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Thread: Big Bamboo - New Machine Project Started

  1. #61
    Member JerryBurks's Avatar
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    Quote Originally Posted by RomanLini View Post
    .... Epoxying the shafts in is unusual....
    Well, this is my first home-built CNC machine, so I am just trying some stuff. Actually, while the rails are set in epoxy to create a conforming mounting surface, I had put some silicone grease on so that the epoxy does not stick and I can remove them if necessary (I hope....)

    Quote Originally Posted by RomanLini View Post
    When you say 5/1000 error is that 5 thou ie 0.005"? That's generally 5 divisions on the dial gauge (assuming a typical dial gauge of 0.001" divisions)?
    Yes, that is what I meant. I will probably map the x-y position error with a microscope across the table surface when the machine is complete. Then I could program a separate postprocessor for the G-Code to transform the coordinates and eliminate the repeatable errors. One of the projects I am planning to use this machine for is plastic gears that need to be pretty precise.

    Last edited by JerryBurks; 10-07-2011 at 12:45 PM.


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    Quote Originally Posted by louieatienza View Post
    I have the same ballscrew assemblies as you, but decided to have the ends machined down. In hindsight, this is the more cost-efficient method; so I'm very interested to see how having the "fixed" end on the opposite side works.....
    I hope this picture explains the concept. Since I don't have real bearing blocks, the bearing pair is pretty much free to self-center when the Y-plate is moved close. They have about 1/10" latitude to move to either side. If the friction turns out not to be sufficient to hold them in place while operating on the other side, I can put a bead of silicone around the outer race. I know, also a bit unconventional and I don't expect all my ideas to work perfectly



    Last edited by JerryBurks; 10-07-2011 at 12:54 PM.


  3. #63
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    Nice diagram! It's looking real sturdy.

    Re the z axis, have you considered lowering the bottom 4 bearing trucks down as low as possible, to near the bottom of the structure? It looks like you could make the 3 lower leadscrew bearing plates smaller and then extend the 2 side plates (that mount the trucks) fully down to the bottom. I think that would improve Z axis geometry a lot without any real costs.



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    Quote Originally Posted by RomanLini View Post
    ..........Re the z axis, have you considered lowering the bottom 4 bearing trucks down as low as possible, to near the bottom of the structure? It looks like you could make the 3 lower leadscrew bearing plates smaller and then extend the 2 side plates (that mount the trucks) fully down to the bottom. I think that would improve Z axis geometry a lot without any real costs.
    I played with this layout for hours on the CAD. Not sure if I got it right but the reason the bearing blocks are rather high is that I want to utilize the 8" gantry clearance fully (I am planning to machine some 3"-4" tall pieces). That means to allow for the spindle and bit to protrude a few inches below the Z-Plate, the lower edge of the Z-Plate (including the rails) must be able to pull up well above the gantry lower edge. But maybe there is another clever way of doing this?

    Anyway, even in the current configuration the bearings blocks are spaced a generous 10" apart vertically (since the rails are 18.5" long) and I hope this will do. I guess this is also more than the usual spacing for routers made from aluminum extrusion.



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    Member JerryBurks's Avatar
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    Next step....Y-plate attached with linear bearing blocks.

    One thing that worries me is the friction/stiction in the linear bearings. It takes 3.5 pounds force to get the x-axis moving (4 linear bearings) and 4.5 pounds to move the y-axis (6 linear bearings). I am wondering if that is because of stresses in the gantry or if that is just a matter of the wiper sealing lips of the bearings rubbing on the shafts.



    Last edited by JerryBurks; 10-08-2011 at 11:00 PM.


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    How much force was required before you coupled the two rails? If it increased, then your rails are not 100% parallel. If it didn't, something seems to be binding/rubbing within the blocks/wipers. Everything is easy until you start to constrain the motion to be true.



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    Quote Originally Posted by PaulRowntree View Post
    How much force was required before you coupled the two rails? If it increased, then your rails are not 100% parallel. If it didn't, something seems to be binding/rubbing within the blocks/wipers. Everything is easy until you start to constrain the motion to be true.
    I did not test that but since you mentioned, I took the remaining 6 linear bearing blocks for the Z-axis and lined them up on a rail. It takes 1.8 pounds to get them moving as a train. Obviously less but not radically so and the other bearings have to carry some weight already. So I guess I should be alright.

    However, when I took these last 6 bearings out of their boxes (I bought them separately from the others, but all from Glacern.com) I noticed they do not have the pre-load grub screw that the others had. Not sure what to think about that. Maybe because it takes only a small amount of preload to make them lock up and it does not really need it?

    JB



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    Quote Originally Posted by JerryBurks View Post
    ...
    Anyway, even in the current configuration the bearings blocks are spaced a generous 10" apart vertically (since the rails are 18.5" long) and I hope this will do. I guess this is also more than the usual spacing for routers made from aluminum extrusion.
    Yeah 10" apart is pretty generous and should work well compared to many builds. I'm just a bit anal I guess about getting the bearing points as close as possible to the point of cutting force (to reduce "bad" leverage). Please don't take my nitpicking as being criticizing of your design, your design is actually very impressive which is probably why I'm watching it with an eagle eye for anything that might lead to any small further improvement.

    As for 3 or 4 pounds of force to move those assemblies, in my opinion (for what it's worth) that would be close to perfect.



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    Quote Originally Posted by RomanLini View Post
    .......Please don't take my nitpicking as being criticizing of your design......
    Not at all.....besides just sharing success and failure the major reason to post here is to get recommendations and (while betting on a few new ideas) to learn from other's experience. I have already made several changes based on feedback I got here and I definitely don't want to come over as the smarta$$ who believes he knows everything.

