DIYbanter - View Single Post

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