Production accuracy requires extra care when preparing mounting holes and installing bushings/liners. The following factors should be considered: 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.
Interference holds press fit bushings in the jig plate. Too much interference may cause a number of problems:
- jig plate distortion;
- bellmouthing (bushing/liner walls bow inward);
- tool seizure, or
- trouble with slip-fixed renewable fitting into the liner. Too little interference may allow slippage, thus causing inaccurately drilled holes.
Mounting Hole Preparation
It is recommended that all mounting holes be jig bored or sized with a reamer to assure roundness. Bushings tend to assume the shape of the hole they are pressed into. See chart below for recommended size.
The recommended installation hole size should be in accordance with metric tolerance class H7.
Press Fit Bushing Installation
First, lubricate the inside diameter of the mounting hole and the outside diameter of the bushing/liner.
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. Maintain centerline perpendicularity.
• All bushings that are ground on the O.D. are held within .012 mm concentricity T.I.R. (Total Indicator Reading).
• Longer lengths than standard or excessively counterbored bushings will meet the above tolerances at the drill exit end only.
• All Acme bushings are manufactured with a blended radius at the drill entrance end unless specified otherwise.
• All Press Fit bushings have a chamfer and ground lead on the exit end.
• Standard and Special bushings will be manufactured from thru-hardened tool steel and heat treated to 61-65 Rockwell “C” unless instructed otherwise by the customer.
• No-Counterbore bushings will be supplied unless requested otherwise. No-Counterbore bushings allow for truer alignment, more accurate holes, less tool wear, and less problems with chips packing up in the I.D. of the bushing.
Inside Diameter Tolerances
The minimum and maximum allowance for the Inside Diameter of standard bushings is in accordance with metric tolerance class G6. Special bushings will be supplied with the tolerances shown, unless specified otherwise.
Standard Drill Bushings:
Reamer Bushings are considered specials and are priced accordingly. When ordering specify the letters “RM” after the I.D.
Outside Diameter Tolerances
The minimum and maximum metric tolerance for the Outside Diameter of standard press fit bushings is class s6 and class h5 for slip-fixed renewable bushings.
To order metric bushings please provide the quantity required, ANSI Symbol, and inside diameter. The order for 26 pieces of a 38 mm I.D., by 55 mm O.D., by 30 mm long, Type PM bushing should appear as follows: