How to correct a tapered gib?
Still working on tuning up my boat anchor Chinese lathe mill/drill. The end to end tapered gib about 9.5" long in the cross slide has the wrong taper by about 0.003" over its length. It appears to have been made with a shaper and possibly ground on the sliding side. Removing material is not an option because it will become too thin and run out of adjustment range.
I am in the process of attempting to soft solder a 0.003" steel shim to the stationary side and plan to carefully draw file it to the correct taper. Getting a uniformly thin solder bond is turning out to be a problem.
Any suggestions on how to do this? Perhaps there is a better way. I do not have access to a surface grinder.
Help appreciated, John.
John, are you trying to get a very thin solder line?
If so, maybe this would work, get a covering of molten solder on each surface then wipe it off with a rag. You should be left with a very thin layer, do the same for the other surface then sweat the two together.
Perhaps there is some modern adhesive that would be easier to use than the solder?
It would seem to me that it might be possible to secure a uniform shim to one side and then correct the taper on the opposite side. You might also consider attaching the shim to the dovetail rather than the gib. A uniform bond thickness would probably be easier with epoxy.
Only fear I would have is if any of the soft solder comes in contact with the ways it will gall up pretty quickly.
Maybe im crazy but if the shim is consistant in width, why does it need to be attached?
otherwise yea I recommend coating in solder and wipeing off as much as you can (Desoldering wick?), then holding the two parts togethor and heating untill the remaining solder melts and letting cool. Basicly just letting the tining layers join.
the gib would be cast iron wouldn't it - is the solder sticking at all? what if you forgot about shimming and just fit it....would you lose too much in the long direction? if so, consider getting a new piece of cast iron....the gib should ideally support along the length not just the ends. imo its not surface grinding work, its mill, file and scraper work. fit it to the stationary part first, then install the moving part, and via blue and scraping fit it to the moving part
Last edited by Mcgyver; 03-01-2010 at 09:54 PM.
If you can get solder to stick to the gib, then thats all you need.
The shim is not worth the effort.
The solder will be on the inside where it sees NO MOVEMENT, only compression.
Mike your gib on both ends, or better yet, lay it on a surface plate and get the thickness.
Now get a perfectly flat piece of Aluminum.
Wet your gib with solder the full length and place a thin shim at one end to match the desired thickness. (.003?)
Heat the aluminum and place the hot gib with shim down on the aluminum.
The solder will flow out, but stay near the shim end giving your a tapered lead shim without filling or machining.
Clean the edges after confirming the thickness desired.
The reason cast iron is difficult to solder is because of the surface graphite. Bead blast the surface to be soldered and solder it immediately after removing the blasting residue with a soft new wire brush.
Use an ammonium chloride flux. Acid flux may etch the iron slightly exposing graphite. A soldered steel shim if well done can work very well on a machine tool gib provided the bond thickness is constant and the shim was originally straight and relatively flat without curled edges.
Soldering the shim to the gib sandwiched between straight flat heated aluminum "cauls" (to borrow a woodworking term) really is the best way to do it. Leave a little extra shim sticking out for later dressing. File only towards the gib so you don't peel the shim off. I think it's best to use a strong silver bearing tin solder.
Once the non-sliding face of the gib is shimmed you can proceed with the fitting. This kind of work proceeds with a scraper and a file working on the sliding face first but only enough to make it flat. The shimmed face is left alone once it's been scraped/filed flat to 8 points but there may be an exception. Normally gibs are peened to straighten them for scraping but this will damage the solder bond. There's no satisfactory solution for this outside of scraping both sides of the gib hoping you don't break through the shim.
Last edited by Forrest Addy; 03-02-2010 at 12:27 AM.
I am a little intrigued here.
Originally Posted by machinist60
It seems that the so-called "boat anchor" (POS and "rat-$hit"?) mill has very flat and parallel dove-tails on the "male" (saddle) as well as very good dove-tails on the upper/female slide. It seems too that the gib is straight and flat too.
There is no mention of making any adjustments to other than the gib which has a taper error of 0.003" over its length.
I am going to be quite surprised if the gib does not "move"/distort while enough localised heat is applied to cause solder to tin the face/s of the gib.
Further, if the 0.003" shim is sweated to the gib and the gib distorts there is going to be quite a problem in straightening the gib without dislodging or buckling the shim.
It also seems that the sweating material has no thickness - but it does - say 0.001" - in which case the shim will need to be 0.002" and not 0.003".
I had a "lack of adjustment" (ran out) on the gib on the milling head on my HF-45 square column mill. I just removed the gib and machined another adjusting slot for the head of the adjusting screw - job done.
I also bought a spare gib - and a spare "Z" lead-screw as well - "just in case". It was not expensive at all.
Just about all gib adjustment is a compromise between "too tight" and "too loose" - so a "Goldilocks" ("just right") compromise is all that is left.
If it were my mill, I'd leave the gib as it is where it is and put in some drilled and tapped holes for "gib adjuster" screws as is the case on many lathe cross and top slides (mine too) as well as all of the slides on my "Sieg" X3 mills - and all of them work just fine.
Why not just get longer gib screws? Or just install the shim to the non-sliding side without any solder? Friction tends to hold things pretty solid.
The solder itself could act as a shim too. Depending on your soldering skills, you may find that you get .003 or more from just the solder. If it is on the non-sliding side wear should not be an issue. I keep thinking of Babbit bearings and how they are alloys of mostly tin, lead and antimony and, although strong, are quite soft. Not that much different than many solder alloys. It is the lubrication that keeps them intact. Maybe a build up of solder or brass braze, filed to size, and liberally lubed everyday could be the solution.