“In ship-building, you are always finding that you need two more clamps than YOU actually possess.”
That quotation is applicable to many other woodworking activities besides building boats. Not all activities are as demanding of clamping devices, but there are many occasions when you will find you need more clamps or special ones to take care of individual cases where the standard clamps will not do. Fortunately, there are many ways of clamping parts together that can be improvised or simply made. You cannot manage without C clamps or bar clamps, but the other arrangements can supplement them or take their place for special jobs.
You can arrange for the actual method of construction to pull parts together. Screws will ensure a close contact. Tapered pegs through staggered holes will pull tenons into mortises. Dovetail joints will automatically tighten in one direction. These are useful devices that avoid clamps, but they still leave many woodworking constructional situations where some outside arrangement has to be used to draw parts together, particularly when using glue. Even where glues are described as gap filling, that often only means a few thousandths of an inch.
Like the holding devices described in section 3, many of the clamping devices you can use depend on wedging. An enormous amount of pressure can be exerted by a wedge, but it does not move an object very far. You can expect a movement of several inches with a screwed clamp, but with a wedge, the movement is probably not as much as 1 inch—packings therefore have to be used.
The amount of pressure is related to the angle of the wedge. A very shallow angle produces the most pressure, but very little movement. A steep angle gets more movement, but each hit produces less pressure. With too steep an angle, the wedge might slip out. It is usual to cut wedges by eye; a slope of 1 in 9 is a typical shallow slope, while you might get away with a 1 in 3 for a steep angle. An acceptable compromise would be 1 in 6.
Graphic 4-1. A single wedge will provide thrust (A,). A pair of wedges gives a parallel thrust (B). Wedges can force laminations around a form (C, D).
A wedge used alone (Graphic 4-1A) might push against a scrap wood pad, but each hit on it tends to move the pad and the job. There are several situations where this is acceptable, but if you want a straight parallel thrust, use a pair of folding wedges (Graphic 4-1B). The two wedges should have identical slopes, although they need not be the same thickness. If they are tightened by driving alternately or at the same time, the thrust obtained is parallel. The power exerted should be at least as good as a jack or other screwed device.
You can use scrap wood to make wedges in anticipation of future needs. You can build up a stock of wedges of many sizes and slopes, preferably paired.
A typical application of a wedge is in forcing laminations around a form. If you pivot a block on a single screw into the base board, it will take up an angle to suit the wedge (Graphic 4-iC). Wedging is a simple and very effective way of closing up laminations if you use shaped pressure pads (Graphic 4-1D).
WEDGED BAR CLAMP
Clamps with a long reach are expensive. You probably do not have enough for puffing large framing parts together or for securing several glued boards to make up a width.
Graphic 4-2. A wedge can be used in a fixed bar clamp (A). A pair of wedges and a movable head can form an adjustable bar clamp (B, C).
You could improvise the clamp in Graphic 4-2 for a particular job, or you could make one that is ready for any occasion. For a one-off bar clamp, screw on the stops to suit the job (Graphic 4-2A). Allow for packings against the work to spread pressure and prevent damaged edges. One block is cut at the same angle as the wedge, unless you will be using folding wedges.
For this or any other long clamp, use a bar deep enough to resist bending under pressure. A shallow bending bar will allow the parts being clamped to also bend.
A bar clamp that can be adjusted has one fixed block to take the wedges and another that can be located at many positions along the bar (Graphic 4-2B). Make the blocks as deep as anything you expect to clamp. For general purposes, blocks 1½ inches deep on a bar, preferably hardwood, and 1 1/2 inches x 3 inches and 36 inches long would be adaptable to many applications.
Glue and screw the fixed block at one end. Make the other block long enough to resist tilting. Make cheeks with plywood and put a piece to spread the pressure across these parts (Graphic 4-2C). To allow for the limit of wedge movement of only about 1 inch, drill holes in the bar at 1-inch intervals. To suit the suggested sizes, they could be holes for a ½-inch dowel rod. The peg could be just a piece of rod, or you could turn a wood or metal peg, as suggested for the traditional vise (Graphic 3-10H).
DOUBLE BAR CLAMP
The bar clamp described in Graphic 4-2 will suit many purposes and is the type to use when assembling a frame when there are parts projecting. When you want to join several boards edgewise to make up a width, there is the problem with this or a metal screwed bar clamp. The assembly will bow under pressure, and you have to use weights or other means to keep the assembly flat.
