[misterroy]

Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I
can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The
part has to withstand a pressure of 260 psi while a gasram is
pressurised. It looks like it is very close, will it be ok?

thanks

[Steve Lusardi]

No.
Steve

"misterroy" wrote in message ...
Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I
can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The
part has to withstand a pressure of 260 psi while a gasram is
pressurised. It looks like it is very close, will it be ok?

thanks

[Bob Engelhardt]

misterroy wrote:
Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I
can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The
part has to withstand a pressure of 260 psi while a gasram is
pressurised. It looks like it is very close, will it be ok?

thanks

It depends, as usual.

How many threads will be engaged? "psi" doesn't describe force on a
fastening. Do you mean "lbs"? If not, what is the setup? What is the
diameter of the threaded part?

Bob

[misterroy]

On Aug 28, 1:36*pm, Bob Engelhardt wrote:
misterroy wrote:
Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I
can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The
part has to withstand a pressure of 260 psi while a gasram is
pressurised. It looks like it is very close, will it be ok?

thanks

It depends, as usual.

How many threads will be engaged? *"psi" doesn't describe force on a
fastening. *Do you mean "lbs"? *If not, what is the setup? *What is the
diameter of the threaded part?

Bob

around 13 threads will be engaged. The part is an adapter from a
bicycle shock pump to a gasram in an air rifle. The inlet to the ram
is 36 tpi and the air pressure needs to get to around 260 pounds per
square inch whilst the ram is being repressurised, then the adapter
will be removed. The diameter is 5.2 mm.
The thread is here http://www.gewinde-normen.de/en/valve-thread.html
its vg 5
thanks

[James Waldby]

On Sat, 28 Aug 2010 06:10:05 -0700, misterroy wrote:
On Aug 28, 1:36Â*pm, Bob Engelhardt wrote:
misterroy wrote:
Hi, the lathe I have access to cuts only metric threads, I think. I
want to cut a thread of 36 tpi a pitch of 0.706 mm and the closest I
can get is a pitch of 0.700 mm, which gives 36.3 teeth per inch. The
part has to withstand a pressure of 260 psi while a gasram is
pressurised. It looks like it is very close, will it be ok?

It depends, as usual.

How many threads will be engaged? Â*"psi" doesn't describe force on a
fastening. Â*Do you mean "lbs"? Â*If not, what is the setup? Â*What is the
diameter of the threaded part?

around 13 threads will be engaged. The part is an adapter from a bicycle
shock pump to a gasram in an air rifle. The inlet to the ram is 36 tpi
and the air pressure needs to get to around 260 pounds per square inch
whilst the ram is being repressurised, then the adapter will be removed.
The diameter is 5.2 mm. The thread is here
http://www.gewinde-normen.de/en/valve-thread.html its vg 5

It seems unlikely that a part with .7mm thread will engage even half a
dozen turns with a 36 tpi threaded part. Still, that might be enough
engagement. A rule of thumb that I've heard (but conceivably might have
misunderstood) is that about 3 turns bear the load anyway, even if more
are engaged.

Are you planning for the thread contact to act as a pressure seal
(in which case I would be surprised if you could get even half the
pressure you want) or do you have an O-ring or a gasket to take the
pressure?

Do you plan to single-point cut an external thread?

What is the make and model of the lathe? For lathes with change gears
rather than a quick change gear box, you may be able to get some
unlisted tpi settings by proper gear selection.

If you have easy access to the lathe, just cut the .7mm thread on a
piece of scrap bar stock and see if it works.

--
jiw

[Bob Engelhardt]

misterroy wrote:
around 13 threads will be engaged. ... The diameter is 5.2 mm.
....

Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13
threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling
is that would work. Be a bit stiff, maybe, but doable. And you could
could cut your threads small by .003/13 (.0002) for an easy fit. That
is the width of the teeth would be .0002 small.

The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so
that's no problem.

These threads don't seem to be tapered, if so leak-stopping has to be
done with some sort of seal. Otherwise, even a perfect thread would
leak. And your slightly-off pitch is irrelevant.

Bob

[misterroy]

On Aug 29, 1:42*am, Bob Engelhardt wrote:
misterroy wrote:
around 13 threads will be engaged. *... *The diameter is 5.2 mm.

