9.6 Start the wheel rotation and immediately lower the lever arm carefully to allow the specimen to contact the wheel.
9.7 When the test has run the desired number of wheel revolutions, lift the specimen away from the wheel and stop the sand flow and wheel rotation. The sand flow rate should be measured before and after a test, unless a consistent flow rate has been established.
9.8 Remove the specimen and reweigh to the nearest 0.001 g (0.0001 g for Procedure C).
9.8.1 Wear Scar Observe the wear scar and compare it to the photographs of uniform and nonuniform wear scars in Fig.
11. A nonuniform pattern indicates improper alignment of the rubber rim to the test specimen or an unevenly worn rubber wheel. This condition may reduce the accuracy of the test.
9.9 Preparation and Care of Rubber Wheels Dress the periphery of all new rubber wheels and make concentric to the bore of the steel disk upon which the rubber is mounted. The concentricity of the rim shall be within 0.05 mm (0.002 in.) total indicator reading on the diameter. Follow the same dressing procedure on used wheels that develop grooves or that wear unevenly so as to develop trapezoidal or uneven wear scars on the test specimen (Fig. 11). The intent is to produce a uniform surface that will run tangent to the test specimen without causing vibration or hopping of the lever arm. The wear scars shall be rectangular in shape and of uniform depth at any section across the width. The rubber wheel may be used until the diameter wears to 215.9 mm (8.50 in.). New rubber rims may be mounted on steel disks by the qualified source (6.2).
9.10 Wheel Dressing Procedure The preferred dressing procedure for the periphery of the rubber rim is to mount a diamond-cut file9 in place of the specimen in the holder and run the machine with load until the wheel is clean. Another dressing procedure for the periphery of the rubber rim is to mount the
wheel on a lathe, and machine the surface with a tool bit especially ground for rubber applications. Grind a carbide or high-speed steel tool bit to very deep rake angles (Fig. 12). Feed the tool across the rubber surface in the opposite direction from that normally used for machining steel. This allows the angular surface of the tool bit to shear away thin layers of rubber without tearing or forming grooves in the rubber as would occur when using the pointed edges of the tool. The recommended machining parameters are: Feed 25 mm/min (1.0 in./min); Speed 200 rpm; Depth of Cut 0.254 mm (0.010 in.) to 0.762 mm (0.030 in.). The dressed wheel should be first used on a soft carbon steel test specimen (AISI 1020 or equivalent) using Procedure A. This results in a smooth, uniform, non-sticky surface. An alternative dressing method involves the use of a high-speed grinder on the tool post of a lathe. Take great care since grinding often tends to overheat and smear the rubber, leaving a sticky surface. Such a surface will pick up and hold sand particles during testing. If the grinding method is used, not more than 0.05 mm (0.002 in.) may be ground from the surface at one time so as to prevent overheating.