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Thread: Yankee Spiral Screwdriver Bits from Hex Bits

  1. #1
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    Yankee Spiral Screwdriver Bits from Hex Bits

    I found that the #2 Phillips bit for my spiral screwdriver was rounded in the drive direction, so the bit would cam out prematurely. After finding that adapters may not be the best option (reports of slop, and some only support short bits) and that old bits are expensive to buy used online, I decided to find a cheap alternative.

    For middle size drivers (9/32 in or 7mm shank, North Bros or Stanley 30 and 130, Miller Falls 61 series), modern hex bits are the perfect diameter. The bit needs two notches to be cut to seat properly (both aligned with a peak of the hex):
    - Square notch at the far end from the tip
    - A little more than halfway through
    - 1/8 in (3.175 mm) from the back end
    - Triangular notch near the middle
    - 95.5 degree triangle
    - 0.65 in (16.5 mm) from the far end
    - 0.05 in (1.27 mm) deep (to get a 0.11 in, 2.8mm wide notch)

    I just used a triangular file (cut the depth, then angled the file to get the width) and a mill file. After chamfering the edges, it fits well into the chuck and drives well.

    This could be used to fit a driver type, hex bit size, or hex shank accessory that weren't originally available and/or wouldn't fit the hex adapters. To speed up the cuts with a mill, an end mill and a chamfer bit could be used to make the cuts.

    If anyone tries this, a few tips on fine tuning:
    - Start with a square notch halfway through the width of the bit at the end, and if the back end doesn't seat cut the notch a little past halfway. It'll add some rotational slop, but the old bits also had slop to allow an easy insertion.
    - If the middle notch isn't seating, the collar at the front will be loose in the axial direction and may turn less easily.
    - The triangular notch isn't deep or wide enough for the lock to seat fully. Increase the depth, angle, or width.
    - The notch isn't aligned (refiling the notch, or removing some material from the back notch, will help).
    - If the bit is catching internally, make sure the edges are chamfered. The back square notch originally had a large chamfer, as shown in the pictures, to allow the chuck internals that mate the middle notch to slip past.

    Hope this helps as a cheap solution or a solution in a pinch. A few pictures are included to make the solution a little clearer.

    Side Profile of Notches.jpgOriginal Bit In Focus.jpgNew Bit In Focus.jpg

  2. #2
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    Thank you Greg for this informative post. I had to call up my Law of Sines calculator and using a 1/4" bit the overall diameter is 7.33mm which is close as dammit to 9/32".
    "If you have all your fingers, you can convert to Metric"

  3. #3
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    Thanks, Greg.

    This is handy.

    Stan

  4. #4
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    Last edited by lowell holmes; 11-29-2015 at 10:45 AM.

  5. #5
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    Quote Originally Posted by lowell holmes View Post
    For the #30 and #130 (OP's size), LV only has the square bits.
    "If you have all your fingers, you can convert to Metric"

  6. #6
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    Actually, Lee Valley lists both 7mm and 8mm bits.


    My 2 Push screwdrivers (never knew anyone made metric size) bought at an electrical supply house 30 years ago. I never measured their diameters, but they work!
    One is a Millers Falls and the other is Klein. Why would anyone be concerned as long as the bits work.
    Last edited by lowell holmes; 11-29-2015 at 6:20 PM.

  7. #7
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    Good info! Thanks for the tip.

    However, if the bit is soft enough that you can file the notch with a file, it is way too soft to bother using.

  8. #8
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    Quote Originally Posted by lowell holmes View Post
    Actually, Lee Valley lists both 7mm and 8mm bits.
    Yes but the 8mm bits don't fit into the #30 and #130. The basic gist of this thread. If you can't get these then the information provided by the OP is invaluable.
    "If you have all your fingers, you can convert to Metric"

  9. #9
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    The bit was made of S2 tool steel, and while cheap diamond needle files and a mill file worked slowly a fine toothed bi-metal hacksaw blade didn't cut at all. I assume the bit (a common brand) was hardened some, but I have yet to put the bit through a lot of use to see how well it lasts.
    Last edited by Greg Krummel; 11-30-2015 at 12:22 AM.

  10. #10
    I'm way too lazy to modify every kind of bit I might want to use. I recently welded a hex adapter to the shank of a bit that fits a bit brace. When I get around to it, I will modify the shank of a hex adapter to fit my Yankee 30. Only have to make one modification to use any hex-shanked bit or even small hex-shanked sockets..

    As far as the filing/milling described above, a micrometer and a scale make it easy to duplicate the original geometry of a shaft--if you have those tools and an original Yankee bit..

  11. #11
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    Quote Originally Posted by Greg Krummel View Post
    The bit was made of S2 tool steel, and while cheap diamond needle files and a mill file worked slowly a fine toothed bi-metal hacksaw blade didn't cut at all.
    Ah, diamond file. Now it sounds more reasonable.

    A grinding stone in a Dremel would save some effort.

  12. #12
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    Google Yankee 30 screwdriver bits. You will find numerous sources.

    I bought a new set of drill bits online for my Yankee drill some years back. I've never regretted it.

  13. #13
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    Lee Valley sells a hex bit adapter in three sizes so there is one to fit all Yankee models.
    Only $7.50 and you can use all your hex bits.

  14. #14
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    Quote Originally Posted by Ron Kanter View Post
    Lee Valley sells a hex bit adapter in three sizes so there is one to fit all Yankee models.
    Only $7.50 and you can use all your hex bits.
    Yes but is it Impact ready?
    "If you have all your fingers, you can convert to Metric"

  15. #15
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    Quote Originally Posted by Hilton Ralphs View Post
    Yes but is it Impact ready?
    Why would you need impact ready bits for a Yankee Screwdriver?

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