Thread on BA threads

I am a newbie… building a Stuart Victoria at the moment.

I have a set of “BA” tap and dies but find Stuart supplied parts and even external threads cut with these dies will not thread unless I drill out to #37 instead of the specified #39.

Is this an issue of “new” vs “old” BA? Old was apparently 47.5 degree included angle while new is 55 degree at same thread pitch?

If this is the case, I can’t locate “Old BA” taps for purchase- in most cases it is not specified as to included angle.

ANY TIPS on whether this is the issue and where (if at all) one can find old BA T&D sets would be MUCH APPRECIATED-

You don't say what size thread.

I am no expert but my chart gives #37 drill for BA5 . #39 drill is not mentioned anywhere on the chart.

See this link for BA sizes and tapping drills

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.

I'm confused you say external threads cut with these dies. Then say if I drill out...........

It could be you need to close up th edie a bit more if they are split dies or your taps are cutting undersize.

I tend to use a 2.7mm tapping drill for 5BA which about the same as a No36 as I don't use number drills.

Also if you are threading stainless steel rod you can get some extrusion and the OD of the rod ends up larger than when you started so make sure that die is closed down

Edited By JasonB on 02/05/2023 10:06:31

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

I was screwcutting stainless recently 3/16 x 40 and the crests formed up well over 3/16 and I had to turn them off before continuing. I try to spare my dies from stainless.

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

Whitworth form threads, (BSW, BSF, BSP, BSB ) are Imperial sizes as are Unified threads (Althopugh the thread forms are different, Whit d form is 55 degrees, Unified and matric are 60 degrees)

Britash Association is 47.5 degree thread form, and has been since inception, although bthe pitches are basically Metric, changing by a factor of 0.9 as the diameter decreases .

You drill to cut an Internal therad with a TAP

A DiIE cuts an External thread. Most Dies are split, and the holder provides a facility to either open or close th e Die, slightly to achieve to correct size of thread. (I usually adjust my Dies to a commercially made setecew or bolt. (The industrial machine will have been set with much more accurate gauges than the average amateiur can have in their workshop. )

If you are drilling to produce a thread,, you are using a TAP.

To use a DIE you turn the diameter, before cutting the thread.

If you want BA Tap and Die sets, go to either Tracy Tools of The Tap and Die Company.

Although more costly, my npreference is for High Speed Steel.

AND you should use a tappiung lubricant such as Trefolex or Rocol RTD.

As a newbie, you might find it worthwhile to buy and study No 32 in the Workshop Practice Series; "Drills, Taps, and Dies" It could save you broken taps or danaged external threads.

Howard

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.

Engineers Edge says 2.65mm (#37). Contrary to popular opinion, slightly bigger won't affect strength significantly. Using a drill the same as the core diameter is a guarantee of broken taps.

I've got 2 Zeus books, 1970 and 78/79. Both quote 2.65 mm

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

Regards Nick.

BA has not changed at all, neither have any of the other thread standards. -

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