What a Difference a Lathe Makes
This week I took the introductory class on using a metal lathe at Urban Workshop and decided to spend the weekend playing around with the lathe. My instructor taught me some important things to remember:
Machine by ear - if it sounds right, it's probably cutting right.
Look at the chips to see if your speeds and feeds are right. They should be long and curling, but not too thick. They should be the same color as the metal, otherwise they're probably heating up from too much force at the cutting edge of the tool.
Remember your part is a cantilevered beam, so chuck up as much material as you can to create the shortest, stiffest beam possible. Also understand that the end of the part farthest from the chuck will deflect the most - the tool will push it away and consequently cut less material. Consequently, you can expect your part to have some taper along its length.
Rotate the part while you chuck it up.
I tried to make a pen today. I made it in two parts - the front nose and the back handle. I started with the back handle. I drilled out the center to make a hole for the pen, but made the mistake of loading the 1/8" drill bit too much, and it got jammed into my part.
So I took the part out, flipped it around and used the remaining material to make the front nose instead. I drilled it out with two diameters - one larger diameter for the main pen and then a smaller diameter in the nose for the tip to extend through. I was much more careful to peck while drilling instead of driving the entire bit through the part!
Then I figured out that I could rotate the tool post and the slide around. At first I took the bolts fixing the post out completely and it took me a minute to see that they screwed into threaded washers that pulled the tower against the base. Once I got that figured out I rotated the tool post and set the cutter at an angle. I fed it in using a combination of x and z slides - the cut was not very smooth, I had trouble figure out how to feed the tool in to make a nice cut.
Finally, I flipped the front nose around in the chuck and turned down the last 1/2" and cut 3/8-24 threads with a die. My threads were pretty shallow, so I think I need to leave more material on the part. A quick Internet search suggests that I should leave the major diameter of the thread on the part. I used the tail stock to brace the die against the part and keep it perpendicular to the part's z-axis.
Now it was time for a second try at the back handle. It was pretty straightforward - I turned down the outer diameter, drilled out the center and added 1/2" of internal threads for the nose to screw into. I had trouble with turning down the OD. I turned down one half and then flipped it around in the chuck; there was some runout in the part and there was a mismatch when I tried to blend the OD's together. I should probably be more careful about chucking up the part and minimizing the runout.
The pen didn't end up threading together, so tomorrow I'm going to try to remake the front nose with better threads. I'm also going to try to add a brass grip that slips over the aluminum pieces.