Graphic 4-3. A double bar clamp with wedges is suitable for a greater width.
If there are bars above and below the assembled boards, the tendency to bow is minimized and the double bars themselves prevent movement. The double bar clamp in Graphic 4-3 can be closed onto the work or packings put under the top bar, so there is no risk of boards moving in their thickness.
As the bars are not subject to bending loads under pressure, they can be lighter, say 1-inch- x -2-inch strips, 36 inches long. Make the wedges as thin as the thinnest boards you expect to clamp or have a supply of wedges of different thicknesses.
Bolts can be ¾ inch in diameter. The clamp is shown with one bolt in a single hole and the other in alternative positions, but you could arrange a series of holes at each end. If the rows of holes are staggered in relation to each other, you will get a greater choice of settings. Suppose the holes at each end are 2 inches apart. If you make the distance between the end holes of each group an odd distance, say 11 inches, you get a choice of overall settings in 1-inch steps.
If you use this clamp tight across glued boards, put paper between the bars and the wood to prevent the clamp from becoming glued in place. Varnishing the bars will also reduce the risk of glue adhering to them.
FLOORING CLAMP
When floorboards have to be nailed across joists, or you have a similar constructional assembly, each board has to be pressed tightly against the previous one as it is nailed. There are special flooring clamps made for this purpose, but you can make one to use a wedge action.
For just a few joints, you might nail a block on the joist and wedge against that (Graphic 4-4A). Of course, you have to lift the block and re-nail at each place.
Graphic 4-4. Floorboards can be forced together with a wedge against a block (A) or with a clamp that can be moved along a joist (B-H).
A clamp that can be moved along a joist or from one joist to another is more convenient. This flooring clamp is held against the joist by friction (Graphic 4-4B).
Make the block a little wider than the usual 2-inch width of a joist. There could be a piece across its end for the wedge to bear against (Graphic 4-4C). The bottom piece can be 1 inch x 2 inches with a ½-inch dowel through it (Graphic 4-4D). This will probably have enough friction against the sawn undersurface of a joist, but you could glue coarse abrasive paper to it if necessary.
Even if the other parts are softwood, the two straps (Graphic 4-4E) are better made of hardwood—a section ½ inch x 1¼ inches is suitable. If you prefer to use metal, make the pegs from 5 rod and the straps about 1/8 inch x 1 inch.
Put a peg through the top block. Allow plenty of extension on both sides of both pegs. Taper the ends slightly so that the straps are easy to put on.
Drill a series of matching holes in the straps. For flooring work, you are unlikely to meet joists less than 6 inches deep. The straps do not have to finish vertical to a joist. The clamp holds best with the straps at a slight angle (Graphic 4-4F). If they have to be taken to a wide angle (Graphic 4-4G), a point will be reached when the risk of slipping under load is too great.
With the clamp, you can have a wide piece that acts as a pressure pad against the floorboard. With its other edge sloped to match a wedge, you get the effect of folding wedges (Graphic 4-4H). In use, position the pressure block with the thin end of the wedge against it and the clamp head tight on that. Hook on the straps and push or tap the lower block along the underside of the joist, then tighten the wedge.
ROPE CLAMPING
Simply tying with rope or cord might not prove tight enough to draw wood parts together, but there are ways of increasing the tension of the squeezing action enough to exert considerable pressure where it is needed. This is possible with almost any sort of cord or rope, but the extra strength of modern synthetic fibers makes feasible more pressure than would be possible with natural fiber cordage.
The only modern cordage to avoid is nylon, which is elastic. You need rope or cord with minimal stretch, and all the other synthetics and most of the natural fibers are with out appreciable stretch.
For most methods of rope clamping, you need to get the first application of rope fairly tight before applying any special tightening techniques. You could join the ends with a reef knot, positioning it where it will bear against the wood. For lighter cordage, the packer’s knot allows you to get a good initial tightening. Take the end around the other part, then under itself to go around in a figure-eight manner (Graphic 4-5A). This makes a slip knot, which you pull tight. When you are satisfied with the tension, twist a half hitch with the part you have been pulling (Graphic 4-5B) and slip it over the figure-eight end. Pull it tight to complete the knot (Graphic 4-5C).
