Preferably, {TAIL ~ the grinding wheel} starts grinding from the object portion which is located nearest to the home position of {HEAD ~ the second wheel head}, and then grinds the remaining object portions while the first wheel head is returning to its home position 
Preferably , { TAIL ~ the grinding wheel } starts grinding from the object portion which is located nearest to the home position of { HEAD ~ the second wheel head } , and then grinds the remaining object portions while the first wheel head is returning to its home position
OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH