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William Waters III - N7IPY

Building the Motorized Ear Posts

The Ear Posts for this B9 are to be motorized so that meant fitting small motors into each Ear Post or using motors inside the Radar Section with flexible drive shafts.  I decided to use the motor in the Ear Post method.


To do this I decided to use plastic.  I used the method I saw on another member’s web page where he used wood to build the parts.  I machined the plastic on a small hobby lathe/mill to the dimensions found on the club web page.  I used Delran for the two tapered parts and found 1.25 inch Plexiglas balls at TAP Plastics when I was purchasing the Delran. 


Machining Jig for the plastic ball made from scrap wood and epoxy putty.  Click Picture To Enlarge


Jig with plastic ball mounted on the Lathe with some of the motor hole milled out.  Click Picture to Enlarge

The motor with reduction gears is fitted into the 1.25 inch plastic ball.  To do this the ball must be machined with a rectangular hole the size of the motor.  To hold the plastic ball a fixture was constructed using a few scraps of wood and epoxy putty.  I drilled an oversized hole through the wood base then glued a thin 3/32 piece of hobby ply to the bottom.  I then mixed the epoxy putty and put it into the opening, spreading it out to allow the plastic ball to fit in and form a perfect fit.  Each time I pressed the ball into the hole, I covered it with tire powder (a shop powder used to help fit inner tubes into tiers) to help it to not stick. After the putty hardened, I sanded it smooth on a belt sander and drew alignment marks.  To hold the ball in the base, I used a second piece of 3/32 hobby play and cut an undersized hole in it, drilled a hole in each corner and then drilled and tapped 6-32 threads into the base piece.  When tapping wood, I always use a high viscous (runny) Super Glue to harden the wood threads and then re-tap to clean the holes.


Delrin in the Lathe being turned to the proper dimensions.  Click Picture To Enlarge


The Main parts of my motorized Ear Post.  Click Picture To Enlarge


Next I mounted the ball into the fixture and mounted both to the lathe/mill and drilled a pilot hole for the motor and a second hole 90 degrees around the ball for as a reference for the horizontal part of the ear posts.  Then I milled the rectangular hole for the motor.  When working with Plexiglas care must be taken to keep it from getting too hot or it just melts and gums up.  I used a light spray of air from the shop air compressor and frequent pauses to keep the melting to a minimum.


The top and side parts of the Ear Posts are machined out of Delran plastic.  This plastic material is great to machine but has the challenging property that nothing sticks to it without special processes that are not easily done at home, more on this later.   These parts were turned on the lathe to the documented dimensions.


Motor attached to the top support via 2 small screws.  The drive shaft has not been cut to length yet.  Click Picture To Enlarge


Dry fit of parts with the motor in place.  Other unit in primer and ready for final paint and assembly.  Click Picture To Enlarge

The method I used to attach the motor output to the drive shaft was to crimp a brass tube around the keyed motor shaft.  A brass sleeve that fits over the drive shaft was cut and press fit into the top taper cut part.  The drive shaft is held in place by the crimp and the brass skeeve.  The drive motor is attached to the top part with 2 small screws.  Then I slid the entire assembly into the machined ball. 


It is not possible to glue or paint Delrin without taking special steps, non of which are things that I can do so I came up with my own method.  First I roughed up the outside surface of the Delran with 60 grit sandpaper.  Then I coated all exposed outside surfaces with a 20 minute epoxy.  After the epoxy hardened, I then lightly sanded it.  This now allows paint to stick to the surface.   To glue the large Delran part into the plastic ball, I used a dremal tool with the milling bit and dug groves into the Delran and plastic ball. I then used 5 minute epoxy to glue the parts together.  The epoxy does not bond the parts together, but it fills in the groves and when hard creates a strong joint.


Ear Post in primer and ready for final paint and assembly.  Click Picture To Enlarge


Both Ear Posts with Sensors Ready for paint.  Click Picture To Enlarge

The final assembly was done using a 6-32 set screw in the plastic ball to hold the motor in place and epoxy putty to fill any gaps in the motor assembly.  I decided to use the Bill K. & Rod R Sensors and Resin Holders.  For the price is was worth it.

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(C) 2007 - 2018  William Waters   Last Updated February 2018