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IndustryArena Forum > MetalWorking Machines > Uncategorised MetalWorking Machines > CNC rotary fixture , modular , belt driven
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  1. #101
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    Nobody participates in the chat area, they had to go somewhere and "help", if that is what you call it.
    A lazy man does it twice.

  2. #102
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    help???......whom?

    This is an area where ideas are exchanged hopefully to gain new ideas.....you can also show off your latest creation and answer queries pertaining to the design or useage so that others can see how clever you are and admire your work.

    Apparently someone thought it was purely a one way window to show off an idea and not attract questions relating to it....re post #98,

    I hadn't realised that was the case, my apologies for being inquisitive.
    Ian.

  3. #103
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    Quote Originally Posted by handlewanker View Post
    Hiiiiyyy Mac, belt drive not being the very best solution generates a desire for a better solution....etc.

    Sure, the build by Katran is neat and tidy and I think even he would tire of the superlatives describing the build quality now it's up and running, and eventually it gallops off into the sunset as another thread dies never to be revived again, meanwhile back at the ranch the faithfull of the 4th axis god debate all the other possibilities that could be an alternative drive to the general design Katran is using.

    He's already been encouraged to keep the third bearing for the spindle end support, so it does pay to chew the rag sometimes, and we all can benefit from a general discourse as long as it stays more or less on topic.

    I'll be going into the 4th axis build scenario soon so I want to get all the info pertaining to a successful solution that has been used to date.....ie, plan twice, cut once is my motto.

    Katran has already made his choice in the drive method and will tolerate any inadequacy in the drive mechanism if or when it occurs, but some of us are leaning on the side of perfectionism and would like to have a better way.

    Did you have something to say, or are you just saying something?
    Ian.
    There's alreeady a thread on backlash-free rotary axes in the linear motion section; of which you've been a great participant. Why hijack Katran's build thread, when these great ideas can be shared there, and just direct others to that thread? Also using "underhanded" comments like "will tolerate any inadequacy in the drive mechanism if or when it occurs" may not be so constructive. Katran has been receptive of possibly implementing some ideas, but do we need to beat him and others over the head as to one's idea of "perfection?"

    The "art" of engineering is designing to meet the specific needs and tolerances of the end-user, within that end user's means and budget. This is the practical aspect; and is something most mere mortals still working have to consider, as opposed to those who are retired and have the time on their hands. As to perfection, there is no such thing, as we can only build to a certain level of tolerance that our tools will allow. The rotary as Katran has designed it is simple and practical in its design. If the need arises for upgrade I'm positive he is up to the task!

    As to hydraulics, yes they can be pretty "stiff" and exhibit zero backlash, like "hydrostatic" lead screws (although their cost must be stupendous...)

  4. #104
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    Hi Ian

    The hydraulic motor can quite small for tons of force and can also be held stalled in any position, and with an encoder give accurate positioning in both directions without missing a step or encodec line.
    You know, this represents a significant change in strategy, one which has not been examined very closely. Let me explain.

    Case A: Use stepper motor or servo motor with encoder on motor for both drive AND position sensing, and gear down. Spur gears, worms, belts, whatever - it does not matter: the position sensing is being done at the motor end of the chain.

    Case B: Use servo motor for drive, gear down (any way you want), and sense final movement/rotation at the rotary table.

    We are all very familiar with Case A: most CNC machines are built that way, despite the probelsm with backlash. But Case B does not have anywhere near the backlash, and in some cases it has essentially zero backlash. By way of example, we do have some small ultra-precision CNCs which use laser interferometry to measure the X, Y & Z movements of the table. You are talking sub-nanometer precision here.

    It's a bit more tricky with rotation: you have to use something other than interferometry, but it is possible. I do have some old Selsyns somewhere ... put a 20-bit sigma-delta ADC on one and you have 1 part in a million or ~1.2 arc-secs resoluion. Hum ...

    Thing is, once you start sensing the rotation of the table directly, how you drive the rotation simply does not matter. To be sure, a smooth low-backlash drive helps, but it does not affect the measured rotation.

    Just a thought
    Roger

  5. #105
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    Hi Caff, I bow to your superior knowledge.......I'm not very up on sensing mechanisms, being a purely mechanical orientated person, but I'm learning at a rapid rate of knots how movement is sensed and the outcome of high technology down at our level.

    Having worked on mills and horizontal borers for a good part of my working life, one thing you don't do with those machines is climb mill, not unless you have a hydraulic buffer to resist the grab of the cutter etc, but with ball screws, climb milling, (providing the set-up is rigid enough) is just another way to get the cutter to do the job.

