A New Method of TESTING AND GRADING Fine Abrasives 400158
THE difficulty of selecting the proper abrasives or “stones” for Superfinishing machines made by his company, because of the “apparent inability of stone manufacturers to grade the hardness of the bond properly and uniformly,” led to the development described, Mr. Hemingway explains.
Reciting the difficulties encountered in Superfinishing operations due to lack of correct bond grading he declares that, when a stone of too great hardness is applied to a ground surface, its too-rigidly-held grits are not removed to dress and sharpen its face and the cutting action slows down much sooner than desired. When the stone used has too little bond strength its action on the hill peaks results in an extremely rapid removal of metal as the stone breaks down very fast.
After describing various preliminary tests, he reveals that a Rockwell hardness tester with a ⅛-in. steel ball and a 60-kg load finally was selected for grading Superfinishing stones. A curve is included that indicates the proper stone to use in Superfinishing a given hardness of steel under standard conditions. He explains that such graphs have been used for five months with satisfactory results, and reports that the former difficulties have practically disappeared.
In conclusion, Mr. Hemingway contends that every user of fine-grit bonded abrasive, no matter what their application may be, has at his disposal in this method an accurate means of predetermining their cutting characteristics.