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  1. #101
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    I think that once you get longer than about 5-6 ft, you're better off going with servos when using ballscrews. Servos have a couple advantages when using high lead ballscrews.
    One, their high rpm allows you to use a belt reduction, which minimizes the affect that the much higher inertia of the screw will have on acceleration.
    And you also get much higher resolutions, which you lose when using steppers with high leads screws.
    I would think that a 2525 ballscrew would be fine up to about 6ft and 1000rpm (1000ipm), but I'd probably want to use either a spinning nut, or a larger screw for an 8ft machine. Remember, for an 8ft machine, you'd typically need at least as 9'-10' screw.
    Unfortunately, 2525 seems to be the largest commonly available chinese screws. Anything larger, and the price tends to go up by about 5x.

    Gerry

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    Quote Originally Posted by dmalicky View Post
    The max deflection (for simply supported at each end) is (5*w*L^4) / (384*E*I).
    - w w =
    A Ball screw is not a simply supported beam at both ends so I don't think it's the right equation.
    > I think it's [W*L^4/(384 EI)]: 5 times less than the above prediction.

    A ball screw is more clamped at both ends or maybe in some cases clamped at one end (fixed bearing) and a hybrid at the other end (floating bearing). Nonetheless there are implied moments due to the bearing blocks which will reduce the deflection vs a simply supported case.

    Last edited by MI370; 02-01-2014 at 03:36 PM.


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    Glad it's useful! Yes, a ballscrew on the Y has a lot of advantages over a rack: linearity, repeatability, packaging, easy of installation (unless the rack is spring loaded), longevity (pinions wear, esp with spring loading), backlash (unless rack is very accurately placed or spring loaded).

    Yes, we did direct drive with a 2020. Ours is from the ebay site of Bob LaLonde's (he's on cnczone)--he had a bunch of Taiwanese Gten ~zero-backlash 2020s for a reasonable price; he may still have them. Depending on accuracy needs, Gerry's point on stepper resolution is important for high lead screws. 20mm lead results in a ~0.002" half-step size. 50mm lead would be 0.005". (Higher microstepping is usually done, but the steps aren't as evenly spaced). For ease of use, we tried to ~match the X and Y step sizes. With 4.2:1 and a 20T pinion, a half-step moves the X ~0.0019".

    MI370, yes, that's correct, take away the '5' for perfectly fixed-fixed end conditions. My calc was a worst-case, since I wasn't sure how rigid the BK bearings actually are (and I was sleepy). From what I have seen, the usual config is BK-BF (fixed-simple), but BK-BK is also done. For fixed-simple, the 5/384 or 1/384 changes to 2/384. But even a preloaded double back-to-back AC BK is not 'rigid'. Nook shows End Fixity 'D' is for "both ends mounted with a double preloaded angular contact bearing spaced apart by least 1.5 times the diameter of the mounting journal", or the catalog shows a quad AC on each end. So 'D' is probably as close to 'fixed' as is possible, and would result in acceptable deflection.

    I wonder how rigid the low $ BK and BF bearings are. The few BKs I've seen are just 2 600Xs next to each other: a big step down from a Nook BK. A BF with a single 600X normal clearance bearing would be simply supported initially, but would offer an increasing moment if angular deflection were high enough (seems unlikely a ballscrew would get there).

    Rereading the pros of ballscrews, I took another look at if or how to make a 2525 work at 8'. Nook's 'length' is between the supports, so for 8' travel and a nut, about 100". (The ballnut-to-farthest-support distance is probably more accurate, and you might subtract off whatever distance is needed to accelerate to speed.) If you used preloaded back-back ACs on each end, you get end fixity C. Plugging in 21.7mm, 2540mm, and 'C' gives 600 rpm, 600 ipm. Or if you separate the bearings by at least 30mm, that's end fixity D, giving 900rpm. That's about as fast as the Nema34 will go anyway.

    Another issue with a long screw is the column strength; Nook has that calculator, too, and plugging in 21.7, 2440, and 'C'.... Nook says "You have exceded the Slender Ratio for this Diameter." Their max safe length for 'C' is about 1550mm (61"). For 'D' it's about 2150mm (85"). Then I checked our 2020 screw--it fails the calculator but runs fine. So their slenderness check is likely pretty conservative; it's hard to say how the risk ramps up.