    JB



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    Installed one of the lead screws on the Y-axis for test purposes and it appears to work quite well. The idea of using belleville spring washers to preload the lead screw bearings was not so great: the load is way too high, causing a lot of friction. But a simple solid washer works fine, the pre-load nut just must be tightened very carefully.

    I repeated the torque test for the stepper motor with a serious flywheel attached (6.3 lbs lead casting) and the difference was stunning. If I run into problems with resonance stalling later I am sure such a flywheel will cure that. The effect on the achievable acceleration should be moderate, I estimate a 20% reduction. I could not run the test faster than the equivalent of 250ipm because the unbalanced flywheel was shaking the setup in pieces.

    Makes for a nice hand wheel to move the lead screw, too....





    Last edited by JerryBurks; 10-10-2011 at 11:15 AM.


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    That's definitely the biggest flywheel I have seen on a stepper!




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    True it up on a lathe and then balance it on a suitable balance stand by drilling shallow holes on the heavy side until it balances "neutral". That should cure the shaking.

    It seems to me that the maximum accel number in Mach3 will be limited somewhat by the heavy weight, but other things like a heavy gantry may affect that even more. The combination could make the machine sluggish even with this much torque.

    CarveOne

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  13. #73
    Member JerryBurks's Avatar
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    That was a very unscientific experiment just for purposes of torque testing according to the "more is better" principle . I am not sure if that much mass would be needed and on the machine I really hope I don't need it at all. Some of the behavior may also be related to the 2 motors coupled together.

    I had a closer look and according to the CAD software the moment of inertia of my monster flywheel should be 15 lbs-in^2. Given the 15mm lead screw pitch that would be the equivalent of another 160 pounds of mass on the gantry. That may be indeed a little too much; because the gantry itself should be only 80-100 pounds.

    I will just wait until everything is together and see if the motors still have resonance issues and then add flywheel mass and maybe some damper rubber as needed.



  14. #74
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    If you use Gecko drives you probably won't need to worry about resonance and dampers. Well worth the price.

    CarveOne

    CarveOne
    http://www.carveonecncwoodcraft.com


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    Quote Originally Posted by CarveOne View Post
    If you use Gecko drives you probably won't need to worry about resonance and dampers. Well worth the price.

    CarveOne
    Now you tell me

    No kidding, I bought the drives with the motors as a package and did not know that Gecko even existed. Shame on me for not doing more research but I am an enormously impatient person.

    Talking about impatient...I could not stand it anymore and installed the Y-motor today just to see something moving. Lot of fun and indeed it moves!

    Good news:
    - my Kuroda lead screws are essentially backlash-free (as advertised). I can see movements smaller than 1/1000" back and forth on the dial indicator when applying single steps.

    - the Gantry is enormously rigid as I had hoped. Applying 60 pounds of load to various points of the Y-plate makes the dial indicator budge only 2-3/1000". That is about 40 times stiffer than the machine I am using currently.

    Bad news (kind of):
    I can not get beyond 100 ipm before the stepper stalls and it gets already very weak beyond 80 ipm. For some reason the monster flywheel that made a big difference on the torque test rig not help on the machine; actually makes it worse. These big NEMA34 steppers are also much noisier than the NEMA23 on my old machine.

    Not that I am planning to machine often with speeds beyond 80 ipm but it pi$$es me off and transversing at high speed makes some sense. I would not mind buying one of the Gecko drives but I would be ticked off even more if it does not improve it. Anybody has a loaner?



    Last edited by JerryBurks; 10-11-2011 at 01:01 AM.


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    It sounds like you are back to similar issues you described in post #41?

    Are you still running with 1/8th microsteps? 1/16th microsteps will produce a lot less resonance. You said before your drivers will run quite fine microsteps?

    Also I would suggest a much smaller flywheel, even one of those rubber hockey pucks with a hole drilled in it and pressed onto the stepper shaft.



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    Quote Originally Posted by RomanLini View Post
    It sounds like you are back to similar issues you described in post #41?

    Are you still running with 1/8th microsteps? 1/16th microsteps will produce a lot less resonance. You said before your drivers will run quite fine microsteps?

    Also I would suggest a much smaller flywheel, even one of those rubber hockey pucks with a hole drilled in it and pressed onto the stepper shaft.
    No, not really. I am not losing steps until it stalls (as far as I can tell) and I tried both 1/8 and 1/16 microsteps. Does not make a big difference although the 1/16 may be a tad smoother when running really slow (e.g. <10 ipm).

    I will try the hockey puck and if that does not help probably buy one of the Geckos. I could also increase the DC from 48 to 80V but I guess that will only be an incremental improvement.



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    There are two major features of the Gecko drives that others don't have (as far as I know. Some of the newer drives may have caught up by now if patents haven't prevented it). A very effective resonance reduction circuit, and automatic morphing of the step pulses per revolution. The very popular G540 and G203V drives both have this.

    All of my G203v drives are in use, and my G540 is on the way to Gecko to fix a self inflicted blown fuse or something else I may have done while tuning the X axis potentiometer.

    CarveOne

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    You might try adding weight to your carriage, similar to what it would be fully completed with router, and see if the motors still stall.



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    Quote Originally Posted by louieatienza View Post
    You might try adding weight to your carriage, similar to what it would be fully completed with router, and see if the motors still stall.
    Good point, I will try that. The total y-move weight is supposed to be 55 pounds when loaded with the spindle and that may make a difference.



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