convey rotational power from one gear to the other. But if they were a straightforward direct coupling, they could have been injection molded as a single piece. Their multi-piece assembly hinted at something more, so I picked it up and started playing with it. I quickly found that I could rotate the top gear (as viewed in picture above) counterclockwise while holding the bottom gear static, but if I try turning the top gear clockwise they would lock up and turn together.

Aha! It’s a way to ensure something only turns in one direction. A concept implemented several different ways (example) all inside this gearbox. I also knew this concept from the rear wheel of my bicycle, allowing me to coast without having to pedal in sync. I knew there had to be a term for this common concept but my search efforts came up empty. I ended up asking my friend Emily Velasco (who has bike hacking among her many talents) for help and she told me it’s called a freewheel. Unlike the rear wheel hub of my bicycle, this pair of gears didn’t make the clicky-clack noises of a ratchet mechanism. It was smooth and quiet so I had to see how they implemented it. Maybe it’s like one of the illustrations on Wikipedia, a series of spring-loaded ball bearings all around the perimeter? Maybe a clever arrangement of many layers of friction material?

I popped the two gears apart and between them I found only a single metal coil. Wow. How did this work?

I first focused on this detail: both ends of this coil stuck out beyond the coil diameter. I believed it would allow them to smoothly coast along a surface in one direction, but dig in when moved in the opposite direction. A little bit of this dug-in force would expand the diameter for this coil, relaxing its grip on the inner cylinder (half from one gear and half from another) and allowing gears to turn independent of each other. As soon as the direction reverses, the coil contracts back down to its normal diameter and its grip keeps the two gears moving in sync. Torque transmission would have been limited by the friction of the metal coil against slick white plastic, but it’s more than enough to resist my finger strength and evidently enough for this application.

On further examination, I changed my mind. I looked inside the coil housing for any marks of surface damage from coil ends digging in, and found it completely smooth to my eyes. Also, for long-term durability, it would make sense to avoid any mechanism that destroys the surface over time. Perhaps friction against the coil interior, without any wedge dig-in action, is enough for this design to work. [UPDATE: Indeed it is! Emily Velasco told me this is an example of capstan effect.] If so, the fact that the coil ends stuck out beyond its diameter may merely be an artifact of its manufacturing process.

One data point supporting the “friction is enough” hypothesis is the fact this coil is wound from thin metal wire with a square cross section instead of the typical round wire. This would help maximize contact surface area.

Another supporting data point can be found on the big black output gear, where one end (the <1cm length of metal stub) is held static at all times and the other end has nothing to dig into. This is enough to allow counter-clockwise (as viewed in this picture) rotation but resist clockwise rotation.

I found a slot for that metal stub on the gearbox lid that holds the stub in place.

Getting this functionality from a single precision-manufactured coil of metal is a feat of mechanical engineering and manufacturing that impressed and amazed me, and I almost missed it entirely. Given that, I’m sure there are many other details in this gearbox that has gone completely over my head without me noticing, because they are designed to priorities different from mine.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.