Magnetic catches with ferrite or ticonal nickel alloy magnets (the latter have greater holding power in small diameters) have a practically unlimited life with a probable strength loss of not more than 1 per cent over ten years. Figure 274:1 is the smallest type for gluing in with a pull of about 21/2 lb (1.134 kg) and suitable for very light doors and flaps; 274:2 is a stronger type with ribbed brass or nylon case and about 41/2 lb (2.041 kg) pull, and 274:3 is a face mounting type in a range of pulls from 4 1/2 lb (2.041 kg) to over 40 lb (18.143 kg) according to the size. A mortise version suitable for light doors, etc. is shown in 274:4. All these catches require intimate contact with the metal striking plate. and are therefore not suitable for bowed or warped doors which require a more positive catch to pull them in. In fitting, the magnet catch is fixed first with the striking plate placed in position on the magnet. The back of the plate is then rubbed over with chalk and the door pressed against it to register the position. To check that both magnet and plate are in perfect contact throughout, a trace of oil is smeared over the magnet face, and if the oil fails to register exactly to the ends and sides of the plate, it should be shimmed out accordingly.
Ball, bale and double ball catches are shown in 274:5, 5A and 6; the double ball type is useful for the heavier type of door or one which has bowed slightly, for the compression on each spring-loaded ball can be adjusted by the retaining screws. Single ball catches, either barrel (274:5) or plated (bales, 274:5A), can be fitted at top or bottom or on the side, and either to the door or to the carcass framing. However, with the striking plate (274:5) and ball on the door, the edge of the carcass will be crushed with the passage of the ball, while the lip striking plate (274:5A) can be cut off flush with the outer edge and gently tapped down to lead the ball into the socket. Alternatively, the ball can be located in the carcass framing, but here
again a slight depression must be gouged out under the striking plate to receive the ball and this will be noticeable on the door edge if below eye level. In effect, therefore, the position of ball and plate should be chosen to give the neatest appearance. As to whether one or two ball catches are necessary per door (at top and bottom) depends entirely on the size and power of the ball (3/8 in [9.5 mm] is a good average), the size of the door and whether it requires pulling in. However, doors in slight twist are usually corrected by moving out either the top or bottom hinge, and generally speaking a perfectly flat small- or medium-sized door will only require one catch. In attaching ball catches the barrel should be a reasonably tight fit in its prebored hole and tapped home with a hammer on a wood block to protect the ball. Normally, the projecting rim is hammered flush, but some woods tend to crush badly and it is advisable to score the exact diameter of the rim with the wing tip of a sharp bit before the hole is bored for the barrel.
available and manufacturers' catalogues should be consulted.
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