So, after owning the lathe for just over a couple of weeks, I saw the need to mill arcs. Usually this is accomplished by means of a rotary table, but unfortunately I do not have one and am not able to shell out some $300 for one. Fortunately, I do not need the precision a normal rotary table offers, so I ventured off and tried to build one myself from readily available hardware-store components (balls for ball bearings, a bold, a nut and some brass) and some scrap aluminum. Overall budget - somewhere just over $10.
The idea was to have two plates that could rotate freely while being very parallel to each other. I have some large steel balls rolling on steel washers ensuring the parallelism, with some smaller balls ensuring that I can tighten up the two plates tightly towards each other, while still being able to rotate them freely. That kind of makes two ball bearings from scratch. The axis for the plates is actually made out of two co-axial brass tubes that can rotate within one another fairly freely, while preventing any other direction of motion, thus making a sort of a sleeve bearing.
Hmmm... I think I just realized that I am not all that good with word descriptions, and a picture is worth a 1000 words, so
here we go!
Machining the plates. That was a bit loud and painful because of the lack of proper tool. But it worked.
All parts. The inner bushing and the bolt haven't been trimmed to size yet.
Top plate with outer brass bushing press-fit.Do you think that "press-fit" was easy? NO!!! The bushing was about .003" too large!. So, naturally, I tried to heat the plates to expand them, like that:
Unfortunately, even when the plate gets too hot to handle, they still don't expand enough to make up for the .003" shortfall. Duh! I had to put the bushing back on the lathe and meticulously dial it in with a sub-thou precision so I could shave off that 1.5 thou off the radius and still have things cocentric and round (there is a really good chance to get an elliptic-looking bushing in that case). That was a really painful zeroing-in process. Lesson learned! Make sure things are finished to the specs and within tolerances before removing the piece from the chuck. Arghhh...
Packing smaller ball bearing. Hope white lithium grease works well here.
Hope it works for the main bearing too. There are steel washers on the bottom of each groove, so the balls will roll on steel. The idea is that this setup will be less prone to loosing parallelism in the orientation of the plates, and more resistant to wear. We'll see if that will work.
I don't have a picture where I drilled and tapped a bunch of 10-32 holes and milled a couple of slots in the top plate for mounting the work pieces, as well as drilling a bunch of holes in the bottom one to attach the table to the t-slots in the milling attachment. But at least I have a picture showing that the plates are quite parallel to each other (and you can even see the balls between them - I should really cut a PVC washer or something to protect them from chips.) I don't have a picture of the whole thing Now, that the table is assembled, I wanted to test it on something simple. The first victim for the test will be a small bracket that mounts over the tailstock dead end and chuck that allows me to measure the depth of holes I drill with a dial indicator. I made the bracket a couple of weeks ago, but it has been waiting for the arcs to be milled.
Finally, I put some markings on the table so that it is easier to mount workpieces on it in the future.
And after that I tried milling some arcs in a 1/2" thick aluminum with a 3/8" end mill. It would certainly help to have a little worm gear to rotate the table, but, as long as I take thin cuts (about .020" at a time) and have a steady hand things seem to work out OK. I just need to pay extra attention to the setup to ensure that if I can't hold the table, and the milling bit jams into the workpiece, things won't go flying all over the place, and my fingers won't get screwed onto the milling bit. This attachment can be quite dangerous if adequate thought is not paid to the setup, but if you think things through, it is really quite fine (as safe as any other part of my lathe, really).