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Gears

Does anyone have an idea as to where I might find a plastic gear six inches in diameter with 57 TPI? I want to put an arm turntable on my robot base and turn it with a DC motor. I will build a servo channel into the P2 and use a multi-turn Pot as my position feed back. Or if I can get it to work I will use an AS-5040 that I have for the position information. I haven;t tried to make a 5040 function with the P2.
Thanks,
Jim

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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,185
    edited 2021-12-22 18:51

    I can make you one. How thick (1/16", 3/32", 1/8"), and what kind of hole in the center? Material will be black acetal copolymer (like Delrin). I assume 6" is the pitch diameter and not the overall diameter, and that 57 is the total number of teeth? The pressure angle will be 20° (most common) unless you need something different.

    Also, if you need mounting holes for the arm, I can put those in, too.

    Finally, I can also cut the gear from 1/8" MDF. MDF makes great gears, since the laser cutting caramelizes the edges, resulting in very durable tooth surfaces. In fact, if you're meshing with a plastic gear, I would recommend MDF over acetal. Just don't get it wet!

    -Phil

  • I have a library function for the AS5048a unit that I use in a weather station to determine wind direction. The library is in C though.

    Mike

  • ercoerco Posts: 20,068

    @"Phil Pilgrim (PhiPi)" said:
    I can make you one.

    I love PhiPi! He's the most helpful guy around, even when it's not Christmas!

  • Thanks Phil. I will get back to you on the details.
    Jim

  • RS_JimRS_Jim Posts: 1,570
    edited 2021-12-25 00:42

    Phil,
    further info the gear would need to be 1/8" thick. it would be mating with a spur gear that is 48P. The large gear mounts on a lazy susan turntable from Home Depot. I will have to get one to get the hole placement I plan on using a nylon or plastic screw in the center if I can find one with a hex head to easily mount the circular magnet for the AS5040.
    Does this help? I realize I need to get you all of the info befote you could make the gear.
    Jim
    Edit, Maybe it would be easier just to make a set of gears. 1":& 6"?

  • Jim,

    Are you sure about Home Depot? The only bearing I see there is too big for a 6" dia. gear. I think my local hardware store might have something that'll fit, though. I'll have to check. If not, I know McMaster-Carr has something.

    Also, 48P is probably too fine to laser cut from 1/8" material. Can you get a coarser pinion gear, say, 24P or even 16P ?

    -Phil

  • Duane DegnDuane Degn Posts: 10,507
    edited 2021-12-26 03:14

    Also, 48P is probably too fine to laser cut from 1/8" material. Can you get a coarser pinion gear, say, 24P or even 16P ?

    Is this something a 3D printer could handle?

    I don't know the terminology well enough to know if this is something printable using a 0.4mm nozzle. If someone produces an STL file, I'd be happy to try to print it.

    Edit: If a 3D printed solution would work, I would also be willing to attempt to model the part in Fusion 360. I think a set of gears is within my present 3D modeling skill set.

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,185
    edited 2021-12-26 04:26

    Is this something a 3D printer could handle?

    Unlikely. The gear teeth, in addition to being so fine, have to maintain that involute shape. I've got a 3D printer, but this would not be an appropriate app for it.

    Truth be known, I think a continuous-rotation servo would be adequate for this app. It's already geared down, and I can make a pinion gear to mate with it that meshes with the larger spur gear. I've already done this with the robot arm I designed for the local high school:

    https://forums.parallax.com/discussion/171391/new-robot-arm-with-blocklyprop-documents-online

    You can see the lazy-susan bearing in the photo. I used a Parallax Servo360 for the drive, so I didn't have to add an encoder. But the base only rotates 180° and is geared 2:1. That way, with proper placement of the gears, I didn't have to keep track of zero crossings with the Servo360's encoder. But zero crossings are not an insurmountable obstacle. I was just being lazy. :)

    BTW, the robot arm assembly manual provides some additional detail about the gearing:

    http://pthsrobotics.org/blockly/docs/PTHS Robotic Arm Assy.pdf

    The download is huge, though. Sorry.

    -Phil

  • Phil,
    Wow, I downloaded the manual onto my laptop as I am running out of room on the iPad. Yes, the file is huge! It took me a while to figure out that you had cut the gear for the base on the inside of the turntable. My idea is to use a gear that goes on the turntable with the teeth on the outside. This way I can use solid center with an AS5040 magnetic position encoder and a cog in the prop2 as my servo and have very accurate positioning of the arm base. I think I can get a turntable from my local HD that is about the same size as the MC unit you are using. Hopefully I can check this week. I won't be off until Wed again so that may be the earliest I can get there.
    Regards,
    Jim

  • My idea is to use a gear that goes on the turntable with the teeth on the outside.

    Yup, got that. I just wanted to illustrate how a continuous-rotation servo could be used to drive a pinion gear that would mesh with the larger spur gear. That way, a coarser pitch is easier to achieve, since you don't have to find a coarser pinion gear for sale somewhere.

