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  1. #1
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    Default Tapping Drill Chart

    I have a copy of the Engineer's Black Book. It has tables with tapping drill sizes for most of the thread standards. There are two problems with it. Firstly, the text is too small for my ageing eyes. Secondly, all the tapping drills sizes given are for 77% thread engagement. If you want the size for a different % engagement you need to use the formula given. I decided to make a table for the metric sizes a hobbyist might use for various % engagements.

    It's much easier to read. And as I don't have every drill size made, it allows me to work out how much thread engagement I will get for the drill size I do have on hand.

    I hope someone finds it useful. If you would like any extra info added to the table, let me know and I'll see if I can oblige. I'm working on a UNC and UNF version too.
    Attached Files Attached Files
    Chris

  2. #2
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    For metric threads I always use hole diameter minus thread pitch equals tap drill.
    Gold, the colour of choice for the discerning person.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Get yourself an ecopy of the 100 page, Workshop practice Series #12 by Tubal Cain. "Drills, Taps and Dies" . The Appendix has about 10 pages of easy to read, drill sizes and thread engagement charts in the back for all types of threads.
    [QUOTE]For metric threads I always use hole diameter minus thread pitch equals tap drill.[/QUOTE]
    According to the ISO Metric formula this approach will give >85% thread engagement.
    This can be hard on taps and hands especially in harder materials, even when using a good lube.
    Eg M10 1.5, would work out to be a drill size o 8.5, whereas I typically use 8.8mm for a 75% engagement which will work for most things I do. When I made my DP table with the 25 x 5/16" holes I used a 0.1mm larger drill than recommended which produced 70% engagement. Those thread/holes are 20 mm deep and that's plenty of thread @70% to clamp down objects.

  4. #4
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    Quote Originally Posted by .RC. View Post
    For metric threads I always use hole diameter minus thread pitch equals tap drill.
    That will give you 77% thread engagement. Probably fine for most applications, but maybe too much for tougher materials?
    Chris

  5. #5
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    Quote Originally Posted by BobL View Post
    M10 1.5, would work out to be a drill size o 8.5, whereas I typically use 8.8mm for a 75% engagement which will work for most things I do.
    According the formula in the Black Book, 8.8mm would give 60% engagement.
    Chris

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by jack620 View Post
    According the formula in the Black Book, 8.8mm would give 60% engagement.
    That is closer to Bolt Thread engagement, which is slightly different to Nut thread engagement. The Tubal Cain books gives both and specifies an approx average biased slightly in favour of the Nut thread engagement.

    In the Tubal Cain book 8.8 mm will produce a 77% nut thread engagement and 66% bolt thread engagement for an overall effective engagement of 75%.

  7. #7
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    Quote Originally Posted by BobL View Post
    That is closer to Bolt Thread engagement, which is slightly different to Nut thread engagement. \
    Which is relevant to tapping?
    Chris

  8. #8
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    This web based calculator gives exactly the same numbers as my table. OSG | Taps | End Mills | Drills | Indexable | Composite Tooling | Diamond Coating | Die Products
    Chris

  9. #9
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    Chris, thanks for going to the trouble of posting your guide. I have others, but the extra info in yours will be very useful!
    Much appreciated��

    Michael

  10. #10
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    I've made up THIS table for work, I printed it on 4 A3 pages and I carry a copy around on my phone for quick reference. It doesnt have your % engagement request, but some people might find it handy. It covers metric, fractional inch, decimal inch, gauge / letter drills, metric coarse and fine, UNC, UNF, UNEF, BSW BSF from 0 through to 1 inch.

    If anyone has a suggestion for more detail that would suit my table though, feel free to point it out.

  11. #11
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    Chris thanks for sharing your chart.

    In this day and age I tend to use one of the online calculators or an app for the phone to look that kind of info up. I don't have the smarts to calculate my own. Easier to print the chart from the magic box. One example. Theoretical Machinist

    Pete

  12. #12
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    Quote Originally Posted by QC Inspector View Post
    In this day and age I tend to use one of the online calculators or an app for the phone to look that kind of info up.
    Thanks Pete. The online calcs/apps are great, but I tend wear gloves in the workshop (I know, I know!) and I have to remove them to use the phone. Plus I don't like greasing up my new iPhone. My dream is to have a dedicated iPad mounted to the workshop wall for just this purpose, but that's an expensive option. My old surplus iPad wont run the latest OS sadly.
    Chris

  13. #13
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by jack620 View Post
    Thanks Pete. The online calcs/apps are great, but I tend wear gloves in the workshop (I know, I know!) and I have to remove them to use the phone. Plus I don't like greasing up my new iPhone. My dream is to have a dedicated iPad mounted to the workshop wall for just this purpose, but that's an expensive option. My old surplus iPad wont run the latest OS sadly.
    I just have an H&F drill and Thread size chart behind a large acrylic sheet mounted on my shed door. Also tucked behind the acrylic sheet are several tables inside plastic sleeves from the Drill, Taps and Dies book.

  14. #14
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    Quote Originally Posted by .RC. View Post
    For metric threads I always use hole diameter minus thread pitch equals tap drill.
    thats simple

    will try to remember that

  15. #15
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    Quote Originally Posted by scottyd View Post
    I've made up THIS table for work, I printed it on 4 A3 pages and I carry a copy around on my phone for quick reference. It doesnt have your % engagement request, but some people might find it handy. It covers metric, fractional inch, decimal inch, gauge / letter drills, metric coarse and fine, UNC, UNF, UNEF, BSW BSF from 0 through to 1 inch.

    If anyone has a suggestion for more detail that would suit my table though, feel free to point it out.

    I can't see accurate data for 4-48 ? Only because I've just used that size and your chart gives 5.3 mm and 0.2090 diameters.
    Which is wrong.
    Best Regards:
    Baron J.

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