The plate has two sides that are straight, and two sides that are ground and shaped to produce sharp teeth. These teeth should be kept sharp, for they dig into the wood placed against them and prevent slipping. If the dog is to be used with a piece of wood that must not show these toothmarks, the dog may be removed from the holder and turned so that the straight side will come in contact with the wood.
The cam and shaft should be oiled occasionally to permit easy operation. The cam should be tightened by means of the screw just sufficiently to hold the dog in position; excessive tightening may break the cam.
When planing or sandpapering the surface of a piece of wood that cannot be held in the vise because of its shape or size, the bench stop should be used. When using the bench stop, the dog should be raised just high enough to hold the wood in position. It should always be below the top surface of the stock. Failure to keep these two points in mind may result in splitting the wood or ruining the cutting edge of the plane.
When not in use, the bench stop should have the dog set down into the recess of the holder. A dog with its sharp teeth projecting may cause a serious cut on the hand. A dog with dull or broken teeth should be put back into condition by grinding the teeth to the required shape. Dull teeth will not bite into the wood placed against them, and the wood may shift when being worked.
In the absence of a bench dog, a thin strip of wood can be fastened to the working surface, by means of nails, brads, or screws, and used as a substitute.
The bar clamp
The bar clamp consists of the bar, the head jaw, the sliding jaw, and the screw. The head has a threaded shaft which passes through the head casting. To one end of the shaft is attached a metal plate that forms one of the jaws; the other end of the shaft has a crank fastened to it. As the crank is turned, the head jaw is moved.
The sliding jaw has a small dog which engages grooves in the bar and maintains its set position regardless of the pressure exerted. Bar clamps-can be obtained in various lengths for various uses. We shall discuss here only one common type.
As with all tools with moving parts, lubrication of the bar clamp is necessary. All moving parts, such as the threaded shaft, the socket in the head jaw into which the shaft sets, and the dog in the sliding jaw, should be lubricated occasionally.
Since the bar clamp is often used on stock that is being glued together, a certain amount of glue will adhere to the clamp. This should be cleaned off after the clamp has been used. A bar clamp is designed to apply a certain pressure; for this reason the crank is of a fixed length. Excessive pressure produced by using a wrench on the handle will result in the bending or breaking of the handle or sliding jaw.
Bar clamps are used to hold work together during assembly. They can be made to apply a steady pressure between two or more pieces of wood or to hold these pieces securely in their same relative position for an indefinite time. This is especially important when gluing up stock. When bar clamps are applied, the bar should be at right angles to the surfaces of the wood that are in contact with the jaws. Failure to do this will result in the glued-up units being forced out of square.
The metal jaws of the clamp should not come into direct contact with the wood members being held together; a piece of scrap wood should be placed between the jaw and the stock. When preparing a bar clamp for use, the head jaw should be brought back as far as possible by turning the handle in a counterclockwise direction.
The sliding jaw should be set by releasing the dog, and then moving the head in the proper direction so that the distance between the two jaws is a little greater than the material to be held in the clamp. The dog of the sliding jaw should be engaged in the groove cut into the bar. The clamps should be placed on the stock, with a piece of scrap wood between each jaw and the surface of the stock.
Bar clamps that have the jaws sprung out of parallel should not be used, for they cannot hold securely. The main causes of jaws being forced out of parallel are excessive force used and wear on the bar and sliding jaw.
An emergency device that can be used in place of a bar clamp can be made by fastening two blocks of wood to a plank, placing them about one inch farther apart than the length of the stock. The stock is placed on the plank, between the two blocks, and a wedge-shaped piece of wood is driven between it and one of the blocks to hold it tightly in place.
The C-clamp, often called a carriage clamp, is used to hold two pieces of work together temporarily. This clamp has a C-shaped frame, one arm of which has a hole drilled and tapped in it, and the other arm of which is flat, forming a bearing surface for the work. A screw passes through the tapped hole in the frame. Fastened to one end of the screw is a swivel head. At the other end of the screw is the handle.
C-clamps come in a great variety of sizes depending on their use. The depth of the throat — that is, the distance between the center line of the screw and the frame — and the maximum opening between the jaws determine the size of the clamp.
C-clamps should be used only as temporary holding devices. The small jaw surface does not cover a great area, and a great amount of pressure, will dent the work. The handle of a C-clamp is so small that only the fingers are used to turn the screw; this makes it difficult to exert excessive pressure between the jaws. For such work as sandpapering, the C-clamp is useful to hold a piece of wood to the bench top.
The C-clamp can be used to advantage in construction. In boat building, for example, it is used to hold the planks tightly in position against the frame of the boat when driving nails or screws in place.
The handscrew, also known as a parallel clamp, is used to hold tightly to the bench top, material that is being worked. It is also used to hold, two pieces of stock together when being glued. The large jaw provides ample surface to distribute pressure over a relatively large area. There are two types of handscrews, one made entirely of wood and the other with jaws of wood and spindles of metal.
Handscrews are opened and closed by grasping the handles, one in each hand, and revolving the entire clamp. The left hand grasps the middle spindle and the right hand holds the outside spindle. By revolving the entire clamp around the middle spindle, with the hands holding the spindles securely, the jaws can be opened and closed, keeping them parallel. If one spindle is moved independently of the other the jaw will shift out of parallel. With the all-wood handscrew, this may result in a broken spindle.
The adjustable handscrew may have the jaws slanted to hold stock that does not have parallel sides. This can be done up to certain limits; to tip the jaws beyond a certain point will result in bent spindles. When applying or releasing the pressure of the handscrew, the used first.
The handscrew should be opened sufficiently to fit freely on the stock to be clamped together. The handscrew is then placed in position. The middle spindle is turned so that the jaws will come in contact with the stock; then the pressure is applied and the jaws brought back parallel by turning the outside spindle.
When applying or releasing the pressure, the outside spindle should be used. When opening and closing a handscrew, hold it far enough from your body so that there will be no danger of striking your face as you revolve the jaws around the middle spindle.