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One of the features lacking in the 9x20 lathes is the ability to do left hand threading.  For standard threading you set up the gears to engage the lead screw to get your thread pitch.  Then when you lock the half nuts the lead screw will move the carriage from right to left.  This produces a nice right hand thread.  With practice  :).  Great but I want left hand threads and buying dies to get them is expensive!!

So how do we do this.  Its actually quite simple.  Its nothing more than a standard gear train.  So adding a gear will make the lead screw spin the other direction.  This means the carriage will move from left to right and we get a left hand thread.  NO - running the lathe spindle in reverse will not do the same thing since you will also reverse the lead screw.  :)

It looks worse than it actually is.  On the bottom is the new plate that will carry the existing large gear and the added 42 tooth gear.  The gear on the bottom will continue to ride on the existing stud in the head stock.  The stud for the large gear must be removed from the head stock and installed on the carrier plate along with a new stud to hold the 42 tooth gear.  The parts to the right are my version of a lock to hold the gear plate in forward, neutral or reverse position.

This is the back of the carrier plate.  The plate was cut from a piece of 1/4" x 3" x 6" steel.  On the left is a 1/2" id bushing I welded to the plate to hold the lock handle. I decided to remove the threads from the large gear stud.  I made an identical stud for the 42 tooth gear.  Then welded these into the carrier.  On the bottom is a 3/4" id bushing.  This rides on the  lower gear stud that remains threaded into the head stock.  I had to remove the stud and turn the OD of the stud to match this 3/4" bushing.  

Here is a bad picture of it mounted to the lathe.  It is currently in the neutral position.  This means that neither of the upper gears are engaged with the spindle gear.  The lead screw will not rotate.  

Once mounted I scribed the arc of the 1/2" bushing that will hold the handle.  Then cut a 9/16 slot through the lathe.  The L shaped piece from pic 1 slips through the slot into the bushing.  It is then secured with an 8/32 set screw.  A Piece of 3/8" bolt was machined to slip through the hole in the handle.  

The lock plate is not yet finished.  More to come.

1 19 08 - Getting to some updates.  I was not happy with the setup.  You can see in this pic I have moved the selector to the outbound side of the lathe.  Also note the bearings in the 2 upper gears.  The lower gear does not have a bearing.  The gears were not running smooth on the home made studs.  I cut off the studs drilled and tapped for 1/4" bolts.  Then made some Aluminum inserts for 5/16 ID bearings.  Bored both gears for a press fit on to the bearings.  I went a little to far on the large gear.  some jb weld seems to have solved that problem.  I was under for the small gear.  It split when I pressed the bearing.  Add one to the parts order next time.  It does run a LOT smoother since this was done. 

   
 

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