Screw Thread diameter Help

[Martin King 2Participant @martinking2]

Need to diameter to match internal thread

[Martin King 2Participant @martinking2]

Hi all,

Just sold my ML7 and have got a very nice Super 7 with Gearbox and VFD which is a joy to use.

For the first time I can get around to the many thread jobs that I have waiting. Have cut a few practice external threads using the gearbox with no problems.

Now I have to make several screw in filler caps for various oil cans that are missing them, most are 26tpi brass. My question is how do I calculate the diameter that I machine the bar to before cutting the thread? These are all different sizes, the first is 0.866" diameter for the internal female part as shown.

I presume there is a formula for this? Will it be the same formula for Whitworth Metric BA etc?

Cheers

Martin

[mechman48Participant @mechman48]

Look in the Zeus book… in the thread tables it gives O.D / maj.dia. pitch, effective depth etc. for various threads.

George.

[ASFParticipant @asf]

Wouldn`t you add 0.0246" x 2 onto any internal diameter?

See **LINK**

[Brian OldfordParticipant @brianoldford70365]

Posted by ASF on 24/11/2016 15:49:13:

Wouldn`t you add 0.0246" x 2 onto any internal diameter?

See **LINK**

To get a full thread depth. Yup.

[Neil WyattModerator @neilwyatt]

If the thread isn't full depth you may get problems.

Screw bit of softwood into the hole, it will form to match the thread then measure that.

Neil

[Clive FosterParticipant @clivefoster55965]

Basic dimensions for brass threads here :- **LINK** .

The essential "wot thread is it" identifier here :- **LINK** in text format which may need bit of massaging for easy reading.

Prettier version of the same data here with a link to an Excel file version **LINK** .

A PDF version arranged to print out on A4 here **LINK** . Something I need to get round to.

With grateful thanks to Andy Pugh, Ian Wright and Micheal Jones who did all the hard work of compiling this data and sorting it all into a format making it easy to identify almost anything. My library is pretty comprehensive in such matters but nothing else comes remotely close in comprehensiveness and range covered. There are some missing but you'll have to have majorly offended the gremlin master if anything of unknown provenance containing such comes your way.

This list has kept my tail out of the fire on more than a few occasions and given me a totally uwarranted reputation for cleverness in certain naive quarters.

Clive.

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