On Sunday 28 September 2014 21:54:25 andy pugh did opine And Gene did reply: > My lathe is rubbish. That's just a fact. Luckily the lathe is a rather > clever mechanism and even a bad one can make decent parts due to > fundamental precepts of geometry. > > What I noticed today was that the saddle can twist. I think I have the > rear gibb a bit tight for anywhere more than 5" from the chuck. The > fact that there is a gibb is my own upgrade, the OE arrangement was > some cast iron plates with screws that were not quite tight onto a > painted surface (did I mention that my lathe is rubbish?) > > So, I started thinking, and it occurred to me that whilst the > traditional lathe solution to the fact that the leadscrew is offset > from the point of action of the tool is to have a saddle with long > wings, you could have a short saddle that only controls in X with dual > ballscrews to keep X perpendicular to Z. > > I _think_ that this makes some things easier, as you can have a short > saddle which makes missing the head easier, and leaves more room for > the screw covers. And, you can adjust the X-Y squareness, which is > normally impossible. > > Apart from the extra cost, what is the drawback?
The requirement for a totally backlashless coupling between the screws? I too have a similar problem with my toy 7x12. And because the gibs are a cast iron bitch to adjust, I've considered pulling the no longer used rack, and adding an at least as long as the H wings bar front and back and putting small ball bearings on some sort of adjusters to ride the bottom of the front lip and back lip, which should reduce the friction from trying to keep the gibs tight enough. Its slightly compounded on this early 7x12 as the angle faces on the front of the way, and the angle cut in the saddle didn't match by several degrees. With a front gib pulled up tight, and the rear one off, the back of the H is lifted, originally about 3/16", now maybe 5 thou. I milled that out to a better fit and its wearing in nicely over the last 5 years, nearly stabilized. But with a QC toolpost, the offset from the hold down bolt causes the tool to overhang the front of the H and heavy cuts lift the right of it a thou or 2. OTOH that does seem like an awful lot of work as I'd still have a sows ear lathe. Bolton has an 11x28 (23 between centers) with a variable speed motor that sells bare for about $3000 USD. The B290VF. Marked down some now. $2721 w/6" 4 jaw & backplate. What can this group say about it? Good or bad? Is it worth putting ball screws in it? > This isn't the same as a central leadscrew, by the way, that's nice, > but in that scenario too, the scew can't keep the saddle square. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS ------------------------------------------------------------------------------ Slashdot TV. Videos for Nerds. Stuff that Matters. http://pubads.g.doubleclick.net/gampad/clk?id=160591471&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
