The Gibs of the Mini Lathe
How to make them work again

Misbehaving gibs are a common problem for the mini-lathe. Even if the slides and gib have all been scraped flat, you might find the gib does not do much to prevent the compound from rocking when sideways force is applied. This is not necessarily because the design of the gib is bad. It seems simple on the surface but it has subtle features.

Created July 22 2018
Updated August 4




Soon after I got my 7x12 mini lathe I decided the cross slide gib wasn't doing its job. It could have been my fault. After all, what do I know? I'm not a machinist. But it seemed like an interesting problem, so I went through several iterations of trying to modify the gib to improve it. I was advised not to mess with it, just use the factory system as-is because it was adequate. Conspicuously absent was a satisfying explanation of the gib design. My gib was doing this:


cross-slide-malfunction


What no one could explain to me was, what prevents the gib from rotating in this manner? Nothing, as far as I could tell. It was easy to observe on my lathe. The factory design must be flawed, I thought. I couldn't find a convincing argument otherwise, except folks who said that they never changed theirs, and it worked great! What was I to make of that?

After much discussion and speculation, I realized that the gib was meant to be clamped against the "roof" of the slide. This clamping force can be considered to be a preload which keeps the gib seated securely against the slide. This preload is the ONLY force preventing the gib from coming unseated and rotating.


factory-gibs


It is commonly heard that cutting forces on a lathe push the compound down and that the sideways forces have little effect. This would lead to the conclusion that the rigidity of the gib is unimportant.

This is not really the case. Facing cuts and threading both produce side loads which have a cantilever effect on the compound, dependent on the gib to restrict movement. This is very important if you are forced to use the cutter with a large overhang off the compound, such as when boring. If these forces are sufficient to cause the gib to deflect or rotate, then the compound will move, the cut will go wrong and the freedom of movement will invite chatter.

Furthermore, interrupted cuts cause backlash which is a violent lifting force. If this movement is not controlled somehow, the cutter will "jump" into the cut and damage itself. This requires a rigid gib with as little play as possible. If the allowed movement is smaller than the oil film thickness on the slide, the unbroken oil film will have viscous damping properties which absorb some energy from the backlash (if you remove the gib and pull up on the slide, you will feel it stick and then suddenly come loose due to the oil film breaking). If the movement is enough to break the oil film, the compliance of the compound increases and the backlash or chatter will worsen.

The aim of this gib design is to prevent the gib from becoming unseated from the roof of the slide. The ability of the gib to resist this is completely dependent on the preload forcing it against the roof. The unseating forces are not weak, therefore the preload should not be weak either. The stronger we can make the preload, the better.

If you have not modified your gibs from their factory condition, they should already have some preload, although sometimes there are manufacturing errors. But if you have sanded or machined your gibs flat or modified them in other ways, you may have removed the preload! Furthermore they easily come out of place if the user tries to reassemble them without understanding exactly how they are intended to work. This design works for specific reasons, depending on conditions which must be met:

  • 1: The gib must have a preload against the roof of the slide in order to prevent it from rotating.
  • 2: The force of this preload must be sufficient to prevent it from unseating during a cut; it is not enough to merely prevent it from falling out due to gravity.

With a simple inspection of the slide we can determine the health of the gibs based on these requirements.





Gib Inspection



If you discover the preload is absent or weak, then you can add shims between the gib and the roof of the slide. This will force the gib down against the screws to generate the preload. Use metal shims, not plastic or paper as weak shim materials will allow the gib to rock. I have measured the thickness of a soda can wall to be around 0.0041" with very little variation, and used one to cut out a shim for my gib.

You may need to cut new tips onto the gib screws. They are crudely formed from the factory and are often not concentric enough. You will have to work out what is the best diameter for the cylindrical tip, and it is good to hold the screw in a collet or in a pipe in a chuck shimmed so that the screw threads are centered well. If the screw tip is not concentric to the threads, then you may not be able to get a consistent preload. It is also good if there is a slight undercut away from the tip, so that the contact point is further inside the gib. Make sure the tip is long enough that it contacts the end of the hole in the gib, rather than the flank contacting the back face of the gib. My gibs now look like this:


gibs-final




In Summary



Here are the key points collected together. For the factory standard gib system (I assume my vision is the same as the lathe designers. That's where all the clues point, but of course I could be wrong):

  • 1: The gib is clamped flush to the roof of the slide by a preload provided by the screws in a sideways direction from their tips.
  • 2: As long as the gib is flush with the roof, it behaves like a solid part of the slide. It is almost as rigid, depending on how well the mating surfaces match.
  • 3: What is more rigid than the slide itself? Nothing. The slide sets the limit.
  • 4: While the gib is flush with the roof, the roof bears all the vertical load on the gib, not the screws! Therefore, increasing the stiffness of the screws or using pins is not entirely productive unless the preload is already failing to keep the gib seated on the roof.
  • 5: At the point the gib is pried out of flush with the roof, it becomes a lever on the screws, which act as a spring, and so the whole compound becomes a spring. Only after this occurs does the gib no longer behave like a solid piece of the slide. (Okay, there are forces in line with the screws pushing the gib sideways, but the dovetail angle turns these forces into vertical forces and the screws are weaker vertically than horizontally, and so it is the vertical strength that is causing problems).
  • 6: The amount of force at which this occurs can be known based on the lever relationship and preload.
  • 7: While the gib is flush with the roof, it does not matter how stiff the screws/pins are as long as they have a sufficient side load to prevent the gib from being pried away from the roof. The rigidity will be close to that of the slide itself if the gib and roof are mated well. Only after the gib is pried off the roof does the rigidity of the system decrease. It becomes springy, and this increases the chances of chatter. Therefore increasing gib preload or gib-to-roof matching is the first priority when improving this system. Screw/pin stiffness is important in as much as it may increase preload, and improves rigidity in situations where gib movement cannot be prevented.
  • 8: However, the intent of the system is that the gib never leaves the roof of the slide in the first place, and so having an adequate side load on the screws to clamp the gib to the roof will ensure that the spring constant of the screws/pins does not need to become a factor in how well the system performs in normal usage!


Gib Modifications



When I finally understood how the gib was originally meant to work, I realized none of the modifications I had dreamt up would actually improve it. Now I'm not so sure it really needs to be improved.

But for those that want to, Mario from the 7x12minilathe Yahoo group (now Groups.io) suggested a mod that makes the most sense to me. Instead of relying on the adjustment screws for a preload, the gib is clamped to the roof of the slide directly by a cap screw. We can now just flatten the adjustment screws and create a flat for them in the gib. With this modification very high amounts of preload are possible and so it should be very rigid. The shim is only there in order to shift the gib down to get more contact area with the dovetail on its left face.


gibs-final





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