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"Milling Machines"
Milling Machine Maintenance
Milling Home

The milling machine lathe is a precision machine tool and must be treated with great care to maintain the machine’s accuracy. Regular cleaning and maintenance will help to assure that the milling machine will maintain its service life and accuracy for many years. This unit will cover basic milling machine maintenance. The procedures you find within this document should be able to be performed by apprentice or beginning machine tool students. Milling machine maintenance that requires more extensive disassembly should only be done by, or under the supervision of, qualified personnel.

Drive Belts

The drive belts supply power from the motor to the spindle. Access to the drive belts is gained by opening the side door on the column of the milling machine (Figure 1). Make sure that all power is locked out before removing any guards.


Figure 1. Side Access Door.

Drive belts come in matched sets and should only be replaced with a matched set of belts. Visually inspect the drive belts for excessive wear and cracking. If you notice that one or more of the drive belts appear to be excessively worn or cracked, bring this to the attention of the instructor. Check the belt tension by applying finger pressure to each belt at a point midway between the two pulleys (Figure 2). For correct tension a deflection of about 3/4 of an inch should be evident in each belt. If the amount of deflection is more than 3/4 of an inch in any one or more of the belts, bring this to the attention of the instructor.


Figure 2. Each belt should have approximately the same amount of tension.


Gib Adjustment

All milling machines employ precision slide ways. The saddle, table, and the knee all ride along a box slide way or dovetail slide way. After time the parts that ride along the slide ways begin to wear. To compensate for this wear, machine tools are equipped with adjustable parts called gibs that allow you to eliminate the space that has been created by the wear between the slide ways (Figure 3).


Figure 3
. A tapered Gib located on the table of the milling machine.

There are two types of gibs, straight gibs and tapered gibs. Straight gibs are adjusted by screws spaced out along the length of the gib. The screws push the gib in to create more contact with the sliding mechanisms (Figure 4).


Figure 4. A knee slide utilizing a straight gib.

Tapered gibs use one or two screws. The screws are located in each end of the tapered gib. One screw acts as an adjustment while the other screw acts as a locking mechanism. Because tapered gibs are wider on one end than the other, they slide in or out creating more or less contact between the sliding mechanisms (Figure 5).


Figure 5. Tapered gib with screws.


Table Gib Adjustment


Figure 6. Table Gib Screw.
Wear in the table slide ways must be adjusted by using the screw on the left face of the table slide. (Figure 6).

On larger machines, the procedure is to first loosen the similar gib screw on the right side of the table, then re-tighten the opposite gib screw to lock or adjust the Gib in its new position. On small ram-type milling machines there is only one adjusting screw on the left-hand side of the table. The procedure is identical to the larger machines only there is no locking screw. After the adjustment, traverse the table over its entire length to be sure of smooth, even operation.


Saddle Gib Adjustment

Wear in the saddle slide ways must be adjusted by using the screw on the front face and rear faces of the saddle slide way. On small ram-type milling machines there is only one adjusting screw on the front of the saddle. The procedure is identical to the larger machines only there is no locking screw.  (Figure 7).


Figure 7. Saddle Slide Way Gib Screw.

The procedure is to first loosen the rear gib screw on the saddle slide way, then re-tighten the front screw to lock or adjust the Gib in its new position. On the small ram-type milling machine adjustment can only be made from the front gib screw. Note if a chip wiper is present, you will have to remove it before any adjustments can be made (Figure 8). After making the adjustment, traverse the saddle over its entire length to be sure of smooth even operation.


Wipers Pads

Most milling machine are equipped with Wiper pads (Figure 8). Wiper pads are typically made of felt that will hold oil.


Figure 8. Rubber wiper pads with felt inserts.

Wipers are designed to keep out small chips and dirt between the slides and the ways. Wipers are saturated with oil to catch the fine particles of dirt or debris before they get between the two sliding surfaces. The wipers should be removed, cleaned and re-saturated with oil regularly. You should never use compressed air for cleaning a milling machine. Compressed air will push the fine particles trapped in the wiper between the mating surfaces of the slides, and causing premature wear on these precision surfaces.


Knee Gib Adjustment for Ram Type Milling Machine

Ram style-type milling machines are equipped with a tapered gib on the knee. To adjust the gib remove the wiper and adjust the top gib screw until smooth movement over the entire travel is attained (Figure 9).


Figure 9. Knee gib for a ram-style milling machine.


Knee Gib Adjustment for Large Knee-Type Milling Machines

Large knee-type milling machines are equipped with a straight gib on the knee. To adjust the gib a series of bolts with lock nuts must be adjusted individually (Figure 10). This gib adjustment should only be done under the supervision of a trained machine tool mechanic or instructor.


Figure 10. Straight gib on a large knee-type milling machine.

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