Wednesday, December 23, 2015

Santa -- You Can Take That Leaf-Blower Off My Wish List

The main active part -- the turbine fan (in light yellow) -- shows
extensive around the shaft attachment.
It's Fall in South Texas, and that means the leaves are falling. I recently pulled out my battery-powered lime-green leaf blower (bought 4 years ago from you-know-where) only to find it make lots of racket and rattling but no rushing of wind. We have a large yard (this is Texas, after all) and lots of trees, so after some time with a rake gathering leaves I decided it was time to replace the leaf blower. But after a review of my favorite web sites I found that Fall apparently is not a good time to purchase leaf blowers.... And, after perusing a few repair web sites, I came to the conclusion that battery-powered leaf blowers are designed to be disposable items (making me a bit skeptical about how "green" they really are...). So I went to the fall-back plan and called in the go-to guy -- Santa! Surely I've been nice enough to deserve at least a new useful yard tool (note: my wife has agreed not to post on this particular blog...).

But wait -- I'm a Maker with a lot of 3D design experience. I most often apply these skills to robotics, but why not appliance repair too? It would be a tough application with all that rotational speed, but what's life without a good challenge (or two)?
Close-up of the turbine fan with shaft-mount cracking

So I dove into a bit of device dis-assembly and problem analysis. (Note: always remove any power source BEFORE attempting to open power tools and equipment.) Removing a few screws quickly revealed the source of the problem: the turbine fan shaft mount had fractured in multiple places, so that it was no longer able to rotate. Fortunately the motor appeared to still be in good condition, and all the electrical components still operated fine. So, if I could design and 3D-print a replacement for the turbine fan, there was a good chance this tool would blow again.

I chose to 3D-print the replacement turbine fan because I had multiple 3D printers available in the makerspace capable of printing a variety of materials with potential to stand up to the mechanical stresses. But this would be a challenging design, since there was no large flat surface in the original part structure to support reliable 3D printing. And there are many, many curves in the turbine fan structure!

But with some design decomposition I landed on a 2-part component, both with the requisite large flat areas for 3D printing: (1) a flat shaft collar on the bottom,  and (2) the working part of the turbine fan structure with all the curved blades and functional cavities. I decided to use ABS as the 3D printing material for best wearability and glue adhesion.

Now came the "fun" part: designing the turbine fan section! I decided to use TinkerCad (an excellent free online 3D design tool designed for beginners but with 80% of the power of top-end Autodesk tools, in my opinion) so I could share this model with students in the future. With TinkerCad it was relatively easy to create and shape the curved parts -- it just requires a little bit of mental dexterity with the concept of cutting primitive shapes with other shapes. Intermediate and final results of the design are shown in the pictures.

The 3D-printed replacement turbine fan
Perhaps the most difficult part of the 3D design was implementing the reverse blunt threads needed to install the 3D printed parts onto the motor shaft. TinkerCad has a nice library of user-created shape generators that yielded a reverse-thread shape, but it took 2 test prints with smaller test pieces to get the fit right despite my use of a micrometer to measure the shaft. The successful dimensions were then applied to the turbine fan design.

Now it was 3D printing time! Despite the large flat surface on the bottom, I was concerned about how well the bridging (unsupported horizontal surface areas) would print given the all the curves (printing the curves reduces 3D printer speed, which increases printing sag -- which is not good). I decided to try the PolyPrinter 229 I'm currently testing because of its high bridging speed and was very satisfied with the results -- OK, I was actually surprised. The replacement turbine fan was good to go after the first print, needing just a little bit of cleanup with a sharp knife.

After test-fitting the 3D-printed parts onto the motor shaft, I decided they'd operate better with additional bonding and applied a good amount of resin epoxy glue. Both parts (the shaft collar, in orange, and turbine fan) went on well and fit correctly. And everything fit back into the leaf blower just fine (yielding an initial stage of relief....) Then I aligned parts to reduce wobble when rotated and gave the epoxy an hour to fully set.

After reassembly it was time to test. A few power cycles confirmed the 3D-printed parts were indeed attached to the motor shaft. And the tool now generated a rush of air -- enough to scare any lingering dust bunnies in the Techno Chaos makerspace. But would it be enough to move a Texas-sized accumulation of leaves? After taking it home and trying it in the "real world" I can confirm the answer is "YES!!!!"

So Santa -- I know you're busy right now -- but if you're listening you can take that new leaf blower off my wish list because I have one that works just fine now! (I estimate it produces about 75% of the air speed of the original part, probably due to the increased weight of the turbine fan and slightly-thicker fan blades for the 3D-printed part. But as the video shows it definitely works!)

And in case you and the Lead Elves get bored after the Big Day, we have some classes in 3D printing coming up soon! You'll find them useful for many things back at the North Pole...    ;-}

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