Patent GB-662756-A (GB)
Title: Improvements in or relating to safety guards for power operated machine tools
IPC: B27G19/02
Filed: 1948-12-10
Abstract: 662,756. Machinery guards. HUNT, A. W. Jan. 10, 1950 [Dec. 10, 1948; June 13, 1949], Nos. 32022/48 and 15681/49. Class 80 (iv) A guard for machine tools (e.g. grinders or milling machines) is shown applied to a circular saw 1, and comprises a pivotal outer guard 16 and inner guard 7, and operating means such that the inner guard is opened automatically after raising of the outer guard provided that the work to be sawn is in position under the outer guard. In the construction shown in Fig. 1 the inner guard 7 is raised or lowered through a high reduction gear (which prevents manual operation of the said guard) by an electric motor (not shown) controlled by two double-throw switches 18, 19 carried by the outer guard 16. The inner guard is secured to a shaft 5 on which is loosely pivoted both the outer guard 16 and an operating member 8 which comprises two plates interconnected by a curved toe 8a resting on the bench 2 between the two guards. The outer guard is partly counterbalanced by a weight 15 and is held clear of the bench by a stop (not shown). The switch 18, which is spring- biassed to connect its lower contacts 18b, is operated by a set screw 20 carried by the inner guard, while the switch 19, similarly spring-biassed, is operated by a set screw 11 carried by a plate 10 secured to the member 8. A set screw 13 carried by the outer guard 16 and having its head lowermost passes through a hole in the plate 10 so that upon lifting of the guard 16 by means of the handle 17 the member 8 is also raised and the work may be inserted beneath it until stopped by the inner guard 7. On lowering the guard 16, the toe 8a contacts the work and relative movement between the guard 16 and member 8 closes the contacts 19a. This (contacts 18b being closed) energises a lifting winding 22, Fig. 3, on the motor, so raising the inner guard until the set screw 20 breaks contacts 18b and (movement continuing due to the inertia of the gearing, etc.) makes contacts 18a. When the work is withdrawn, member 8 drops, contacts 19b are made and (contacts 18a remaining closed) the winding 23 of the motor is energised to lower the guard 7 and, therewith, the guard 16 and member 8 until the guard 16 engages its above-mentioned stop. Continued movement of the inner guard breaks the contacts 18a and (moving again under inertia) makes the contacts 18b. In a wholly mechanical form of the invention, the inner guard 7, Fig. 4, is raised by an adjustable spring 32 when work has been inserted under the outer guard 16 and operating member 8. As in the first-described form, lowering of the guard 16 and toe 8a on to the work causes relative movement therebetween, but, in this form, the movement actuates a toggle 46, 47, which moves a rod 44 engaging a non-toothed part of a spring-loaded arcuate plate 40 centred on the axis of shaft 5 and provided with ratchet teeth 41. Movement of the plate disengages the teeth from the edge 42 of a slot in the inner guard so allowing the spring 32 to raise the guard until a set screw 29 engages the outer guard 16. An adjustable spring 35, partly counter-balancing the outer guard, is of insufficient strength to prevent the fall of the outer guard (bringing with it also the inner guard) and the member 8 upon withdrawal of the work, the teeth 41 ratchetiiig over the edge 42 in this movement. It is stated that a pedal may be used to raise the outer guard, that the springs 32, 35 may be replaced by leaf or rubber springs or by weights (e.g. suspended bags of lead shot) and that, where powerdriven means are employed, other forms of motor may be used-e.g. hydraulic or pneumatic motors, valves then replacing the switches 18, 19.