Focusing rack for view camera build

Discussion in 'Camera Building, Repairs & Modification' started by Andrew Moxom, May 20, 2008.

  1. Andrew Moxom

    Andrew Moxom Member

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    As I start to work on the focusing rack for my medium format view camera build, I realize I need to know a bit more about my options on gears/racks available. So based upon feedback here, I went to the Small Parts website and am looking at either 32 pitch or 48 pitch spur gears and respective racks in brass. My choice right now is the 48 pitch with either 12 or 15 teeth as this will give me a smaller overall height requirement, and also as this is not going to be a bohemoth sized camera, should suffice. What I am asking for is whether this is adequate. What are others using for their view camera builds and why?
     
  2. makan

    makan Member

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    It is an interesting fact - as long as the front standard isn't too loose on its rail (doesn't wiggle)and can move more or less smoothly it's not so important if you move it by hand or by the gear with teeth. In fact, if you had a small ball bearing for the standard you could focus it moving it by hand as well as with anything else. I found this when I made my first 4x5 and decided in favor of a simpler non geared variant. Works with no problems at all even witha heavier lens.
     
  3. Colin Graham

    Colin Graham Member

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    I've been using brass 20 degree pressure angle, 48 pitch for cameras up to 8x10. If I remember right, teeth per inch in racks is relative to pressure angle- 20 degree is 15 tpi in this size. It always seemed like it would be finer and smoother to focus than 14.5 degree. But it's been ten years since I actually researched this and may have misremembered. Regardless, I still buy the same sizes because they do work very well.

    For spur gears I use 18 teeth- 3/8" pitch diameter- mainly for size considerations. But I usually rebore the hub to take a 3/16" shaft- 1/8" has a lot of flex in it.
     
    Last edited by a moderator: May 22, 2008
  4. Steve Smith

    Steve Smith Member

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    Does anyone know where to buy racks and gears like this in the UK? I have only managed to find short lengths of rack.


    Steve.
     
  5. panoramic

    panoramic Member

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    pressure angle doesn't determine smoothness

    In a nutshell:

    The pitch number is the number of teeth in a one inch diameter gear. (it is actually the pitch circle and the gear is slightly bigger because the circle size is measured between the bottom of the tooth and the top of the tooth)

    A 32 pitch gear that is one inch (1.03125 inches) in diameter has 32 teeth and a 48 pitch gear that is one inch (1.020833 inches) in diameter has 48 teeth.

    14.5 degree pressure angle gears are steeper than 20 degree pressure angle gears. It was originally thought that the 14.5 degree PA was smoother and the 20 degree PA gear was stronger. I'm not certain but I believe testing has proved this is not important.

    So a 32 pitch rack will have 10.186 teeth per inch and a 48 pitch rack will have 15.279 teeth per inch regardless of the pressure angle used.

    If you are using a really small diameter pinion gear, you will have more problems because of leverage even though the ratio would seem to be in your favor, try to use a bigger pinion gear because it is easier to set the tooth depth and make it run smooth. Too loose causes gaps as there is a point where no gear contact happens until the next in line tooth makes contact, too tight causes binding.

    I've made hundreds of gears for cirkut panoramic cameras.

    Hope this helps.

    Ron Klein in Alaska
     
  6. Colin Graham

    Colin Graham Member

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    Great info, thanks Ron.
     
    Last edited by a moderator: May 23, 2008
  7. Andrew Moxom

    Andrew Moxom Member

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    Thanks a lot.
     
  8. Frank Szabo

    Frank Szabo Member

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    Go to http://www.bostgear.com/ and check out what they have.

    Their goodies should be available through your local industrial supply outfit (WW Grainger, etc.)