Problems with tapping drill sizes for 2BA bolts.

[Nick TParticipant @nickt]

[Nick TParticipant @nickt]

I have a set of HSS BA taps and dies from Tracy Tools and have successfully tapped 4 & 6 BA threads using the drill sizes recommended in Tubalcains's Model Engineers Handbook.

Today I was tapping 2 BA bolts and the recommend 4.1mm drill was way too big. I checked the bolt diameter and it is 3.89mm in diameter. I checked tap drill sizes elsewhere and 4mm was recommended.

Any advice would be welcome. Thanks. Nick.

 

Edited By Nick T on 11/09/2016 18:24:32

[JasonBModerator @jasonb]

Sounds like your bolts are not 2BA more like an unersize 3BA. Have you checked the tpi?

I use 4.0mm as a tapping size for 2BA but for general use your 4.1 should be OK.

[Nigel BennettParticipant @nigelbennett69913]

Might even be M4?

A 2BA screw should fit (very loosely but it should work) in an M5 nut.

[Nick TParticipant @nickt]

Thanks Jason & Nigel. I have just remeasured and they are 4mmm in diameter, the 3.89mm one was a damaged bolt. I will check with the supplier tomorrow as it looks as though the packet might be wrongly labeled. Nick

[Gary WoodingParticipant @garywooding25363]

The over-thread diameter of a 2BA screw is 4.7mm

[roy entwistleParticipant @royentwistle24699]

In proper measurements use a 22 drill

Roy

[KWILParticipant @kwil]

Roy,

Some people may ask 22 what? Most might guess at a Number 22!

[Tim StevensParticipant @timstevens64731]

Many people who ask for advice here might have difficulty finding a 22 drill, if there was nothing that size in what they inherited. According to Machinery's Thread Book the tapping sizes for 2BA are:

3.8mm (93%), 3.9mm (83%), 5/32" (75%), 4.0mm (74%). No number drills are mentioned in this table, which dates from 1972, and offers a clue about what was considered 'proper' 44 years ago.

There is no such thing, I suggest, as a correct size for a tapping drill. All authorities agree in saying use a size a bit larger than the minor diameter, but how much bigger is left to the judgement of the user (and depends mainly on what she has in the drills drawer).

Cheers, Tim

[Nick TParticipant @nickt]

Thank you all for your help. Supplier has just rung and confirmed that they were mis-picked, which is obvious in hindsight but BA is all new to me – quite happy with metric! Nick

[Neil WyattModerator @neilwyatt]

Posted by Tim Stevens on 12/09/2016 14:30:46:

There is no such thing, I suggest, as a correct size for a tapping drill. All authorities agree in saying use a size a bit larger than the minor diameter, but how much bigger is left to the judgement of the user (and depends mainly on what she has in the drills drawer).

Tubal Cain suggests 65% engagement in his handbook and gives the drill sizes.

I printed out a table for metric and BA which sits behind my lathe.

Neil

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