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IndustryArena Forum > MetalWorking Machines > Uncategorised MetalWorking Machines > Rotary 4th axis build componants & tech ?
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  1. #1
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    Talking Rotary 4th axis build componants & tech ?

    I am ready to CNC my totary table and need help in selecting motor (servo or stepper) & sizing of motor to have appropriate torque for holding, & being able to achieve a useable feed rate that will keep up with, rather than slow down the "x" & "y". I have a 65V 20A power supply driving my IH servo'd mill. I will also need some advise on the selection of encoder, appropriate enough to break down the degrees of rotation fine enough to handle whatever need arrises. I don't know if i should concider gearing motor down or go direct drive, would direct be too fast for worm ? The rotary table i have is an 8" import. but seems plenty solid. I plan on obtaining gecko appropriate for motor selected. Now what am i not concidering, I don't know. Any assistance I can achieve here will of course be greatly appreciated. I would like to get this project underway soon. It must have been done numerous times already but i sure couldn't find any links. I don't want to end up under powered and won't mind being maybe a little over powered. I would like something that will do for a few yrs or more ..... Thanks !
    Don
    IH v-3 early model owner

  2. #2
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    I think the 90-1 reduction is enough? If you have servos already then get another one. And a rotary will only use X+Z+A afaik. Mine is a mere 1.8NM and has no problems. hth
    Keith

  3. #3
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    I must admit that i favor servo's, I don't quite know what 1.8NM (newton meters?) would be for torque but it sounds small. must be deceptive then, and applied to a 90/1 it could be quite appropriate, but that is why i'm asking as i am completely out of my line here and don't understand many of the factors involved with motor selecting and understanding the lingo describing the science involved. I would like to read any shortcomings of your system tho and if you did it over again what would you do differant ? I really would like to build this thing smartly, rather than wish i'd done it differantly later on .... I'm sure that there are others looking on wanting to do the same thing without paying for a ready to go system. My primary concern is not having enough torque or resolution, for whatever may end up requiring this machine's capability's. Can you provide some more details of what your rotory will do for you ? and thanks for your reply ! I do appreciate it !
    Don
    IH v-3 early model owner

  4. #4
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    Quote Originally Posted by Cruiser View Post
    .... I don't quite know what 1.8NM (newton meters?) would be for torque but it sounds small. must be deceptive then,...
    It is deceptive.

    The force is 1 Newton at a distance of 1 meter from the axis of rotation.

    A crude approximation is that a Newton is about 1/4 lb force; actually it is 0.23 or something like that, 1/4lb gives you 4 ozs.

    Another crude approximation is that a meter is 40"; again the correct figure is 39.370" but 4 and 40 are much easier to do mental arithmetic with.

    So you have a force of 4 ozs at a distance of 40 inches. I am pretty sure you can see this is a reasonable torque when expressed in oz-ins. And please express it correctly; force first then length of lever arm as in oz-in, NM.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  5. #5
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    Dec 2003
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    The two common methods of operating a rotary table is the axis always active, if you want to interpolate the rotary with the linear axis, the other is to disable the servo at position and use a brake, (non-interpolated machining).
    The former requires more torque due to machining pressure.
    If you figure out the maximum rotation speed you require and then calculate the maximum reduction you can use, based on the max RPM of your motor type, DC servo's usually max out at 3000rpm 4000tops.
    Remember reduction multiplies the torque by the ratio of the reduction.
    The other thing to consider, is, if you have decided on a reduction and the encoder is on the motor, you will need to select a resolution that is within the reading (speed) capability of the system you are using.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
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    Thank you Geof, So if i reason this out correctly then moving the fulcrum in to 1" would be 40 times the force to hold, so 40 x 4 would be 160 oz in ? and at 1/90 then it could be 14,400 for use ? I sure i blew it here but it sounds good. I think (bewildered) How about the encoder, would a 1000 line be too much ? 100 to little ? I haven't broken down the units into minuits or seconds of rotation yet, but applying it to the motor ?
    Don
    IH v-3 early model owner

