Once the magic smoke comes out, things don't work any more.
John Kasunich
jmkasunich@fastmail.fm
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Mon, 18 Dec 2006
Scraping, lapping, cleaning, done.
Well, I now remember how much I dislike Dykem Hi-Spot Blue. The stuff manages to get everywhere. However, the scraping is done.
When I started, with the gib adjusted moderately snug, the saddle would move over about three-fourths of the length of the bed, with some noticable variations in drag, not terrible but noticable. But when it got to the right hand end, it would lock up solid.
After lapping the gib flat and cleaning it, I put the saddle on the machine, and hung a 0.0001 test indicator out 4" to the right of the saddle, bearing on the front dovetail. As I moved the saddle to the right, the indicator read pretty close to zero most of the way, but the last 4" or so showed an increase of about 0.0015. Same thing on the back showed a smaller increase, maybe 0.0007 or so. So I scraped pretty heavily in those areas, until the indicator showed only a couple tenths of rise. Then I started the fitting - slide the saddle until it finds a tight spot, mark it, scrape a little, clean off the scrapings, oil, repeat.
After several hours I got to the point where there are just a few places where the saddle has a tiny bit of extra drag (this is using thin oil, no screw, and sliding it very slowly so momentum doesn't carry it over the stiff spots). Still a bit snug at the tailstock end. Then I put a tiny drop of 500 grit (17 micron) Clover lapping compound on the bed, and ran the saddle back and forth a few dozen times. That nicely marked a high spot or two at the tailstock end, which I scraped down. Then I cleaned all the grit off, ran the saddle back and forth a few more times, cleaned again, repeated, then took the saddle off and scrubbed everything down to make sure no residual grit is left. Later this evening I'll put the saddle back on, lube it with very light oil, (or maybe just WD-40), run it back and forth a bunch to dislodge or break down any remaining grit, then take it apart, clean it again, and finally assemble with proper oil and adjust the gib.
A lot of work, but I think the machine will perform a lot better. Since the saddle leadscrew isn't centered, any slop in the gib allows the saddle to twist a little as the screw drives it one way and then the other. Not much of a problem when turning with the tool near the center, but if I'm boring with the bar extended out toward the headstock, it definitely affects the accuracy. Before I couldn't get the gibs really snug, because there were tight spots and loose spots. Now its very consistent.
(posted: 18 Dec 2006 18:03) (permalink)