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Cross Slide, Carriage & Feed Gears

   
An earlier, simple modification.

Made a ring for the outside of the cross slide hand wheel. Moved the handle out for more leverage. Used three, M5-.8 set screws (same thread as the handle's shoulder bolt). I milled the screw ends flat to remove the sharp ridges so they would not gouge the original knob. Don't make it too big or it will interfere with your hand on the carriage & compound handles.

Cross slide, lead screw upgrade.

When I modified the Taig mill for CNC, I had high-quality, precision ground, ½-20 left-hand
lead screw with bearing plate & matching bronze nut assemblies as spare parts.
The Jet OEM lead screw (top) is a flimsy, left-hand ¼-20 with a crude thrust/radial bearing/bracket.

First squared-up the nut on the mill then turned the bronze nut to fit the cross-slide table counter bore (on the bottom).
Drilled & tapped for a 10-32, 100° flat-head cross-point screw that holds the nut in from the top.

Bottom of the cross slide with the bronze lead-screw nut mounted.

Machined a solid-aluminum bracket that uses 3, M6-1x75mm cap-head bolts & 6.5mm clearance holes.
Had to tap a 3rd, bottom thread. The steel was about 5mm thick & the drilled hole was 5mm for the M6-1 tap.
Machined bracket from 2.5" round stock 2.87" long. There is a 9/16" clearance hole for the ½" lead screw. This is a super rigid design.

Adapted the threaded Taig mill OEM bearing plate. The lead screw with its bearing, thread into the plate.
The back of the Taig dial has holes for a pin-spanner wrench that I made.

 
Tapped the Taig hand crank end for M5-.8 to accept the Jet OEM handle that rotates on a shoulder bolt. Super smooth cross-slide control;
no flexing or binding. The precision ground ½-20 screw with a quality bearing & bronze nut are massive when compared to the Jet OEM screw.
Retained the black sliding swarf cover over the lead screw. The dial scale position is resettable though not usually used in lieu of the DRO.

Made a large, 3.25" cross-slide hand knob for the Jet 9x20. Like the other knobs, there is no knurling; it would load-up with dirt.
The knob is 0.485" thick at the center; same as the Taig OEM crank.
The knob needed a precision-reamed 5/16" hole with a 1/16" keyway. See broach cutting for more information.
Center hole reamed at low RPM on the lathe. For comparison, the rack & pinion carriage hand wheel is 4" in diameter.

 A small M5-0.8 set screw in the back secures the knob's shoulder bolt in place to hold its adjustment.
The re-zero-type dial scale was retained to preserve the parts spacing. The 0.001" gradations are still correct.

Bottom view showing the cross-slide's massive bracket with the 3rd (added) bolt.
The semi-round bracket has a milled flat on the bottom.

A custom brass bushing was made. It is threaded M5-.8 so it has to be screwed onto the bolt (captured) but
it rotates freely in use. A small collar inserts into the handle base. There is also a shallow, 15/64" diameter
 counter bore (using a 5/32" pilot) that the bolt shank sits in to allow a close but non-binding fit. To make the counter
bore, I have also used an A-sized drill & squared the bottom  using a small, hand-held boring bit.
A little grease on the bolt & bushing makes it extra smooth. Using dissimilar metals (steel, brass, aluminum)
reduces galling. All handles now have this brass bushing & a 5mm locking set screw.

Metric (M6-1) die cast zinc ratcheting lever from MSC. 
The brass spacer allows the lever to work smoothly due to the contact of dissimilar metals.

Made new, front & back carriage apron tension bars out of thick brass (the original was thin steel, peened to fit). The cross slide must be removed to access the middle mounting bolt. The front bar is metric tapped for the retaining bolts and the rear bar is held on by M6-1x17mm bolts to the carriage underside. The rear holes, tapped into the carriage, required cleaning out with a metric tap. Milled a 0.002" step along the bolting surface to take-up the carriage play at the bearing surface. Required step size determined using trial & error as it likely varies among machines. Used a 45° end mill to heavily chamfer the edges. Lubricated it with white lithium grease. Greatly reduced carriage play.
The lathe DRO is mounted off the back of the carriage.

Feed Gears
In order to obtain cross-slide feeds much slower than shown on the chart, use these gear combinations:

At position A of the gear train, use the 28teeth gear
At position C of the gear train, stack the 30t gear on the outside and the 127t gear on the inside
At position B use the 120t gear lined up with the 30t gear of the banjo

Note: Shown below is a Jet BD-920N on its stand which raises the lathe high enough for the big gear to  clear mounting surfaces.
A lathe mounted directly onto a flat surface would require spacers. This gear chart is for a Jet BD-920N lathe.

These gear combinations allow very slow feed rates.
The nine quick-change gears provide a useful range of different speeds.

Drive timing belts are identified by three parameters & are marked as follows:

170 XL 050 NG

The first number is the belt length (17"). The belt has 85 teeth.
The second number is the pitch (XL=1/5"), e.g., the peak to peak tooth spacing
The third number is the belt width (050= ½")


Belt/gear guard housing notch cut to allow the big gear to clear when the cover is closed.
Gear protruding through the notch. Making a safety guard.