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...