Cross-Slide Table Modifications

Photos 5 and 6 show the extra gib holes drilled and tapped to take the ball handled locking screws. Once tightened, these keep the positioned table perfectly stationary. Very accurately spaced holes can be achieved.

Thrust Bearing
I wanted to try light milling on the drill press since I didn't have a milling machine at the time. Experiments showed that there was just too much uncontrolled table vibration. For milling, the table should be as jiggle free under movement as when locked down. Once the gibs are properly adjusted, the only other component allowing uncontrolled motion is the lead screw.  I started with the thrust bearing where I assumed (incorrectly) all the backlash to be.

The original was just a steel disc with lots of non-adjustable backlash determined by how (not very) close to it  the outer sleeve was pinned to the shaft during manufacture. The new one carries a ball bearing and spacer so the backlash is determined by the precision of the ball/race fit. If I were doing it again, I'd make this block thicker and mount two ball bearings with a spacer between and some bearing preload. It's amazing how much play there is in a single ball bearing. I never thought of this until later but couldn't have done it originally anyway because I was fitting it to the original leadscrew and was limited to the thickness of the original thrust plate.  

The new plate had to be larger in diameter to make room for new mounting holes outside the bearing recess.

Leadscrew and Nut
The new thrust bearing helped but didn't completely eliminate the problem. The table could still be moved perceptibly back and forth. So I took the whole thing apart to have a look at the leadscrew assembly.

The original leadscrew has a 1/2-10 Acme thread and a very loose nut. The second nut in Photo 9 shows how I tried to tighten up this fit by adding screws to bear on the leadscrew. It was a half-vast fix and didn't really work. Making an adjustable nut for an Acme thread is problematic and, anyway, the cost of a 1/2-10 Acme tap is close to $100.  So back to the drawing board.

Here is the heart of all these modifications, a new leadscrew and nut. The leadscrew is 1/2-20 threaded rod and the thrust bearing end was machined completely on the Taig lathe. For some reason, at the time of making this particular part I made step-by-step photos which are in the appendix for those who are interested. The nut is adjustable following the practice of the Taig  CR mill and is somewhat larger than the original which is shown for comparison. Also shown is one of two spare blanks made in case of a screw-up along the way.

The new lead screw works like a charm and is as smooth as silk.While the thrust bearing could be improved, it's just not worth the trouble because the backlash is quite acceptable.


Dials and Handles
The original dials were intended for a 10 pitch leadscrew and were too small. The new ones have 50 divisions to suit a 20 pitch leadscrew and are the same size as the new thrust bearing carrier. The dial carrier was made the same length as the original pinned spacer. The new dial is held in place by a circlip which is bent and rides in a groove wide enough to let it press lightly on the dial so as to damp its rotation.   

The increase in leadscrew pitch  means more turns per inch table travel so rotating handles ease the wear and tear on fingers.

Table Stops
Adjustable table stops make certain kinds of repetitive operations much more convenient. The stops ride in standard T-Slot Tracks (Lee Valley 12K79.22) used in tablesaw mitre gauge slots. The stops are just drilled aluminum blocks secured by 1/4-20 T-bolts and finger nuts. The stop posts are made as shown.

Now cut a length of threaded rod about 1" longer than the lead screw to be made. Grip it in the three-jaw and support the outboard end with the just-made round riding in the steady rest, a kind of substitute for a tailstock dead center.