Originally Posted by charleyy okay, my background is architecture/woodworking, so i am not conversant with mechanical practices, theory, and common solutions that many of you pros and high-end amateurs are experienced in, so don't laugh too hard at my stupid newbie questions... (you can laugh some...) |
Well, then you're already an engineer. That will help you tremendously! You understand things like deflection, and real world tradeoffs.
1. on many designs -including the 7th sojourn of JOHN CONRAD KLEINBAUER (inventor of the homebrew cnc (sic))- there is both an end support where motor(s) and/or end bearings for the drive shaft are attached, *and* a motor mount plate of some sort... (here's the stupid part) what function does the motor mount plate serve ? why can't you just mount the |
OK, the job of the motor is to *turn* the lead screw...nothing more. Having to support the screw as well, means that undue stresses are put on the motor bearings...stresses a simple "mounting plate" would have solved.
Think about it this way. First you build a machine which could be worked with handcranks. That means that your screw would have to be supported on both ends. THEN you attach a motor to it, and make it move under computer control.
2. for the delrin/whatever 'nut' to attach to the backplates being driven by the threaded rod, is there any benefit to making that nut somewhat (or much?) thicker ? would it help absorb potential 'whipping' actions of the rod (especially as it got longer) ? would it be more betterer if you had TWO such 'drive nuts' at each end of the moving table (or the other axes) ? would that distribute forces better ? |
The thickness of the nut, after a point, is of no relevance. From the context of your question, I'm assuming you want to know if making it longer (more threads) would be of benefit.
The honest answer is yes, and no. Remember those real world tradeoffs I mentioned? Well, the cost will increase...the friction will increase (bigger motors to accomplish the same thing = more cost, again)...the complexity will increase (more parts interacting, means more things can go wrong). It's an unnecessary expense, and an unnecessary complication.
So long as your axis is reasonably well supported in the first place, the problems you mentioned (except for whipping) are more or less theoretical.
BTW, having a bearing at each end of the screw allows you to tension the screw...reducing whipping. Sizing the lead screw correctly will practically eliminate whipping, anyway!
3. in contemplating the problem of attaching motor shaft to threaded rod, is there a possibility that making a coupling out of delrin or other high density plastic stuff would stand up to the forces involved, yet provide the -apparently necessary- flexibility to allow for misalignments ?
I am thinking that taking a block, drilling for the diameter of the motor shaft on one end, and for the rod diameter on the other end, then just glopping it up with some epoxy *should* (?) work okay... besides the drawback of being made a 'permanent' connection with the epoxy, would such an arrangement work okay ? |
I don't remember anyone doing it this way, but it seems as if it could work. The tradeoff is that your lead screw and motor shaft would be permanently attached to each other. If you want to change motors, or upgrade your lead screw, you are SOL.
If you want a simple and effective way to couple them, make sure they're about the same diameter, and use a piece of fiber reinforced air hose with some hose clamps to join them. It's so cheap that it's practically free, and its flexibility will allow for some minor misalignment -- you can make a more elegant mount, later, if you so choose.
5. don't laugh too hard, but i am thinking about using a set of drawer slides (actually, they were used for a trash rollout) as the 'linear bearings' for at least the sliding table. they are of a high quality and have virtually zero slop in both axes (side-to-side and up-and-down), and is already in a pre-made frame that i just have to stiffen up with a couple pieces of plywd, slap on a table, drill for the rod to come through one end, and i've got a sliding table for little effort and expense... it *does* have a slightly variable resistance when one portion of the full-extension slides pull out far enough for the other portion to engage, but i'm not sure if that is an insurmountable problem or not... |
Why would we laugh? Lots of people have built machines using drawer slides. They work pretty well, too!
http://www.pathcom.com/~vhchan/cnc2/cnc2.html
Just remember about those tradeoffs, and plan ahead for upgrades.
-- Chuck Knight