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:
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.
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
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.
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.
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:
The gib must have a preload against the roof of the slide in
order to prevent it from rotating.
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.
The screws should NOT turn easily. There should be reasonable
resistance to turning; more preload means more resistance. The
more preload, the larger the force required to unseat the gib.
Remove the handle and dial hardware and pull the slide off,
while holding the gib in place so it doesn't get bumped. Once
the slide is off, does the gib stay in place and not fall out?
It should be held against the roof of the slide and should not
wiggle or come out without first sliding away from the screw
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:
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.
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.