Hi Kerry,
Specifically on the ball screw/ acme (original lead screw) choice:
The low-dollar, conservative approach (IMHO) would be to stick with the original lead screws initially, since you can always go to ball screws later.
The main advantage of this approach is obviously financial, since you can be making chips under cnc while you're deciding on whether to upgrade to ball screws. If or when you do decide that you want to go ball screw, you have an operational CNC to help fabricate the parts (bracketry etc.) for your ball screw conversion.
It also removes (or postpones) one level of difficulty, or source of frustration from your conversion.
disadvantages of this approach are that the original leadscrews will have many times more backlash than the ballscrews would, acme screws are going to require more power than ballscrews, much in the same way plain bearings require more power than ball bearings. (this is the screw systems' efficiency)
Neither acme nor ballscrews are likely to have zero-back lash
How is backlash dealt with by the cnc? Every software package I've heard of uses backlash compensation. This applies to commercial units as well. Even brand new full size cnc machines have backlash (usually in the .00001's or .0001's range) that is compensated for by the controller.
With proper backlash compensation, your circles will not have .022" "steps" at the noon,3,6, and 9:00 positions on your logan, using the original lead screws. Your actual results will depend on lot's of other factors of the machines condition also, which ball screws can't always fix.
I don't think any of the sherline, taig, size machines use ball screws, can they make "good" circles? How?
I can also understand the opinion of installing ball screws from the get-go, I guess it's sort of a preference type thing. There are also certain milling applications (intricate profiling, engraving etc.) that would incur a severe speed penalty on cycle time with a large backlash machine, since the backlash compensation has to actually move the screw (in your case .022") every time an axis changes direction. Rapid positioning rates are very important for some jobs, less important for others. Your logan might rapid significantly faster with ball screws than with acme, all other things being equal.
Again, just my two cent's worth.
The 38 volt ametek motors are at surpluscenter dot com item # 10-2351
$32.50 us dollars. These are almost encoder ready. I guy had a set of three on ebay a while back with encoders installed. It seems they didn't sell, I think he was wanting 150 or so for the set. I made a short shaft extension , and a small aluminum adapter plate to mount the encoder on mine. Also, the 5/8 PTO end is smooth, no keyways or flats, so you'd have to make a flat, or a keyway or use a tapered bushing type pulley or sprocket. I'm pretty sure these motors have been discussed on other posts. The G340 or g320 should be able to drive these. They're probably on the smaller end of the scale for servos applicable to a bridgeport sized mill.  |