(cast iron or unhardened steel jig plates)
Production accuracy requires extra care when preparing mounting holes and installing bushings/liners. The following factors should be considered: diametral interference fits, alignment, chip clearance, and how close the bushing is to the workpiece.
Because of the many variables, no definite rules can substitute for the skill and judgment of the experienced toolmaker. The following suggestions should be helpful to others.
Interference holds press fit bushings in place on the jig plate. Too much interference may cause a number of problems: (1) jig plate distortion; (2) bellmouthing (bushing/ liner walls bow inward); (3) tool seizure, or (4) trouble with slip-fixed renewable fitting into the liner.
Too little interference may allow slippage, thus causing inaccurately drilled holes. In most cases, interference of 0.0005 to 0.0008 inch is sufficient to properly install press fit bushings and liners.
Mounting Hole Roundness
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. White lead is a typical lubricant.
Second, use an arbor press to press the bushing/liner into place. If an arbor press is not available, draw the bushing/liner into place by tightening two steel plates connected by a nut and bolt. A bushing/liner should never be hammered into place! Why defeat the main purpose of the bushing? – Accuracy!
One school of thought suggests a space between the workpiece and the drill bushing. The other suggests direct contact. In order to decide, you should consider the abrasiveness of the material being drilled, the drill size, and the type of bushing/liner being applied.
Chips produced from drilling will be ejected either through a space or through the bushing. If your drill clogs or heats up, try increasing the space between the end of the bushing and the workpiece. Normal spaces vary from one-half the drill diameter for small chips (as from cast iron) to one and one-half times the drill diameter for long stringy chips (as from cold-rolled steel).
Vary the length of Slip-Fixed Renewable bushings to achieve proper spacing for your operation (see number 4 below for a more detailed explanation). A rule of thumb is, the greater the space between the bushing and the workpiece, the greater the chance for error. Where possible, test your conditions and continue to check for accuracy throughout your production run.