CNC Router for Hardwoods: Evaluation and Questions - Page 2


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Thread: CNC Router for Hardwoods: Evaluation and Questions

  1. #13
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    Its been a long time since I took the easy way out of something! I now understand that my comparison did not reveal quite what I thought it had. I reworked the assembly as louieatienza suggested. I adjusted the position of the spindle collet in relation to the bottom of the z carriage, Moved the fixed blocks to the bottom of the Z yoke and increased their spacing a bit. Should I increase the spacing more?

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-33-jpg

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-34-jpg



  2. #14
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    I've found it beneficial on my machine to have the spindle nose a fair bit below the bottom of the Z plate - this makes it easier to clear clamps etc (as you only have to worry about the small sized collet mutt and tool, not a wide plate)

    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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    The second illustration above shows the problem with your idea of attaching all that hardware to the moving spindle. You've not only given it more weight to move, but you're dragging all that stuff down with the spindle, so that it's more likely to foul on any projecting parts of your work. If you leave all that attached to the Z-axis, and just attach the spindle to a plate with the blocks mounted to it, then just the spindle drops down, not the whole Z-axis assembly. So if you're carving a relief, like the Himalayas, for instance, the screw won't hang up on Mt. Everest while you're trying to carve the river valley below.

    Space the bearing blocks as widely as you can without compromising travel. And like someone else said, make a plate to mount your motor on that has a few different heights it can be fixed at, to accommodate longer tools or maximize rigidity with short ones.

    Andrew Werby
    www.computersculpture.com



  4. #16
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    I do see the difference between the two systems:

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-35-jpg

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-36-jpg

    They both have major ramifications on the design of the other components. I plan on trying to refine both approaches as a part of my CNC's evolution. For a start I will have to space the blocks with greater care on the second example. I will consider giving myself the benefit of adjustability on both choices.



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    Quote Originally Posted by Sapele View Post
    I do see the difference between the two systems:

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-35-jpg

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-36-jpg

    They both have major ramifications on the design of the other components. I plan on trying to refine both approaches as a part of my CNC's evolution. For a start I will have to space the blocks with greater care on the second example. I will consider giving myself the benefit of adjustability on both choices.
    Well let's consider the spindle fully down as the worst-case scenario. On the top configuration, there is more of a spread between bearing trucks. Also the rails themselves become part of the structure of the carriage. Note the lowest point the spindle is mounted to in both drawings. With the lower configuration, the relationship of the lower trucks to the collet (or workpiece for that matter) would always be in a worst-case scenario, regardless of spindle position. I would argue that this also increases the "lever" action against the gantry since the bearing blocks will be higher and farther from the center of the gantry beam.

    Again, if a majority of your cuts are as pictured in the second configuration, that might be fine, but then again if that was the case there would be absolutely no reason to have such a high Z clearance or travel.

    Bpth designs have merits and pitfalls, and there always will be tradeoffs between either designs. Just remember that just because a rail and block system has a max load rating, they still will deform under load (even preloaded rails) and there will always be a tradeoff between bearing truck spacing and ridgidity. Obviously the closer the bearing trucks, the more of a disadvantage they are mechanically.

    I also don't understand the fear many have about the weight of the carriage. The weight can be beneficial since there can be more resistance to vibration and deflection. Stepper motors are cheap enough that sizing up for the job won't break the bank. If you were making 1000 machines and were looking to save money MAYBE you'd consider a lighter and cheaper design.



  6. #18
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    I have been working on the Z with the fixed bearing trucks. I flipped the ball screw assembly around so that the screw, bearing blocks and stepper motor are now fixed. The rails and the ball nut are mounted to the Z carriage with the spindle. I dropped the spindles relationship to the Z carriage plate for more clearance as pippin88 suggested. The end result is just about 7" of travel from the bottom of a 2" cutter. I could reposition the spindle for further clearance via an adjustable mount.

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-37-jpg

    CNC Router for Hardwoods: Evaluation and Questions-jpg

    Happy New Year to all



  7. #19
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    That looks much better. Still not sure about wrapping the slide around the gantry. I think you would be better to leave it open as others have suggested.


    Steve



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    Thanks Steve.

    I like this configuration so I think it is time for me to consider the orientation of the X carriage trucks. I set them up this way so that the spindle would be a bit closer to the X beam. The bearings are also spaced a bit further apart than they would be if mounted to the front of the beam. Are these valid considerations? If not then moving the trucks to the front of the beam will allow the Y beam to get a bit lower and it will make mounting the E-chain easier.



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    I would not be worried about the distance to the spindle if you can get a wide stance on the trucks. The further they are spaced in both directions will give you more gains than the shorter distance to the spindle.


    Steve



  10. #22
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    The spacing of the trucks gets closer when they are moved to the face of the 8020 beam. I feel this is a negative. E-chain mounting gets easier which is a plus.

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-40-jpg

    There is less interference with the clearance under the gantry, it looks like I pick up a half an inch in clearance, another plus

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-38-jpg

    I am not sure how to score this? I feel the bearing spacing is a big issue



  11. #23
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    They are close, I overcame this limit by using 2 extrusions. Then the spacing can expand considerably. All of your cutting forces are getting transmitted through this area so extra support is always good.


    Steve



  12. #24
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    As Steve said, I think the single extrusion is the weak point of everything you're doing. It'll twist before the Z axis flexes.

    Gerry

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


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CNC Router for Hardwoods: Evaluation and Questions
CNC Router for Hardwoods: Evaluation and Questions