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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > [automated] Gantry alignment with dual ballscrew setup?
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  1. #1
    Join Date
    Dec 2013
    Posts
    7

    Lightbulb [automated] Gantry alignment with dual ballscrew setup?

    Hello all,

    I've been a member for some time but never posted before. I just finished a colossal build of my machine that I've been designing, building, redesigning and rebuilding for many years now. It is a gantry router type setup but it's geared toward metal machinning.

    Attachment 275238

    Everything works great and unlike my previous machines where working on metal was more like grinding (the chips were dust) and I could only take light cuts, this machine is biting in and making actual milled chips and I can finally take aggressive cuts and do high speed machining tool paths with high feed rates/aggressive cuts.

    On to my question:
    I currently align the gantry to the table (the two 1.5x4.5 80-20 extrusions in the middle of the machine) by clamping a dial indicator to the z-axis and jogging the x axis back and fourth turning individual ball screws by hand to "skip" motor steps until it's perfectly aligned to the table. It has to be this way because I do relatively decent precision metal work and things have to be square.

    Currently both of my stepper motors are driven by the same pulse/dir signals but they have two separate drivers.

    I noticed that if I jog the gantry into an obstacle (or otherwise do something irregular) which causes the Y axis (gantry forward and back motion) to skip steps.... this immediately throws alignment out of whack and I have to tear down my current setup, clean table, clamp dial indicator and do all of that again which is a big pain.

    Now I could in theory create limit switches and align them perfectly but limit switches don't have good repeatability.

    My absolute best idea (but as much as I like the geeky and over-complicated vibe of the idea I cringe at the idea of having to write the macro to do this):
    -Wire the controllers to have independent pul/dir controls
    -Setup one of them a SLAVE A axis
    -Write a mach 3 macro that will use a touch probe to jog to the table and actually:
    ---measure the angle of the gantry vs table,
    ---disconnect slave axis and jog it or the master axis to adjust for the angle.

    Once again, I LOVE the thought of doing this but really don't want to actually code the macro (I smell a lot of grinding sounds and crashing gantries, bent touch probes...so on).

    Any other ideas? Does anyone else do this kind of stuff?

    Thanks for looking.

  2. #2
    Join Date
    Mar 2010
    Posts
    1
    I contemplated the exact same thing on my plasma table build. I ended up going with a tensioned timing belt and pulleys using one stepper. The stepper torque was increased to drive both screws. Granted the plasma table doesn't require the torque needed for mechanical cutting, a proper timing belt can transmit quite a bit of torque. I marked my pulley and belt at the home position for quick reference of a slipped belt. So far so good.
    Good luck!
    -Brian


    Quote Originally Posted by Ruso View Post
    Hello all,

    I've been a member for some time but never posted before. I just finished a colossal build of my machine that I've been designing, building, redesigning and rebuilding for many years now. It is a gantry router type setup but it's geared toward metal machinning.

    Attachment 275238

    Everything works great and unlike my previous machines where working on metal was more like grinding (the chips were dust) and I could only take light cuts, this machine is biting in and making actual milled chips and I can finally take aggressive cuts and do high speed machining tool paths with high feed rates/aggressive cuts.

    On to my question:
    I currently align the gantry to the table (the two 1.5x4.5 80-20 extrusions in the middle of the machine) by clamping a dial indicator to the z-axis and jogging the x axis back and fourth turning individual ball screws by hand to "skip" motor steps until it's perfectly aligned to the table. It has to be this way because I do relatively decent precision metal work and things have to be square.

    Currently both of my stepper motors are driven by the same pulse/dir signals but they have two separate drivers.

    I noticed that if I jog the gantry into an obstacle (or otherwise do something irregular) which causes the Y axis (gantry forward and back motion) to skip steps.... this immediately throws alignment out of whack and I have to tear down my current setup, clean table, clamp dial indicator and do all of that again which is a big pain.

    Now I could in theory create limit switches and align them perfectly but limit switches don't have good repeatability.

    My absolute best idea (but as much as I like the geeky and over-complicated vibe of the idea I cringe at the idea of having to write the macro to do this):
    -Wire the controllers to have independent pul/dir controls
    -Setup one of them a SLAVE A axis
    -Write a mach 3 macro that will use a touch probe to jog to the table and actually:
    ---measure the angle of the gantry vs table,
    ---disconnect slave axis and jog it or the master axis to adjust for the angle.

    Once again, I LOVE the thought of doing this but really don't want to actually code the macro (I smell a lot of grinding sounds and crashing gantries, bent touch probes...so on).

    Any other ideas? Does anyone else do this kind of stuff?

    Thanks for looking.

  3. #3
    Join Date
    Dec 2013
    Posts
    7

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Thanks Brian,

    I am actually switching away from any belts in the drive system because it's designed for metal. The last iteration of the machine used belts and they caused a lot of issues. Granted the ballscrews require very little torque to turn and produce a lot of it so I don't see problems with resistance from cutting making the belt flex but I still would be weary of having a belt connecting the two sides.

    I've definitely thought about this during the design phase (actually more toward a tensioned chain and sprockets to reduce issues with belt stretching across that type of distance) but made the decision to have two motors instead.

    Thanks for the input! Definitely glad to see others ran into this .

  4. #4
    Join Date
    Sep 2010
    Posts
    717

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Hi Ruso,
    My machine also has two scews, but drive by one servo and connected with a belt (actually 2 at the moment until I fix it).
    Anyway, to the point, I made a couple of "sync discs" and put them on the screws to ensure they are both rotating the same. Mine have a bit of tolerance, a few degrees (of ball screw rotation) until E-stop.
    They could be made with higher precision if needed.

