embodiment shown in fig7 where the wheel 43 is bonded to the plate 41 with adhesive is the depth of the centering tabs 45 thus in each of these embodiments only a small part of the abrasive material of the grinding wheel 14 or 43 is not usable once the abrasive material of the grinding wheel is exhausted the backing plate wheel combination can be replaced without removing any of the other components of the grinding head assembly in the replacement procedure bolts 19 are unscrewed sufficiently to allow the flange 12 to be slipped downwardly to disconnect the backing plate 11 from the backing plate holder 18 a new backing plate with a grinding wheel attached is then installed as described above the bolts 19 do not have to be removed from the backing plate holder 18 and neither the backing plate holder 18 nor drive shaft d is disturbed in the replacement procedure the procedure thus takes a relatively short time and is easy to perform it is clear from a consideration of the foregoing disclosure that the invention provides a backing plate assembly for a grinding head of simple and economic construction that can withstand the stresses associated with the grinding process can be automatically centered to ensure balance allows use of the grinding wheel to its maximum or near maximum depth may be reused once the abrasive material of the grinding wheel is exhausted and is quickly and easily installed and replaced while specific embodiments have been disclosed in detail it should be understood that modifications and variations may be resorted to without departing from the spirit of the invention for example the shapes and angles of the various flanges and centering tabs.