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  1. #1
    Full Member Behemothrex's Avatar
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    Angry Teeth on a curve?

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    Hey Guys,

    Once again my class is asking me to draw something I never have, with zero instruction. Did I mention the textbook has zero drawing instruction in it LOL.

    So I am drawing this part that has teeth, which is no big deal, but the teeth need to be wrapped around the wheel. And for the life of me I cannot find a way to bend them around the curve of the wheel.

    Polar array will not do it, at least like I am using it because if I draw the teeth in their normal form, a rectangle, then use polar array it arranges them fine, but they do not conform to the circle.

    I have enclosed a bad scan of the drawing, but you can see the teeth.

    Any suggestions would be appreciated.

    Thanks!
    Attached Images

  2. #2
    Super Member nukecad's Avatar
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    Polar array will not do it, at least like I am using it because if I draw the teeth in their normal form, a rectangle, then use polar array it arranges them fine, but they do not conform to the circle.
    Can you explain more what you mean by "they do not conform to the circle".
    Do you mean that the number of teeth do not fit in a whole number with the diameters?

    Unfortunately we can't see how many teeth there should be, the note is off the top of the scan, or what the diameters are.
    Are the teeth supposed to be all the way around the part?

    Polar array should do the job.

    It might help if you think about how teeth like this would be cut on a machine.
    (Think of it as grooves rather than teeth).

    e.g.
    Put the part in the centre of a turntable.
    Mill one groove across the edge.
    Rotate the part.
    Mill the next groove.
    Rotate the part.
    etc.

    That's basically what you are doing with a polar array:
    Draw one groove.
    Copy the groove and rotate it a bit.
    and again, and again. (But all done at once by the polar array).
    Last edited by nukecad; 30th Jul 2015 at 09:40 am.
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    Seems to me we're missing some information re: inner/outer diameters of the ring the teeth are cut into.

    Either the landing or the gear tooth would have to taper as you go from the O.D. to the I.D. right?

    BTW...are you drawing this in 2D or 3D?
    Last edited by ReMark; 30th Jul 2015 at 11:41 am.
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  4. #4
    Full Member Behemothrex's Avatar
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    Hey guys,

    Well the outer circle is a diameter of 1.98 the inner is 1.62. And I am trying to put them on the top view, so this is 2D I will enclose my drawing. I guess I could draw each groove manually, but there is no indication on the drawing itself what the arc should be, they show the structure off the teeth at the bottom and give all measurements so you can draw them. But of course this is in a straight line, not curved to conform to the diameter of the circle. And since the book gives no info on how this is normally drawn, or represented in 2D I came here. Okay let me upload my drawing.

    Thanks again!

    Doh hey nukecad, just rereading your post and yes I think polar array could do it, if I did one tooth at a time, I was trying to do the whole row of 5 at a time, face palm! Let me give ti a shot.

    Update: Okay doing each tooth individually and using polar array I got a result, is it correct according to the measurements, probably not, as I just had to fudge with the polar array numbers until it looked similar to the drawing. As this project is about sectioning not drawing. But I would still like to know how to do it so they match the drawing of course LOL. But this drawing gave no angles, other than the diameters of the circles, and it gave the width between each tooth .020 but obviously they are going to have to be farther apart at the outside and closer at the inside. Is this normally noted?
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    Last edited by Behemothrex; 30th Jul 2015 at 04:57 pm. Reason: Enlightenment

  5. #5
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    In the image drawing this in a isometric type 3d view (If described wrong please correct) or illustration drawing. Which has a bit of artistic license, you would now draw it real full 3d, slice it and view from a point. Back to the teeth because its a 3d illustraion your teeth are actually going along an ellipse so the way to draw them would be to segment the ellipse say by pitch angle they will actually vary as you go around hence why they only draw a few.
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  6. #6
    Super Member nukecad's Avatar
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    Quote Originally Posted by ReMark View Post
    Either the landing or the gear tooth would have to taper as you go from the O.D. to the I.D. right?
    Quote Originally Posted by Behemothrex View Post
    but obviously they are going to have to be farther apart at the outside and closer at the inside. Is this normally noted?
    If you think about it logically they will probably be mating with a spur gearwheel, which will have teeth cut straight across the OD at the same width.
    So the 'grooves' on this part should be cut straight across. If they were cut at a taper then they would not mate correctly.
    As Remark points out there will have to be a taper somewhere and this would be on the top lands of the teeth.

    If you think about the example I gave above - using a milling cutter would leave the bottom of the grooves the same width all across with a slight taper on the top lands.
    That would be the easiest way to cut these teeth.

    I'm guessing a bit here, but from the jpg you attached I would say that the tooth profile shown is at the outer diameter and the top lands would be smaller at the inner diameter.

    Normally the only time that gear teeth would be specifically cut on a taper is for Bevel gears.
    If that were the case then there would need to be a lot more info about the teeth.

    BTW, those shown are very simple profiles for gearteeth.
    If teeth need to transmit any kind of power then the profiles would be involute curves rather than just straight sided grooves.
    Gear tooth profiling is a pretty specialised subject though and I would not expect it to be covered in a basic CAD course, you only tend to learn about it if your company is actually manufacturing gears.
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    Full Member Behemothrex's Avatar
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    Thanks for all the input, the exercise was a sectioning exercise, so the teeth did not have to be greatly detailed, I was just trying to figure out how to get them on the curve. Obviously if you were to build that part you would have to get more technical when it comes to the angles, etc. But I was just frustrated because I could not get them to align, but that is because I was trying to do all 5 teeth at once instead of one then using polar array and then copying. And it was just a simple multiview drawing nothing 3d.

    Thanks again.

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