Tapping drill sizes for BA threads in steel

[Ken BevanParticipant @kenbevan42381]

I need to tap some small BA size threads in steel. I do remember reading some advice on increasing the size of the tapping drill size, over & above the recommended size without incurring any detrimental effect on the thread surfaces in contact; but can't remember the advice offered. Can anyone suggest an approach on how to proceed based on experience? I know I could experiment, but it could result in broken taps etc.or thread sizes with insufficient contact.

[Ken BevanParticipant @kenbevan42381]

[RobboParticipant @robbo]

You might find this chart on Harold Hall's website useful, giving 3 different sizes for varying degrees of thread engagement. – **LINK**

If I remember correctly, Tubal Cain in his Workshop Manual used to suggest going for 65 degree engagement. (Now ducking into bunker!)

[Thor 🇳🇴Participant @thor]

Hi Ken,

I don't use BA threads myself, but Tubal Cain in his book: Drills, Taps and Dies recommend using a larger tap drill in steel, a 60% or 65% engagement should be Ok. A few examples (drill dia. in mm) that may be of help:

BA No   60%   65%   70%

8            1.89   1.86   1.84

6            2.44    2.4    2.38

4            3.15    3.1   3.05

 

Thor

Edit: Robbo beat me to it

Edited By Thor on 13/07/2017 17:39:29

[JasonBModerator @jasonb]

One of the biggest problems is as the thread size gets smaller you will have a job finding drills that cover the small differences of 5% engagement and even if you find them do they cut dead on size. I tend to use one size for all materials.

[GeorgineerParticipant @georgineer]

Posted by Thor on 13/07/2017 17:36:46:…Tubal Cain in his book: Drills, Taps and Dies…

Excellent book, that. I keep it on my bookshelf, next to Eats, Shoots and Leaves.

George

[Jon GibbsParticipant @jongibbs59756]

Posted by Georgineer on 13/07/2017 23:09:55:

Posted by Thor on 13/07/2017 17:36:46:…Tubal Cain in his book: Drills, Taps and Dies…

Excellent book, that. I keep it on my bookshelf, next to Eats, Shoots and Leaves.

George

Brilliant! Really made me smile.

[Ken BevanParticipant @kenbevan42381]

Thank you all for the speedy response to my request. The advice is most helpful and exactly what I was looking for, particularly the reference to the depth of engagement. Will go ahead today -Thanks.

[Author]

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