You guys all had some very good info...

Here's a link to some photos I took earlier:
http://pg.photos.yahoo.com/ph/cj1210...548scd&.src=ph

Right now trying to get the thread chasing dial working is not my
biggest concern. I've got to get an accurate TPI first. For some reason
though I think this may be all intertwined...

As you can see from the photos I've strictly kept the setting
(according to the charts and gears) to 16tpi. The leadscrew is exactly
16tpi (not anything metric as you can see from the picture). When I
turn the chuck over one rotation by hand I don't get a complete
rotation in the leadscrew.

This obviously points to some kind of gear miscalculation on the
manufacturer's part... (at least that's the only explanation I can
think of at the moment).

Now, as far as the thread chasing dial, I've tried pulling it away from
the headstock while cutting in order to take all of the play out. This
doesn't do anything. I've also tried giving it resistance with the
carriage wheel.

Maybe the thread chasing dial can't work properly if the gears are
messed up. For example maybe when it's set to 16tpi and it's really
cutting close to 17tpi this will then proceed to not let the half nut
engage at the proper time, which will defeat the whole purpose of the
thread chasing dial.

There is about .007" play in the leadscrew itself which I can probably
fix with a thin washer. There is about .014" play in the half nut unit.
This I'll have to pull apart to see if I can tighten it up. This could
definitly be part of my problem with not getting the same cut to line
up and is just not right, but something tells me there's more to it.
Like I said up above I've tried keeping pressure on the carriage (away
from the headstock) and it hasn't done any good...

Any other ideas!?

Thanks,
Carter

Eric R Snow wrote:
On 10 May 2006 14:08:37 -0700, wrote:

Hello,

First of all, I have an Acra (now Enco?) 9x20 Chinese bench lathe. This
lathe was given to me by a friend a few weeks ago and it didn't get
much use at all from him.

For some reason the thread cutting system seems way off. When I cut one
pass (say 16tpi) and measure the light cut with my thread guage, I'm
getting closer to a 17tpi cut. When I try to do a 26 tpi pass I get
closer to a 28tpi cut. I've checked and rechecked all of the gear
combinations listed on the chart and the are correct.

Additionally, when I try to use the thread chasing dial, I can never
get the same cutting pass. For example, I'll use strictly the number 1
on the dial and my second cutting pass will be right in between the
first cutting pass. I'll try it for a third pass and I'll get a whole
different cutting pass.

This is really frusterating! Has anybody ever heard of this before? I
was actually impressed with the quality of this Chinese manufacturing,
but now I'm beginning to wonder...

If pictures would help at all I'd be glad to take a few and post them.

Thank you very much,
Carter
Greetings Carter,
It sounds like it may have a metric leadscrew. Another thing to check
when looking at the gears is to see if paired gears are used and if so
maybe one of the gears is on the shaft the wrong way. What I mean is
this: the lathe may have a 120 tooth gear paired with a 127 tooth
gear. These gears will be on the same shaft and both gears will be
keyed to the shaft so they rotate together. There should be a gear
driving the 120 tooth gear and the 120 tooth gear should drive another
in turn. So the 120 tooth gear is acting as an idler. OR, it could be
the 127 tooth gear that is acting as the idler. But if the lathe has
an inch leadscrew and you want to cut metric threads then one gear
will drive the 127 tooth gear, which spins the 120 tooth gear, which
drives another gear. This is how the metric conversion is done. Also,
when cutting metric threads using this method you must keep the half
nut closed until all threading passes have been made. This means that
at the end of the cut you must withdraw the tool and stop the lathe at
the same time. Then with the tool clear of the part, reverse the
spindle to back the tool up, then so on and so forth. If your machine
is equipped with a metric leadscrew and the gears are set up for
cutting inch threads then the same thing happens. By the way, when you
figure out what is really going on please let us know.
Eric