Largeish bsw taps

[Ian ParkinParticipant @ianparkin39383]

i have had a problem this last few days with 1&3/8” bsw taps

I have had one a presto brand that i chipped a few teeth off being greedy with cutting under power in the lathe.

I always single point cut the thread and then pass the tap through to clean it up.

i bought some nice looking ones off eBay.

came to use them yesterday and i cant get my head around how different they are in sizes

so theres a RLL brand spiral cut one that measures 1.400 across the tips

a presto one that is 1.380

and a CEJ thats only 1.314 across

none of them look worn but the CEJ has in very faint etching “. 1/2 SER. “

Why are these all different sizes and if i try to use the spiral cut one it cuts completely wrong..it wont pick up the screw cut thread and seems to cut a new form completely.

Any ideas on what’s wrong?

[Ian ParkinParticipant @ianparkin39383]

[Clive FosterParticipant @clivefoster55965]

I think the CEJ is intended to be used like a serial tap to reduce the tapping loads on these big BSW threads.

Start with a taper tap and run in until it gets to be really hard work. Probably no more than a turn or so at full depth thread. Run the undersize CEJ in and finish off with a full size bottoming tap. If a set were made it would replace the second tap. But I don't think serial sets were a thing in those days.

Being undersize the tapping loads are lower and the tendency to pull sideways is reduced.

Big coarse BSW taps are a pain to keep true when used by hand. Doesn't help that the taper on the taper tap is often excessive so its difficult to get them cutting well enough to pull in. I have one biggie that is basically a reamer. Have to step drill the end a thread or two deep to get it to start by hand. Leaning on a tapping head in the big Pollard drill teaches it manners tho'.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian Parkin on 29/12/2021 09:52:48:

Any ideas on what’s wrong?

.

It doesn’t help, and I must assume that it’s an optical illusion, but …

that Presto tap looks much more than 20thou smaller than the RLL

MichaelG.

[Author]

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