Very low clamping force compared to traditional clamps. Very quick to use. Can be useful for holding things in place while fixing them, but often several clamps are needed to give enough holding force.
Its possible to make basic spring clamps by cutting plastic pipe into C shaped pieces. See Make Things from PVC Pipe.
Quick release clamp
A locking type of plier. Not made from moles.
Gravity is enough for some jobs.
Rope & Wedge
A rope is tied round the items to be clamped. A wedge is inserted under the rope to pull the rope tight. Using one wedge on each side is preferable, as it avoids asymmetric pull.
When a wedge is not to hand, scrap wood, food tins and so on can be used.
A dog clamp has 2 spikes that are knocked into the pieces of wood to be held together. Inserting the clamp pulls the wood together.
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PVC Pipe Clamp
A PVC pipe]] clamp is nothing more than an inch of pvc pipe slit open. It is a simple type of spring clamp readily made from plastic pipe scrap. If you need greater clamping force, use 2" pieces of pipe instead of 1".
Here are some with some fancy wooden jaws added. This also increases the spring force.
Tape can be used as a crude clamp. The tape is puled tight as its applied, and clamping force builds up with each added turn of tape. An initial layer of paper or polythene can be used to stop tape sticking to the workpiece.
A screw in an inconspicuous position is sometimes good as a clamp.
Simple minimal cost spring pegs. Small jaw opening, too small for most work. Oversize pegs are now available.
A vacuum clothes storage bag is connected to a hoover for light clamping, with the workpiece inside. Force is exerted on the workpiece in all directions.