1. Process for control of the cutting, by means of a machine tool, of a series of workpieces having surfaces of different characteristics to be machined by variation of the feed speed of a cutting tool of the machine tool, comprising a teaching mode carried out on at least one sample workpiece, follwed by an adaptive cutting mode, carried out on each of the workpieces of the series, wherein the teaching mode comprises the operations of: (a) subjecting at least one sample workpiece to cutting by means of the machine tool with the cutting tool advancing at a constant feed speed; (b) generating a first signal C proportional to the working torque of the cutting tool; (c) generating a second signal Q proportional to the depth of cut along the whole of the sample workpiece; (d) processing the first and second signals C, Q to provide a third signal C represented by: EQU C=C.spsp.k.sup.1 .multidot.k.sub.2 .multidot.Q, where k.sub.1 is a constant which is predetermined according to the tehcnological characteristics of the cutting tool, the material of the workpiece to be machined and the cutting conditions, and k.sub.2 is a constant which is predetermined according to the number of cutting edges of the cutting tool; (e) generating a fourth signal P which is a function of the relative position of sample workpiece and the cutting tool; (f) storing in a memory progressive values of the third signal C as a function of the relative positions of sample workpiece and the cutting tool; and wherein the cutting mode comprises the operations of: (g) generating a fifth signal P.sub.i representing the instantaneous relative position of the workpiece and the cutting tool; (h) extracting the third signal C, stored during the teaching mode and corresponding to the instantaneous relative position between the workpiece and the tool, and generating a sixth reference signal C'.sub.m proportional to said extracted third signal; (i) generating a seventh signal S.sub.i proportional to the actual depth of cut along the workpiece surface to be machined by the cutting tool; (j) generating an eighth signal C.sub.a having a predetermined constant value, representative of an additive value of torque; (k) processing the sixth signal C'.sub.m, the seventh signal S.sub.i and the eighth signal C.sub.a to generate a ninth signal C.sub.c represented by: EQU [C.sub.c =C'.sub.m .multidot.k.sub.3 +C.sub.a ].multidot.S.sub.i, where k.sub.3 is a predetermined constant proportional to the ratio, derived experimentally, between the cutting forces of a new tool and the cutting forces of the same tool at the end of its working life; (l) generating a tenth signal C.sub.i proportional to the instantaneous working torque of the cutting tool; (m) processing the ninth signal C.sub.c and the tenth signal C.sub.i to generate an eleventh signal .DELTA.a represented by: ##EQU4## where k.sub.4 is a corrective constant which is predetermined so as to optimize the response rate of the machine tool control system, and k.sub.5 is a constant which is predetermined according to the technological characteristics of the cutting tools, the material of the workpieces and the cutting conditions, and (n) controlling the feed speed of the cutting tool by means of the said eleventh signal .DELTA.a.