Lathe mod - Saddle fitting/lapping
7x10 Mini-lathe - Fitting the Saddle to the Ways
[Note: This procedure is not original with me, but I have been unable to relocate the link I based it on. If and when I rediscover that link I will post it here.]
After making some initial saddle gib adjustments, I found that the saddle was still not sitting firmly on the ways. I removed the slides, apron, and gibs, and with the bare saddle sitting on the ways, I grasped it firmly with both hands and tried to rock it back and forth on the ways (as if I were facing the headstock and trying to rotate the saddle on the ways); I could a tiny but perceptible movement of the saddle.
An inspection of the underside of the saddle (photos at left) showed that the machining was, if not exactly rough, not of the highest quality either.
I put a light coating of marking fluid on the saddle surfaces which contacted the ways, and ran the saddle back and forth on the ways to find the low spots.
As can be seen on the photos to the left, only the outer ends of the saddle were making any contact with the ways.
I attached wet-dry sandpaper to the lathe bed, using double-sided tape, as shown at the far left. I started with 80 grit, and placing the saddle carefully on top of the sandpaper I worked the saddle back and forth with both hands applying an even pressure.
Initially I ground the saddle dry, but I found that applying WD-40 to the sandpaper as I was working helped the process by "washing out" the grinding dust. I ground in five minute segments, stopping to "wash out" the dust with WD-40. When the paper became worn out (I could feel the grinding action diminishing), I would apply fresh paper. Changing the paper required removing the old paper and double-side tape, cleaning off the WD-40, and then applying new tape and paper.
In the photos to the left, you can see the state of the saddle part way through the 80 grit grinding. By sighting down the length of the surface, the low spots can be clearly seen.
After 3-1/2 hours of grinding with 80 grit, a uniform surface was achieved (photos at left). It's a bit difficult to see in the photos, but although the surface was uniform, there were fine grooves in the surface left by the 80 grit paper.
Successive grinding with finer grades of wet dry paper removed the grooves and left a smooth surface.
A table of grinding grits and times is given below.
The lapping (or grinding if you prefer) was done with increasingly fine grits of wet-dry paper (obtained from the local auto parts store), with each grit being used for the grinding times shown in the table below. The actual elapsed time was about a week.
Results
Following this treatment, the saddle fit was greatly improved. On a scale of 1 to 10 (with 1 = horrible and 10 = perfect), I would say that the saddle before lapping was a 6.5, and after it was an 8.5-9. Had I been willing to spend even more time on this, I think I might have gotten the fit even better. All in all, though, it was a good effort and a big improvement.