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Thread: Big Bamboo - New Machine Project Started

  1. #21
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    No matter how true you make the machine, you should expect to do a minor surface skim of your waste board because it also may not be perfectly flat.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


  2. #22
    Member JerryBurks's Avatar
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    Good point. Maybe I should not worry about it too much and just check if still reasonable when the 80/20 slats are installed. Actually I don't even know how straight/flat those are (I have the triple-wide 1545 type).



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    I have a 6' known straight aluminum rule that I use for checking things no longer than that. If you see a small gap in the middle flip the rule to the other edge and the gap should be the same if the rule is straight. If you see a bow in the extrusion you will see a hump in the middle if you flip it to the other edge.

    For longer things I use a sewing thread taped tightly to one end and pull the thread taut from the other end. You will see a bow or hump this way also.

    I also use two sewing threads taped diagonally from corner to corner to check for twist in the frame. The threads should barely touch each other where they cross in the middle.

    Most of us make a best effort to align it properly within our means of measurement and then surface skim the spoil board. At that point you can attach a dial indicator to the router mount and measure the flatness over your working area.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


  4. #24
    Member JerryBurks's Avatar
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    Good progress today!

    - Mounted new bigger wheels to the cart with brakes and much easier to turn
    - Attached the 80/20 extrusions and x-axis rails (96 carriage bolts with washers and nuts installed from below )
    - Adjusted x-rails to be parallel and twist-free. Parallel required two 5-mil shims between the 80/20, twist free was already very good.

    So far I am quite happy. Minor mistakes and annoying problems but no show-stopper...looks like it may become a nice machine



    Parallel measurement contraption:


    Worst case rail parallel error 2/1000 due to a slight bow in the middle but I guess that is acceptable. Never mind the white socks ;-)


    Diagonal better than 20/1000" flat (estimated from 0.017" diameter fishing line):


    Last edited by JerryBurks; 09-04-2011 at 01:04 AM.


  5. #25
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    Looks like a very good start. The rails can be straightened by inserting thin shim stock between the rail and base at the proper points. How tight the mounting screws are torqued can also be used for adjusting very minor amounts.

    Now comes the most fun part (to me) of building up the gantry and Z axis assembly.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


  6. #26
    Member JerryBurks's Avatar
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    Quote Originally Posted by CarveOne View Post
    ....Now comes the most fun part (to me) of building up the gantry and Z axis assembly.....CarveOne
    I guess what you are saying is that it now gets really complicated

    Getting rid of the remaining 2/1000 parallel error would be nice but that is approaching the limits of my measuring capability/repeatibility.

    I also took out my old engineering text books and estimated the thermal (bi-metal) bending of a 20mm steel rod firmly attached to a 4.5" wide aluminum bar (accounting for the air channels in the 80/20 profile). Over a 10 degree C temperature span (which can easily happen in my shop) there might be a whopping 6/1000" bending in the middle under worst case conditions. I suppose practically it is much less because the steel rod may be sliding on the supporting extrusion but I will check the remaining error at different temperatures.



  7. #27
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    It's all been fun to me, but cutting and welding tubing is less fun than machining the parts. I wouldn't be happy if I had to make a living making the same things over and over. It's the novelty of doing something different each time that interests me more.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


  8. #28
    Member JerryBurks's Avatar
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    No practical work today, just too hot and muggy (my shop is in the garage, extending into the driveway...)

    But I did incorporate a few suggestions into the design today (torsion box gantry and z-axis thrust bearings) and made many more detail changes and corrections:

    Complete rendering (but without the cart)


    X-Plate and Z-axis from "inside"


    The Z-Axis ballscrew is completely enclosed. X and Y axis will get sliding curtains to keep the dust out. Well, at least that is a the idea.

    I got to cut a few more aluminum parts for bearing supports and stepper mounts and then then I can start the gantry structural pieces.



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    It looks nice and beefy!

    Does that bamboo plywood need lacquering to stop it absorbing moisture and warping etc?



