Pm 923 M Gib Adjustment

[JR49]

Could someone please explain the princible behind the gib adjusting system on this mill. It has a big headed screw at each end of the gib which, from what I can see only slides the gib end for end. How does sliding the gib end for end make the X, Y, or Z gib tighter against the ways?? Thanks, JR49 EDIT: sorry, the title should say 932 M

 

[Chipper5783]

The gib has a shallow taper. Setting it in further reduces the clearance. It needs a screw on each end so that it does not get tighter/looser as you move the slide back and forth.

 

[coolidge]

JR here's how you adjust the gib...the gib is wedged shaped and adjusting the gib 'in' takes up slop and tightens up the clearances, adjusting the gib out loosens up the clearances. You turn the two adjustment screws in unison, putting a slight bind on each other so that the gib cannot move when the table moves you want the gib locked in place but don't over tighten, if you over tighten the two screws against each other you can bow the gib putting the table in a bind. Now how tight should you adjust the gib? You want to adjust the gib until you just start to feel a light bit of drag when the table moves. This should be consistent throughout the tables travel so you need to run the table back and forth to make sure the slight drag feel is consistent. Depending on the quality of the mill the ways may not be exactly true, so if you hit a tight spot back off the gib.

A really poor quality mill can have ways that are ground very poorly indeed, with multiple tight and loose spots especially on Z, or even ways ground to a taper so the table binds at one end of its travel but is loose at the other end. I find its easiest to adjust the gibs with the ACME feed screws disconnected so that you can just slide the table back and forth by hand along the ways, you can really feel the drag/no drag point, or poor ways that are tight/loose/tight or tapered, it sticks out like a sore thumb vs the torque you have with a hand wheel cranking it back and forth. But of course disengaging the ACME feed screws is a major operation.

 

[joshua43214]

I like to back out the bottom screw til it is about to fall out, then advance the table so it drives the wedge in. you will feel when the table just starts to bind, then run the bottom screw in and go another 1/2 to 2 turns until it stops binding. Then snug down the top screw.
I find doing it this way helps overcome the film from the way oil better than just trying to push the wedge in deeper with the screw. It also makes adjusting the gib for the z-axis much easier since tightening the gib actually lifts the head/table.

 

[JR49]

Thanks to all for the great explanations. All are much better than the manual provided. I do have one more question. Is this "set up" of gib adjustment standard on milling machines, or is it specific to Precision Matthews (or should I say Chinese mills) ? I'm not new to adjusting gibs on my Logan lathe, and feel a little embarrassed that I was unable to figure this out. Thanks, JR49

 

[lpeedin]

The tapered gibs have .010" of taper per inch. It almost looks like they are flat gibs when you pull them out.

 

[coolidge]

JR I can't think of a new mill I have purchased that did not require setup including adjusting the gibs, tramming the head, adjusting the ACME screw nuts to reduce backlash, etc. I just consider this part of the cleaning/setup process and don't expect the factory to take the time to do this properly. Its when I have to start drilling and tapping holes, installing replacement parts, and fixing factory screw ups that I get annoyed.

 

[JR49]

JR I can't think of a new mill I have purchased that did not require setup including adjusting the gibs, tramming the head, adjusting the ACME screw nuts to reduce backlash, etc. I just consider this part of the cleaning/setup process

I totally agree with your statement Coolidge, and also expect to make all these adjustments, as a process of setting up my new mill. If my last post sounded like I was complaining about having to make these adjustments, then I mis spoke. I'm actually looking forward to the process, and the learning that will be gained.
What I was trying to ask (as part of my learning from you guy's), is what everyone's opinion is on the tapered gib system of adjustment, as opposed to the only other type I have any experience with, which is just a straight gib that is pushed against the dovetail by a number of grub screws. Thanks, JR49

 

[lpeedin]

I can tell you based on screwing up a tapered gib on my machine that getting a new gib to fit properly is a royal PITA! I am still using the one that I messed up for now which is shimmed and works fine. Matt did send me a couple of new gibs from China so I could get a new one fit. After messing around for a while with it, I am going to wait until I get a couple of other fun projects done before I try to tackle the actual fitting of a new gib. The nice part is that the replacement gibs already have the taper cut on them from the factory at the .010" per inch. I need to do some scraping to insure flatness and oil holding space in order to properly install the new gib. I do think it would be a nice touch when I do the new installation to drill and tap my saddle to also have the other style of adjustments with a set screw and lock down nut like my lathe and little X2 milling machine have. I think this would give the ultimate in adjustability and insure that the gibs are evenly distributing the load.