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  1. #13
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    Default Re: Precision high resolution machine build

    What are the speeds, acceleration's and forces needed? If the speeds and acceleration's are low, using secondary measurement is much easier, as the feedback loop will be easier to tune satisfactorily (has more time to settle.) C7 screws might work if you don't mind going very slowly. I'm not sure why you don't want to go to at least C5 ground screws, their cost won't be much compared to the scales, linears and frame.
    Will there be any forces (other than acceleration and friction) on the movement? Machine mass needs to go way up if there are (think tons.)



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    Default Re: Precision high resolution machine build

    I will just be mounting a laser
    Are you sure the laser beam it self is that accurate, most times laser beams aren't round but oval!
    During direction changes there is acceleration and deceleration resulting in X and Y forces on the gantry. To avoid deflection, the gantry setup must be incredible rigid and free from play! I think that even lubrication will spoil the result.
    Have you considered how to measure the finished products and how expensive these measurement tools are?
    Great respect if you can do this.



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    Default Re: Precision high resolution machine build

    Quote Originally Posted by skrubol View Post
    What are the speeds, acceleration's and forces needed? If the speeds and acceleration's are low, using secondary measurement is much easier, as the feedback loop will be easier to tune satisfactorily (has more time to settle.) C7 screws might work if you don't mind going very slowly. I'm not sure why you don't want to go to at least C5 ground screws, their cost won't be much compared to the scales, linears and frame.
    Will there be any forces (other than acceleration and friction) on the movement? Machine mass needs to go way up if there are (think tons.)
    what is secondary measurement?.

    I have no problem in going for the C5 ground screws, I was just trying to understand why a decent ball screw like C7 which is considered as better than Acme thread wouldn't make the cut and I was banking on the electronic systems to bail me out.
    Last i found they were around US200 for a 400mm length and C3 were around USD500 for 500mm length on Aliexpress.
    No not much forces on the axis apart from what you are saying and speed I would be happy with 100mm / sec,
    Any idea where i would get a prebuilt gantry so I can accelerate the build .



  4. #16
    Community Moderator ger21's Avatar
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    Default Re: Precision high resolution machine build

    I believe that you need to use screw mapping. You can't just use a screw with poor accuracy and expect the scales to fix the error.

    Screw mapping would require you to measure the actual distance traveled along it's entire length.

    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)


  5. #17
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    Default Re: Precision high resolution machine build

    Quote Originally Posted by hfjbuis View Post
    Are you sure the laser beam it self is that accurate, most times laser beams aren't round but oval!
    During direction changes there is acceleration and deceleration resulting in X and Y forces on the gantry. To avoid deflection, the gantry setup must be incredible rigid and free from play! I think that even lubrication will spoil the result.
    Have you considered how to measure the finished products and how expensive these measurement tools are?
    Great respect if you can do this.
    Exactly, that was my intention of this question to understand what kind of tools, equipment, actuators and sensors are required to get this kind of job done.
    I'm open to any kind of suggestions..
    I do understand now the importance of system rigidity. I hope the esteemed folks here will highlight more issues so i can focus attention on these.



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    Default Re: Precision high resolution machine build

    Quote Originally Posted by ger21 View Post
    No.
    I think LinuxCNC can do it.
    Possibly the KFLOP and KMotion?
    Other than that, you'd probably be looking at $5000-$10000 for a control that can do it.

    This project is far more complex (and costly) than you think. If it was so easy, others would have done it.
    Quote Originally Posted by ger21 View Post
    I believe that you need to use screw mapping. You can't just use a screw with poor accuracy and expect the scales to fix the error.

    Screw mapping would require you to measure the actual distance traveled along it's entire length.
    Yes, I guess I read about it yesterday somewhere on these forum's. Hopefully Kflop / Kmotion has the screw mapping feature built into it.



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    Default Re: Precision high resolution machine build

    Quote Originally Posted by ZeroBacklash View Post
    I'm looking for 1um in 500mm resolution and also accuraries in the order of 1um.
    What does it take to achieve this.
    I was looking for a 3 axis gantry type build, with 500mm x 500mm work area, is that the way to go?

    I have read this forum post exhaustively https://www.cnczone.com/forums/linea...60578-cnc.html

    And from what I gather from that post the minimum requirements are

    1. C1 / C3 ground ball screw with 3mm lead.
    2. Ultra precise encoder 24bit+
    3. AC Servo motors or Linear motors
    4. Good air conditioning
    5. Good measuring equipment

    Also I have read user hanermo's post https://www.cnczone.com/forums/linea...ml#post1656188 and he gives quite interesting advice.