  10. #30
    Member JerryBurks's Avatar
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    Quote Originally Posted by RomanLini View Post
    It looks nice and beefy!
    That is the idea. Given that the bamboo has only 20% of the stiffness of aluminum you must make an element almost twice as thick (due to 3rd power) to get the same bending resistance as aluminum (and more when using plywood where not all fibers go in the preferred direction). My goal was to achieve a setup at least as rigid as a typical aluminum machine of that size. The total weight of this machine will probably come in around 300-400 pounds.


    Quote Originally Posted by RomanLini View Post
    ...Does that bamboo plywood need lacquering to stop it absorbing moisture and warping etc?
    Yes, that is recommended for Bamboo. Apparently it is susceptible to moisture from the cut end-grain. Therefore I spray-coat all wood parts twice with oil-based polyurethane.

    On the other hand, we have a Bamboo plywood cutting board with built-in strainer sitting on one of the kitchen sinks for more than 5 years. It gets wet with water, fruit juice and other spills and washed off all the time. It is still flat and no signs of disintegration so I guess it can not be that bad with the humidity.

    Last edited by JerryBurks; 09-06-2011 at 08:15 PM.


  11. #31
    Member JerryBurks's Avatar
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    I found an interesting material comparison database ("MakeItFrom.com") here. Looks like my material choice is not totally stupid.

    Stiffness-to-weight ratio of the bamboo according to this is indeed identical to 6061 aluminum and understandably much better than MDF.

    Unfortunately it does not have data for other more common plywoods.



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    Quote Originally Posted by JerryBurks View Post
    I found an interesting material comparison database ("MakeItFrom.com") here. Looks like my material choice is not totally stupid.

    Stiffness-to-weight ratio of the bamboo according to this is indeed identical to 6061 aluminum and understandably much better than MDF.

    Unfortunately it does not have data for other more common plywoods.
    In my parent's home country (Philippinbes) as well as other Asian countries, we use bamboo for scaffolding as well as construction material.

    Although the stiffness-to-weight ratio maybe the same, the statistic is a bit deceiving, as obviously the higher density of aluminum means a bamboo part of comparable stiffness will have a way larger cross-section... though the damping qualities of bamboo are probably way better than of aluminum.



  13. #33
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    Quote Originally Posted by louieatienza View Post
    In my parent's home country (Philippinbes) as well as other Asian countries, we use bamboo for scaffolding as well as construction material.

    Although the stiffness-to-weight ratio maybe the same, the statistic is a bit deceiving, as obviously the higher density of aluminum means a bamboo part of comparable stiffness will have a way larger cross-section... though the damping qualities of bamboo are probably way better than of aluminum.
    Sure, larger cross-section is needed and that is why the gantry of my machine is rather bulky. But, the tensile modulus is only part of the story and there is an upside to that. Due to the 3rd power dependency of the "area moment of inertia" a solid bamboo sheet of 1.8 times the thickness of a solid aluminum sheet resists bending the same while weighing only 36% of the aluminum. To get to similar structural bending stiffness you would need to change from solid aluminum sheet to a sandwich or space frame structure.



  14. #34
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    Quote Originally Posted by JerryBurks View Post
    Sure, larger cross-section is needed and that is why the gantry of my machine is rather bulky. But, the tensile modulus is only part of the story and there is an upside to that. Due to the 3rd power dependency of the "area moment of inertia" a solid bamboo sheet of 1.8 times the thickness of a solid aluminum sheet resists bending the same while weighing only 36% of the aluminum. To get to similar structural bending stiffness you would need to change from solid aluminum sheet to a sandwich or space frame structure.
    Yes, that's true, but having more "mass" can be advantageous, since it would have more "inertia" so to speak (sorry the last time I held a physics textbook was almost 20 years ago!) which inevitably leads to smoother cuts. Of course the downside is that more powerful motors are needed to move the machine.



  15. #35
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    I spent the day making aluminum chips (not my favorite, as a woodworker). But some machine parts just got to be metal....

    Bearing and ballnut support plates and motor mounting brackets now complete:


    Heavy-duty coupler for 14mm stepper shafts to 15mm ball screw:




  16. #36
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    Nice work on the plates! Did you just drill the bores in the couplings?

