Q: Mini Mill Gib Replacement?

[Deviant]

Hello.

I've finally gotten moved into my new house and it's time to pick up the conversion again.

With that said.

I've tried and tried to adjust the play out of my lil mini mill. I'm beginning to wonder if they might be a problem with the gibs. Or possibly the table?

Is there a proper way to tighten the gibs. I've attempted a couple different ways of screwing it all the way to one motion then tightening, then moving it to the other end of it's motion and tightening. Also I've moved it to the middle and adjusted it while moving back and forth slowly.

It will be nice and smooth at one end of it's motion and bind/stiff at the other. When I adjust out the stiffness, there will be a slight side to side play in the table that I don't like.

Slightly frustrating to be getting stuck on something that would appear to be very minor. *shrugs*

[BobWarfield]

Deviant, below are my notes on gib adjustment. YMMV.

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Adjusting Your Gibs
Gibs are adjustable pieces whose purpose is to take up the slack when the sliding pieces become worn.
Some accounts I have read refer to spending “hours” adjusting gibs. I hope you are getting the idea that achieving the highest levels of performance is going to require some patience and detective work. Before talking about their adjustment, be sure you have some sort of locking mechanism so that you can maintain that adjustment despite machine vibration. My lathe has a jam nut that holds the gib adjustment, for example. Products like Loctite should also be considered.
The first question is, “How would I know whether I need to adjust my gibs?” Unfortunately, the best answer I have here is you won’t know if you don’t try. Be careful not to tighten them too much or you’ll really lock down the axis and generally make things worse. For some kinds of cutting you may find it advantageous to lock an axis in this way. I have seen the practice recommended, but the one time I tried it the result was no improvement and a badly adjusted gib so I steer clear.
Some folks limit gib adjustment entirely to a degree of tightness for the slide. They back off all screws and start on the centers, tightening just enough to create a slight drag on the slide when there are no leadscrews attached. A similar approach involves tightening that center screw until it is “just snug” and then following up with the ends. The latter is an approach I used with some success on my Lathemaster lathe.
I have seen one source that recommends lapping the gibs, as they are often of somewhat poor or rough finish.
CNC machines make gib adjustment a little more difficult. Without handwheels, you can’t really feel how tight they are. Commercial CNC machines make the adjustment by measurement how much amperage is drawn by the motor during traversal. One account I read indicated it should be about 30% to 50% of the maximum rated amperage for the motor.
An approach that is more analytical can be used for both manual and CNC machines. This is supposedly the factory procedure used by Bridgeport to set them. Use a 0.0001 reading indicator and measure the slop in the slide. Example: For the X axis, place the mag base on the end of the saddle and put the stylus on the table. At that end of the table push and release. Then pull and release. The differance is the amount of clearance in the slide. Repeat at the other end of the saddle. Adjust gib in a like new machine with little wear to give a reading of 0.0005. A machine with more wear will have to be checked with the table closer to the end of travel. The same procedure is used to set the saddle to knee gib. There must be some clearance for the oil film and that film also helps dampen vibration. On a machine with hardened and ground box ways and turcite on the moving member the procedure is to set the clearance to almost nil. 0.0001 is a good number.
Another approach was based on trying to minimize backlash by making the gibs as tight as possible. In this case, it was advocated to overtighten the gibs until backlash due to flexure against a locked table was measurable, and then gradually back off in ¼ turn increments until that backlash was minimized.