From the remits of an investigation of the principal laws of rotary percussive drilling of blast-holes, we have determined [1] the optimum characteristics for rotary percussive machines for blast hole drilling, and have also given the results of our first experiments on the drilling of blast holes of small diameter in an experimental test rig with remote pneumatic hammers and independent rotation of the drill rod by a separate motor.In 1960-1965, using the test rig and also with sample rotary percussive drills of types P-29 (BU-70), AB-2, and VP-80 which were undergoing industrial tests at Krivoi Rog mines, we continued our experimental drilling of small diameter blast-holes in rocks with hardnesses from f = 6-8 to 2 c --20. In the table below we give our chief results, from which we can determine the optimum axial feed thrust, the energy per impact, the number of impacts per rotation of the drill rod, and the torque, in relation to the hardness of the rock.The test rig for rotary percussive borehole drilling permitted us to set up pneumatic strikers with various energies of single impacts and impact power (in the experiments these ranged from 5 to 20 kg. In), and also, by means of hydraulic cylinders, to vary the axial feed thrust from 100 to 3000 kg and the rate of turn of the drill rod from 60 to 720 rpm.All the main experiments were carried out under main conditions in rocks of varying hardness, to that the results were reliable for determifiing the optirntum force parameters of drilling (the axial thrust, impact energy, numbet of impacts per revolution of the drill rod, and torque).For experimental drilling of boreholes in the faces of blind workings, drilling chambers were fitted up with compressed air, water, and electricity supplies for the test rig. The operation of the machines was measured by control and measurement devices: the air flow rate was measured by an air meter tube with interchangeable disks of 18, 24, and 32 mm diameter and a DS-50 differential manometer; the number of impacts was measured by a VA-1 vibrograph and a resonance vibrotachometer; and the rate of rotation was measured by a hand operated centrifugal tachometer, type IO-10. The feed rate was measured by a scale gauge and timer. The axial thrust, transmitted to the drill rod,was measured by a DC-5 or a DC-1 compression dynamometer, which was compressed by the forward end of the drill rod with the drilling bit removed. The compressed air pressure was monitored by a gas manometer with 10 atrn scale divisions. The electrical energy expended on rotating and feeding the drill rod was measured by a three phase meter; wear of the drilling instrument was determined from the width of the end surface due to blunting, which was measured under mine conditions by means of a magnifying glass with tenfold magnification. Under laboratory conditions, the blunting of the bit was determined with a large instrumental microscope of type EMI to within 0.005 ntm.We aimed to establish experimentaUy the main laws of rotary percussive drilling of small diam...

Extemive measurement of wear on the cutting edges of rotary-percussive drilling bits has shown that they are exposed to impact-load wear over the side edges (leading and trailing), while the end faces of the blade are exposed to static and peripheral forces. Where the impact energy and axial feed force are correctly selected for a particular rock hardness, the profile of the face blunting should approach a fiat profile or the profile of blunting in rotary drilling. Hard-metal wear at the end faces of the blade in the bit due to the impact load will always be in advance of wear due to static and peripheral forces, i.e., hard-metal wear from impact loads always reduces the width of the face area of the blunted part; it therefore increases the penetration depth and has no effect on the intemity of wear on the end face part of the bit blade.Axial and peripheral forces in a rotary-percussive drilling system are continuous, whereas the impact load is intermittent. Hence it follows that the size and nature of the operative forces governing the wear of the cutting edges of the bit in rotary-percussive drilling remain the same as for rotary drilling, i.e., axial and peripheral forces. Therefore the derivation of an analytical formula for bit life will resemble such a formula for rotary drilling with free tool feed (pneumatic or hydraulic).How tool geometry affects its life is determined by a formula describing a relationship between the base of a triangle and its height [1]. For a cutter shaped like an asymmetrical wedge, with a leading negative angle B, bit blade wear in terms of height is determined by the formula /z "-at (1) tan (7 --}) +tansThe same is true for a one-sided wedge shaped tool with a leading angle 8 = O:tan 7 where h is the height of blade wear on its periphery in ram, a t is the width of the end face of the blunted part on the bit blade 8 mm from its periphery in mm, y is the dressing angle of the bit in deg, B is the leading negative dressing angle in deg.We will now derive a formula for the life of a bit in free feed (pneumatic or hydraulic) on to the hole socket. Wear of the cutting blades on the bit will be related to its peripheral portion where value, a t and h meet their /) Fig. 1. Deviation of the bit life formula.maxima. All the reasoning involved in deriving this formula relates to volumetric attrition at optimum, or virtually optimum, bit penetration depth per revolution.To determine the elementary friction ( Fig. 1) we ~11 use the following notation: to is the coefficient of volumetric wear of hard metal in cmS/kg, dA is the elementary work done by friction in kgm, d~ =atd p is the elementary area of friction on the end face surface of the bit blade, d~ is the elementary angle of turn of the bit, and /~ is the coefficient of external friction.The elementary friction energy in tl~ case is: dA = ~al%d~?. The work of friction per revolution of the bit is found by integrating Eq. (3) with respect to p and ~o Moscow.