I posted before about a few modifications to my minilathe. These cheap lathes are fun and accessible to many, but they have a few weaknesses. Now that I'm taking the machine shop course at Princeton, and learning a lot more about machine work, I've realized a few more deficiencies.
In particular, since a lathe provides several degrees of freedom to the tool holder, it's possible for the tool holder to drift along these dimensions while cutting. Decent lathes provide a locking mechanism along each degree of freedom to prevent this, but for the most part, the minilathe does not.
My minilathe's tool holder has five degrees of freedom: left-to-right (X), forward-backward (Y), rotation (Theta), compound (Z), and tool orientation (Phi). The factory stock lathe provides locks for rotation and tool orientation, however the remaining three cannot be locked.
It turns out it's pretty easy to add locks for the X and Y dimensions.
To add a lock to the X dimension, one needs only to drill and tap a hold along the X-carriage so that a bolt will press against the X-ways.
A very similar approach will work for the Y dimension, though care should be taken to prevent this bolt interfering with the rotation of the tool holder, or from coming in contact with the work piece or chuck.
The Z dimension is trickier because of the shape of the tool post, and I don't have an answer yet for how to add a lock. In the best case, I can't figure out how to provide a lock over more than a small region of the Z-travel. Even then, there are two counterbored holes through the Z-ways (to lock the rotation) which would make this more difficult. If you have any suggestions, I'd like to hear them.
Also, I've been putting a lot of thought into rotation. With a stock minilathe, you have to half-remove the compound to unlock the rotation, adjust the rotation, and then replace the compound. This is really a pain in the butt, and I've been thinking about a way to add a dial to rotate the tool post. My plan is to cut threads along the perimeter of the rotation-clamp part, and then add another screw that will slowly rotate the tool holder. This is still in the works, and I'll let you know when I get it working.