Is there any way to cut a 1/8 BSPT thread apart from a taper turning attachment or a CNC lathe? (dies are no good as it isn't exactly a standard thread form -close to 1/8 BSPT but not quite). Just fishing to see if anyone has any bright ideas that I've missed.
Regards
Kevin
1/8 BSPT seems pretty close to 1/8 BSPT to me
You can get taper thread chasers for Coventry die heads, I've got a couple (not 1/8") but never tried them so can't comment on their effectiveness. Also I think special die heads were available to do the job.
There must be other ways?
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
In message , Kevin Steele writes
Kevin, Unless I am missing something 1/8 BSPT is the taper form of 1/8 BSP - The thread pitch is the same (which isn't exactly clear from your wording above... which is why I say I might be missing something)
When I was with the gas industry we cut 1/8 BSPT on tubing using a die. The die is of course tapered..... These items were common enough and I probably have some in the workshop.
This will only cut a taper portion equal to the thickness of the die but that is adequate to engage in a BSP pipe fitting.
Regards,
are you 100% sure is BSPT and not NPT... the reason i sak, is i have just been bitten by NPT, assumed i needed to buy 1 BSPT gauge, but really needed a NPT one, and it just wont seal, not with all the PTFE tape in the world!
anyone know where you can get some 6, 25, and 28 bar 40Ø rear entry NPT gauges?
OK, let me explain this fully. I need to cut a core for an injection mould tool that will produce a 1/8" BSPT thread in the finished part. So the core must be a reverse form of the thread, plus a shrinkage allowance.
If I were to cut it using a die although I might get away with the shrinkage I would have a male thread form, and when the part was moulded around it and the core removed the part would have a male thread form in a hole (which doesn't work very well!). It is always a pain doing threads in mould tools, but being tapered just makes it harder.
Regards
Kev>
A pipe threader will give you an approximation to BSPT male, and the better ones will be adjustable so you can make it oversize.
Does the thread absolutely need to be tapered? a lot of applications use a tapered male into a parallel female. Are you concerned about the crests/roots of the threads being wrong?
How about working between centres with an offset tailstock or a boring head type offset centre holder?
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Screwcut it using one piece of a 28 tpi coventry die head held in a holder and slewed on the piss to the angle required to form the taper. infeed to suit specs.
Something this small and short it would probably be as quick to turn the chuck by hand with the key
-- Regards,
John Stevenson Nottingham, England.
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How much shrinkage? and does the pitch need to be accurate enough to take it into account?
Mark Rand RTFM
About 1% in this case, and you can usually get away with the pitch being slightly out. What you can't normally get away with is using a die to cut it, as the rounded/truncated sections of the thread end up in the wrong place when the moulding is made from the part. On a male thread the crest would be truncated and the root rounded, but obviously if you then mould off this it goes the wrong way round. Clearances have to be swapped around as well, otherwise the threads don't fit together -it's all a bit of a bugger really.
Regards
Kevin
< snip >
Hi.
BSPT is a 55 degrees thread and NPT is a 60 degrees one.
Conical treading has been developed to be hermetic without any kind of seal devices.
Minimum and maximum Inner and outer diameter are normalized.
Of course taps and dials are a bit smaller and longer than exactly one to compensate wear and material nature.
Threadable only by tap, die or CNC machinery (EDM).
BSP and BSPT is not a good solution as so only one (1) thread is the right one.
WinXP, from Venice, Italy
Set your tailstock over.
In that case grind three flutes off a tap and put this in a holder on the piss and use as a screwcutting tool.
Or am I missing something ?
-- Regards,
John Stevenson Nottingham, England.
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John Stevenson said
Unlike you John but yes I think you are missing something.
The thread form on taper threads is still 'right' to the axis of the work - using a tap or chaser 'on-the-piss' would put the thread form 'right' to the taper.
JG
Yeeeesssss. OK then, get a tapered tap grind the lead off and 3 flutes. Mount it parallel and use this to screwcut the thread. Correct pitch, right roots and crests and the right angle.
-- Regards,
John Stevenson Nottingham, England.
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John Stevenson said
That is more like the answer I would have expected the first time around :)
JG
"Conical treading" (sic) has been developed to be hermetic without any kind of seal devices."
Could that be an assumption that has allowed Venice to become so damp over the years? :-)
Tom
This is an important point, that I had missed. just because the fitting that has to go into the threaded hole is tapered does NOT mean that the hole has to have a tapered thread. Apparently it is quite acceptable (indeed, common practice) to fit tapered fittings into parallel threads.
Anyway, after getting them to agree to us putting a parallel 1/8" BSP thread in they have now changed thier mind again and gone to a push in fitting. Bit of a bugger that as it means the hole must have perfect surface finish, and be sized to +/- 0.05mm (that's a tight tolerance to hold on an injection moulded part). Bloody customers, never can decide what they want.
Offsetting the tailstock is an obvious (if tricky to set up) option that I hadn't thought of -I knew there would be an answer somewhere.
Regards
Kevin
Why didn't you suggest it then
-- Regards,
John Stevenson Nottingham, England.
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But whaterer you cut the thread with wouldn't it have to move along an angle (along and out at the same time) in order to cut a tapered thread. This was the problem I couldn't work out without a taper turning attachment or a cnc lathe. The screwcutting gear on my lathe will only move the saddle along, and the cross slide has to move out at the same time.
I think setting the tailstock over is a good suggestion, and although tricky to set up I can't see why it wouldn't work.
regards
Kevin
Yes, but unless the tap is being fed at the correct pitch using the screwcutting gear you will just cut a tapered workpiece with a series of grooves around it. And once you feed the tap along the workpiece, as the screwcutting mechanism moves the saddle along parallel to the workpiece it doesn't matter what angle the tap has on it the job will be cut to the largest form.
To cut a tapered thread on a lathe the cutting tool (single point or tapered multiple point) must move in two axis at the same time.
Or have I got in a terrible muddle here?
Regards
Kevin
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