    If the column buckling issue can be resolved, and some custom bearing blocks were made to reproduce fixity 'D', a 2525 for 8' may be reasonable.

    David Malicky


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    More info on column buckling. Nook's slenderness limit appears to 100:1, as it's consistent with this article: Fixed-end, supported-end, free-end ball-screw support conditions | Machine Design "A length-to-diameter ratio (L/D) exceeding 100:1 requires special design consideration; consult the manufacturer regardless of anticipated loads or fixity."

    But, Thomson's own ballscrew engineering pdf shows no 100:1 limit, nor does THK mention it.
    page 86: http://www.thomsonbsa.com/pdf/bsa_en...og_section.pdf
    https://tech.thk.com/en/products/pdf/en_a15_030.pdf
    Thomson's chart for a 100" 2525 screw (fixed-fixed) shows a limit of about 1000 lb; they don't mention what safety factor is accounted for. THK's equation calcs to 7kN (1500 lb), including a safety factor of 2. Either are plenty for our needs.

    As for the 100:1 limit, I'm not sure what to make of it. A 2525 x 100" would be about 117, so only a little more, and "100" was almost certainly an arbitrary choice. My guess is they had some bad experiences and want engineering to look at the longer ones.

    On end conditions, Thomson has a nice figure indicating the rigidity of BF, BK, and the spaced-BK types:
    CNC Router for Hardwoods: Evaluation and Questions-ball-screw-end-mounted-jpg

    I can think of three potential disads of fixed-fixed:
    - Mounts are harder to make or more $.
    - A tensioned screw is usually needed, so it doesn't buckle when it heats up (or if mounted to an aluminum frame, differential thermal expansion could make it buckle). But, no tension is needed if the far mount used a pair of needle bearings to do the angular fixing (or make a sleeve mount for ball bearings that slides axially). Tension only has a minor benefit to critical speed, so I'd rather use needles and avoid the whole issue. Or, if the 117:1 ratio is a concern, a tensioned screw should alleviate that.
    - The alignment of that 2nd fixed mount is much more important, as it needs to be very co-linear with the screw. An easy way to do that is to make it a face-mount (like FK) on a plate; then drive the gantry car over to it and tighten the face bolts.

    So, with 2 mounts using spaced bearings, an 8' 2525 is looking pretty reasonable to me.

    David Malicky


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    I think the big problem with using a ballscrew on the long axis would be racking or skewing of the axis. A lot would depend on how far apart you spread the bearing blocks, or even which bearing blocks/rails you choose to use. I don't know if FK/FF supports would work easily because they would have to rely on your two end plates being perfectly coplanar to each other, othewise when you tighten the bolts you'll pull the block out of alignment; and getting them aligned over about 10" distance is no easy task. A stiff transverse plate or beam connecting both gantry uprights could help keep the bearing blocks in the same relationship to each other which could minimize skewing. the other way to prevent skewing would be to use two screws, run by two motors, one slaved to the other. Or you can run two screws with one motor via timing belt. This of course increases the cost and complicates the build slightly. I feel if you want to do any serious work with your machine (and hardwoods require as ridgid a machine as possible) you might want to seriously consider driving both sides of the gantry, rather than driving the gantry from the center.

    The reality is, it all depends on the timeframe you have for your machine. Sometimes, a "canned" or other proven solution is better than spending time on "discovery." You can use Nook's or Thomson's calculators all you want, but it's only a guideline as you may not be using their screws. The China ballscrews are generally fine, but I've seen mixed reviews on their support units. I bought a set of three that I wouldn't use on a machine, and don't want to spend the time to fix.



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    Yes, definitely a ballscrew on each side--sorry that was not clear before. That's a good point on the FK blocks needing parallel surfaces at each ends of the X. One idea is to use a ~4.5' long square or rect tube mounted transverse across the lower frame: one at X=0', and another at X=8'. If the tubes are straight, faces are flat, and the corner diagonals are measured equal, the mounting faces should be parallel. Then there is the JBW conforming shim method.