    I hope they are still axially concentric when clamped, i had trouble with some couplings of that type which had clamps on opposite sides. When they were done up each side compressed in 0.001" or so, and being on opposite sides it misaligned the 2 shafts by double the per-side compression.

    The only couplings I will use now are bored to a very snug fit on the shaft and use a grub screw onto the flat of the shafts. With both grub screws lined up on the same side. Anyway I hope the couplings do work well enough, and you should still consider putting a grub screw onto the shaft flat as steppers will produce enormous short-term peak rotational loads.



  17. #37
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    Quote Originally Posted by RomanLini View Post
    Did you just drill the bores in the couplings? ..........
    Well, yes and no. The motor shafts are 14mm and I drilled it with a new 14mm bit all through on my wood lathe (type 2011 aluminum alloy rod). That means both sides should be reasonably concentric. For the ball screw I had then to ream out one side to 15mm. I did that also on the lathe but I am not sure if that will do the trick. If it is not good enough I will do it over and have somebody at work bore it out on a real lathe.

    I may add a grub screw for the motor side but for the lead screw side I want to use only clamp friction to avoid marring the open screw end. With three M6 bolts per coupler end I hope to achieve enough friction to hold up with my big steppers. I suppose the edges of the shaft flat and the screw thread grooves may dig into the aluminum surface and help increase the torque limit.

    I will have to see how the opposite side clamping action works. I did that mainly to avoid imbalance but you are right, it may have unwanted side effects.

    Oh, and drilling the holes into the plates almost drove me nuts. The spindle is way too fast for that and the bits always gummed up with that 6061 stuff. I ended up routing all the holes with a 1/8" end mill. If god had meant for man to do metal work he would have created metal trees

    JB

    Last edited by JerryBurks; 09-11-2011 at 12:56 PM.


  18. #38
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    Nice work on the metal! Those are some beefy couplers...

    As for the comment on metal: If God didn't intend us to build CNCs, we wouldn't have given us teh ability to work metal!



  19. #39
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    Quote Originally Posted by louieatienza View Post
    Nice work on the metal! Those are some beefy couplers...
    Thanks, I am quite happy with the plates but I suspect RomanLini has a point with the couplers. I had one spinning on the motor today and the runout is pretty bad. I probably should have tried one first before doing the whole set. Oh well.

    I purchased my steppers before stumbling on this forum and in my infinite naivite got some pretty fat NEMA34 motors with rather high inductance (20mH). Since I have only 48V supply I got concerned about the performance at speed. So I built a torque test stand today from plywood, string and rollers with a luggage scale. I used 2 coupled motors spinning synchronously (same pulse inputs) and can measure the force it takes to make the top stepper housing skip one step while the shafts are spinning at any desired speed. Works like a charm, here a picture:


    The results are encouraging, although I could not go beyond the equivalent of 80ipm feed rate due to the unloaded steppers getting in resonance and stalling out. I will try to re-run the test withe some kind of harmonic damper or flywheel but at least I know now the cutoff feed rate where the torque decline starts. I guess for my typical feed rates they still have ample torque to move the machine and after running for 40 minutes got only to 120 degrees F:


    Last edited by JerryBurks; 09-12-2011 at 12:02 AM.


  20. #40
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    Hmm, sorry to have been the bearer of bad news with the couplings.

    Great improvised setup for motor testing too! I like the pulley and fishing scale for measuring running torque.

    So that is your own chart? I guessed so as it is in IPM not motor revs. If I got the calcs right;
    10 IPM = 17 RPM = 0.28 revs/sec (RPS)?
    which means you topped out at 80 IPM = 2.25 RPS? That's not bad since you have a worst case resonant motor driving a resonant load and no mass or viscous damping of any type in there. It will be a lot better driving a leadscrew.

    Also don't get too "torque focused". At your 80 IPM the motor is producing 1/3 the torque but at 3x the speed, so it's still producing the same shaft power even at 80 IPM!

    20mH is not bad for Size34 motors, and your motors have rear shafts so you can put dampers on there, even your belt pulleys and belt will damp a lot of resonance.

    May I ask where you got the nice big motors and how much you paid?



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