BA IS 47.5* ! Whitworth form is 55* and metric (& others) are 60* ! I am unaware of ANY new BA at 55* ? A tapping chart will give you the sizes, be it in Number drills or metric, but make sure you use good sharp drills that are to spec on size , there is some VERY poor quality about. Noel.

There is no old and new BA – the standard has never changed. Which size BA thread are you cutting? #39 doesn't seem to be useful as either a tap or clearance drill in the BA series.

My guess is the recommendation to use #39 (2.52mm) is a mistake on the plan or in the instructions.

Tubal Cain's highly recommended "Model Engineer's Handbook", which has much to say about sensible thread engagements, suggests:

• 2.75mm (#36) for 5BA
• 2.34mm (#42) for 6BA

I refuse to have anything to do with Letter and Number Drills, because they're bonkers! So for tapping 5BA I'd use either 2.8mm or 2.7mm. The first gives a loose engagement (easy to fit, but weaker), the second a tight one (hard to fit, but stronger). For practical reasons (see Tubal Cain), I prefer loose engagement. Same logic with 6BA – either 2.4mm or 2.3mm.

Beware Model Engineering plans! For all sorts of good and bad reasons they often contain mistakes.

Dave

As per SOD, plus that a 55 degree angle would be Whitworth thread form? BA and Whitworth thread pitches are two entirely different series.

Zeus Charts give the tapping size for 5 BA as No 39 (2.56 mm or 0.1008" )

A No 37 drill is 2.65 mm or 0.1043"

Tubal Cain's "Model Engineers Handbook" (A very useful reference book ) gives the core diameter for 5 BA as 2.49 mm or 0.09803" dimater, so a No 39 drill will result in a thread truncated by about 0.001" (0.0127mm) a side..

Not a bad thing, since it will prevent root / crest interference.

You may fgind it a good idea to read other books in the Workshop Practice Series. They will extend your knowledge and understanding, to augment your practical esxperience.

Howard.

Hi, there's quite a bit of info on various screw threads in this Babani No. 6 paperback.worldradiohistory.com

Regards Nick.

Tubal Cain's book Drills, Taps and Dies (No. 12 in the Workshop Practice Series) goes into even more detail. For 5 BA he gives:

• 2.80 mm for 59% thread engagement
• 2.78 mm for 60%
• 2.75 mm (No. 36) for 65%; this is his recommended size
• 2.7 mm for 70%
• 2.65 mm for 75%
• 2.60 mm for 80%

While it may seem a good idea to go for maximum thread engagement it gives very little extra strength, and the rate of tap breakages goes up dramatically. Somewhere between 60% and 70% is generally reckoned to be appropriate for general purposes.

This is the real Tubal Cain by the way, not the other one of the same name. I keep Drills. Taps and Dies next to Eats, Shoots and Leaves in my bookcase. Well, it makes me giggle.

George