Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?


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Thread: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

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    Default Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    Why can't I just mount stepper motors on the manual handwheels to do a CNC conversion?

    The context of the question is that I used to do milling, 90% CNC and 10% manual, at the Tech Shop workshop, but they filed Chapter 7, so now I need something for my small hobby robot parts. This is 90% 6061 T6 and 10% Delrin and Nylon 6/6 work.
    I have about 36"x24" floor space in the garage that I may press the wife into letting me use, so no used Bridgeport for me.
    I'd love a 2 kW / 10000 rpm spindle (there's two-phase in the garage for driers and the car charger,) but that isn't to be -- and honestly, I have the patience and limited travel needs that the 3990 would be sufficient, and cost is a concern.
    I like the idea of speed controlled direct drive motor (from LMS) rather than belt drive (from Grizzly) and the 2800 rpm of LMS is slightly higher than 2200 of Grizzly.
    Also, if I had my druthers, I'd run LinuxCNC. I used Mach 3 and the PathPilot (Tormach skinned LinuxCNC) before, and preferred the latter. But that won't make or break getting a machine into the garage.

    I have significant robotics and software experience, and enough machining to make finished parts. While I could probably build the entire CNC conversion myself, I'd much prefer more-finished kits because I'd rather spend my time building robots than building robots to help me build robots.
    So, the range of options seems to be:
    1) Buy manual LMS 3990 for $800, and over time, convert to CNC. Hence: Why not just turn the handwheels with steppers?
    2) Buy the ready-made CNC G0704 kit from Automation Direct for $4k.

    That's a long way of asking the short question that was actually in the subject line :-)



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    Ball screws aren't strictly necessary for CNC conversion, but they're one of those things you'll wish you did if you didn't. There are 3 big issues with the typical acme lead screws in these Chinese machines when CNCing. First, they're slow. The 16TPI screws might be able to do 60 IPM if you directly couple them to a stepper, but that would require spinning them at 1000 RPM and they might whip at those speeds. Cheap ball screws are usually 5mm pitch (about 5TPI.) If you can get that same 1000 RPM you can get 200 IPM rapids (you would need pretty big steppers to be able to do that though.) Second, they're inefficient. Depends upon a lot of variables, but acme screws are typically between 10-50% efficient. Ball screws are typically 85-90% efficient. So a much smaller stepper will do a lot more with a ball screw. Third is backlash. This is the big one. Acme screws almost always have some backlash, as eliminating it makes their efficiency even worse. Plus, the lead screws typically have very basic bearings that have even more backlash. I had an earlier version of the LMS mini-mill and I remember the Y axis didn't even have a ball bearing, it was just a bushing. That much backlash is tough for backlash compensation to deal with, and it makes climb milling impossible.
    Only way I'd recommend using the stock screws in a CNC conversion is if it's an interim conversion so you'll be able to make the hardware to convert to a proper ball screw system later. Also the Z axis on those is terrible for CNC. Sloppy worm gear and rack and pinion.



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    Thanks @skrubol -- that's quite illuminating.
    The question then becomes -- backlash and rigidity is also a problem when manual milling.
    Why isn't there a version of these mills that already use the ball screws? Then I wouldn't have to pay for the ACME lead screws, and I can imagine that factory building with good screws would be significantly cheaper than the onsies-and-twosies after markets.

    Anyway, this is good info, perfectly answers my question!



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    There are usually workarounds in manual machining. You are usually only using one axis at a time, so you can usually lock the gibs on a manual machine on the axes you aren't using, then conventional mill, and backlash becomes a non-issue.



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    Yes, but I've found that actually locking the gibs is a real hassle.
    I once used a Jet mill where I had to literally walk around the table to get at the Z height locking handles.
    The quill got more extension that it should have on that one, I'm afraid to say ...



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    I had a LMS 3900 and did a cnc conversion from scratch and it was a fairly simple process. It was a good machine to learn to do a CNC conversion on and you can build the parts with the mill manually before tearing it down for the conversion. Considering the low cost of the import ball screws/kits on ebay and elsewhere it's simply not worth not making the upgrade. The imported balls screws and nuts provide pretty good accuracy. Much much better (tons less backlash) than the original acme screws. Backlash isn't a huge issue when manually milling, but it is when CNC controlled. I later moved up to a G0704 and CNC converted it. I'd suggest getting at least a similar size mill. It would fit in your space and the cost is not that much more while the capabilities differences are huge. There are lots of screw kits (automation technologies, ebay) and tons of conversion threads on the forum and elsewhere for these mills. There are also kits with everything including motor/bearing mounts and electronics etc if you don't want to mess with making your own. Some setups leave the existing manual handles. Some don't, but using dual shaft steppers and putting handles on the outside stepper shafts allow manual milling. As another alternative, getting a pendant (like the ones Vista CNC make) allow you to control any axis with continuous or step control at any speed or step increment. You'll find you can control a cnc mill "manually" with the pendant for those tasks you don't need to actually cnc, easily and with perfect control with a fraction of the effort of manual hand wheels. Good luck!



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    Thanks, I have learned a lot!

    Looking at the SX2 and SX2.7 mills from LMS (3990 and 5500) it ends up being about $3k to get started manually on them (even though the initial price is $900/$1500) because of all the extra struff I need (tool holders, vise, DRO, shipping charges, etc.)
    Then another $2k for the CNC conversion (drivers, steppers, power, screws, control computer, and so forth.)
    I can maybe shave a bit from that, but the end result is $3k to get started, and $5k all in. At that point I have a very small mill with a 2000 rpm spindle. I'd also need to figure out cooling -- probably some mist system. Or just stand there with a spray can of WD-40...
    I could also get the pre-converted G0704 at $4k from automation technologies, the benefit is that I get a ready-made mill. I'd still need to add shipping, and vise+collets+etc. And cooling is still something to work on. Call it at least $5k, if not more.

    I was pretty set to get at the $3k option just to get something that I can make chips with, and then upgrade over time, or perhaps replace it when I need to go CNC for real.
    But, thinking really hard about it, and digging deep, I think the right choice for me is a Tormach PCNC 440. It's a 10000 rpm spindle, and has a very nice table/motion/controller setup.
    It also has a nice full enclosure so I don't need to worry too much about spraying coolant around the area. (However, the enclosure -- five sides of 16 gauge and polycarbonate AFAICT -- is $1.5k! When the entire mill, with steppers and screws and spinde and all that, is $5k! My guess is they don't make many of the enclosures, and it thus has a hand-made premium price.)

    So, I'm getting a PCNC 440. Wish me luck :-)



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    I don't think you'll regret going with the pre-built machine (unless you really wanted the experience of building up a CNC machine.) Tormach owners seem to be pretty happy in general.
    The stand/enclosure is definitely something you can probably do yourself for cheaper and possibly better. There are tons of basic plans and videos on building enclosures for them, and I think their stands are almost all sheet metal. If you buy from Tormach, the stand is $595, Chip pan is $495, enclosure is $1495. More than half the price of the machine altogether.
    Are you sure the 440 is going to be enough machine for you though? 10x6.25" travels is pretty small, but I guess you do have a specific application for it.



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    I'm comparing to a LMS 3990 which doesn't have more travel. The 5500 has a little more X travel, but my parts are typically 4"x2" at most.
    My main constraint other than money is floor space. I have 48" x 36" total, and the 440 is 40" x 32" plus change.
    The 3990 would certainly have been an even better fit, but that 10000 rpm spindle and enclosure for flood cooling sealed the deal for me ...

    I'm wondering whether I could build an enclosure out of four pieces of polycarbonate that I rip on the table saw? And some angle profile to put it together, and some magnets to make the front easily removable.
    Anyway, that's for a different forum -- I learned what I needed about the LMS mills and the differences between ACME and ball screws, so thanks all!



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    Default Re: Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

    I'm sure you'll love it. From what I've seen on youtube etc. if your parts fit it, the 440 is a great machine. Another thing about the Tormachs is they're heavier (stiffer) than the typical Chinese machines of similar size. The 'typical' weight (maybe includes stand/enclosure?) of the 440 is 450 lbs. Shipping weight of the 5500 is 262 lbs.



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Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?

Why do SX2 / LMS 3990 mills require ballscrews for CNC conversion?