An Old Project: The Snakebite Extruder

There's recent interest in the forums on different ways to drive the filament through the hot-end, especially as it seems that the teeth in some gear driven extruders result in artifacts visible in the surface of the prints. 

Here's an example of the problem:

The wood-grain looking waviness in the print surface seems to be coming from the gear teeth in the extruder.

There's an interesting thread on the subject at the Duet3D forums here.

Here's one designer's idea about a different way to drive the filament. 

Here's a recent design that looks very interesting- similar to the one above:

Both of those extruders rely on rolling threads into the filament to drive it through the hot-end. I explored that concept in a crude way several years ago.

Back in 2014, 1.75 mm filament was a new thing, and extruder jams were everyone's biggest problem in 3D printing. I thought that what was needed was a very strong push-force extruder that would be able to force filament through the hot-end and nozzle under almost any circumstances, including a partial blockage of the nozzle.

My new design came about when I found myself a little bored at a makerspace meeting and and started fooling around with a piece of filament and a 6-32 nut I found on the table where I was sitting. I noticed that the nut could be threaded onto the 3 mm filament. That got me thinking that I could use that idea in an extruder to drive the filament by spinning the nut with the motor. 

One problem with that idea was that the nut fit tightly on the filament and caused the filament to twist when the nut was turning. I needed something to prevent the filament from twisting. I decided to add a second nut, rotating in the opposite direction, figuring that if one nut twists the filament, the second one will untwist it. The second nut would have to have the opposite (left-hand) threads. Hmmm.

I did a little research and found that one can buy left-hand threaded 6-32 taps very cheaply (about $7 IRIC), so I ordered one and used it to make a left hand threaded 6-32 nut from a small piece of mild steel.

The next problem was the gears to drive the two nuts. A little shopping found sets of plastic gears for about $2 at American Science and Surplus that would do the job. I added some 5mm diameter brass tubing, and some small bearings to fit the tubing and it was almost done.

Total invested funds- about $30, most of which went to the Budaschnozzle hot-end that, believe it or not, was considered one of the more reliable designs available at the time. I can't say enough bad things about that hot-end but that's not what this post is about, so I'll just leave it alone.

The final final snakebite extruder, assembled. The green printed part is 3 pieces, all indexed to each other to ensure proper alignment when they are assembled. The red and blue gears are just press fit onto 5mm brass tubing and there are bearings at both ends of both pieces of tubing.

Did it work? Yes and no. It was fine at driving the filament, but retraction proved problematic. If the threads in the two nuts didn't match exactly and/or the hole diameters were a little different, one nut experienced more friction with the filament than the other. It wasn't enough to allow the filament to twist but it interfered with retraction. Also, the nuts had fixed diameter, and in those days, filament diameter was poorly controlled, so it would occasionally run into a blob on the filament that wouldn't fit through the nuts.

I thought about using threaded collets that would allow adjustment to fit different filament diameters, or even spring loaded collets to allow automatic adjustment, but ultimately abandoned the project when 1.75 mm filament became the standard to allow higher speed printing. 

The first prototype of the snakebite extruder. I went from this to the "final" design in about a week. The nuts are soldered to the ends of the brass tubes running through the two gears on the sides of the extruder. The mechanical force tended to push the two side gears apart, so I redesigned the top cover to help hold them together.

Here is an intermediate version assembled with the hot-end for print testing. I eventually used smaller gears to drive the nuts. With this 1:1 gearing, a full rotation of the motor drove the filament about 0.8 mm, so with 16:1 ustepping and a 200 step/rev motor, it was about 4031 usteps/mm. The result was slow but very smooth extrusion.

Here is the very first test print made using the snakebite extruder. You can see that there were some retraction problems (and ringing which is not the problem that started this post).

The top of the extruder, with smaller filament drive gears, opened so you can see the gears and bearings. The green part is actually two pieces that fit tightly together to capture the bearings.

Comparison of an early version to the final, size-reduced version.

The three printed pieces of the extruder assembled.

Side view of the final extruder showing how the gears mesh. The large green gear was press fit onto the motor shaft and was able to fit through the hole in the printed base of the extruder.

A print made using the snakebite extruder. It had excellent surface quality except for the random loop-blebs scattered over it.

A close-up of the blebs. I suspect it was part of the retraction problem.

Here are some videos I made of tests of the snakebite extruder:

More Bike Stuff

I've been riding the Priority Continuum Onyx (PCO) daily for about 4 months now (see my post here) and have made a few changes for comfort and convenience. I've also, finally, had to do some maintenance/repairs. I had a flat tire on the rear wheel, and the front brake developed a horrible squeal. 

The flat tire repair wasn't hard, but getting the shifter cables off the rear wheel proved to be a little difficult. However, once they were off, the repair went as any flat tire repairs does, and putting the wheel back on the bike and reattaching the shift cables was no problem.

The brake squeal was fixed by cleaning the brake disc and pads with disc brake cleaner followed by scuffing them with sand paper and a wipe with IPA. A total of about 15 minutes was all it took.

Other than those two minor issues, the bike has performed flawlessly. Shifting is always instantaneous and silent, the generator that powers the head and tail lights keeps humming right along. 

I list the stuff I added/removed from the bike, below:

Safety gear

I finally replaced my 20+ year old Giro helmet with what I have found to be a much more comfortable Specialized Echelon II helmet in flat black to match the bike. Back in the Giro helmet days, helmets came with a bunch of different thickness foam pads to adjust the fit. Now they have a single thin layer of foam and a knob on the back of the helmet that you turn and you can feel the helmet hugging your head as it tightens up.

