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    Discussion in 'The main mechanical design forum' started by Freddy15, Mar 2, 2015.

    1. Freddy15

      Freddy15 New Member

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      Hi,

      I'm currently working on a school project in which I have to design a portable ski sharpening device. The product will consist of three main parts : The base part which will be in contact with the ski, a sliding part with a spring which will apply a constant pressure on the edge of the ski and a top part which will hold the motor and grinding wheel (See drawing).

      [​IMG]

      The top part must also be adjustable so it can grind a different angles (from 90 et 85 degrees with a 0.1 increment) and I can't figure out a simple way to do so. Does anyone have a mechanism idea or any standard part that could adjust the angle by increment?

      Thank you,

      Fred
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      Hi Fred,

      The key thing in any design - is to have a look out there and see how everyone else has solved this problem - otherwise you can end up re-inventing the wheel - or wasting a huge amount of time.

      Additionally, it is always worth checking the design requirements/goals are sensible/realistic.

      So if you have a look at the video below - you will see they are using shims to set the rotation angle by 0.5 degree increments. I have no idea why you would want to achieve a 0.1 degree increment - to me it appears too precise - and as a skier myself - I don't think you would be able to decide whether 85.3 degrees was better or worse than 85.4 degrees.

      Hope this helps.

      https://www.youtube.com/watch?v=WZb9c_MEgHU
       
    4. godbehere

      godbehere New Member

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      Hi Freddy15,

      There may be something better suited to your task, but my first thought is to use some sort of precision adjustment screw such as this:
      http://www.mcmaster.com/#97424a770/=wfv5w9

      Then you can adjust the height (let this length = y) and the length of the angled "top part" (let this length = H) is known. You can calculate the angle using arcsine(y/H). Then you can fabricate a scale with the appropriate angles and place it to align with a needle on the end of the "top part".

      [​IMG]

      You'll probably also need to incorporate some sort of locking mechanism to ensure the angle doesn't change while the motor is running.

      Don't know if this helps at all. Let me know how it works out.

      Best Regards.
       
    5. Freddy15

      Freddy15 New Member

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      Thanks for your reply.

      This is almost exactly the mechanism I decided to go with. However, as the angle increase, there will be a misalignment between the screw and the threaded hole. I was thinking about making a welded assy of a threaded rod, a bellow coupling and a shaft which would hold the knob, so that the assembly can be flexible (see pictures). I was also planning on using a positioning indicator (http://us.misumi-ec.com/vona2/detail/110300086220/?Inch=0) and place the lifting rod at a distance which would make one turn equal one degree.

      [​IMG][​IMG]


      If you have any other ideas to solve this angular misalignment problem please let me know! I will be looking more into adjustment screws like the one you suggested, they seem to be made exactly for this purpose.

      Fred
       
    6. Chris Cantrell

      Chris Cantrell Member

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      Freddy15, your problem reminds me of a Dremel attachment I saw on TV yesterday. You can find it here. The Dremel solution only works for preset angles, but a slight modification to the design would make something similar work for any angle desired.
       
    7. Bill @ ERG

      Bill @ ERG Member

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      It is possible to include a drilled rod with flats for the adjustment rod and nut to seat against in the moving plate. If you install a stiff compression spring to hold the assembly against the nut (open) you may not need any other locking device.

      There are also ball seat wachers available, or you might use a lug nut from a car wheel (or machine a nut with a spherical end) and spherical seat - most of the lug nuts have a spherical seat profile.

      Because of the small angular change (only 5°) properly designed you only have a ±2.5° variation to contend with.
       
    8. Dana

      Dana Well-Known Member

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      A bellows coupling is expensive and overkill for what you want. You might consider what is sometimes called a "barrel nut". I can't find a picture, let's see if I can describe it:

      Take a piece of round bar, say 1/2" diameter by 1/2" long (the barrel). Brass works well for these. Drill and tap across the middle of it (i.e. perpendicular to the axis of the bar) for your adjusting screw. Now make another one just the same, but instead of tapping it drill a larger hole so the screw can spin freely in it. One of these mounts to the base and the other mounts to the moving part; in both cases the axis of the rod is parallel to the hinge line and they're free to rotate. The screw threads into the tapped one, and at the other end it's just retained from moving but allowed to spin (in its simplest form a couple of jam nuts and washers will do to retain it). Because the barrels can pivot, they stay in line with the screw, and rotating the screw gives your adjustment.
       
    9. Bill @ ERG

      Bill @ ERG Member

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      I checked on the ball seat washers, and they tolerate ±3° of motion, and are manufactured to DIN . I wish to correct my comment above - common lug nuts are conical rather than spherical bearing surfaces, so the lug nut probably isn't appropriate. Dana better explains my suggestion for the barrel arrangement. The washers conform to DIN standard 6319, and are available in metric and imperial sizes from many suppliers. For best results, you might wish to put a shallow counterbore at the washer seat to prevent movement and thread binding. Either a barrel nut, or a washer and an anti-rotation feature would be required on one end.
       

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