You can use a single turn of fairly stout rope, but in general, the best clamping is with several turns of thinner rope or cord. One way of clamping is with a wedge. Tighten the rope on several turns of cord around the parts to be clamped. Drive a wedge under (Graphic 4-5D). Hold the turns square to the work at first until you have driven the wedge as far as it will go. Get an extra bit of tension by now knocking the wedge and rope further along the edge (Graphic 4-5E).
Graphic 4-5. A simple rope clamp is made by joining the ends (A, B, C) and using a wedge to force the parts tightly together.
SPANISH WINDLASS
If turns of tight rope are twisted with a lever, they will pull even tighter. This is seen in the method of tensioning the blade in an old-fashioned bow saw (Graphic 4-6A)—almost obsolete in America, but still seen for saws of many sizes in some European countries. This use of rope or cord to produce a pull is called a Spanish windlass, although there is another action that has the same name.
To draw parts of a frame together, turn the cord several times. To prevent the cord marking the wood, put pieces of scrap wood where pressure will come. There is no need for excessive initial tightening. Put a piece of scrap wood through the turns (Graphic 4-6B) and start twisting. Continue twisting until the wood parts have been drawn close, then add maybe one more turn. It is possible to twist so much that you break the cord, but modern fibers are remarkably strong, and you would have to twist far more than normally necessary to break the cord. With a comparatively light assembly, you might break the wood before the cord.
Lock your clamping by jamming the end of the lever against the frame (Graphic 4-6C). If that is impossible, use another cord loop (Graphic 4-6D), which does not have to be very tight.
A similar technique can be used to pull a bundle tight. This could be just a bundle of sticks or a cylindrical shape made up of many staves coopered together. Turn the cord two or three times around the object and join the ends, but leave a little slackness. Put a lever through the turns (Graphic 4-7A) and start twisting. When you have achieved the tightness you need, secure the lever by tying it to another turn of cord (Graphic 4-7B). There is an advantage in having a long lever.
Graphic 4-6. Cord twisted as a Spanish windlass can exert considerable pressure.
Graphic 4-7. A twisted rope will pull a bundle tight.
The same method can be used around other shapes. An example is a picture frame. Make four identical blocks to fit over the corners (Graphic 4-8A). Make the inside angles square but make cuts into corner holes to prevent damaging the extreme corners of the frame (Graphic 4-8B). Groove the outside corners (Graphic 4-8C) so the cord cannot slip off.
Put the corner blocks in place and fit a double turn of cord around them with enough slack to admit a lever. Use a lever in the same way as the example in Graphic 4-7 and twist the cord (Graphic 4-8D). After tensioning, turn the end of the lever under the cord and lodge it against the frame (Graphic 4-8E).
All of these applications of the Spanish windlass are providing compression. The other rope-work application that is called a Spanish windlass provides a pull. It could be that something you are making requires a strong pull, such as bending a substantial piece of wood around a form, but the technique can also be used to move a heavy weight, such as a machine to a new position. One man can move a car, and no elaborate equipment is involved.
Graphic 4-8. With suitable blocks, a twisted rope can draw mitered corners together.
Graphic 4-9. Another form of Spanish windlass can be used to move a load.
Use a piece of rope strong enough for the load. With it you need a spar about 5 feet long and about 2 inches in diameter, although that size is not crucial. A lighter lever of about the same length must be strong enough to withstand considerable bending load. Hold the spar upright, but not rigidly so. Prevent its foot from possibly slipping by letting it a few inches into the ground. Have an assistant hold the top.
Attach the rope to the load and to a strong point in the direction it is to be moved. That could be a part of a building, a post driven into the ground, or anything that will not move when you start pulling. Give some slack to the rope and position the spar about midway between the load and the anchorage.
Twist a few turns on the end of your lever (Graphic 4-9A). Put the lever against the spar and start twisting it around (Graphic 4-9B). As the load moves, the spar will begin to tilt towards the anchorage. Do not let it slope too much and see that its foot does not move. Your assistant should press down on the spar. If you want to move the load very far, release the twists and shorten the rope, then start again.
For a first time it might be advisable to try a light temporary load. Run through the drill with your assistant to see what happens before using the considerable tension that comes from using a Spanish windlass on a really heavy load.
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