Family Home Page Machinist Home Page Shop Details Lathe Cleanup Projects

Telescope Lens Cap (Step 1)

One goal for our amateur astronomy hobby is to be able to control the observatory and telescope equipment completely remotely, eliminating a long walk out to the observatory (which is especially annoying on cold winter nights). Building a motorized lens cap for the telescope Is one step toward this goal.

There will be two lens caps – one for the main telescope and a second for the smaller guide scope. Each lens cap will be attached to a motor which is controlled by a microcomputer connected to the home Ethernet network. Sending a command to the microcomputer will cause the motor to rotate and open the lens cap. Sending a second command will rotate the motor in the opposite direction to close the cap.

The first part of this project involved machining brass shafts that will attach the lens caps to the motors. The motors are surplus automobile ventilation damper motors purchased from All Electronics (part number DCM-276, if it's still available). Since they came with a hub containing an internal spline, I had to machine a mating spline on one end of each brass shaft.

Machining the splines

To machine the splines, I removed the three-jaw chuck from the lathe and installed it on a rotary table attachment for the milling machine. I used a regular end mill to cut the spline. After each cut, I rotated the shaft 45° and made the next cut. Eight cuts and the spline was finished. The result probably isn't up to industry standards, but it fits well into the motor shaft.


Here's a close-up of the splines. The shafts are ¼" round brass stock. The angle between faces of each cut is 90°. Some angles appear more acute, but this is an illusion caused by the camera angle. A square-end mill can cut only 90° angles.


Updated October 12, 2017