Patent GB-1043199-A (GB)
Title: Blasting
Inventor: ASHWORTH STEWART IVES
IPC: B24C9/00
Filed: 1962-10-16
Abstract: 1,043,199. Blasting. ABRASIVE DEVELOPMENTS Ltd. Oct. 3, 1963 [Oct. 16, 1962], No. 39054/62. Heading B3D. [Also in Divisions B8 and F2] A method of blasting comprises entraining abrasive in a stream of gas and directing the composite stream through a blasting nozzle at an article or surface to be abraded, the abrasive being fed to the nozzle through a vibrating sieve forming part of a resiliently suspended vibrating system which is vibrating at its fundamental resonant frequency, the amplitude of vibration of the system being controlled to control rate of flow of abrasive through the sieve. A boom 13, Fig. 1, of wood, e.g. ash or Canadian fir, secured at one end to a fixed column 11 carries at its free end a container 16 for abrasive. A series-wound electric motor 41 mounted by a bracket 40 on the container 16 has an eccentric bob weight 43 on its spindle. The container comprises portions 17, 18 between which are located a perforated metal plate 19 and a nylon mesh sieve 22. The plate 19 carries a tube 20 provided at its top with a gauze 21; the tube 20 allows breathing between the portions 17, 18 and reduces undesirable damping effects caused by the abrasive above the sieve tending to act as a piston. The upper end of the container is provided with an inlet surrounded by an 0-ring 32 co-operating with a closure member 24 held closed by a spring 31 and openable by depressing a knob 30. A compressed air line 36 has a branch 38 leading to the top of the container and a branch 35 leading to a T-piece 33 at the bottom of the container and having a discharge line 34. A compressor 44, Fig. 3, feeds air to a dessicator 46 containing calcium chloride, silica gel etc. or including a condenser. Dessicated air passes to a junction 47 from which branches the line 36 to the container 16. The discharge line 34 passes through a pneumatically operated valve 48, junction 49 and line 50 to the nozzle 51; air for operating the valve 48 is supplied from the junction 47 through lines 54, 57 under the control of a solenoid valve 59. From the junction 47 a line 52 passes through a pneumatically operated valve 53 to the junction 49; air for operating the valve 53 is supplied from the junction 47 through lines 54, 56 under the control of a solenoid valve 58. The nozzle 51, Fig. 2, comprises a body 60 into which the line 50 is threaded. A rubber seal 63 is mounted in a holder 62 screwed into the body 60. A nozzle tube 64 has one end received in the seal 63 and is secured by a holder 65 screwed into the holder 62 so as to compress the seal 63, whereby the tube 64 is gripped. The controlling electric circuit is shown in Fig. 4. When the main switch S1 is closed, a lamp 67 shines and a rectifier bridge 67 charges a capacitor 76. To start blasting, the fast switch S2 is closed, energizing relay coil C1, closing contacts C1R1, causing the capacitor 76 to discharge through the motor 41 so that a surge of power passes through the motor and the latter starts to rotate, the surge assisting in overcoming the inertia of the bob weight 43; after the capacitor 76 has discharged, the motor 41 is supplied from the rectifier bridge 67 through a circuit including tapped resistors 72, 74, a variable resistor 73 and a milliammeter 76a. Energization of relay coil C1 also closes contacts C2R1, (a) energizing the solenoid 59S of the solenoid valve 59 opening the pneumatic valve 48 allowing abrasive-laden air to pass to the nozzle 51 and (b) energizing relay coil C2. After a predetermined time delay, e.g. 3 seconds, the relay coil C2 closes the contacts C1R2 shorting out the resistor 74 and rendering the variable resistor 73 operative to control the current to the motor 41. The speed of the motor being such that the assembly comprising the boom 13, container 16, motor 41 and parts movable therewith, vibrates at its fundamental resonent frequency (which is 200-800, and preferably 600 vibrations per minute), small variations in the power applied to the motor vary the amplitude of vibrations with no appreciable alteration in the motor speed, thereby controlling the feed of abrasive to the nozzle. The milliammeter 76a may be calibrated to show the amount of abrasive passing to the nozzle in unit time. To stop blasting, switch S2 is opened, stoping the motor 41 and closing the valve 48. If it is desired to blow the abrasive away from the workpiece, footswitch S3 is closed, energizing solenoid 58S of solenoid valve 58 opening the pneumatic valve 53 allowing air to pass from the junction 47 to the junction 49, line 50 and nozzle 51.