I'm having my second go at a piece of torture.

The item in question is the screw threading for the Nano.
The 0.1cc diesel published in Model Engineer some years ago.

I've built one of these before that should still be sitting on the end
of Craig Wall's desk as a paperweight. In that instance we spark
eroded the ports and stuffed them up with all ports ending up at the
same height in the cylinder.
This incarnation wont make the same mistake.

I dont recall just how I did it before but in this torture I'm having
trouble getting good threads. If I look at them under a 10 power
magnifier the threads look like a wire pulled through clay. tear smear
tear smear tear smear, you know the stuff.
the real killer is that I'm using 1214 free machining steel this time.

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?
I'd welcome a description of how you get really schmick threads at
this size.

(other than the steel threading it all comes out ok.)
Stealth Pilot

Stealth Pilot wrote:

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?

7mm shouldn't be a real challenge. Or is it the mix of imperial and
metric? :-)

HSS bit, experiment with cutting oil, supporting with a live center.
Other things that have some influence:
Clamped top slide, cleaned bead and lead screw all freshly oiled and moved
back and forth a few times. Slightly clamped saddle.


Nick
--
The lowcost-DRO:
http://www.yadro.de

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?
I'd welcome a description of how you get really schmick threads at
this size.

The problem you're fighting is cutting speed. The SFPM on that small a
diameter is too slow unless the lathe is running at high RPM.

Karl

On Sun, 7 Oct 2007 08:57:56 -0500, "Robert Swinney"
wrote:

Karl sez:

"The problem you're fighting is cutting speed. The SFPM on that small a
diameter is too slow unless the lathe is running at high RPM."

More'n likely if he is running that fast, the problem is the snaggle-toothed tool bit resulting from
crashing into the chuck at end of cut. 7/25.4 = 0.2756 inch is not a "small" diameter for
threading. I would seriously consider taking a close look at the helical angle involved. There may
not be enough relief below the nose of the tool.

Bob Swinney
"Karl Townsend" wrote in message
tanews.com...

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?
I'd welcome a description of how you get really schmick threads at
this size.


Karl


of course! gods I never even considered that aspect at this size.
hmmmm grind a wedge out of hacksaw blade to cock the tool over.
easy squeezy!

I'll have another go tomorrow night.

Stealth Pilot

Robert Swinney wrote:

I would seriously consider taking a close look at the helical angle
involved. Â*There may not be enough relief below the nose of the tool.

That's about a 0.5mm pitch. That is a fine pitch for 7mm diameter.
Standard pitch for M6 is 1mm!


Nick
--
The lowcost-DRO:
http://www.yadro.de

On Oct 7, 1:32 pm, Nick Mueller wrote:
Stealth Pilot wrote:
apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?

7mm shouldn't be a real challenge. Or is it the mix of imperial and
metric? :-)

Nick

I suspect he meant 0.7mm. I can't imagine 7mm fasteners on a 0.1cc
displacement engine or looking at 7mm threads with a 10 power
magnifier.

Dan

wrote:

I suspect he meant 0.7mm.

Uh!? 0.7mm diameter and a 0.5mm pitch!? Now *that* is really a
challenge. :-))


Nick
--
The lowcost-DRO:
http://www.yadro.de

wrote:

On Oct 7, 1:32 pm, Nick Mueller wrote:

Stealth Pilot wrote:

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?

7mm shouldn't be a real challenge. Or is it the mix of imperial and
metric? :-)


Nick


I suspect he meant 0.7mm. I can't imagine 7mm fasteners on a 0.1cc
displacement engine or looking at 7mm threads with a 10 power
magnifier.

Dan

For a guess I would expect that is the thread to adjust the upper face of the combustion chamber, in small compression ignition I have used which are larger the upper combustion chamber face which the piston comes upto meet is moveable to allow adjustment of the compression ratio. I could well be wrong but 7mm x 48tpi sounds quite reasonable for that adjustment on such a small engine.

On Oct 7, 5:55 am, Stealth Pilot
wrote:
I'm having my second go at a piece of torture.

The item in question is the screw threading for the Nano.
The 0.1cc diesel published in Model Engineer some years ago.

I've built one of these before that should still be sitting on the end
of Craig Wall's desk as a paperweight. In that instance we spark
eroded the ports and stuffed them up with all ports ending up at the
same height in the cylinder.
This incarnation wont make the same mistake.

I dont recall just how I did it before but in this torture I'm having
trouble getting good threads. If I look at them under a 10 power
magnifier the threads look like a wire pulled through clay. tear smear
tear smear tear smear, you know the stuff.
the real killer is that I'm using 1214 free machining steel this time.

apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?
I'd welcome a description of how you get really schmick threads at
this size.

(other than the steel threading it all comes out ok.)
Stealth Pilot

I've done 50 tpi for some oddball gun parts. I made sure the HSS tool
edge was SHARP and polished the working faces with a black Arkansas
stone. Need to run the lathe at high speed, too. This might be the
time to think about a rear tool post and thread things in reverse so
the carriage goes from left to right. Otherwise, make your threaded
section much longer to allow for reaction time and trim to length
after threading.

Stan

On Sun, 07 Oct 2007 12:44:22 -0700, "
wrote:

On Oct 7, 1:32 pm, Nick Mueller wrote:
Stealth Pilot wrote:
apart from having the tool exactly on centre height, sharp edges to
the cutting tool, and experimenting with cutting speeds there are no
other tricks to getting polished threads at 7mm x 48tpi are there?

7mm shouldn't be a real challenge. Or is it the mix of imperial and
metric? :-)

Nick

I suspect he meant 0.7mm. I can't imagine 7mm fasteners on a 0.1cc
displacement engine or looking at 7mm threads with a 10 power
magnifier.

Dan

no it is 7mm by 0.5mm pitch.
but since it is a custom engine with little liklihood of there ever
being interchangeable parts I took the easy course and approximated
the pitch. 48 tpi saves mounting the 127 tooth wheel.

on the nano the cylinder screws to the crankcase with this thread.

Stealth Pilot

Robert Swinney was right on the mark.
I tilted the tool to approxmately the thread angle and saw an
immediate improvement.

one other suggestion locally was that a tool out of 1/8th square HSS
was never going to be rigid enough even when supported by another
regular tool.
I'll grind a new one out of some parting off tooling.

if you want to see this item of torture there is an article on
building nano at www.modelenginenews.org/nano/index.html
the model isnt mine.

thanks Robert. I knew it had to be something really simple that I was
missing.
Stealth Pilot