Polar table design

New Home Forum ZenXY Advice – ZenXY Polar table design

This topic contains 77 replies, has 13 voices, and was last updated by  Dajosco 2 weeks, 4 days ago.

Viewing 30 posts - 1 through 30 (of 78 total)
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  • #77995

    karltinsly
    Participant

    I just adapted a bunch of sisyphus designs for my sand table using Sandify. All those beautiful round designs make me want to build a polar table. I’ve been googling around, but haven’t run across any polar designs yet that don’t require the drawing surface to turn. Anyway, can anyone point me at a good polar design? Thanks!

    #78010

    Jeffeb3
    Participant

    If you build it, they will come. :D.

    I haven’t seen anything like that (I am far from an expert though). I would guess the mechanism in the polar tables is something to spin a rod, and something that rides along the rod. The part that seems most mysterious to me is how the thing can spin 3000 times in one direction, and still have anything attached to it. I wonder if the radius axis is moved by a stepper at the center, so everything that spins could be passive. There must be some way to do that with a couple of gears and a continuous belt. Sort of like the corexy. If you spin just the gear motor, the thing turns and the magnet moves closer, if you do them both, you can move one or the other. I’m not making any sense, but it works out in my head :).

    OK, I couldn’t help it, I had to draw what I was thinking.

    polar

    The radius motor moves a belt, which moves the gantry. The gantry slides on some track, which is held at a particular angle by the large gear. To change the theta, the theta motor turns, and to keep a constant radius, the radius motor needs to turn the same angle. The mounts connect the track to the large gear, and both motors are stationary.

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    #78015

    Jeffeb3
    Participant

    How would you get to zero radius though? The gantry and the middle motor need to be able to not collide. Maybe there’s something with a string instead of a GT2 belt, so you can wrap it around a pulley instead…

    #78016

    Jeffeb3
    Participant

    I guess if the gantry was connected to the belt on the “bottom” in that drawing, it could move past zero…

    #78017

    Jeffeb3
    Participant

    And, you don’t really need the left side of the slider either :). This is really interesting to me (in case you couldn’t tell).

    If you put a 608 bearing on the left side, and a 608 bearing out at the end with the normal smaller pulley on the motor, you probably have clearance to get past in a straight line. You wouldn’t have to have a continuous belt, the zip ties could be on the gantry. I wonder if EMT is the right choice for the linear rails? I suppose it all depends on the scale of things. That gear would get pretty big if the linear rails were big. Could one piece of 2020 work?

    #78018

    Jeffeb3
    Participant

    It’s just like this, but with the arm mounted on the platter, and the middle of the platter is cut out for the arm motor.

    With a top/bottom configuration on these rods, EMT could probably easily be used.

    #78022

    karltinsly
    Participant

    Hi Jeff – yes, I think you’ve got the idea right. Magnet goes back and forth on a bar that in turn rotates like a big clock hand. The motion is already built into Marlin, and maybe GRBL too (my present table uses GRBL).

    The problem is – how do we get the precision of a cartesian machine into a polar machine? The axis arm that spins is simple enough – it’s basically the X-axis of just about any cartesian – a carriage that slides along a length of profile, or on some smooth rods – or in the case of the ZenXY, on EMT conduit. Put a stepper on one end with a belt or leadscrew and you’re golden. But how do you spin it and keep it rigid? Spinning with precision is fairly simple – another stepper and belt, or even string – but the rigidity is the challenge, in my mind at least.

    Here’s an example on youtube of a diy polar sand plotter. It uses some parts from an old printer or scanner. It works, but the precision isn’t there. https://www.youtube.com/watch?v=X7liebJP0JM&t=196s

     

    #78023

    karltinsly
    Participant

    I was thinking 2040, mainly cause I think I might have a piece lying around.

    #78025

    Jeffeb3
    Participant

    That build looks like it’s reinvented the wheel (no pun intended). It’s reeling out wire for the gantry, and it looks like it really just needs a stronger magnet to stop the jerkyness.

    I don’t think rigidity is going to be that big of a deal, because it’s got almost zero drag.

    A GT2 belt is 2mm/tooth, and if you had a 16 tooth pulley, and 1/16th microstepping, that’s 16 steps/ whole step * 200 whole steps/rev * 1 rev/ 16 teeth * 0.5 tooth/mm = 100steps/mm of belt.

    If you had a circumference of the big gear at 180mm, you’d have 50 steps per degree. That would be about a 60mm diameter gear, totally printable. The larger, the more resolution you get.

    If you had a 500mm linear rail on top of that, 1 degree would mean a change of about 9mm at the edge. So that’s about 0.15mm/step. That might be cutting it a little close, but I think these numbers are worst case. Making the gear bigger would help with strength and rigidity too, so something in the 120mm diameter would about as big as everyone could print and end up with a lot of resolution. 500mm is also pretty large radius, IMO. It would be twice as good at 250mm.

    #78026

    Jeffeb3
    Participant

    I was thinking 2040, mainly cause I think I might have a piece lying around.

    I haven’t got any of that lying around. I do have a bunch of conduit, 5/16″ bolts, and 608 bearings though.

    #78032

    karltinsly
    Participant

    Reeling out wire to the gantry – that’s crazy.

