Wee Walker Wagon V1 and 2

Getting back to baby furniture, I have seen different kinds of little pusher wagons that kids learning to walk can use to help them build muscles and coordination. A co-worker mentioned that his daughter had a plastic store bought walker that she was learning to walk with, but that it was so light that it would shoot away from her. I had him measure her height to the shoulder to act as a rough starting point

I will probably build one for us and a few for friends having kids. That means making use of templates to repeat the work once I figure out what I want it to be like. I broke out the compass and thin bandsaw blade and got curvy.

I added holes to the templates and used a transfer punch to copy those holes over to each part. That way the wheels will connect in the same spot, and the handles will go on evenly. To add a little color and flare I painted green accents to the handle and wheels and a racing stripe for +2 speed. When the paint was all dry I coated everything with polyurethane.

I was worried the walker would go skidding across the floor if placed on anything other than carpet. I printed thin TPU bands to act as tries and glued them to the wooden wheels. The TPU isn’t as grippy as a rubber tire, but I was able to produce it in house and it will grip better than the bare wood.

I spent some time fiddling with the arms, and probably built them a little too short. I will give this to the co-worker and have him test it out on his daughter.

The finished product looks nice. It is heavy enough that a child just learning to climb up will not have it shoot away, but light enough they can still push it. Lock nuts set the tension at the wheels, so there can be more or less resistance as needed.

I gave this one to the co-worker that was complaining about the plastic one his daughter was using. The only catch was that he had to report back how it worked, and let me tweak it if need be. A few weeks later the reports were in. The bigger kids liked to play with it too! They treated it like a bumper car and crashed it into a lot of things. Also they wrenched on the handle hard enough that the arms were flexing where they screwed into the body. Lastly, it was a little on the wide side.

No problem, this is why I gave it to him! I didn’t like how the old arms turned out, so I went ahead and remade the template. Instead of freehanding some curves I got more systematic. I drew two circles that were the size I wanted each end of the arm to be, and drew them further apart than the original arm so I could move the base down lower on the cart. Then, to connect them, I set my drawing bow to a nice curve and connected the tangents of the two circles. The results look a lot better than my first arm template.

I assembled everything with pocket hole screws from underneath, and a few visible screws on the side. That let me take everything apart for alterations. I trimmed the center width down from 16 inches to 12. That should lighten the look and load of the thing and make it more maneuverable.

The bumper car comment had me wondering. I had left the bolt heads stick out beyond the wheel. If they caught a piece of furniture or baseboard, the bolt would probably fair better than the target. Also, there wasn’t much cushion to the tires. I mostly added them for grip, not as a bash protector. I counter-bored all the wheels, to recess the bolt heads, and upgraded to 5/16″ bolts as that was what I had available. The old tires were cut off and replaced with thicker ones that wrap around the outside face of the wheel. Now they shouldn’t be so offensive to fine features in one’s house.

With those upgrades complete I reassembled everything and gave it back to my co-worker. If there aren’t any more changes needed I can go into production and make a few for myself and other co-workers that are having kids.

TPU Printing Tips

Printing flexible filaments holds a mixture of horror and delight. On one hand making something squishy and flexible allows for custom grips, cushions, dampers, and all sorts of fun wiggly toys. On the other hand it can print like a nightmare. I had enough bad experiences early on I nearly completely swore off printing flexibles. Eventually I had someone requesting some flexy parts, so I got back into it.

Most printable flexible filaments are either TPE (Thermo Plastic Elastomer), or TPU (Thermo Plastic Urethane). TPEs tend to be a little softer, but you can print a structure with more infill to make it stiffer, or less to make it more complaint.

Problem 1: Bed Removal

Normally when 3D printing it is a hurdle to get things to stick to the bed. Not so with TPU. It sticks to the bed sheet material with such fervor that it will actually peel the PEI layer off the bed. I did this in the past and was upset about it. I found spreading down talc powder helped, but never fully alleviated the issue. Then along came my little purple friend.

I have heard people talk about glue sticks to help print adhesion. I never had any luck with normal hard plastics. But it acts as kind of a barrier or release agent with TPU. Sticky enough that it won’t come off during printing, but soft enough the part can be peeled off. It builds up over time, but is water soluble, so just wash it away in the sink. A thing layer is all you need. I never heat the bed when printing TPU.

Problem 2: Loading

Once I tackled the removal issues a little while back I was happy to perform occasional prints. Loading flexible filament could be a pain though. Half the time it would jam and wrap around the extruder. I did some reading and experimenting and came up with a two part solution.

When loading, increase the temp 10-20 degrees. It makes the TPU flow better. My Prusa has an automatic filament loading function. It detects the filament, loads quickly, then slows down to extrude new filament and flush the old residue. That initial quick load gets me in trouble about half the time. Instead I insert the filament, but not all the way to the extruder gear. I let it perform the fast load, then push it in the rest of the way when it slows down. It hasn’t jammed yet using that method.

Problem 3: Stringing

My TPU prints had crazy stringing. It was annoying, but a set of side cutters would take care of it. I was just so happy to be printing flexible materials I didn’t care at first. Now I am annoyed at the post process work required. I started googling and came up with some things to try.

First up, a number of places talked about the flexible materials not working well with retractions. It is flexible and might jam, but I thought that would make the stringing worse. Sure enough, it was a lot worse.

My original settings were what came with the Prusa Slicer. No retraction caused considerably worse stringing. Ok, how about instead we lower temperature, that was supposed to help out.

Sure enough, that worked. It makes sense, the stuff wants to solidify faster. I checked the manufacturer of my filament, and 240C is a little hotter than they suggest. I tried one at 220C, but was starting to get poor layer adhesion. I also slowed down the retraction speed from 35mm/min to 20. Some stringing still exists, so I will try to retract further.

Even better! There are some nubs left (hard to see with black filament), but this is so much better than before. I will call this good for now, but might go back in the future and do more turning.

While I was testing these I was slowly increasing my initial Z off set. They were coming off the bed more easily with 100um of increased Z height without significant impact to the bottom layer quality. It turns out you can alter the first layer Z height in the slicer, so I made that change for my flexible material profile.


TPU sticks too well?

  • Use a layer of purple glue stick on the print surface, it acts as a release agent
  • Increase initial Z height so first layer doesn’t get pressed in so hard

TPU strings badly?

  • Reduce temperature till you have layer adhesion or jam issues, then back up. Lower temps have less stringing.
  • Longer retractions at slower speeds might be needed.

What does one print with flexible material? Well Those test prints were feet for my macbook laptop. The originals keep falling off and getting lost, so I have been printing new ones and gluing them on.

Speaking of feet, I printed a set of vibration damper feet (thingiverse link) for the printer. The first set looked so ugly I figured I had to spend some time tuning to get rid of all this stringing. The before and after tuning results are pretty dramatic.

Other fun prints are a set of flexible hex bit holders (thingiverse link). They grip well enough to keep the bits from slipping out, but are easy to insert and remove. I tried making a set from hard plastic, but they just don’t compare.

Lastly the support arm on my tablesaw had a rubber guard to keep you from catching the edge. It got lost ages ago and and I have found it to be very uncomfortable when impacted at walking speeds. I made a push on flexible cap, but it too got lost after a few months. This one bolts into place in such a way as not to interfere with the fence. I bet I won’t be losing this one. Hopefully I won’t be racking my hip on it either.