2 Speed Focuser for Meade LX 200 10"

The focuser on my Meade LX200 10" is a little to coarse for me when I am using higher power eyepieces. I had been planning to make one from a ball style reducer but have not been able to find an inexpensive one suited to the project. I ran across the "differential screw" by tkircher which inspired this project.

I wanted to retain the existing focuser holes in the OTA. This determined the maximum size of the housing and in turn the maximum size of the threads that could be used for the differential thread. This resulted in the original focuser screw being too large a diameter to use in the deign - so i made a new one out of brass with a slightly smaller diameter.

This project requires PLA (ABS does not print the threads with an acceptable finish).
I found the PLA-CF gives a good enough finish for the threads to work.

I wound up making the "Screw" and "SmallCyl" from brass and the "BigCyl" from aluminum in order to get a good "feel" to the focuser. Used short pieces of a coat hanger to make the spline (2.04mm). Drilled holes in blanks (prior to machining) with a 2.5mm drill. Glued the splines into the "screw" using loc-tite red.

Design Flaw: The design is inherently flawed as the fine focus knob engages the differential threads and has more friction than the coarse focus knob that runs in the roller bearings. My soulution (very ugly) was to pack the area between the housing and the BigCyl with some paper towel wrapped around the BigCyl and held with tape. This provides enough friction to allow the fine focus knob to move without turning the coarse knob at the same time.

The 3d printed parts served as models for machining the parts.

The collar is a very stiff press fit to the BigCyl - recommend giving it more clearance.

I will put the focuser in the Meade this weekend. I'll let you know how it works.

simplethreads.scad needs to be in the same directory as the focuser file.