1/8" BSPT

We are getting there. First bit. Yes the screwcutting will have to be set to the same pitch as the tap. Because the TAP is tapered the lead or start part, ie. the thinnest part, will finish up at the thick end of your male thread.The tail end or thick part of the tap will produce the thin end of your male part.

Imagine a chamfering tool ground to 45 degrees and traversed along the bed. It will produce a chamfer on the work at 45 degrees. Now tilt this tool so it is at an angle of 5 degrees and traverse that across and all the while it's on the face of the tool it will produce a male spigot with a 10 degrees included angle. That's the part you are trying to duplicate with your tap but not at

10 degrees, that's just an example.

-- Regards,

John Stevenson Nottingham, England.

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John Stevenson
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John Stevenson said

I am still 'lurking' and re-learning the (dormant) skills I originally aquired some 45 years ago. Threads were my 'forte' for some 3 years while at Herbert Small Tools (Coventry Dieheads - amongst others) so I was confident on the point that I raised but needed time to think the whole issue through.

Incidentally, (and not directed at you, John) setting over the tailstock would only be any use for an external thread and even then the issue of the form being 'right' to the axis would need to be addressed in the chaser setup.

JG

Reply to
JG

JG said

Duh!

We are ALL missing the very obvious!

I know that this is now academic since the customer has changed the design but . . .

We are dealing with a TAPER thread here which by definition gets larger the further into the work you tap. The only difference from the standard is that it needs to be 1% larger.

When you measure a taper-threaded hole you screw the gauge in and measure how much of the gauge still protrudes. In the case of 1/8 BSPT the dia at the top of the large end of the thread is 0.383" and the gauge should protude 5/32". In this case we need a top dia of 0.3868" (1% oversize) so . . . . with a taper of 3/4" per foot (on the dia) or

1/32" per inch on radius we need to wind the tap in an extra 0.06128" beyond the 'Standard'. At 28 TPI this amounts to turning the tap an extra 210°.

To measure the thread, the gauge should only protrude 0.095" rather than

0.15625"

Voila!

I said I needed to get a 'handle' on the real problem :)

There is still the problem with the pitch but the discrepancy would be about 0.0003" (per thread) and over the 3/4" (standard length of 1/8" BSPT) it would be 0.013". I'd hate to try to determine the gearing to overcome this issue though!

JG

Reply to
JG

Tom said

Machinery's Handbook - Screw thread systems - British Standard Pipe Thread - length of thread in coupler.

Length of thread on Pipe end is 3/8" where the error would be half what I quoted -

I did arise at a silly hour when the thought came to me :)

Since then I have also been thinking :

How is the part to come off the mold if it is threaded? Surely threads ought to be cut after moulding - but I'm sure this a simple matter of split moulds and has already been sorted.

JG

Reply to
JG

The axis of the tap is parallel to the work, that way the taper tap form is transferred, in the cutting, to the work. Because of the multiple cutting faces, the thread could be cut in one or two passes.

As for usung the tailstock setover, I'd be surprised if it would be practicable.

Tom

Reply to
Tom

Yes

Reply to
Tom

We are talking about an internal thread in the finished component, which means cutting an external thread on the core that the thread will be moulded around.

The only way to remove the threaded core after the part has been moulded is to unscrew it. In this case it would have been manually unscrewed (as the volumes are quite low), on a fully automatic mould tool it would have to have an unscrewing mechanism built in -these are often hydraulically driven from the hydraulic system on the injection mould machine.

The thread could be cut into the material with a tap after moulding, but this is undesirable in some materials (especially glass filled) as it destroys the nice hard skin on the outside of the component -in the case of glass filled components exposing glass strands which can cause problems.

Regards

Kevin

Reply to
Kevin Steele

I was thinking that as you traversed the tap it would cut to it's largest size, and you would end up with a straight thread at the smallest diameter. I think what you are pointing out is that if the tap is withdrawn at a point (probably with a quick-retract toolpost?) then you would indeed have a tapered section of thread.

Regards

Kevin

"clonk" -that must be a penny dropping

Reply to
Kevin Steele

Yes, easier with the die head as it automatically releases at the right point. If I were to try this (although I don't need to now) on a lathe using a tap mounted in the toolpost I think I'd be templet to turn the spindle by hand -especially as I don't have a quick return toolpost.

Regards

Kevin

Reply to
Kevin Steele

"over the 3/4" (standard length of 1/8" BSPT)"? Where do you get this dimension?

Tom

Reply to
Tom

If one used a coventry diehead attached to the saddle with 1/8" BSPT chasers in it, it would cut a tapered thread, in this case it is the same as using one chaser instead of 4 in the diehead. As with using a diehead to cut tapered threads, the length cut is critical.

Tom

Reply to
Tom

I didn't mean to use a diehead, I was just illustrating how a tap would duplicate the same action. As for using a tap in the, toolpost, no different than cutting with a single tip tool.

Turn the spindle by hand? What are electric motors for?

Tom

Reply to
Tom

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