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Thread: For those of us that are mathematically challenged, is there a formula

  1. #1
    Join Date
    Feb 2007
    Idaho Falls, Idaho

    For those of us that are mathematically challenged, is there a formula

    I am trying to make some pseudo inside out ornaments. I drill the holes into the blank, then round the blank the on the lathe. The piece in the pics started as a 2 X 2 piece of ash.
    IMG_2775opt.jpg IMG_2776opt.jpg

    It seems to me that if I want a .25' inch side post, the hole would need to be 1.5". The larger hole here is 1.5 inches. When I tried to round the blank, the inside tried to become larger than the outside ; traditionally thought of as bad form. I didn't get the blanks even close to round. I tried a smaller hole at 1.25", and it worked, but by the time I got the blank round, the sides at the center of the hole are so thin that the pressure from the tailstock crushed the blank. I also don't want them to be so fragile for sanding, and handling afterward. Obviously I need a smaller hole, 1 1/8", or 1", which is easy enough, and I can determine how small by trial and error. But with different sized blanks, I would have to start over guessing for each one. I will never get anything finished if everything is trial and error. I am also wasting my wood stash. So my question is, if you have made these pseudo ornaments, is there a formula you use to determine what size hole is needed, exterior size vs hole, or a percentage of hole size to the blanks exterior size? I want it to look delicate without being too fragile. If I master this with a 2" blank, I want to do other sizes, and don't want to try trial and error on larger more expensive blanks. TIA for your help. By the way, I now realize ash was a very poor choice on wood to experiment on these. A chippy, splintery, tearout prone, mess.
    Last edited by Brian Brown; 09-05-2023 at 9:12 PM. Reason: Ash comment

    Sawdust Formation Engineer
    in charge of Blade Dulling

  2. #2
    Join Date
    Nov 2007
    Houston, Texas

    Mr. Walt Wager did a demonstration of these at SWAT this year, but he didn’t mention any sort of ‘formula’ for the hole vs. the blank, and I don’t recall anyone asking for such a ratio. Probably would have been good to inquire about that. He also published an article on this very thing in the November 2019 issue of Woodturning Fundamentals. If you’re an AAW member you can look it up. In that article, he used a 1-3/8” (35 mm) Forstner bit on a 2”x 2” blank. I’m about to try my hand at a few of these myself, so I’m glad you raised the question — I now have something else to think about before it start them .

    Best of luck to you. Let us know how they turn out.
    Don't let it bring you down,
    It's only castles burning,
    Just find someone who's turning,
    And you will come around

    Neil Young (with a little bit of emphasis added by me)

    Board member, Gulf Coast Woodturners Association

  3. #3
    Join Date
    Feb 2007
    Inver Grove Heights, MN
    I have done several Christmas ornaments that way. I admit to using the eyeball formula where I hold the forstner bits up to the blank and use the one that looks good. With a 2 inch blank I might end up with a maximum 1 3/4 round. The section where the holes are is usually square when I drill so 1/4 inch for each vertical piece beside the holes leave room for a 1 1/4 inch hole. That ends up with the vertical piece being 1/4 inch across the face of a hole, or 3/8 inch tangential to the surface of adjacent holes. So, if you really want a formula mine would be the width of the stock minus 3/4 inch gives you approximately 1/4 inch beside your holes. It might be better to use the expected diameter of your stock after you make it round, but you really don't want to drill round stock if you can avoid it. Good luck, ornaments are well received done this way or done by inside out turning. The difference is in how quickly you can produce them this way.

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