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Cutting left-hand threads on a Chinese small lathe?

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sb327:
Here is an adapter currently unavailable



It seems it would be simple to build.

Dave

happymecanic:

--- Quote from: sb327 on February 04, 2023, 02:41:27 PM ---It’s the same. Consider this.

Example.

Drive is 10 tooth

Idler is   5 tooth

Driven is 20 tooth

If drive turns 1 turn, it  ‘outputs’ 10 teeth into idler. The idler turns 2 times and  ‘outputs’ 10 teeth.

The driven intakes 10 teeth and turns 1/2 turn in the same direction as drive.

Take the idler out and direct mesh, drive one turn, driven 1/2 turn but in opposite direction.

So no matter the arrangement, the idler only ‘output’ the number of teeth the drive ‘inputs’ into it.

Hth,
Dave

--- End quote ---

Oh yes it helps Dave, thanks A LOT!

happymecanic:

--- Quote from: JPSAXNC on February 04, 2023, 04:39:15 PM ---You don't need different gears to cut a left hand thread. If you push the thread (cutting toward the head stock) you get a right hand thread, if you pull the thread ( cutting away from head stock) you get a left hand thread.

--- End quote ---

On my lathe the lead screw is driven by the spindle. If I ''pull'' the thread the spindle rotates counter-clockwise, cutting right-hand threads. I actually cut my threads that way since I don't have a foot brake. I set the tool bit upside down when I do this. I really need to reverse the rotation of the lead screw to cut left-hand threads. I know what you mean though, this is how they do it at the shop where I work. I wish I could do that at home, but I only have a small hobbyist machine. See the picture in the link Dave posted ;).

WobblyHand:

--- Quote from: JPSAXNC on February 04, 2023, 04:39:15 PM ---You don't need different gears to cut a left hand thread. If you push the thread (cutting toward the head stock) you get a right hand thread, if you pull the thread ( cutting away from head stock) you get a left hand thread.

--- End quote ---
At least on a Grizzly G0602 type lathe without an idler, this is not true.  As a matter of fact you will cut a right hand thread if you put the lathe in reverse and cut outward.  I did that to cut the threads on a cast iron back plate for a set tru ER40 chuck.  I used a left handed tool, turned the lathe in reverse, and I made a perfect 8 TPI right hand thread, that exactly matched my spindle thread.  To cut a LH thread, you need an idler gear to change the rotation of the lead screw relative to the spindle.  If you reverse rotate a RH screw, it doesn't turn into a LH screw.  If you call Grizzly and ask them if the stock G0602 will cut a left hand thread, they will tell you it is incapable of cutting left hand threads.  You need an idler gear.

That being said, instead of installing an idler, I designed an electronic lead screw of my own.  It can cut any thread I like, metric or imperial at the touch of a touch panel display.  No changing gears at all.  Wrote all the code myself.  This thing works great.  The first picture is of the electronics being assembled.  It has a 3.2" touch panel display.  It also integrates DRO's if you have them.  Second picture is it fully installed on the lathe and cutting a 5/8-11 thread, just because I could.  (If you zoom in on the picture, you can see 11 TPI on the display.)  If I wanted to, I could program it to cut a unique thread, oh, like 3*pi TPI, or 1.89mm, if needed.  By touching the Right Hand button, I can toggle it to Left Hand, or toggle it back to Right Hand.  Pretty convenient.  (But not on the fly!  Only when the spindle is stopped, do I allow changing thread or feed pitch, or handedness.)  Even managed a 4 TPI thread as a test, and 100 TPI while I was testing this, along with the equivalents in metric.  Feeds are selectable by touching the display as well.

I made that cast iron back plate threading outwards and in reverse, as I described in the first paragraph.  I machined the back plate it before making the ELS.  Started out with a raw chunk of cast iron.  The back plate locks up to the spindle beautifully.  No run out that I can measure, since I machined it on the spindle.  The ER40 Chuck has 0.0002" TIR mounted on the back plate.  I could probably do better, but chasing the last few tenths is hard.

Rob M:
good topic.. and its the same on my g4000 , i dont even bother , i just buy the chinese dies on amazon , especially for small size left hand threads.

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