    My point is, you can drive the table around or in a straight line with whatever mechanism takes your fancy and sense the movement with whatever sophisticated methods are currently available to an extremely fine degree, but if there is any self feeding aspect in the drive train without any form of hold back mechanism, like a cutter in a climb milling mode, it will play havoc with the set-up and overload the cutter.

    That is the reason I am pursuing the alternative drive methods and consider a belt drive as having a limited although quite suitable application for most needs.

    Good enough is not good enough.

    A toothed belt drive, without argument, can be considered as a very flexible rack and pinion, but with a compromised material and tooth profile for accurately moving the table.

    The point on that score is, if you can have a linear expansion from rest to tension you don't have accuracy as it's randomly variable.

    It's possible that the toothed belt is 99% of the story, but only a test program would reveal it.

    No matter how it is applied, there has to be a better way.

    If it's true that the sensing mechanism senses the ultimate position for the input stimulus, then a hydraulic drive with sensitive check valves will provide the best of both worlds, IE, move from A to B and back to A without loss of resolution, and move without having a forward or reverse reaction under any load.

    As this thread started out as Katran's project I won't debate further on alternative drive methods.
    Ian.

  6. #106
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    [QUOTE=louieatienza;1368654]There's alreeady a thread on backlash-free rotary axes in the linear motion section; of which you've been a great participant. Why hijack Katran's build thread, when these great ideas can be shared there, and just direct others to that thread? Also using "underhanded" comments like "will tolerate any inadequacy in the drive mechanism if or when it occurs" may not be so constructive. Katran has been receptive of possibly implementing some ideas, but do we need to beat him and others over the head as to one's idea of "perfection?"

    Hi Louie, no "underhand" aspect intended or posted.......if the drive has slackness or lack of resolution then Katran will have to live with it as that is his choice of method to do the job.......he's not going to change the set-up in the near or distant future unless it becomes too inefficient for his needs.

    As far as he's concerned, it works to his satisfaction and exhibits no problems that are apparent.

    The fact that the test piece was a carved wooden object where resolution is of a very low order is irrelevant.
    Ian.

  7. #107
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    Hi Caff, I bow to your superior knowledge
    Dangerous, very dangerous! :-)

    Having worked on mills and horizontal borers for a good part of my working life, one thing you don't do with those machines is climb mill, not unless you have a hydraulic buffer to resist the grab of the cutter etc
    Can I interest you in a very cheap, almost new, 20 mm Dormer square-ended milling cutter - currently in two pieces? And I was milling plastic at the time. Actually, the plastic grabbed out of the vice, but no matter.

    but if there is any self feeding aspect in the drive train without any form of hold back mechanism, like a cutter in a climb milling mode, it will play havoc with the set-up and overload the cutter.
    No argument at all!
    In fact, that is precisely why I hesitate to use a simple toothed belt drive on a rotary table. Even at 20:1 reduction, I am sure you could back-drive it some day. So, ... my conclusion is that for a reliable rotary table I will have to use a worm drive. We know they do not back-drive.

    Over to the other thread: this is Katran's.

    Cheers
    Roger

  8. #108
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    CIAO for now.....I've always wanted to speak French.
    Ian.

  9. #109
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    Ian, in reference to hydraulic drive: my first introduction to NC machines was a Cincinnati Cintimatic vertical NC mill, drill & tapping machine. It had hydraulic servo's in 1963!!! However, when the temperature got hot, the hydraulic fluid got a bit thin. Strange things started happening so we had to put cooling coils in the reservoirs.

    Dick Z
    DZASTR

  10. #110
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    Hi Dick, I'll be over on the rotary table thread.....hydro...hmmmmm.
    Ian.

  11. #111
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    "Only" 31 years!

    Quote Originally Posted by handlewanker View Post
    Hi Wiz, I have a DIY power hacksaw, using standard hand hacksaw 12" blades, built in 1982 for a special purpose job and the main swing arm had Oilite bushes in it.......they only lasted 31 years and now have to be replaced due to some ovality........needle bearings or ballraces would still be going strong.
    Ian.
    That isn't bad at all.


    I've been research router machines and thus have scoured the Internet looking at the different builds out there. One guy uses oilite bushings on his machine machining seems sort of particle producing plastic material from what I can see. Whatever he was doing left the entire enclosed machine covered in a deep "dust". He preferred oillite bushings in this application because they ran so well in the environment created by the machining. Sometimes old school just works.

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