Mach 122: Abrasive Machining Bonded Abrasives – Part 1 Objectives: Review abrasive types Discuss standard wheel marking system Identify and explain abrasive type, grit size, and the grade of a grinding wheel. Created by: Ronald C. Schwartz 8/27/2009

Bonded Abrasives Aluminum Oxide Aluminum oxide works well on steels but poorly on cast iron, as it is somewhat soluble in cast iron at the high temperature of grinding. The wheel dulls quickly as the sharp edges dissolve away into a solid solution with the iron. Created by: Ronald C. Schwartz 8/31/2009

Bonded Abrasives Silicon Carbide Fig. L-39 Silicon carbide. (Courtesy of Steve Karg) Silicon carbide is somewhat harder than aluminum oxide, but has a sharper, more friable & brittle crystalline structure. Works well on cast iron & nonferrous materials such as aluminum- and copper-based alloys. Created by: Ronald C. Schwartz 8/31/2009

Bonded Abrasives Cubic Boron Nitride (CBN) Fig. L-40 Cubic boron nitride. (Borazon CBN) (General Electric Company) tm Cubic Boron Nitride (CBN) Cubic boron nitride (CBN) is much harder than silicon carbide. It was created by the General Electric Co. in the late 1950s. Works best on hardened ferrous alloys, especially cobalt & nickel super-alloys. Created by: Ronald C. Schwartz 8/31/2009

Bonded Abrasives Cubic Boron Nitride (CBN) Fig. L-40 Cubic boron nitride. (Borazon CBN) (General Electric Company) tm Cubic Boron Nitride (CBN) CBN is not competitive with diamond in the grinding of tungsten carbide. It is up to 200 times more expensive than aluminum oxide or silicon carbide. Machine requirements are also rigorous. Created by: Ronald C. Schwartz 8/31/2009

Bonded Abrasives Diamond Diamond is another super-abrasive obtained in either natural or manufactured form, and is the hardest substance known. The single-crystal diamond nib used for the truing and dressing of grinding wheels is a common application. Fig. L-41 Manufactured diamond abrasive. (General Electric Company) Created by: Ronald C. Schwartz 8/31/2009

Bonded Abrasives Diamond Most diamonds used in precision grinding are manufactured Engineered to have controlled crystal configurations not duplicated in natural diamonds. Especially useful for grinding of ceramics & tungsten carbide, but not effective on steels or superalloys containing cobalt or nickel. Fig. L-41 Manufactured diamond abrasive. (General Electric Company) Created by: Ronald C. Schwartz 8/31/2009

Wheel Marking System A standard wheel marking system is used for the identifying five major factors in grinding wheel selection: Type of abrasive Grit size Grade or hardness Structure Bond Fig. L-47 Wheel specification. A wheel marked A 60-J8V indicates the following: Created by: Ronald C. Schwartz 8/31/2009

First Symbol: Abrasive Type Five major abrasives are in common use: A C D MD/SD B Fused aluminum oxide Silicon carbide Diamond Manufactured or synthetic diamond Cubic boron nitride Created by: Ronald C. Schwartz 8/31/2009

Second Symbol: Grit Size (A 60-J8V) Grit refers to size of the abrasive grains. Grit numbering is derived from screen openings used to sort abrasive grains after manufacture. Coarse grits are used for fast stock removal & on soft ductile materials. Fine grit is used for hard brittle materials. The following scale can be used to determine grit: Coarse Medium Fine 4 36 46 60 100 120 240 500 Created by: Ronald C. Schwartz 8/31/2009

Third Symbol: Grade of Hardness (A 60-J8V) Hardness grade is a measure of bond strength of the grinding wheel. Bond material holds abrasive grains together in the wheel. The stronger the bond, the harder the wheel. A to G are softer. H to P are more medium grades. R to Z are harder. Created by: Ronald C. Schwartz 8/31/2009

Third Symbol: Grade of Hardness (A 60-J8V) Created by: Ronald C. Schwartz 8/31/2009

Third Symbol: Grade of Hardness (A 60-J8V) Selection & Identification of Grinding Wheels tab Third Symbol: Grade of Hardness (A 60-J8V) It is necessary to have grains pulled from the wheel as soon as they become dulled, to expose new sharp grains. If they are not, cutting efficiency & finish will be poor. Three sketches illustrating (from top down) a soft, a medium, and a hard wheel. This is the grade of the wheel. The white areas are voids with nothing but air, the black lines are the bond, and the others are the abrasive grain. The harder the wheel, the greater the proportion of bond and, usually, the smaller the voids.

REVIEW Name the four common abrasives found in grinding wheels. How are the different abrasives identified on a grinding wheel? Identify the following abrasive types. A C D B How is grit size identified? Explain importance of grit size selection. What is used to identify the Grade of a wheel? Which wheel is harder? Grade E or Grade T What does the term “friable” mean? Why is wheel grade selection important? Created by: Ronald C. Schwartz 8/31/2009