Side Pulls in the LMP Shop

2.810 uses a plastic gear that can be tough to source, and not all of them have a set screw, which makes them harder to use.  I wanted a mold making challenge and the ability to make gears at will, so I machined the molds.

 

My design goals were:

1. Mold a gear that meshes with existing RC car drivetrain components

2. Insert mold a set screw into the part.

Link to the CAD file.

The first goal was pretty easy. Fusion 360 has an add in gear generator. I knew the pitch of my gear and the number of teeth.  Find this generator under Design – Tools – Add-Ins – Scripts and Add Ins – Spur Gear.

When building the molds for the gear I only scaled the generated gear by 1%, as that’s a low estimate for polypropylene shrinkage. The mold could always be remachined to be bigger, so starting at the low side for part shrinkage made sense.

 

Creating the side pull is mostly about adding an extra parting line. I chose the centerline of this gear because it’s symmetrical, but any line that makes sense for your part is fine.  The side pull mounting holes and the hole that holds the set screw were machined first, then the side pull and rest of the core were machined as a set to avoid steps, mismatch and flash.

Die springs operate the side pull. These were found in a drawer of misc springs in the shop. They’re really strong – I can’t close it without a press. The side pull is closed in the injection molder with an angled plate. There’s a bronze plate behind the side pull to prevent aluminum to aluminum galling as the two parts slide past each other. The side pull has a section of allen wrench pressed into it, with a magnet behind it to retain the set screw.  It’s a little fiddly to put the set screw on the mold, but it stays on there well and releases well in the machine.

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