  7. #7
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    Al, I have a vague understanding of what you say here. I would like this system to be capable of dynamic interpolation, so then i assume a factor should be used for the "what if" and i don't have a what if to factor, hense a bit of over engeneering might be in order here ! as to the encoder selection, I will be tuning it into Mach 3, and the speed ? is this in pc / mach speed ? or ? This is getting just about as deep as i suspected ! Now i'm sure it is going to get much deeper ! Thanks lots guys and keep it coming.
    Don
    IH v-3 early model owner

  8. #8
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    I have not used Mach, but I believe there is also a maximum encoder rate limit that Your drives can operate at, as the encoder goes back to the drives.
    The basic resultant quadrature count of the encoder is multiplied by 4 to increase the resolution. i.e. a 100pulses/rev = resolution of 400p/rev.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  9. #9
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    Quote Originally Posted by Cruiser View Post
    .....This is getting just about as deep as i suspected ! Now i'm sure it is going to get much deeper ! Thanks lots guys and keep it coming.
    Yes, so you take it in bite sized chunks. And don't be afraid to do the ball park calculations I did just to get a handle on things. Also when things start 'not making sense' break off and do something entirely different: Letting the subconcious processor inside your brain chug over understanding things in the background can work wonders.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  10. #10
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    With some basic number crunching i came up with 129,600 units per revolution (@rotary) would equal 10 seconds or .00019" of 25.133" circumferance which would be also equal to the resolution that i understand my IH mill is computed to have and would be very acceptable ! So with 129,600/90=1440 for units @ 1 turn of motor/encoder .... now i'm lost ! ..... my system uses step & direction altho mach has the encoder quadrature capability and i'm sure that this would be useable, i just am not sure how to figure it from here for a proper encoder count selection. But, would a 500 count @ quad 2000 be a good choice then to better my 10sec resolution target ?
    edited in: or 2000 count encoder using simple step & direction hookup, is there even such a thing ?
    Don
    IH v-3 early model owner

  11. #11
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    Quote Originally Posted by Cruiser View Post
    But, would a 500 count @ quad 2000 be a good choice then to better my 10sec resolution target ?
    edited in: or 2000 count encoder using simple step & direction hookup, is there even such a thing ?
    I assumed you were using servo's, if using steppers then you probabally do not need encoders, as the resolution is decided by stepper.
    BTW the correct term is 500 quad count x 4 = 2000. the term (quad)rature does not refer to the count in this context.
    AFAIK Mach does not read encoder input. The feed back is to the drive.
    I make it .002 per deg. for a (500/rev) 2000 counts encoder at 90:1 red.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  12. #12
    Join Date
    Jan 2007
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    210
    Cruiser,
    A couple of questions.
    Where did the table come from and what brand name is on the box? The tables from Harbor Freight are not as good as the ones from Enco or J&L and take a lot more torque to turn.
    What are you planning on using for amps? Due to their design the Geckos don't like really high resolution feedback.
    Remember that this is a worm gear mechanism so you you must run it with at least 2 to 3 thou backlash (slop) at the gear diameter. Worm gears can't be run at zero backlash or they will lockup.
    Max RPM of the worm must be held to under 1500 RPM and at this speed they don't last long.
    I've run 8 inchers with 100 watt AC servos belted 3:1 with 2000 line (8000 count) encoders. This will just turn the table when new and will occasionally lock up once things start to get sticky. I'm building one with a 200 watt motor now.
    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

  13. #13
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    Al, I don't fully understand this, but i'm working on it, and Thanks !
    Bob, I don't remember if i got it through enco or JL, i had it a while sitting and waiting. I played with it some by hand and i feel a bind that makes me think it has crap in it, i intend to dismantle it and see for sure. It is definitely stiff if bumped to a no slop, and it seems to be a bit concentric too. I wish to match motor to my existing power supply so from what i have seen it will have to be a servo, it looks like for a few bucks more i can overpower it and hopefully not be too concerned about the internal drag, but formost i think i need to learn more so that i can order one encoder and do this right. I've always been a wrench, or operator, only dabbled in electronics and then i followed instructions and schematic. I think that the encoder is one thing i really need narrowed down then buy servo and gecko 320/340 etc.
    Don
    IH v-3 early model owner

