Fixing Power Tools With Robots

Here's an example of the how a RepRap 3D printer can be used as a regular household utility, with the ability to fix many consumer products. Step-by-Step after the jump.





In this case, the plastic tensioning bracket of a Craftsman 10in band saw had shattered due to excessive pressure. I was able to design and print out a replacement in under one hour.

The printer used in this article is the RepRap eMAKER Huxley #307, which I built in about three days. To learn about open-source desktop 3D printers, visit reprap.org.

1. Tools/Materials

-3D Printer(RepRap, Makerbot, Ultimaker, Printrbot)
-ABS plastic
-calipers/precision steel rule
-3D design software(Autodesk 123D[free], Google Sketchup[free], Blender[free], Autodesk Inventor[free for students; I use this])

ABS plastic is chosen over other types for its strength and degradation resistance. It is also very easy to machine or finish. PLA(the other common printing material) is not chosen because it is biodegradable(made from corn). PLA is useful for prototyping wherein multiple iterations yield a large number of obsolete parts, which can simply be composted. This becomes a problem for a functional part, however, if the part is exposed to water for an extended period of time, or if the part comes into contact with any strong chemicals. ABS is a fairly standard plastic that many consumer products already incorporate.

Ideally, one would print every iteration in PLA, switching to ABS only for the "final draft." Or better yet, if the service is available, print only in ABS and recycle any obsolete parts.

2. Design a New Part

Thingiverse.com is like a public library of 3D design files. Search for your part there first. If it's a common problem, someone may have already designed a solution. You can download this part from Thingiverse for free.

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Use calipers to precisely measure all of the part's dimensions. Then translate these dimensions into your design.

There are three objectives to consider when designing a new part:
1. Retain all critical features of the original.
-keep holes the same diameter, depth, and position
-retain the placement of supplemental parts(screws, bolts, etc.)

2. Resolve any design flaws of the original.
-determine what made the original part fail
-reinforce any weak points

3. Make it printable.
-support any overhangs
-consider the use of breakaway support material
-use polygons instead of circles to minimize the vertex count

With my bracket, I kept the length and width of the original. The inner diameters of the holes were left unchanged, as was their proximity(they breach at one point). The original cracked when the bolt cylinder was compressed along its axis. I fixed this in the new design by making the cylinder's wall much thicker. I also changed the shape from a cylinder to a hexagonal extrusion to make that section easier to print. Before I started drawing, I chose to orient the part so that I could print it without support material.

3. Print the New Part

When printing parts that serve a functional purpose, strength is the key factor. Due to the thermal expansion of ABS plastic, it is difficult to print a solid plastic part without warping. For this reason, most home 3D printers actually print in a kind of structural foam. By printing a thick shell and filling it with a porous lattice structure, it is possible to create a part with nearly 100% of the strength of a solid, but without warping and with much less material. For functional parts, you should print at the maximum density that your printer will allow, with at least two shells.

I printed this part at 50% solidity with three shells.

4. Assemble

This should be the easiest step. If you made close enough measurements, and the printer is calibrated sufficiently, the new printed part should be a drop-in replacement for the old one. If it doesn't fit, or if something's not quite right, just go back to the design file and modify accordingly.

My new bracket was designed with large tolerances in mind and only required one iteration.

Instructables Members: If you found this interesting, please rate it and vote for it in the ShopBot Challenge. I like robots that make things, and a ShopBot CNC router would make a great addition to my RepRap. Please also vote for this entry in the Mad Scientist Fair.