Segmental grinding wheels for a vertical spindle machine, such as the "Blanchard", are currently manufactured in various shapes and sizes.  These generally fall into two widely used catergories, one of which includes abrasive segments which are angularly spaced and do not form a continuous annular grinding ring.  The other type are abrasive segments which form a continuous annular grinding tool ring when mounted on a chuck, such as those known in the art as the "Sterling" interfitting shaped segments disclosed in U.S. Pat.  Nos.  1,868,492; 2,023,041 and 2,133,009. 
The "Sterling" shape segments form a continuous annular grinding ring by abutting its neighbor in a tongue and groove fashion.  Usually, but not always, the inner concave side of the segment is coated with molten sulfur or other material to seal the abrasive surface and prevent coolant from flushing swarf into the pores of the segment. 
The tongue and groove construction of the Sterling type segment is expensive to produce due to abrasive wear and cost of replacing the molds in order to maintain the precise shape. 
Due to mold wear, curing or firing the size and shape of the segments vary enough to require hand fitting of segments to a chuck by means of rubbing down oversize segments or building up the abutting edge of undersize segments with sulfur or other type of paint or coating.  For this reason they are usually sold in complete sets and not as individual segments. 
However, the composite segment of the invention differs therefrom and is an improvement thereover in that it has an abrasive segment of more simplified arcuate shape, somewhat smaller in size and of less precise dimensions encased in a shell of relatively non-abrasive plastic material molded to precise dimensions and shape with perfectly interfitting tongue and groove sides. 
The shell is formed around each individual preformed bonded abrasive segment by either injection molding, compression transfer molding or casting, in the conventional manner, a thermoplastic or thermosetting composition into the space between the precision mold surfaces and preferably all but the working abrasive surface of the simplified abrasive segment inserted therein. 
Preferably, the material selected to form the shell is in itself a grinding aid or contains as much of any well known grinding aid filler material as is consistent with flowability and strength. 
The composite segment of the invention has many advantages over its state of the art counterparts as follows:
1.  The mold to produce the simplified bond abrasive segment need not be as precise, is less expensive to produce and can wear appreciably before it need be replaced. 
2.  No fitting of the composite segments is required since the exterior dimenions and shape are formed by a precise mold not subject to abrasive wear.  Thus each segment is of such precise dimensions that duplication, mating engagement and interchangeability is assured. 
3.  The shell encasing all but the working surface of the composite segment prevents grinding swarf from filling the pores; and also reinforces the segment. 
4.  The shell contains and provides active filler or extreme pressure lubricant to aid the grinding process. 
Applicant is aware of the prior art disclosing abrasive wheel segments and honing sticks coated with sulfur or cement, encased in metal, or plastic shells containing a filler, lubricant or grinding aid. 
However, the encased abrasive segments or honing sticks do not have a precision molded plastic shell of a complex interfitting shape which can be consistently duplicated to the precise and exact dimensions.  Thus, the interfitting segments of the invention are more uniform in size and shape than the "Sterling" type segments and therefore more readily interchangeable with one another.