The next step is to drill out the holes that I will be pressing the gear posts into. These holes will be about .005” smaller than the diameter of the gear posts. This is to help facilitate a press fit and eliminate the need for fasteners. I centered up the plate with the center finder tool and prepared for drilling.

       I decided to use a 25/64” (.391”) drill to make the initial holes for the posts. These holes will then be precision widened to .399” with an emery cloth hone.

       Below is a honing tool that I like to use to slowly bring a bore up to size. It is basically a rod with a slot in it that emery cloth can be wound onto. I cut the length of cloth to snugly fit the hole I am honing and spin the hone with a drill motor. I pause intermittently to check the bore size and continue as needed.

       The only drawback to using this honing method is that the hole you are honing will tend to widen more at the ends of the bore than in the center. This is not a problem with holes this shallow although it is worth mentioning...

       Once my hole was at .399” I was ready for the press fit. I started one of the posts into the plate with a dead blow hammer and then checked the post for plumb.

       I moved the plate assembly to the 6 ton press and drove the post into place.

         I would guess that the force needed to drive the post into the plate was about 3,000+ pounds.

       And now for a quick fit check. The gears fit beautifully with optimal mesh. It seems they have about .030” tooth end clearance which seems to be about right. I could now check my measurements for the second tumbler gear post and install it the same way.

       The second gear post was pressed into place with the same degree of success. So far so good!!!

       I installed all of the gears for a fit check. The acetal tumbler gears were a bit tight on the shafts but nothing to worry about. These gears will last a very long time granted they will not be exposed to abrasive dust. I shouldn’t have to replace any parts for many years.

       The assembly is now ready to bolt on the headstock for a test run.

       The layout of the 60-tooth spur gear post is crucial to making the this system work well. There are many things to consider including interference with the different change gears and tumbler control handle position. I spent some considerable time figuring out where the best position for the tumbler assembly would be and I came up with 2.2” up from the bottom of the headstock casting by 3.15” back from the vertical face of the front of the casting (as seen below).
       This layout should provide proper clearances and adjustment of the tumbler assembly. 

       I drilled out the center of the spur gear post hole with a 3/16” bit which was followed by a 13/32” bit. I was careful not to drill into any of the control wires in the headstock :0P

       I was now able to fit the tumbler assembly into the gear train and test it’s shifting travel.

       I carefully turned on the spindle motor and ran the lathe at 300 RPM. I was careful to hold the shifting handle as to not allow the gears to come out of contact. The gears meshed very well with little noise if any :0) Whoo-hoo success!!!

       I tested both directions as well as neutral before proceeding to see if the different gear sets would clear the assembly. I put the largest combinations of thread cutting gears on the lathe to see if my calculations were correct and was happy to see they clear the reverse gear post with room to spare. The combination of gears below is from top to bottom: 60/36 - 127 - 60. This was the largest or closest I could get on the inch/thread gear sets to the reverse gear post in the forward lead screw rotation setting.

       You can see below the tolerance of the gear post to the 127-tooth gear. Note: This is in the forward lead screw tumbler position...

       The next test was the closest metric thread gear change combination which was 60/27 - 127/120 - 60.

       You can see below that there is just about the same clearance with this gear change set which now clears all possible gear change selections.

       Now that my tumbler assembly position has been verified I can proceed to building a shifting selector assembly. I decided to use Dennis Atwood’s proven design as it is simple and robust which includes what I call a “slider plate” and locking knob inside the belt change cabinet. This design is clean, simple and out of the way. To start out I cut out a piece of 1/4” aluminum plate about 6” long by 4” wide.

       I installed the plate on my 6” rotary table and marked out a 5.125” radius from the center of the rotary table. This will be the center of the arc I will be using for the lock down bolt on the shifting handle later on.

       Using my center finder tool I lined up with the 5.125” radius mark and installed a 1/4” end mill in the chuck.

       I measured my shifting travel at about 1.115” at the 5.125” radius (linearly). I then added the 1/4” measurement for the locking bolt and came up with 1.365” end to end for the slider slot. I decided to make the slot a bit shorter (1.350”) so it could be fine tuned with a rat tail file once the slider plate was installed.
       I started up the mill and made progressive cuts to form the slider slot as seen below.

       I used my new misting nozzle without coolant to blow the chips away witch worked great :0)

       Once the slot was made I drilled out two 1/4” holes at 1.558” to center from the center line of the slider plate (linearly). These holes will be used to secure the slider plate to the headstock casting.

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