OK, a suggestion. Ballscrews are expensive, and this is your first machine.
Yes, I said *first* machine. It's human nature to redesign stuff during and after the build...you WILL want to build a second one.
The best suggestion I can give you is to keep reading and learning...but in the meantime just get something moving under computer control. Get an old 486, run TurboCNC on it, and build a machine on which to learn. It'll end up being cheaper, in the long run, to build two...a cheap one for learning, and the one you actually want, than it would be to build your final machine, first. It's counterintuitive, but it's true.
Now...lead screws. It's simple. You fix the position of the screw, and then allow it to rotate. If it's not allowed to rotate, the nut will travel along the screw thread. This is the part you hook to the moving part of your machine...the gantry, or the moving table, depending on your design.
ANY screw thread will work for a lead screw...there are a range of options. In the US, there is an inexpensive threaded rod called "allthread" which is used by a large number of first time builders. I's not overly accurate, but is inexpensive, and allows you to get a table up and running using locally available parts.
The next step up in quality is ACME thread. It's the "squarish" thread type found on vises and clamps in workshops. When it's made well (precision ground) it's actually quite good, and is more efficient in transferring power than the triangular thread on allthread. Practically, this means that there are lower losses in the system...smaller motor is needed.
The highest quality thread is a ballscrew. Basically, it's a "helical ball bearing" that has such low drag that the screw will "fall" down the thread, without YOU turning it.
Anti-backlash nuts are needed on all of these theads... First, backlash is the "wiggle" present between the threaded rod and the nut. It can be seen in moving tables, when you turn the screw as much as half a turn, before the table moves. Seriously...go in your garage, go to your vise, and take a look. Now that you know to look for it, you'll be amazed!
How you take care of it is simple. You use *2* nuts, spaced so that they push outwards against both sides of the threads. A common way to do this is to put a spring between them, and force the nuts apart. Other ways include a fixed "spacer" that goes between the two halves of the nut, and some screws that push the two halves apart. There are lots of ways to minimize/eliminate backlash...and doing it doesn't require ballscrews.
Go to your hardware store, pick up a few nuts and bolts, and experiment. Try some of the designs you've seen on this forum...some of them are quite simple, and very effective.
You may decide that you don't need that $100US ballscrew assembly for your 12" X-axis.
-- Chuck Knight  |