Specialized Echelon II helmet. Very comfortable and the matte black finish matches the Priority Continuum Onyx bike.

I like to ride in the evening because it's usually cooler, less windy, less traffic, and I won't get sunburned. Staying alive when riding at night requires being seen. To that end I am using a set of Cygolite Hotrod lights that are small, light, rechargeable, and bright, to supplement the PCO's built in head and tail lights. I put the white light on the left side of the fork and the red light on the back of the bike's rack. Their batteries are good for more than a week of my riding between charges.

Cygolite Hot Rod lights mount on the bike using stretchy rubber straps. I put the white one on the left side of the fork and the red one on the back of the rack on my bike. The straps are secure and seem to hold up in exposure to weather. The lights are rainproof, too.

I need my hands to work, so I bought some gloves to protect them in case I fall, and they are reflective so my hands will be quite visible at night when signaling turns, etc. They aren't great cycling gloves- not much padding for my palms, and they have seams between the thumb and forefinger that make using the twist shifter uncomfortable, so when they wear out I'll replace them with proper cycling gloves.

Reflective glove lit by the flash in my phone. Normally just looks gray with a little color. The colors you see depend on the angle you view them from.

I bought a 247 Viz reflective vest to ensure that I'd be seen at night. This thing is very bright and easy to fold up and carry in my bike bag. It will fit over my coat in winter.

I added reflective tape to the wheels, helmet, and rack, and may add even more.  You can never have too many reflectors on a bike!

I added a Hafny bar-end rear view mirror so I can more easily see if someone is approaching from the rear. The mirror is made of polished stainless steel, so should be more scratch resistant than polycarbonate mirrors, and it won't break if the bike falls over or I crash (though the mount may break). The mount seems well designed and easy to install and adjust. The mirror is convex and gives a wide angle view of things approaching from behind, but the optical quality isn't as good as a glass or polycarbonate mirror. 

Bag/Backpack

While I was waiting for the PCO to arrive after I ordered it in January, I also ordered a Deploy bag from Timbuk2 that was offered at a steep, close-out discount. It is convertible from a pannier to a backpack. It has some plastic clips that lock it to the rack on the bike quickly and easily. The bag is made of some rubberized (?) nylon with a top that rolls up and keeps whatever you carry dry, even in pouring rain. I carry my ancient, heavy, thick, Thinkpad laptop to work in it every day, and it can hold quite a lot of groceries.

I also have a small bag from REI that I've had for many years that fits under the seat for emergency tools. I don't have it on the bike all the time- just when I'm going on longer rides. Someday I'll probably regret not using it all the time.

Comfort

I have a Pearl Izumi bike jacket that I have been using for many years. It's waterproof and breathable, has long sleeves that cover my arms even when stretched out on the bike, and and extended "tail" that covers my lower back in riding position. It's a great jacket, except for one thing- it has no pockets! That is a never ending source of frustration for me. I have looked at many other biking jackets and find that about half of what's out there has no pockets or just one pocket at the lower back. What are they thinking? Real people have to carry stuff when they are riding. There isn't always someone in a sag wagon driving along behind the riders to carry their crap.

I swapped the PCO seat for the Serfas Rx seat from the old Cannondale I was riding because the PCO seat just wasn't very comfortable for my pelvic geometry. The Serfas seat is split down the center and the two halves can rotate to conform to the shape of your (my) ass bones. The padding on the seat is very dense, not squishy, and is comfortable even on long rides.

I also bought a pair of Ergon GP1 grips to replace the ones that came on the PCO. I was finding that my hands were getting numb after a few miles of riding with the original grips.

I added a cheapo water bottle cage - nothing special, but it does the job.

Tools

Fortunately there isn't much to do to the bike, and I was already well equipped with metric tools that I have collected over many years of working on bikes, cars, 3D printers, and electronics. I do carry a small multi tool when I ride for just-in-case. I also carry a CO2 inflator, patch kit and tire levers. The bike has 15mm nuts on the axles and no quick release, so I should start carrying a 15mm wrench, too. Priority Bicycles supplied a decent 15mm wrench with the bike.

Sette multitool with uSD card for scale. I got this tool from Pricepoint many years ago. No, the uSD card didn't come with the tool.

A few years ago I bought a 3D printer that came with a set of hex screwdrivers. I had never heard of the brand- Bondhus- but I was impressed with them. I looked them up and it turns out you can buy them on amazon.com. I have since bought another set of the hex drivers and torx drivers because they are good and inexpensive- much better quality than anything from I ever got at Harbor Freight Tools. I have yet to break or damage any of them and they are much more comfortable to use than the L shaped drivers, though they don't fit everywhere an L shaped tool will fit.

I have to say the best, recent tool acquisition was the Ryobi One+ high pressure inflator that has replaced my crappy old floor pump. OMG it is fantastic! No more struggling with a floor pump to try to maintain tire pressure. A quick 10 second squeeze of the trigger tops off each tire and I'm ready to roll without working up a sweat before I even start to ride. If you have any of the Ryobi One+ tools/batteries, and you have a bicycle in your household, this one should be your next new tool. I paid $25 for it and feel like it was the best $25 I have spent in a long time.

When I repaired the flat tire on the rear wheel, the Ryobi inflator made quick, easy work of refilling the tire. I can't say enough good stuff about this tool!