Interference between the O.D of the bushing and
the I.D. of the hole in the jig plate holds the press fit bushings
and liners in place. Too much interference may cause problems: (1)
jig plate distortion (2) bellmouthing of the bushing where the walls
bow inward (3) tool seizure, or (4) difficulty sliding the renewable
bushing into the liner.
Too little interference may allow slippage, thus causing inaccurately drilled holes. In most cases, interference of .0005” to .0008” is sufficient to properly install press fit bushings and liners.
It is recommended that all mounting holes be jig bored or ground to assure roundness. Ordinary twist drills seldom produce an accurately sized or truly round hole.
First, lubricate the inside diameter of the mounting hole and the outside diameter of the bushing/liner – before pressing into place. Lubrication prevents scoring of the hole wall.
Second, use an arbor press to press the bushing/liner – into place. If an arbor press is not available, draw the bushing into place by tightening two steel plates connected by a nut and bolt. A bushing/liner should never be hammered into place. The same procedures should be followed when removing the worn bushings and liners.
Carefully align the drill with the bushing axis to avoid poor alignment and excessive wear. The radius on the bushing will help center the drill point. National no-counterbore bushings help the operator accomplish this with ease.
Dull drill bits defeat the preciseness of bushings. Sharpen by first grinding each drill with the point in the exact center. This helps keep the drill from “walking” when it first enters the work piece.
Check the coolant label to be sure you are using the coolant for your machining process.
Slip/Fixed Renewable Bushings for Multiple Operations
Slip/Fixed bushings of different lengths provide for both accuracy and chip removal during multiple operations. For example, short bushings during drilling will allow chips to clear away and longer bushings for reaming will provide greater position and roundness accuracy.
Adapt Bushings to Irregular Work Surface
The exit end of the bushing should conform to the work surface. For maximum guidance, keep the clearance space as small as possible. A drill that does not enter the work surface exactly perpendicular will “walk” or wander. In such cases side loads exerted by the drill are concentrated near the exit end of the bushing. This can cause the bushing to wear prematurely. For long production runs it is best to form the end of the bushing to the work piece which provides a longer wear surface in the bushing and therefore longer bushing life.
Long Production Runs
We suggest using Titanium Nitride (TIN) coated bushings or carbide bushings for longer bushing life. Call for details.
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