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Thread: Z Axis Conversion

  1. #1
    Community Moderator Jim Dawson's Avatar
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    Default Z Axis Conversion

    My Eagle 10x50 CNC mill started out life as a 2 axis machine and of course I can't leave well enough alone. So it needed a Z axis. So the question was: What is the best approach? The machine is a manual/CNC machine and I wanted to keep the manual quill capability with a quick change over. So the following is what I came up with. (I also upgraded the controls, which I'll post in another thread.)

    Most Z axis conversions involve adding a ball screw to the quill. There are two problems with that IMHO. 1; you wind up with a big ugly box on the front of the head, and 2; it is not easy to disconnect for manual operation. I took a different approach.

    First I took some pictures of the head, imported them into AutoCAD and just stared at them for awhile until some ideas began to jell.

    Here is the working concept I imported a drawing of a NEMA 34 stepper and moved it around until I found a place that would fit and be out of the way. I tried to find a timing belt combination that would work, but finally settled on a gear train. I'll address the backlash issues later.




    To install the new system, a little surgery needed to be done on the head. So I stripped the head down and borrowed a friend's mill to do the work.




    I also located the mounting holes for the gear case. Mounted to an angle plate in this pic.




    Then I made a new quill stop. This serves three functions: A quill stop for manual operation, a mount for the magnetic encoder readhead, and an attachment point for the anti-backlash system.



    Renishaw 1 um magnetic encoder install




    Making the prototype gear case out of MDF to confirm the tool path and to be able to test fit the components




    And the finished prototype This is made from 3/4 MDF, the real gear case is 2 inch thick aluminum.



    And it fits



    The stepper motor clears as planned. Allows tilt to 45° in each direction.




    The quill shaft to gear adapter.



    The gear is attached with soft machine screws to act as a shear point in case of overload > 150% (just in case the operator does something stupid)



    The next operation is to machine the gear case cover. It's bolted down to the to the gear case blank.


    And the finished gear case cover along with the disengage cam and motor outboard bearing retainer.



    Pocketing the gear case




    And finished with the outside profile.




    Starting to take shape. The center gear set is on a cam so it can be disengaged from the quill shaft gear.



    Shown disengaged. The gear always stays engaged with the motor shaft.



    And the parts fresh out of the dishwasher.




    The anti-backlash system. The cable is about 1/2 of a Yamaha Banshee brake cable. This puts an adjustable upward counterbalance on the quill and takes out all of the backlash by preloading everything. It set for about 50lbs of upward force.



    The other end of the cable is connected to a 1.563 air cylinder, regulated at about 30 PSI.
    The Z axis consistently holds +/- 0.0001. To disengage the drive, just flip the lever to the other position and turn off the Z axis in software and you have a manual quill.



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    Last edited by Jim Dawson; 12-17-2016 at 07:12 PM.


  2. #2

    Default Re: Z Axis Conversion

    Jim,

    Great job with the Z-axis conversion.

    I converted my Bridgeport 12x42 Z-axis both ways. Once with the ballscrew in front, and another time driven from the left side of the machine as you, but not as elaborate as yours. For me, the pinion drive is the best approach, provided you can get the accuracy you desire from it. I abandoned the left side drive for a number of reasons, but the main one was that I could not get the three to four thou backlash out of it.

    The backlash in my build was traced to the pinion shaft assembly. There simply was too much off axis motion loss due to worn pinion parts. The pinion drive gear, the pinion shaft and the pinion bearing must all be in good shape for this conversion to work well.

    There have been conversations regarding this style build here at the Zone. Some guys like it, some do not. But Jim, this is a very viable build conversion. You have done an excellent job with it.



  3. #3
    Community Moderator Jim Dawson's Avatar
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    Default Re: Z Axis Conversion

    Thank you for the kind words William.

    When starting this design I knew that the pinion backlash would be an issue, so I designed the system from the start with this in mind. My quill pinion backlash was closer to about 0.015. The design was not done in a day, I spent about 3 months in front of the computer on the design before ever making a chip. I considered all kinds of crazy ideas before I settled on the air spring. The air spring keeps a constant preload on the entire drive system, so the drive motor is always working against some pressure, but with a gear ratio of 15:1 the back pressure load on the motor is not significant.

    I also oversize the motor with this in mind, and as it turned out I have cut the power to the motor to about 50% of max to reduce the max quill pressure to about 300 lbs. I was pleasantly surprised that I am able to consistently hold 0.0001 on the Z axis, this was not expected. Overall I'm very happy with the results.



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