    I’m thinking of using a slip ring, like this: https://www.amazon.com/GeeBat-Capsule-Electrical-CIRCUITSx2A-monitor/dp/B01MYMM1KT/ref=sr_1_3?ie=UTF8&qid=1543867122&sr=8-3&keywords=slip%2Bring&th=1

    You can get them with different numbers of conductors. I was thinking of mounting the steppers and controller board on the spinning gantry, and then feed them with a simple two-wire slip ring.

    #78034

    Jeffeb3
    Participant

    I think it was Barry that told me that slip rings running any current will have some kind of reaction because it’s constantly making little arcs. I don’t know how long they would last. Maybe it’s plenty of time, and when they fail, it’s not life threatening.

    #78038

    Jeffeb3
    Participant

    Look at this one:

    #78042

    karltinsly
    Participant

    That video highlights some of what I was talking about with rigidity. Note that the arm, which is pretty long, is supported mid-radius with bearings riding on a raised track. Gives me some ideas of how to support mine.

    Darn it, I really don’t have time for this project, but I can tell I’m probably going to do it anyway!

    #78044

    Barry
    Participant

    Yea, slip rings are basically consumables.  They all eventually fail.  The good ones use graphite sliders, and the smoother they start the longer they last.  I’ve seen folks try to use the bearings in home made lazy susans pass current, is never ends well.

    #78045

    Jeffeb3
    Participant

    Yea, slip rings are basically consumables. They all eventually fail. The good ones use graphite sliders, and the smoother they start the longer they last. I’ve seen folks try to use the bearings in home made lazy susans pass current, is never ends well.

    Do you have any idea how long they would last? I mean, if it’s a 1000 hours of run time, that might be long enough for almost anyone.

    #78048

    Jeffeb3
    Participant

    Darn it, I really don’t have time for this project, but I can tell I’m probably going to do it anyway!

    Karl, I just got off the phone with your friends, family, and coworkers. We all agreed. This is more important. Netflix also called, and they aren’t going to release any new shows until this is done.

    3 users thanked author for this post.
    #78050

    geodave
    Participant

    This site may have a movement on it that might help.  At any rate, it is cool site to browse thru. http://507movements.com/

    #78054

    karltinsly
    Participant

    LOL that settles it, then!

    #78067

    Jeffeb3
    Participant

    This site may have a movement on it that might help. At any rate, it is cool site to browse thru. http://507movements.com/

    That is a neat website. If nothing else, it will bang on some pipes and get thing moving in my head. I looked through them, and I don’t see any obvious answers. There’s no corexy machine in there, so we should probably register the url for 508movments.com

    #78068

    Jeffeb3
    Participant

    Maybe the problem isn’t the polar table, maybe we just need better Cartesian focused designs?

    Ah! this problem is just really grabbing me.

    Ryan, what are your thoughts? It must be killing you to not just stop everything and CAD something up.

    #78069

    Ryan
    Keymaster

    packing boxes, prepping orders……thinking of ideas, lots of ideas. Polar loses accuracy the further out you go, though.

    #78070

    Barry
    Participant

    Yea, slip rings are basically consumables. They all eventually fail. The good ones use graphite sliders, and the smoother they start the longer they last. I’ve seen folks try to use the bearings in home made lazy susans pass current, is never ends well.

    Do you have any idea how long they would last? I mean, if it’s a 1000 hours of run time, that might be long enough for almost anyone.

    A good one for something like this should last quite a while, if you can get all the electronics on the moving part, and just push DC power over the slip ring.  Don’t try pushing AC over the ring.  You’d have to use wifi to push gcode to it.  Running power over a bearing will shorten it life span to weeks.

    #78081

    karltinsly
    Participant

     get all the electronics on the moving part, and just push DC power over the slip ring

    That’s my plan. Only thing going through the slip ring would be 12v DC.

     

    #78092

    Bill
    Participant

    Just design the magnet carrier to be offset enough that the magnet itself can ride over to the true zero center. You don’t have to line it up vertically with the gantry. You could also adjust the accuracy with wider circles by moving the ‘theta’ circle out more toward the outside of the machine. The position of that stepper is what determines the spin accuracy. You could also design the theta drive wheel and the circular gear to reduce noise when moving by using gear teeth that aren’t cut vertically but instead at an angle. Perhaps the big gear could be printed as several parts that glue together, or to the outside of a routed wheel?

    #78093

    Bill
    Participant

    When you move the theta motor you’re going to have to do a matching move with the radius motor or else the radius will change, since the radius motor is not connected to the big gear.

    #78099

    Jeffeb3
    Participant

    Yep, yep, and yep.

    I remember one of the firmware letting you configure your own motor equations. We’d have to do that.

    We can design to a precision. Bigger gear, more precise, but it will be slower in the middle. I think gears you screw to a routed circle would be cool.

    What sizes are common in continuous timing belts?

    #78101

    Ryan
    Keymaster

    Did you guys leave anything for me to solve?

    #78102

    Jeffeb3
    Participant

    Did you guys leave anything for me to solve?

    We really need some instructions 😛

    #78103

    Ryan
    Keymaster

    Nope, I’m out, I never get those right.

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