...

Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. *13
threads at .7mm is 9.1mm, giving .08 mm error (.003"). *My gut feeling
is that would work. *Be a bit stiff, maybe, but doable. *And you could
could cut your threads small by .003/13 (.0002) for an easy fit. *That
is the width of the teeth would be .0002 small.

The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so
that's no problem.

These threads don't seem to be tapered, if so leak-stopping has to be
done with some sort of seal. *Otherwise, even a perfect thread would
leak. *And your slightly-off pitch is irrelevant.

Bob

Thanks Bob, I hadn't thought to taper it to get a seal.

[Bob La Londe]

"misterroy" wrote in message
...
On Aug 29, 1:42 am, Bob Engelhardt wrote:
misterroy wrote:
around 13 threads will be engaged. ... The diameter is 5.2 mm.

...

Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13
threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling
is that would work. Be a bit stiff, maybe, but doable. And you could
could cut your threads small by .003/13 (.0002) for an easy fit. That
is the width of the teeth would be .0002 small.

The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so
that's no problem.

These threads don't seem to be tapered, if so leak-stopping has to be
done with some sort of seal. Otherwise, even a perfect thread would
leak. And your slightly-off pitch is irrelevant.

Bob

Thanks Bob, I hadn't thought to taper it to get a seal.

Sounds like you are making an adaptor for a PCP gun.

[misterroy]

On Aug 29, 3:34*pm, "Bob La Londe" wrote:
"misterroy" wrote in message

...





On Aug 29, 1:42 am, Bob Engelhardt wrote:
misterroy wrote:
around 13 threads will be engaged. ... The diameter is 5.2 mm.

...

Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement. 13
threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling
is that would work. Be a bit stiff, maybe, but doable. And you could
could cut your threads small by .003/13 (.0002) for an easy fit. That
is the width of the teeth would be .0002 small.

The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so
that's no problem.

These threads don't seem to be tapered, if so leak-stopping has to be
done with some sort of seal. Otherwise, even a perfect thread would
leak. And your slightly-off pitch is irrelevant.

Bob

Thanks Bob, I hadn't thought to taper it to get a seal.

Sounds like you are making an adaptor for a PCP gun.
close, its an hw90 http://www.sportsmanguncentre.co.uk/...EIHRAUCH+HW90/
To service the seals you have to depressurise it. The adapter puts the
pressure back in.

[Bob Engelhardt]

misterroy wrote:
Thanks Bob, I hadn't thought to taper it to get a seal.

Tapered fittings usually have the male & female tapered to match. E.g.,
the US NPT. Tapering just one would mean that only a couple of threads,
at the most, would be fulled seated. You might get a good-enough seal,
especially since you only need it long enough to get the cylinder to
pressure. Or maybe not.

Made-in-brass might work better - it would deform more easily than
steel, to match the existing threads.

Bob

[Bob La Londe]

"misterroy" wrote in message
...
On Aug 29, 3:34 pm, "Bob La Londe" wrote:
"misterroy" wrote in message

...





On Aug 29, 1:42 am, Bob Engelhardt wrote:
misterroy wrote:
around 13 threads will be engaged. ... The diameter is 5.2 mm.

...

Let's see: 13 threads at .706mm (36 tpi) is 9.18mm total engagement.
13
threads at .7mm is 9.1mm, giving .08 mm error (.003"). My gut feeling
is that would work. Be a bit stiff, maybe, but doable. And you could
could cut your threads small by .003/13 (.0002) for an easy fit. That
is the width of the teeth would be .0002 small.

The 5.2mm diameter at 260psi is only 8 lbs force on the threads, so
that's no problem.

These threads don't seem to be tapered, if so leak-stopping has to be
done with some sort of seal. Otherwise, even a perfect thread would
leak. And your slightly-off pitch is irrelevant.

Bob

Thanks Bob, I hadn't thought to taper it to get a seal.

Sounds like you are making an adaptor for a PCP gun.
close, its an hw90
http://www.sportsmanguncentre.co.uk/...EIHRAUCH+HW90/
To service the seals you have to depressurise it. The adapter puts the
pressure back in.

Gas spring? Like the Browning Nitro Spring?