    And a good point on canned vs new/developed. I lean to the latter for the fun, performance, and often less $, but it does take more time, often a lot more.

    David Malicky


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    Quote Originally Posted by dmalicky View Post
    Rereading the pros of ballscrews, I took another look at if or how to make a 2525 work at 8'. Nook's 'length' is between the supports, so for 8' travel and a nut, about 100". (The ballnut-to-farthest-support distance is probably more accurate, and you might subtract off whatever distance is needed to accelerate to speed.) If you used preloaded back-back ACs on each end, you get end fixity C. Plugging in 21.7mm, 2540mm, and 'C' gives 600 rpm, 600 ipm. Or if you separate the bearings by at least 30mm, that's end fixity D, giving 900rpm. That's about as fast as the Nema34 will go anyway.

    Another issue with a long screw is the column strength; Nook has that calculator, too, and plugging in 21.7, 2440, and 'C'.... Nook says "You have exceded the Slender Ratio for this Diameter." Their max safe length for 'C' is about 1550mm (61"). For 'D' it's about 2150mm (85"). Then I checked our 2020 screw--it fails the calculator but runs fine. So their slenderness check is likely pretty conservative; it's hard to say how the risk ramps up.

    If the column buckling issue can be resolved, and some custom bearing blocks were made to reproduce fixity 'D', a 2525 for 8' may be reasonable.

    I have seen this or similar methods used with great success. This addresses both whip and slender ratio concerns.



    I agree with “The ballnut-to-farthest-support distance is probably more accurate”

    Worst case with this method, ballnut at either end limit while simply supported at screw midpoint.

    Disadvantages – 1) Introduces drag due to friction. 2) Introduces heat to screw and as a result thermal expansion. 3) Wear due to friction.

    1) As long as motors have been properly sized, Load to Motor Inertia Ratio of around 10, then drag would be negligible.
    2) Should be insignificant, given the intended use. I don’t think he needs or will get aerospace tolerances with an 8’ router.
    3) Provided the cradle in made of delrin or some other self-lubricating plastic, shouldn’t be an issue. I would be more concerned with repeatedly fastening material for smaller sized jobs to the same location on a large table. This will result in the same small length of the screw being used repeatedly. Better to move the jobs around so that the screw does wear more evenly.

    Good Luck with your build



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    Our router with 14' screw and spinning nut uses something similar, but more to prevent sag than whipping.

    Gerry

    UCCNC 2017 Screenset
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    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Thanks for the video MeAgain. That is a great solution for sag/whip. I wanted to share some progress on one of my alternative solutions that I have been working on while trying to understand the whole ballscrew/whip/sag issue.

    The link that dmalicky posted to the machine with the vertical gearbox/stepper assembles inspired me to work on that solution. Here is what I have come up with:

    I have moved the rack under the hiwin rail. This solves a few problems:
    The cost of an integrated solution like that from Atlanta Drives
    The fact that I am going to level the 3" x 6" tube with epoxy and the plate can be used to spread out the contact for a more stable situation than if the 20mm rail was mounted directly to the epoxy
    I am able to make the machine narrow enough for my shop!

    CNC Router for Hardwoods: Evaluation and Questions-left-jpg

    This side of the machine is a bit tighter. I wanted to see what the minimum size for the rack looked like. The result is that the motor assemble tucks further in by about an inch. I was thinking of getting a quote from moore gear on a custom rack with holes drilled in it to match the rail. This does not do as much to spread the load out on the epoxy which is a drawback.

    CNC Router for Hardwoods: Evaluation and Questions-right-jpg

    I have just started to work on the base so it is not rigidized as of yet.
    CNC Router for Hardwoods: Evaluation and Questions-iso-jpg

    I need to consider how to tension the pinion

    CNC Router for Hardwoods: Evaluation and Questions-side-jpg



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    MeAgain, thanks for that video! I love the simple solutions and that is a slick one. And far easier than making special bearing blocks, spacing/preloading ACs, etc. I agree the disads are tiny, esp if delrin or uhmwpe is used.

