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Gib construction and PM. (Mike Breen on Preventive Maintenance).

Last month we addressed gib-related faults, such as axis "crabbing" and "slip-stick" as well as how to check and perform PM in this area. This month we're going to finish this subject with a discussion on the construction of the gib.

The Gib

The gib is usually a tapered, wedge-like piece of steel. The contacting surface (the part that contacts the way) is "scraped in" to fit the box-way. Some may be coded with a material called turcite. Turcite is a plastic-like coating that is adheased to the gib. Turcite is available in precut sheets or as a liquid. Both forms are scraped to the slide.

Scraping maximizes the contact of the contacting surface. Initially, the gib is coated with some sort of pressure die. The gib is then inserted into the slide and removed. Where there is contact, die will be removed. This area is considered a high spot. The high spot is removed with a hand held, carbide tipped tool. The scraper removes material at a maximum of 0.0001" at a time.

The scraping itself is done in a circular motion, thus forming tiny pockets. These pockets are used to hold way lubrication to prevent wear. After the high spot is removed, the gib is re-died and inserted again. This process continues until the gib makes full contact (with the exception of the oil pockets) on the entire length of the gib. The reason that the removal of material is so minimal is that the thickness of the gib is extremely important. If too much material is removed there will be no contact. At this point the gib is worthless. Whether the gib is all steel or turcite coated, the mating surface is scraped.

Tightening the Gib

When tightening the gib, the taper is pushing harder to the side of the slide and box-way. If it's too tight, it can lock-up an axis. As a precaution, never move an axis when the adjustment screws are loose. The axis could suck in the gib and wedge itself. At one shop, a vertical axis was stuck so tightly that a crane had to be used to loosen it. Adjustment screws are not always located in the same place. Axes can have adjustment screws on both sides of the gib, a single screw on the large side of the gib or a series of setscrews that put pressure along the backside of the gib.

PM Checks

Now that you have a general picture of the construction of gibs and what is used to set them, let's discuss some precautionary checks that should be carried out prior to gib adjustment.

A wiper protects most gib areas. If the wiper is bad, it indicates contamination in the gib area. If chips or other foreign matter are present, the scraped area will prematurely wear and the oil pockets will be too shallow to hold oil. If turcite is in the area, the chips will become imbedded in the turcite. If the wiper is bad, the gib should be removed, cleaned, oiled, and then adjusted.

Once the gib is removed, inspect it, and look for wear. When cleaning, don't use any type of abrasive cloth or material. If the gib has noticeable grooves, the way may match it. If it's filed too smooth, it may make very little contact at all. A fine polishing stone is all that should be required. I don't advise you to try to shim it or bend the gib. If it is worn, have it replaced.

Prior to any adjustment, contact your machine tool builder for the correct procedure. Rarely will this information be found in the general manuals supplied with the machine tool. In addition, all machine types have unique procedures when it comes to adjusting gibs.

When tightening the side gib use the indicator to the way as I explained in my previous article. Tighten the lower or underside gibs (prevents axis lift) to the specified load on your display. If there is no display, do the "tight and back off procedure" and run it. This is the truest test. Expect to have to re-adjust.

Once your adjustment is complete, check axis backlash. Anytime you tighten one thing on a feed system, it may cause another fault in a different system. If you shift the axis enough, your axis zero point could also be off.

Next month we'll dig into turret alignment. I've had some requests to explain this in more detail. Talk to you next month.

As a certified Electronic Engineer, Mike Breen is presently working as a Field Service Engineer with Ellison Machinery Company of Wisconsin. Mike invites you to share your own machine repair and maintenance experiences with him. For comments and inquiries about these articles, his e-mail address is mbreen@ellisonwi.com.
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Article Details
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Author:Breen, Michael W.
Publication:Modern Applications News
Date:Nov 1, 2002
Words:799
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