I've measured 0.014" of backlash near the middle of the X-axis of
my mill (converted to CNC), and I understand that eliminating that will give me a noticeably better finish in aluminum. I can also see that my circles are not exactly circular, btw.
I need no better than 0.004" error per every 4", (= 0.012" per foot) and from the specs I see on the different options, that is not a problem with any of the options. So eliminating the backlash (as much as reasonably possible) is the only real issue here. Also, I'm going to focus on only the X & Y axes for now.
Existing setup: 23mm (0.91") dia. acme screws. X-Axis total screw length is about 3ft (supported on both ends) and Y-axis is about 2ft. Travels are a bit shy of 20" x 7". Realistically, about 5-10 hrs of use per week, and slow (about 15-20ipm for cutting and 5ipm for finishing). Possible options... (1) Anti-backlash nut. I think I understand how these work
but I haven't found any for my thread size, and it seems like it would wear out fast. It would not be easy to get under the base of a 700lb machine regularly, so I don't want to have to replace or adjust it often (it was hell getting the mill up on the workbench with 3 guys and some jacks, etc).
(2) Get 2 nuts on the acme screw and tension them somehow -- I haven't figured out the details on this yet, but tensioning a nut on an acme screw seems like it would wear it out faster, wouldn't it? And as I understand it, wear occurs mostly in the center of the table's travel where most parts are located on the table, then if the nuts are tensioned, they bind on the ends when a larger part is machined someday. Ugh! Or am I being overly concerned with my low usage level?
This article touts leadscrews being a viable option, but I understand that costs can exceed that of basic ballscrews.
(3) Ballscrews. Seems like rolled would be the better option here, as even C7 or C10 meets my accuracy requirements. But what diameter? I'm fairly sure that a chinese machine wouldn't go above and beyond in their acme-screw selection, so should I go with something at least 23mm (say 1")? Or would 5/8" or 3/4" be fine due to less friction, etc? At my low speeds, say even 30ipm with 5 rpi, that's only 150rpm, so whipping should not be a problem on the y-axis which would have only about 1 ft hanging. (The x-axis is supported on both ends). I'm worried with larger diameters I'd have to start grinding/cutting parts of the mill base for clearance. Also, seems like Ind. Hobbies is using 5/8" or 3/4" dia and their mill is larger than mine (though it seems to be the same motor/head). Tormach uses 3/4" dia. and is pretty much the same size/hp as my mill.
Assuming I'm still on the right track here, what else do I need to know? Ballscrews/nuts still have some backlash, but I see some people change the balls to get rid of the backlash. Is that something I need to do? Or can I get them with zero backlash already. I've not seen backlash specs from any source yet.
Also, I see single-nuts, double nuts and pre-loaded nuts, but not sure what this is. Single-nut seems straightforward, double-nut seems like 2 nuts that should be tensioned against each other, but I don't see any mechanism for tensioning. Or is that internal somewhere? Also what is pre-loaded? Does that mean that the balls are pre-loaded into the nut? Or that there is some pre-load tension on the ballnut mechanism?
Right now, I am looking for what direction to go, and I'll get into the specifics later, such as what type/size/load of bearings, end-blocks, etc I'll need.
Thanks,
-Neil.