INFO - MACHINING CAST STEEL FRONT DISC. BRAKE

REVISION: 02/13/98

THIS BULLETIN IS BEING REVISED TO INCLUDE THE GMC DIVISIONAL LOGO WHICH WAS INADVERTENTLY OMITTED FROM THE ORIGINAL BULLETIN. PLEASE DISCARD CORPORATE BULLETIN NUMBER 76-50-13 (SECTION 5 - BRAKES) ______________________________________________________________________

SUBJECT: INFORMATION - MACHINING CAST STEEL FRONT DISC BRAKE ROTORS

MODELS: 1995-98 CHEVROLET TILTMASTER AND GMC FORWARD W4 MODELS

ALL 1995-98 W4 MODELS ARE EQUIPPED WITH FRONT DISC BRAKE CAST STEEL ROTORS RATHER THAN THE CAST IRON TYPE INSTALLED IN 1994 AND EARLIER MODELS. CAST STEEL ROTORS ARE DESIGNED TO WITHSTAND HIGHER BRAKE FRICTION COEFFICIENTS, AND SINCE STEEL IS MUCH DENSER THAN CAST IRON, CAST STEEL ROTORS WILL GIVE BETTER SERVICE LIFE UNDER EXTREME CONDITIONS.

THIS SERVICE BULLETIN PROVIDES USEFUL INFORMATION FOR MACHINING CAST STEEL ROTORS WITH A BRAKE REFINISHING MACHINE. IT INCLUDES MACHINING CHARACTERISTICS, MACHINE SETUP, AND USEFUL MACHINING TIPS.

MACHINING CHARACTERISTICS

CAST STEEL ROTORS MACHINE MUCH DIFFERENTLY THAN TYPICAL GRAY CAST IRON OR NODULAR IRON ROTORS. CAST STEEL ROTORS REQUIRE SLOW SPINDLE SPEEDS AND CROSS FEEDS TO OBTAIN A QUALITY FINISH. MOST BRAKE REFINISHING MACHINES HOWEVER ARE LIMITED TO ONLY TWO SPINDLE NEEDLES AND TWO CROSS FEEDS, MAKING MACHINING OF CAST STEEL ROTORS MORE DIFFICULT. WHEN CAST STEEL ROTORS ARE MACHINED, THEY GIVE OFF LONG, SPIRAL STRANDS AS COMPARED TO FLAKES OR DUST GIVEN OFF WHEN CAST IRON ROTORS ARE MACHINED. THE REFINISHED SURFACE OF CAST STEEL ROTORS MAY ALSO APPEAR ROUGHER THAN THAT OF CAST IRON ROTORS. THIS ROUGHNESS HOWEVER DOES NOT AFFECT BRAKE PERFORMANCE.

MACHINE SETUP

THE MOST CRITICAL ASPECT OF THE MACHINING OPERATION IS THE INITIAL SETUP. MOST QUALITY BRAKE REFINISHING MACHINES ARE CAPABLE OF TURNING ROTORS TO WITHIN +/- .002 INCH. A MACHINE'S INABILITY TO PRODUCE CLOSE TOLERANCES, IN MOST INSTANCES, IS NOT THE FAULT OF THE MACHINE, BUT RATHER A RESULT OF POOR PREPARATION OR A LACK OF MACHINERY MAINTENANCE.

A SIMPLE, FAST, AND ACCURATE METHOD FOR SETTING UP THE BRAKE REFINISHING MACHINE IS TO MAKE A LIGHT INSPECTION PASS (I.E., A CUT LESS THAN .005 INCH) TO DETERMINE THE AMOUNT OF CONTACT AREA THAT IS TOUCHING BOTH THE INSIDE AS WELL AS OUTSIDE FACES OF THE ROTORS. IF THE SCRATCHES FROM THE TOOL BIT CONTACT 80% OF THE ROTOR ON BOTH THE INSIDE AND OUTSIDE SURFACES, THE ROTOR IS MOUNTED CORRECTLY AND IS READY FOR REFINISHING. IF THE SCRATCHES DO NOT CONTACT AT LEAST 80% OF THE ROTOR, LOOSEN THE COLLAR CLAMP, ROTATE THE ROTOR TO ANOTHER POSITION, AND RECHECK IT AT 90 TO 180 DEGREES (FIGURE 1). IF THIS DOES NOT CORRECT THE PROBLEM, REMOVE THE ROTOR FROM THE MACHINE AND INSPECT THE INNER AND OUTER CUPS FOR DIRT, ABRASION, OR OTHER WEAR CONDITIONS.

FOR HUBBED ROTORS, THE ACCURACY OF THE REFINISHED ROTORS DEPENDS ON THE CONDITION OF THE INNER AND OUTER CUPS. FOR NON-HUBBED ROTORS, IF THERE IS A PROBLEM WITH ROTOR OSCILLATION, A SHIM MAY BE INSTALLED BETWEEN THE ROTOR MOUNTING SURFACE AND THE FACE PLATE TO OFFSET THIS CONDITION. IF THE ROTOR HAS EXCESSIVE LATERAL RUNOUT, THE SCRATCHES WILL APPEAR AS INCOMPLETE CIRCLES (FIGURE 2). A FINAL CHECK WITH A DIAL INDICATOR SHOULD BE USED TO CHECK THE SETUP. SWEEP THE OUTSIDE OR UNWORN PORTION OF THE ROTORS WHERE THE BRAKE PADS DO NOT MAKE CONTACT (FIGURE 3).

MACHINING TIPS

· BE SURE THE TOOL BIT NOSE HAS A KEEN EDGE TO IT. LOOK FOR BURNED, FRACTURED, OR DULLED SURFACES AND CORRECT OR REPLACE AS NECESSARY.

SEE FIGURE 4.

· TOOL BIT MODEL: VALENITE #TNMG 332GF SV315, OR EQUIVALENT.

· USE A POSITIVE RAKE TOOL BIT AND LIGHT CUTS (I.E. BETWEEN .005 -.010 INCH).

SEE FIGURE 5.

· USE A WATER-SOLUBLE CUTTING LUBRICANT WHEN MACHINING THE ROTORS.

· USE THE FOLLOWING SPINDLE SPEEDS AND CROSS FEEDS ACCORDINGLY.

SPINDLE SPEED CROSS FEED RATE/INCH 150 RPM .012 180 RPM .020

· KEEP THE TOOL HOLDER PROTRUSION TO A MINIMUM TO REDUCE DEFLECTION.

· KEEP THE TOOL BIT VERTICAL HEIGHT AS CLOSE TO THE HUB CENTERLINE AS PERMISSIBLE.

SEE FIGURE 6.

· FOR ROTOR AND HUB ASSEMBLIES WEIGHING 100 POUNDS OR MORE, USE A MACHINE WITH A 1-7/8 INCH-DIAMETER BAR OR LARGER.

FIGURES: 06

ATTACHMENTS: 00

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.

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