    Sapele, nice revisions. I like the first design more than the second--the width difference is quite small, and I'd be concerned about mounting something like a profile rail to 2 components -- it will want a stable foundation. The one flexy spot I see is the relatively long shaft supporting the pinion gear. FEA shows that kind of cantilever is very flexible. I would lower the gearbox (and maybe raise the rack a bit), so that the pinion can be attached within 1/8" of the gearbox bearing (i.e., < 1/8" of the 1/2" dia shaft is exposed).

    David Malicky


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    I'm a little confused by your z axis (3rd pic). You have a lot of bit stick out (from the collet) and still are pretty far from the table with the spindle in the full down position. Someone mentioned you may want to be able to touch the table with a 1" bit stick out and clear the gantry with a 2-3" bit stick out.

    or are the rails mounted to the spindle side?



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    or are the rails mounted to the spindle side?
    MI370: The rails are mounted to the spindle and the blocks are mounted to the Y-carriage. That picture is confusing, sorry. I move the carriage over to one side because slight changes in the position of the x-carriage effects clearance for the spindle plate. I can touch the collet to the spoil board but this system will require an adjustable mount to clear the gantry in the way that you describe.

    I'd be concerned about mounting something like a profile rail to 2 components
    Dmalicky: this is also unclear. Sorry. I used the two components to quickly see what size a single custom ordered rack with pre drilled holes would look like. Moore gear offers custom cross sections and CNC drilling. If the price is not outrageous it would probably make for a quicker and more accurate build?

    I will try again with the gear motor mount with the < 1/8" goal in mind. I got sloppy because the keiling gearbox (I am using it because they have a drawing available online) had a 2" long shaft.



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    Back to gantry beams:

    What about tube with bracing cast in?
    I'm thinking of 200x200 steel tube (6mm thick weighs 36kg a metre) with epoxy granite. It would be quite easy to mould so the casting fits the internal shape desired.
    Attached is an example (the attached example is a concept beam for a lathe, the bracing would be better with voids cast at 45º for a router.)

    On the other hand, perhaps we don't need bracing at all with a steel beam like this.
    Based on figures from http://www.steelweb.info/200x200x6.htm and 8020.com, 200x200x6mm steel has 3 times the moment of inertia in the vertical plane, and 10 times in the horizontal plane compared to a 3060 beam from 8020. (Does weight 3x a much though).
    In Australia aluminium extrusion costs ~$18/kg, where as steel costs ~$4 a kg.
    In the US looks like you guys get a much better deal on extrusion at about half the price.

    Attached Thumbnails Attached Thumbnails CNC Router for Hardwoods: Evaluation and Questions-steel-eg-beam-jpg  


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    Quote Originally Posted by pippin88 View Post
    Back to gantry beams:

    What about tube with bracing cast in?
    What do you mean by cast in. Do you intend to have a foundry pour you a beam? If so cast iron might be the material of the day. If you mean a baffle held in place by epoxy granite fill that is a different thing all together.
    I'm thinking of 200x200 steel tube (6mm thick weighs 36kg a metre) with epoxy granite. It would be quite easy to mould so the casting fits the internal shape desired.
    Still not following you with your use of the word casting. Do you intend to cast steel baffles for inside the tube?
    Attached is an example (the attached example is a concept beam for a lathe, the bracing would be better with voids cast at 45º for a router.)

    On the other hand, perhaps we don't need bracing at all with a steel beam like this.
    Based on figures from 200 x 200 x 6 Steel Data and 8020.com, 200x200x6mm steel has 3 times the moment of inertia in the vertical plane, and 10 times in the horizontal plane compared to a 3060 beam from 8020. (Does weight 3x a much though).
    In Australia aluminium extrusion costs ~$18/kg, where as steel costs ~$4 a kg.
    This is why I tend to mention steel often in these forums, it can be significantly cheaper to work with. That doesn't mean it is ideal. In any event I can get steel for anywhere from a buck a pound to a buck sixty as end cuts and drops. I haven't priced buying steel by the full stick lately. Don't forget the possibility of an aluminum tube extrusion instead of the fancy T-slot stuff.