    The other advantage of using a coarser-pitch gear set is that the distance between the two gear axes is not quite so critical to maintain. With super fine teeth, being off even slightly can result in slippage.

    -Phil

  • I asked for the finer because I had the other pinion, courser is ok.
    Jim

  • Phil,
    I got my cheap turntable from HD and find that I will be better off with internal teeth. The only issue with that is I will require two MDF squares instead of just the one with teeth. The inner opening on the TT is 4.66". I am thinking that working with 4" would be good. The blank 4x4 just needs a 0.25 hole in the center. I will use flat washers between layers to provide the necessary spacing. What else do you need?
    Jim

  • Hi Jim,

    I'll need accurate locations and sizes for the mounting holes to the turntable.

    Thanks,
    -Phil

  • Phil,
    The base is a 6 x 6" square. If you draw a line corner to corner diagionaly across the square, there are 4. 1/4" holes on the diagional line centered 3/8" fron the adjacent sides. Clockwise from the 1/4 holes are 4 5/32" holes displaced from the nearest edge by 3/16" and by 9/16" from the farther edge.
    Does this help?
    Jim.

  • Attached is my interpretation of your specs. Print it out, and hold it up to your bearing against the light to see if I got it right.

    BTW, I would recommend a thin layer between the gear and the center hole to prevent your pinion from rubbing on the sheet with the hole.

    -Phil

  • RS_JimRS_Jim Posts: 1,570
    edited 2022-01-01 14:49

    Phil,
    Small holes appear to be perfect, large holes not quite. I will put it on a light table tonight after work, and re-measure .25 holes.
    Jim
    Adit: will use pan head screws w/o counter sink to help between layer spacing.

  • If you have calipers, measure the distance between two adjacent holes, between near edges and between far edges. Then average the two readings to get the center-to-center spacing. That's all I need.

  • RS_JimRS_Jim Posts: 1,570

    Phil:
    I have to correct my measurement on the placement of the .25" holes. They are centered on .203" from the edges along the cross square diagional.
    Jim

  • Sorry this took so long. My internet was down for a couple days. Try the attached.

    -Phil

  • RS_JimRS_Jim Posts: 1,570

    Phil,
    Drawing is perfect. Don't forget small holes only go into the gear layer and the solid layer also needs a .25"hole in the center.
    Thanks
    Jim

  • Here's a rendering of the gear layer, encoder layer, and an optional spacer layer. The first two would be cut from 1/8" MDF. The spacer layer, if you want it, can be made from a thinner material.

    The gear is 24 pitch with 96 teeth with a 4" pitch radius. I think I can do 32 pitch, if that's too coarse. Also, the gear in the picture is non-inverted (external teeth). I'm going to try getting it inverted to form proper interior teeth. However, it should work okay, as-is.

    -Phil

  • RS_JimRS_Jim Posts: 1,570

    Great Phil, lets do all three layers in the 1/8" mdf.I like that better than the screw heads. how about we put the small holes in the spacer layer and counter sink them?
    Jim

  • how about we put the small holes in the spacer layer and counter sink them?

    I don't think we need to. All three layers and the lazy Susan bearing can just be bolted together at the corners.

    BTW, how is the encoder to be mounted? And how does the shaft couple to the center hole?

    -Phil

  • RS_JimRS_Jim Posts: 1,570
    edited 2022-01-05 13:14

    Phil,
    The encoder is a chip mounted on a PCB.That can be mounted to the bottom deck with screws, glue DF tape etc. A 1/4" nylon bolt comes throughnthe encoder layer and then the circular magnet is glued to that.A nut above and below the encoder layer provide the adjustment to get it close but not touching the chip.
    Jim
    Edit, Phil,how big would the outside tooth gear have to be if we went with that and just a single layer? I would have to build on a new base, but I really need to do that anyway because of the size of the turntable.

  • For a 6" bearing, an outside-tooth gear would have to be at least 8.5" in diameter.

    -Phil

  • RS_JimRS_Jim Posts: 1,570

    Oh yeah, I forgot about the length of the diagional. Never mind.
    Jim
    The squaw of the hipopotamos is = to the sum of the squaws of the other two hides.

  • How about using a timing belt? That way the large MDF sprocket can be smaller than the diagonal, with the motor positioned a distance away. Check these out:

    https://www.mcmaster.com/timing-belt-pulleys/for-maximum-belt-width~1-8/
    https://www.mcmaster.com/toothed-belts/width~1-8/

    They're cheap!

    -Phil

  • Also, if you'd rather use a 1/4" belt, I can sandwich two large sprockets together.

    -Phil

  • RS_JimRS_Jim Posts: 1,570

    Phil,
    I need to think about the implications of using the toothed belt. Maybe the best solution is to get one of the Macmaster Kar(sp?) which would reduct the base size considerably and go back to the idea of an outside toothed gear. I think the diameter would need to be around 2.9" and that would work fine for me.
    Jim

  • One nice aspect of a toothed belt is that you can virtually eliminate backlash by adjusting the belt tension. Both my laser cutter and my 3D printer use toothed belts, and there's zero evidence of backlash in the items they produce.

    -Phil

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