Update: 6/16/18 Ok so I got a new set of 28BYJ-48 steppers since I suspected my old ones were weaker than normal. The new ones do seem to have more torque and are resulting in fewer jams. I also tried running them at 9v instead of 5v which seemed to help eliminate jamming even more. But what made the biggest difference was adding a mount for our tank. We have a cheap 30gallon tank with a really flimsy plastic LED hood that has a 62mm x 23mm opening for feeding. So I just made a little mount that replaces the end cap on the feeder fits in there holding the feeder at a slight angle.
That angle seems to have solved most of the jamming issues. In fact I switched back to 5v and while it did jam once going through an entire container of food it cleared itself the next cycle. I've run a full container of food through it 2-3 times now and it only jammed up 2 or 3 times and cleared itself the next cycle on all of those. (I'm still running my test script that runs a dispensing cycle every 2 seconds - and the cycle first starts by running the auger in reverse a few hundred steps to help clear jams before running it forwards.)
I'm added the tank_mount.stl file and since this is most likely to need to be modified I also exported a .STEP file for it. (though again I've included the link to the onshape project which is public so anyone can access the full project directly.)
Original Summary Follows
This "Works" but is still a work in progress as it jams up too frequently for me to trust it right now.
A few years ago my daughter won a few fish at the county fair, unlike most fair fish we actually put them in a decent tank and cared well for them so they've thrived and are now large and healthy. But I'm tired of having to find someone to feed them every time we want to go out of town for a few days, and I'm too cheap to spend $15 on a feeder at the store when I should have enough junk laying around to just build one.
I Initially tried this simple feeder: https://www.thingiverse.com/thing:2539750 but found it tended to jam up on the pellets our fish prefer and just wasn't big enough for the amount of food our fish eat. So instead I started looking at the auger style feeders.
I started with @pcuna's variation of @coberdas's design since I liked the ability to mount a jar on it and I have a bunch of 9g servos laying around. (Though I later realized that the pellets we use are in a larger jar than that design was created for.) However none of my 9g servos have been modified for continuous rotation and I didn't really feel like modifying any. But I have a bunch of 28BYJ-48 steppers on hand so figured I'd try using one of them instead.
I totally overlooked @TeamKRF's 28BYJ version since I only saw the openscad renderings and tend to stay away from published designs that don't show printed versions (I later looked at it and realized he had printed his design...may have saved myself some effort if I had looked closer!)
So instead I fired up OnShape, imported the .ipt file for the tube and auger and modified them to work with the stepper motor. I then spent most of a weekend fiddling with it trying to get it to work. The stepper could turn the auger, but as soon as I added any food pellets it would jam up. Even without pellets sometimes the auger would jam up.
I decided the problem was with the auger only mounted at one end it just wasn't going to be reliable enough.
Since I have a bunch of 608 bearings on hand I decided to redesign it so the auger would be supported at both ends.
At this point I basically started over on the design and only took inspiration from the designs I listed this as a derivative of.
In addition to being supported on both ends with bearings the auger now has a thicker center for extra strength, and a fillet where the screw meets the shaft to help prevent food pellets from jamming up.
Since the bearing blocks the end the tube now has an opening for food to fall out of on the side. The hopper opening is shorter and filleted so it can print without supports. The screw holes for mounting the motor are sized so you can either self-tap M3 screws or use an M4 tap. (They aren't quite small enough to be reliably tapped with an M3 tap...but I tried it and it did work well. Worst case there's room for a nut on the backside if you drill them out or ruin your threads.)
The hopper is a tight press fit - you may have to clean up the opening for it in the tube just slightly - printing the tube with no supports I had a tiny bit of dropped extrusion that I had to cut off at the top of the opening.
The lid for the hopper is also a tight press fit - but if you don't push it on all the way it's still fairly easy to remove. The end cap is a little looser but is really just decorative. Neither cap is necessary just kind of nice to have.
It now works much more reliably - but I'm still fine tuning the hopper inlet because the auger will sometimes still get jammed when pellets feed in. I've tried tweaking my code to run the auger in reverse for half a rotation before dispensing which helped...but still hasn't completely prevented jams with the size of pellets we use. Flakes seem to dispense fine - but our fish don't like them and flakes jam up in the hopper. You could add a stirring system to the hopper like some other designs have to use this with flakes - but since we don't use flakes I haven't pursued that.
Support is needed to print the auger. Everything else prints ok without support including the tube, though it may print slightly better with supports if your slicer is good at supports (I printed without supports and it came out fine.) Parts should all be oriented correctly in the files.
I use 3 perimeters for strength and minimal infill is needed. On the tube it adds a bit of strength in the motor mount area. The hopper can probably be printed without infill as the walls should be self supporting and strength isn't an issue. I left it on just to help give it some internal support as it printed.
Printed in Translucent PLA so I could see inside it while it works, but should work in most materials.
Since the auger needs to print with supports you'll have to remove them. Make sure to clean up any rough edges they leave around the outside of the auger as the auger needs to rotate freely in the bore of the tube.
It's a good idea to test the auger in the tube before installing the bearings - if it doesn't rotate easily you'll want to sand down the edges of the helix a little. You can use a small flat head screwdriver to manually turn the auger when testing.
Add a 608 skate bearing to each end of the auger. If they're a little tight you can sand down the auger a bit - or just use some more force :D If they're a little loose...don't worry the auger should still rotate.
Check the ID of the tube before trying to install the auger. If like me you printed the tube with no support then there might be a few small imperfections around the outlet and inlet openings that will need to be sanded or cleaned up with a razor blade.
Push the auger and bearing assembly into the tube. It should be a tight fit for the bearings and you may have to tap it into place (I used the handle of a screw driver as an impromptu hammer) to get the bearings to fully seat. The end bearing should be flush with the end of the tube when it's fully inserted - but if it sticks out a tiny bit it's not the end of the world.
The motor simply mounts with two M3 or M4 screws. The holes are 3.1mm which should allow you to self-tap the screws if using M3 and provide enough material to use a M4 tap if you'd rather use M4 screws.
I used a M3 tap just to make sure I had decent threads for M3 screws, there isn't really enough material to use a tap but if you're careful it will work. Worst case if you destroy the threads you can tap for M4 or just put a nut on the back.
M6 screws should be sufficient if you thread the plastic. M8 or M10 will give you enough extra length to use a nut on the back.
Install hopper and cap
These just press fit in. Again if you printed the tube with no support you may have to cleanup the hopper opening slightly to get it to fit. It will take some force to get it in but once it snaps in it shouldn't go anywhere.
The end cap is optional and just decorative - it may be a tiny bit loose but should stay on, if you really want it on there and it's too loose a drop of glue will help just use it sparingly so you don't get any into the bearing.
The hopper lid is also optional but can help keep dust and dirt out of the food. It's a little snug but if you don't press it on very far is still easy to install and remove.
The 28BYJ-48 motors usually come with ULN2003 driver boards so that's what I used along with an arduino Pro Mini (though a nano or uno or pretty much any arduino will work just as well, I just happen to have a bunch of pro mini's on had because they're so cheap.)
I used the accelstepper library to control the motor. I'm still experimenting with the settings but right now I'm using:
I then run the motor 800 steps forward followed by 5000 steps in reverse to dispense.
Attached is a small sketch I use for testing that just runs a dispensing cycle, pauses for 2 seconds and repeats.
Once I work out the jamming issues I'll upload a better sketch that runs a daily feed cycle instead.