    In any event adding a baffle inside a steel gantry beam does help some. For the little bit of extra time needed to add the reinforcement I would not skip it. The biggest issue with adding this sort of bracing is fastening it in place. If the gantry tube is long enough no approach is actually easy to install. Which makes me wonder if yo intend to simply hold the baffle plates in place with the epoxy granite fill.
    In the US looks like you guys get a much better deal on extrusion at about half the price.
    There is lots of competition with many different manufactures with varying designs and robustness. That does help pricing.



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    Wizard,

    I mean the epoxy granite is the brace. Epoxy granite is a mix of aggregate of different sizes mixed with epoxy and cast in to moulds.
    In this case the steel (or aluminum) beam forms a permanent mould, and some pvc pipes or similar are used to take up space, reducing the epoxy granite weight and volume. See picture I attached to previous post.



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    Wow! Completely missed that one.

    Quote Originally Posted by pippin88 View Post
    Wizard,

    I mean the epoxy granite is the brace. Epoxy granite is a mix of aggregate of different sizes mixed with epoxy and cast in to moulds.
    In this case the steel (or aluminum) beam forms a permanent mould, and some pvc pipes or similar are used to take up space, reducing the epoxy granite weight and volume. See picture I attached to previous post.
    When you started talking about casting the first thing I thought of was sand molds and aluminum. It went downhill from there.



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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    I am looking to build a 4'x4' machine. Primarily for woodworking but also for aluminum machining as well as plasma cutting. After reading this thread and others, it looks like I will be using mostly steel construction . The biggest set of questions are around the gantry. I am considering a 6x6x.25 steel tube. Adding the diagonal bracing could be tedious. However, what about drilling on the diagonal and placing 1/4-3/8" steel rod that is then welded at the ends to the tube? Alternating the direction, maybe every 6 inches? This would seem to be one of the easier ways to add the gussetting to minimize tube twisting.
    Any comments?



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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Diagonal bracing is probably not needed if only machining wood. For machining aluminum, the stiffer the better. Yes, the diagonal rod design does work pretty well for stabilizing the cross-section, and is probably easier than a diagonal sheet. I'd not recommend welding them in as that would distort the tube. Another way is to use threaded rod and tap the tube on one end (with locknut) and nuts/washers on both sides of the other end (not as pretty, though). Or epoxy, if a lot of bond area. No need to alternate directions (stiffness is within a few %). An unbraced 6x6x0.25 will probably have about 50% the stiffness of an 8x8.0.25; or about 50k lb/in (on track for a machine stiffness of 5k lb/in). Bracing would roughly double that. Keep in mind a gantry of this stiffness is only helpful if all the other components are 'equally' stiff (or stiffer). I.e., the most flexible component always dominates the deflection.

    Steel is great for stiffness but its weight will lead to slower feed rates, or more expensive steppers (which will more than offset the cost save vs alum). Steel does tend to be straighter than aluminum extrusion, but probably not straight enough for profile rail bolted straight to it. So leveling of some method is needed (probably not leveling epoxy as it is soft). There are a few good solutions depending on budget and available shops/tools. I recommend thinking through the leveling plan, as that will influence the gantry tube decision (e.g., steel is harder to machine than alum).

    David Malicky


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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    David
    Thank you. You insights are most helpful..
    The distortion covers one of my fears. I did not want to spend the time welding up the tube and ending up with something resembling an overgrown cheeto.
    Epoxy I know is too soft. Once you add fillers for rigidity, it will not flow for self leveling. I went through gallons of it building my boat.
    Milling the steel rail flat is a problem. The length is probably too much for a bridgeport with my skills..
    However what about a face layer applied to the tube (e.g. MDF) and then machined? Epoxy underneath would provide a stable substrate match the tube undulations. The MDF or other outer layer would be easily machined on a large jointer . I have been able to do 8' lengths for table top glue ups with wide planks on my ancient 16" joiner (<.005 gap between planks over 8' before clamping). Or is the MDF just too soft as well and will deform like the epoxy? This would provide a reasonable alternative to metal machining and most cities have large cabinet shops that could provide this service for very reasonable $$$.



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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Epoxy I know is too soft. Once you add fillers for rigidity, it will not flow for self leveling.
    There's a UK CNC forum that I read where steel frames with self leveling epoxy is the norm. It apparently works pretty well for the members there.

    Imo, MDF is much, much softer than epoxy.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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