Mini Lathe Quick Change Gearbox Project
This is probably the largest and most complicated home project I have had the pleasure of
designing and fabricating.  As many of you mini lathe owners know, it is a bit of a pain and time
consuming to swap out the change gears to cut threads or power feed.  Solution, design and
fabricate a quick change gearbox like the big production lathes have.  Easier said then done!  
After years of off and on designing the gearbox, I finally put my nose to the grindstone and finished
it up.  Next was to make the patterns for the castings and get the parts cast.  Then, let the fun
begin, machining all the parts.

I started by drawing the basic lathe headstock and designed around it using Solid Edge and
later SolidWorks 3D CAD.  Even though not shown in the completed gearbox pictures below, the
original gear cover fits to the gearbox end.  See picture to the left.  The gearbox was also
designed to use the same plastic gears that came with the lathe and are available for as
replacement parts.
Fabricating the gear change levers:
The lever castings for the project are aluminum and were sand cast at a local
foundry using my match plate and core box.  Below are two lever casting. To
the right of the castings are the match plate and core box used to cast the
levers.  I made both from plywood and coated with urethane.  The match plate
makes two levers at a time.  The core box makes sand cores that produce the
hollow center for the gears.
Fixturing to machine the lever casting:
Below and to the right are two pictures of a lever casting clamped into the drilling
fixture. I designed the fixture to hold the parts for accurate consistent machining of the
Parts for the levers were machined on the mini lathe out of steel and aluminum. Below and to the right are the finished
parts and assemblies.
Fabricating the gear box main casting:
I made the pattern for the gear box casting from 1/4 inch plywood and used Bondo to fill gaps and make the corner
radii.  Below are pictures of the pattern.  And to the right is the aluminum casting from the foundry.
Machining the casting:
I used my milling machine to open up the pocket on the front of the castings for the levers to fit through and to
machine square the side of the casting that will attach to the mini lathe. To the right is the first test fit to the lathe
and a picture  of the lever position holes drilled in the casting.
Finished gearbox:
Below is the finished gearbox with some paint, labels and threading chart.

Since completion I have begun testing the gearbox and determined that the lathe is
underpowered to cut many of the largest standard thread above 3/8".  It works great for the lower
threads like 1/4-20 and to power feed.  I guess the next project may be to design a gear reducer
to utilize the lathe's high speed and turn it into torque.
Last update 2/17/2013