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  1. #1
    Join Date
    Aug 2020
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    Wynyard Tasmania
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    78
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    8

    Default Best plywood for gears/pinions?

    I've just bought a Shapeoko CNC which, among other things, I intend using for cutting wooden clock gears, escapements etc. My first tests (on 7mm Tas oak) show that it does this very well. However, most wood clock builders seem to be using plywood for their gears since this offers some obvious advantages (strength and dimensional stability - especially in humid Tasmania). I've spent half a day finding what plywoods are available in Australia and the selction is confusing.

    Can anyone who has already been down this track suggest the best ply to use - Russian Multiply, Finnish aircraft birch ply (only available up to 6mm), birch, poplar or eucalypt laserply and so on? Some are very expensive!

    Less urgently - for visual reasons, maybe after my first clock, I'd like to cut the gears from local hardwoods (possibly from blanks assembled from perhaps 6 pie shaped slices). One problem would seem to be changes in size of the gear in response to ambient humidity. Is this a significant issue? If so, I thought of immersing the gears in a natural wood oil (Tung?) in a pressure cooker connected to a Hivac pump. After several hours at very low pressue, during which the pores should empty of air, I would then return the pressure to atmospheric in the hope that the oil would then penetrate deeply into the wood, replacing the air. I'd have to be careful that I didn't reduce pressure to the point where the oil boils at room temperature, so some prior experimentation would be prudent. Has anyone any comments on this approch, or is it too madecap to merit attention?

    Many thanks for reading this and, possibly, for your comments!

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  3. #2
    Join Date
    Feb 2003
    Location
    back in Sydney, after spending 3 years Covid stranded in Canada
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    67
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    Default

    only your second post,
    so welcome to the "mad house" more properly known as the place to ask questions, get helpful advice and also to contribute your work in progress stories and pictures (including your Duh moments).


    plywood considerations ...
    needs to loo good for appearance.
    needs to have minimal voids in the plies -- the preference is no voids. Voids in a plywood gear will possibly tender the gear unusable.
    up front cost? In reality who cares? -- you will be using so little to make the gears that the cost should be compared to the value of your time invested in the clock project.
    regards from Sydney

    ian

  4. #3
    Join Date
    Apr 2011
    Location
    McBride BC Canada
    Posts
    3,542

    Default

    Dimensional stability for gears and puzzle pieces.
    The gold standard here is "Baltic Birch Plywood."
    Usually available in 30x30 cm squares, void and knot free.
    Most that I have is 12 mm which I use as foundations for wood carving tool strops.
    I would not hesitate to do a glue-up if I ever needed anything thicker.

    I have half a dozen daVinci machine models and other 3D puzzles built entirely of BB plywood.
    In the past, I could buy many different laser cut Christmas ornaments, made of BB, in annual Christmas craft fairs.

  5. #4
    Join Date
    Aug 2020
    Location
    Wynyard Tasmania
    Age
    78
    Posts
    8

    Default

    Thanks Ian. You are quite correct about the cost of the wood being fairly insignificant when weighed against the investment in time, thought and energy.

    BTW - Where in Canada do you live? I also am Canadian (from Vancouver Island) but moved to Tasmania on retirement. I still miss Canada greatly - even the snow!

  6. #5
    Join Date
    Dec 2007
    Location
    Sydney
    Posts
    1,952

    Default

    Baltic Birch Ply or Aircraft Ply would be ok.
    I used to make model buildings for wind tunnel testing and used aircraft ply.
    From memory it was available from about 1mm to 19mm.
    Bruynzeel ply at Brookvale had 1/2 sheets, 1200 x 1200.
    Model shops sell smaller pieces.
    There was also similar ply available which was the same but not Ďcertifiedí for air craft construction which was cheaper.
    Your idea re solid in 6 segments makes sense, thatís how Patterns are blocked up especially for turning as the grain direction is where you want it.
    Also helps stabilise re shape and shrinkage
    Mahogany was used in low volume high end Foundry patterns because of its stability and ease of working.
    Red Cedar even Huon was also used for the same reasons.
    H.
    Jimcracks for the rich and/or wealthy. (aka GKB '88)

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