You have failed me for the last time, Bosun Gibbs

  So I want to bore out some ridges inside my lathe spindle...  This requires my boring bar to reach in from four and half to six and a half inches (and I hope no further) inside.  I recently welded an extension onto my 3/4in HSS boring bar for just this purpose.

 Now an old bogeyman raises his head again: the cross slide of my lathe, as well as the compound rest, have more slop in them than a soup kitchen. This means that when cutting forces try to push the bar down at the business end, the end with the cutter moves instead of staying still.  At best it will chatter and give lousy surface finish, at worst it'll break something.  The longer a bar one tries to use, the worse this situation gets.  The only real answer I know of is to take lighter cuts and have as stiff a setup as the machine is capable of.

 Various repair attempts have been made to both slides, by the former owners and by me with help from a smarter friend, and while these attempts did improve one adjustment screw at one end of one gib, it hasn't done much for the fact that both gibs are bent and badly finished on their bearing surfaces, and the dovetails were left in a horrid state after the most-recent cut was made (at the factory?) and they are now, not to put too fine a point on it, lumpy.  It it likely that stoning the lumpy dovetails will not make them worse...  There is certainly no worthwhile scraping on them now to worry about unintentionally removing.  The bearing surfaces are a dog's breakfast!  Flat or not flat, without scraping it will not hold an oil film worth a damn...*

The bottom of the cross slide showing horrid bearing
surface, worn end of gib, and worn gib screw.

 There are high spots on the body of the cross slide that I will try stoning as well.  I might even attack those dovetails too (with a triangular stone) because the factory's idea of scraping is just dismal; I could literally do better... if I had a straightedge to print them with.  I could print the bottom bearing surface of the cross-slide since it is coplanar (ish) with the bottom surface, just hangin' out in the breeze, no way covers or any of that fancy stuff.

  aside: In this blog, I routinely use the word 'print' to refer
to two different things; 3D printing, and also the practice of
"printing" and hand scraping precision surfaces of machines.
In this post, I am referring to the latter definition. 
There are excellent tutorials on printing and scraping on
YouTube, I especially recommend RobRenz & Keith Rucker.
It is worth noting that no automated process or machine is
capable of producing a surface as flat as hand-scraping, and
scraped surfaces hold oil far better than ground surfaces.

 

shows the gib screw hack engaged
with a notch filed into the gib

  In fact, it occurs to me that the cross slide and its mating surfaces on the carriage would be a good place to learn / practice hand scraping.  I can't make these surfaces any worse than they are now, and the cross slide is not too heavy to repeatedly put on and take off the plate between scraping.  But the carriage is much too big to come off the lathe, nor can it be printed on a plate anyway; I would need a proper (scraped or lapped flat) straight edge to print it in place, and I haven't scored one of those yet.

 Okay so let's talk about the gibs.  The dovetail ways on machine tools are adjustable in order to remove nearly all slack and off-axis motions.  The adjustment is made with a tapered insert called a "gib" which acts as one side of the dovetail pair.  When it is moved back and forth lengthwise, the taper causes the dovetails to get tighter or looser.

 The gibs are adjusted with, and held in the correct position by, gib screws, which have very big heads and a sharp corner at the base of the head, intended to pick up a notch ground into the gib.

adjustment screw hole and worn gib-
it's obvious there is very little metal
under the screw head

 But the screw and the notch can wear, especially if not checked regularly for tightness.  If the gib is allowed to move back and forth with the slide, it is disastrous not only to the work you're trying to do, but also to the gib and adjustment screws themselves.

 And when the gib and screw get worn, attempting to tighten it only pushes the end of the gib outward against the slide, jamming motion or at least making it hard to move near the end of travel...

  Now, in theory I could fill in the notch with some weld or braze, regrind the notch, and replace the screw.  And I'd have something just as bad as what originally shipped.  Well who wants that?  That level of quality is what brought me to this place...

original gib screw and replacement hack

 Some years (decades?) ago, a friend showed me how to do a better job, replacing the gib screw with a hollow tube having an angled finger brazed onto it, and projecting well out of the screw's counterbore and much further into the gib's slot than the original screw head.

 The gib gets an angled notch to receive the angled end of the finger.  This angle will pull the end of the gib closer to its slot when tightened.  Tightening is accomplished with a SHCS that sits inside the hollow tube.  This replaces the wide head of the original screw with something that more positively engages the end of the gib.

  So one of my tasks is to make three more of those gimmicks, one for the cross slide, and both screws in the compound rest. 

 I have a little more to say about the bearing surfaces of the cross slide and the nature of scraping, but this post is already long enough, I think I'll put that in the next post... maybe even later today.

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* I'm aware that modern, high-accuracy CNC machines use ground ways with no hand scraping on them.  They can do that because they have high-accuracy custom-made surface grinders AND because the resulting machines use pressurized lubrication systems which maintain the oil film (and also use a fair bit of oil in a day).