    Here is the disc:
    Attachment 275244

    Just install them rotated 90deg apart and add an optical sensor for each.

    Hub
    Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html

  5. #5
    Join Date
    Mar 2003
    Posts
    35538

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Now I could in theory create limit switches and align them perfectly but limit switches don't have good repeatability.
    A lot of have made our own hall effect home switches, which are supposed to be very accurate, and only cost a few bucks to make.

    Want a much easier method to square your gantry? Add a mechanical stop (adjustable) to each side, and just slowly drive the gantry into it. No macro needed.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

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

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

  6. #6
    Join Date
    Sep 2010
    Posts
    717

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Quote Originally Posted by ger21 View Post
    A lot of have made our own hall effect home switches, which are supposed to be very accurate, and only cost a few bucks to make.

    Want a much easier method to square your gantry? Add a mechanical stop (adjustable) to each side, and just slowly drive the gantry into it. No macro needed.
    Sure, but that won't monitor your screws rotating the same all the time. You will only be able to "home" both.
    Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html

  7. #7
    Join Date
    Dec 2013
    Posts
    7

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Hub:
    That's a great idea! You use it to detect missed steps right? It doesn't help me align the gantry but I really dig the idea and may implement it later on to detect de-synchronization. of the two motors

    Gerry:
    This is also brilliant! And may be the very solution I'm looking for. I could align the gantry once very precisely, jog it past the table and manually bolt on two hard stops. Great and simple solution!

    Although I must say I don't think I would be jogging the gantry into a hard stop (on purpose) to align it :-D. I think I have enough torque and weight on the axis that I would bend/dent/otherwise break parts if I did that. And although I realize that stepper motors are magnetic and probably no damage will occur, I still never like hearing them skip steps after crashing into things.

    Thanks for the input all,

    Any other clever "out of the box" ideas?

  8. #8
    Join Date
    Sep 2010
    Posts
    717

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Ruso,
    No It won't detect missed steps in my case since I have only 1 motor and two screws. But it detects if a belt snaps or slips, in my case.

    Hub
    Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html

  9. #9
    Join Date
    May 2014
    Posts
    182

    Re: [automated] Gantry alignment with dual ballscrew setup?

    With two inductive proximity sensors on each axis, I bet you could expect a .001"-.004" repeatability per axis.

    I wouldn't expect much more than that with a hard stop. If one of your steppers is between energized poles and the other settled in step while you stall out, then they'll be off from each other.

    Is there some rigidity issue with your gantry that allows it to essentially windowpane?

  10. #10
    Join Date
    Dec 2013
    Posts
    7

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Hello UA-Iron,

    Those sensors look great (a bit pricey though). The one drawback I see is that I would have to align the gantry to the sensors and to the table at the same time. I don't think I can get that type of precision with non-contact sensors like that (or even limit switches for that matter)... which is why I like the idea of a hard stop because I can adjust the hard stop when the gantry is already aligned and energized.

    I'm not sure what you mean by window pane issues.

  11. #11
    Join Date
    Sep 2010
    Posts
    717

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Also, IMO, the best possible (most accurate) way to home is to fix an dial indicator at the end of each screw. Carefully adjust each one the first time and fix it to the machine. After that, you can reference to the original setting as precise as the dial indicator is.
    Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html

  12. #12
    Join Date
    Dec 2013
    Posts
    7

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Quote Originally Posted by hub View Post
    Also, IMO, the best possible (most accurate) way to home is to fix an dial indicator at the end of each screw. Carefully adjust each one the first time and fix it to the machine. After that, you can reference to the original setting as precise as the dial indicator is.
    I definitely agree with this in terms of precision. The only thing is that it will take just a little less time than aligning the method I use currently (well that's not entirely true but it will still take some time).

    The reason I like your hard stop idea so much is because it seems to have the best bang for the buck in terms of speed vs precision. It won't be perfectly repeatable but I think it's well within the type of precision I can machine in on this type of machine anyway.

    Although some machined hard stops could be a very good base for two dial indicators because they would already be aligned with the gantry/table and I could use the dial indicators for fine adjustment (in case I need it) so only a little bit is required to speed things up.

    Maybe what I'll do is make mounts in the stops for dial indicators but won't place the indicators until I actually need them and save a few bucks.

    Once again, thanks for all the input from everyone.

  13. #13
    Join Date
    Sep 2010
    Posts
    717

    Re: [automated] Gantry alignment with dual ballscrew setup?

    Quote Originally Posted by Ruso View Post
    I definitely agree with this in terms of precision. The only thing is that it will take just a little less time than aligning the method I use currently (well that's not entirely true but it will still take some time).

    The reason I like your hard stop idea so much is because it seems to have the best bang for the buck in terms of speed vs precision. It won't be perfectly repeatable but I think it's well within the type of precision I can machine in on this type of machine anyway.

    Although some machined hard stops could be a very good base for two dial indicators because they would already be aligned with the gantry/table and I could use the dial indicators for fine adjustment (in case I need it) so only a little bit is required to speed things up.

    Maybe what I'll do is make mounts in the stops for dial indicators but won't place the indicators until I actually need them and save a few bucks.

    Once again, thanks for all the input from everyone.
    You won't need them at all, unless the screws go out of sync for some reason If the machine works properly, it's not often you would have to do it..
    Current build: http://www.cnczone.com/forums/diy-cnc-router-table-machines/264838-new-machine-desing-quot-cnm13-quot.html

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