BSPT hand tapping

[knoba]

I'm after some guidance on manually cutting a BSPT female thread form, for a 'sub 100psi' pressure vessel project, in situ.

The vessel is already prepped with a welded slug in the centre of one of its heads. It will, finally, be pressure tested.

I have a 3/4" BSPT hand reamer & Tap, but am unsure how to proceed next...

Do I drill undersize of the threads' minor diameter and ask the reamer do the rest?
Should I be fabricating a guide jig for the reamer (to keep it straight)?
Is there an easy method of how far to ream, & tap other than trial and error?
When I stop cutting with the tap, won't it leave fluted cut marks on the internal thread?
What is the tightening sequence for this fitting (Turns Past Finger Tight)?

Can anyone offer some advise, direction or recommend some good reading material?

 

[browny289]

hey mate just wrote big response then lost it when I went to post gr:wall:
please ask if something doesn't make sense not very good at written response's.

marks in thread- sometimes, sometimes not, wouldn't worry to much about this as bspt is not a dry seal needs thread tape and this more than lkly take it up...

depth of thread- if you have a male bspt an female fittings screw them together should go 4-5 turns, so when u tap your thread use the male fitting like a test gauge will easily hold 100psi hydraulics go much higher

holding reamer straight- if there is no center on the end of reamer dial it up in the 4 jaw drill a center . place work to be tapped in a vice on a drill press/mill, place a dead center in the chuck, use the dead center in the end of the reamer this should hold it straight with slight downward pressure from the dead center use a tap wrench to turn tap.

drill size - cant help to much here but the bspt taps I have done manually I have got a drill size from a chat then just tapped the hole with success with no reamer

but I do stand to be corrected....
hope im of some help
Browny.

 

[Omnimill]

Tapping sizes here:

http://www.newmantools.com/taps/bsp.htm

I've only cut a couple but I've used trial and error checking with the plug frequently and not had any problems. Do a test on some scrap first if you're concerned. As for keeping the tap straight I found it easier with a plug tap than a normal one but if you can guide it so much the better.

 

[browny289]

just read the in situ. doh

 

[knoba]

Yeah, sorry about that, browny, there was a lot of info to digest...

Anyway, with much thanks to you guys, it's going well. All's good!

*beer*