    I have a few questions
    1. Can I get away with C7 rolled ball screw, my thinking is with ultra precise encoder and servo's and maybe a optical linear scale, the controller should be able to accurately position itself even though the screw is inaccurate?
    2. Can I get a off the shelf gantry structure with decent accuracy so that will speed up my build.
    3. For encoder/ linear scale Renishaw Resolute what kind of price range does one expect?
    4. Servors DMM ac servo's ok? I mean with their integrated 16bit encoders? Can steppers do the job, by running it in closed loop mode?
    5. What kind of motion controller / software would I need for achiving these accuracies?
    6. How does the optical linear scale work in conjunction with the integrated servo encoders? I mean does the optical linear scale output go in seperately to the motion controller, and then some software needs to be written for closed loop control?

    Thanks in advance
    1. No & Why would you be trying to cut corners it will just make the job harder. Don't forget you are going to have to map the screws.
    2. No You may be able to get an off the shelf gantry from China But you will probably need to regrind it.
    3. $5K per axis
    4. DMM's are 16bit so if they can provide that in positional accuracy then they should be fine. 3mm/65535 = 0.0000457 this should be enough for mapped screws.
    5. Personally I would use a FPGA to control the positions of the machine I'm not sure software alone could hold this kind of accuracy at any kind of useful speed.
    6. They are just a secondary check to ensure nothing has gone wrong with the Servo's & temperature compensation is working.

    You will also need a coolant filter & chiller. You will also need to monitor the component temperature & compensate for the differences between the expansion/shrinkage of the components material & the machines.

    Having said all of that I worked to 0.0001" On a thin walled 36" casing 30 years ago on an old NC VTL.
    If you are working to tight limits you are are going to need to check dimensional accuracy mid program & compensate for tool wear.

    Good luck

    Last edited by Bovski; 11-24-2018 at 09:14 AM.


  8. #20
    Member Jim Dawson's Avatar
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    Default Re: Precision high resolution machine build

    I would go with DMM servos, a Galil motion controller (40x0, or 18x6 series), Renishaw or Ditron magnetic scales or your choice of glass scales, 0.1um. I have had good luck with that combination. Properly tuned it will hit +/- 1 encoder pulse every time, normally 0

    Given the small working area, a used CMM with dead controls and a granite base would make a great machine frame. Would also include precision ball screws.

    By using the scales as the encoder input, you close the loop at the controller and that turns the servo drive into a slave to the controller rather than using the drive's intelligence. Doing it this way automatically compensates for any leadscrew error or backlash and gives you a true position.

    The one thing that is not clear to me is the end use of the system. Router? Milling machine? CMM?

    Jim Dawson
    Sandy, Oregon, USA


  9. #21
    *Registered User* men8ifr's Avatar
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    Default Re: Precision high resolution machine build

    Zero - your lack of knowledge is made up by your ambition but sadly I suspect your budget will not support the latter...

    Also consider:

    Steel thermal expansion is ~12 (um/(m K)

    So for every 1 deg degree of temperature change you get 12 um in 1m or 6um in your 0.5m.

    So options are:
    1 - temperature controlled room/box with accuracy 1/6 of 1 degree - but even that has used up your entire 'budget' of tolerance... Plus you need to run the machine so slowly heat generated in ballscrews in negligible - here the problem lies - to remove backlash you need tight preloaded ballnuts... tight ballnuts generate temperature.. so with very tight ballnuts I would expect a speed of a small number of mm/min to avoid any temperature effects....
    To be honest none of this is feasible.
    2 - Run closed loop control to 1um scales using kflop, mach servo drives (Australian look on ebay) or linux CNC. But factor in 1um scales accuracy (if true) will be +-0.5um so, again that's all of your required tolerance used up in the scales and I bet movement of the workpiece relative to laser is not identical to the scales. also I would only trust a reputable brand e.g. mitoyoto and you need to study what their accuracy is, I doubt a 1um glass scale means it is within 1um everywhere and again I believe these are temperature sensitive so you need to understand how their measurement is changing with temperature.



  10. #22
    *Registered User* men8ifr's Avatar
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    Default Re: Precision high resolution machine build

    Sorry just read Jim's post - you can add Galil to the options of closed loop control (and probably the best but $$$)



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