  14. #14
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    Jan 2007
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    Cruiser,
    Is the binding once per rev of the handle? This is normal when new as the worm and wheel have to break in to each other. Unlike most geartrains worm gears will actually increase in accuracy with a little use. If the binding is in one direction of rotation more than the other this may indicate that the worm is moving in and out and the thrust bearing (buried inside the dials) needs adjustment. This adjustment is very touchy and you'll need to build a couple of spanner wrenches to get it right.

    Don't try to adjust for zero slop. Bring it to zero then back it off until you can just get a little movement on the table. If you don't allow this clearance it will soon make it's own.

    If you want to disassemble it and it's the type of table I think it is there is a small pull dowel (a dowel pin with screw threads in it) on the side that holds the worm assembly in place. Pull this pin out and the entire assembly sides out of the table. The spanner nut on the bottom that holds the table in has two small screws in it. Loosen these screws before trying to remove the nut.

    From my very limited experience with the geckos they seem to work best with a 500 line encoder. This should give you plenty of resolution while not limiting your top speed. Am I correct in assuming that you are going to attach the encoder to the motor?
    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

  15. #15
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    Feb 2006
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    794
    Bob, the binding appears to move some and it also has a concentric bind. The lock nut to bottom of table spindle has three screws, and another two slotted nuts going into counterboars of casting base. i'm fairly sure i bought this through JL last yr. I was hoping it would be a little better than the other units but recall the fear it would be identical and only costing more ! The unit has no identification on it at all, but the book says tsl 200mm. As with everything else i've seen from china, i'm sure it needs deburr and cleaning inside so it will get opened up this morning. Right now i think i'll order a servo motor & encoder & a gecko too. I wonder if there would be any advantage to getting the new 340 unit ?
    Don
    IH v-3 early model owner

  16. #16
    Join Date
    Jan 2007
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    210
    Cruiser,
    Yup three screws (oops). I assume you have it apart by now so you realize that that the third piece is a thrust washer with an oil groove on it. Maybe a dumb instruction but make sure the internal notch is on the little locating pin on the table shaft when reassembling.

    This same table is private labeled by just about all the tooling supply houses.
    I've seen it called a Phase II, Value brand, and unnamed. The earlier ones had a little symbol that look like the planet Saturn.
    I watch for Enco to put them on sale and buy 3 or 4 at a time. IMO these are the best tables you can get in this price range.

    Have you figured out how you are going to attach your servo yet? You will want to get rid of the handle and dials as their inertia will make tuning a small servo just about impossible.

    From what I understand the difference on the 340 amp is the step multiplier option. This allows you to set the drive to move the motor more encoder counts with one step input. As you can't stop in between these counts I don't see much advantage to this. It might allow you to use a higher res encoder without running into a speed limit from the pulse train. I don't know as I don't have enough experience with these drives. (I use mostly AMCs). For the extra 30 some bucks it might be worth it just to have the option to play with. You may be able to get a tighter servo loop this way.

    On the small tables I don't generally see many binding problems and when I do it usually goes away when I remove the handle and dials. We do see it sometimes on the larger dia. tables (12 inchers) that we use on our b-ports as manual tables. We chuck a socket in a hand drill and drive the table to "break in" the gears for a half hour or so. (we use the same trick to drive the table when milling round parts). After break in we change the oil and everything seems to be fine.

    If you haven't seen it yet there is a rather interesting "discussion/argument" going on in the Tormach PCNC forum about lubrication for these tables.

    Bob
    You can always spot the pioneers -- They're the ones with the arrows in their backs.

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