Choosing clamping tools
The list above is what exists. This is how to decide which of it you actually need, what separates a tool worth keeping from one that ends up at the back of a drawer.
Holding still and closing a joint are different
Holding a workpiece for sawing, routing or drilling wants as much grip as the tool will apply, fast on and fast off. Nothing is harmed by over tightening a scrap board to a bench.
A glue joint is the opposite. It wants enough pressure to close the gap and squeeze out a thin even bead, and no more. Over clamping forces glue out of the joint and leaves it starved, so a joint clamped brutally can be weaker than one clamped moderately.
- Holding: grip hard
- Nothing is harmed. Fast on and fast off matters more than fine control.
- Glue up: even and moderate
- Enough to close the gap and show a thin bead. More than that starves the joint.
- A thin even bead
- That is the target. No squeeze out means not enough, a flood means too much glue.
- Pressure direction
- Along the joint line, not across it. A clamp pulling at an angle racks the assembly.
How many clamps you need
More than you have. The common rule is a clamp every 150 to 200mm along a glue joint, alternating above and below the panel so the pressure does not bow it. A three board panel a metre long therefore wants six or seven clamps.
The practical version of that rule is to dry fit and count before mixing glue. Glue has an open time measured in minutes, and discovering you are two clamps short halfway through a glue up means the joint sets while you improvise.
- One every 150 to 200mm
- Along the joint. Fewer than that and the pressure between them drops off.
- Alternate above and below
- All on one face bows the panel into a curve. Alternating cancels it out.
- Dry fit and count first
- Glue open time is minutes. Finding you are short mid glue up cannot be recovered.
- Buy pairs
- Clamps work in opposing pairs. A single odd clamp is the one that never gets used.
Stopping clamps marking the work
A steel clamp jaw on softwood leaves a dent that shows through a finish, and glue squeeze out reacting with a steel bar leaves a black stain on oak and other tannin rich timber. Both are avoidable and both are noticed only after the finish goes on.
The answers are pads on the jaws, offcuts under them, and tape or paper between the bar and the joint. A caul is the other half of this: a stiff batten across the work that spreads one clamp's pressure over a much longer span.
- Pads or offcuts
- Between jaw and work. A dent in softwood shows through paint and every clear finish.
- Tape the bar
- Glue and steel stain oak black. Masking tape on the bar prevents it entirely.
- Cauls spread pressure
- A stiff batten across the work turns one clamp into pressure along a whole edge.
- Wipe squeeze out
- While wet with a damp cloth, or scrape it once rubbery. Dried glue will not take a finish.
Clamped tight and out of square
Clamping pressure pulls an assembly toward whatever is easiest, which is rarely square. A carcase can be clamped hard, look right, and be a few millimetres out across the diagonal, which is impossible to correct once the glue sets.
So measure the diagonals rather than trusting a square in a corner. If they differ, angling one clamp slightly across the long diagonal pulls it back. Check while the glue is still open, because that check is the last chance.
- Measure both diagonals
- Equal means square. A square in one corner tells you about that corner only.
- Angle a clamp to correct
- Skewing one clamp along the long diagonal pulls the assembly back into square.
- Check before it sets
- Once the glue goes off nothing can be done. That window is the whole opportunity.
- Flat as well as square
- Clamp on a flat surface, or a twisted assembly sets twisted.
Vices, and where a clamp cannot go
A vice is a clamp fixed to a bench, which is what makes it usable one handed while both hands work the tool. That is the real difference: a clamp has to be positioned and tightened, a vice is already there.
The two common types are the woodworking vice, wide and flush to the bench with soft jaw liners, and the engineer's vice, narrow, hardened and with an anvil. Using an engineer's vice on finished timber marks it every time.
- Always there
- That is a vice's advantage over a clamp. No positioning, no second hand needed.
- Woodworking vice
- Wide, flush to the bench top, wooden liners. Holds a board without marking it.
- Engineer's vice
- Narrow, hardened serrated jaws, an anvil behind. It marks anything softer than steel.
- Soft jaws
- Aluminium or fibre liners that let an engineer's vice hold a finished part.
Frequently asked questions
- How many clamps do I need for a glue up?
- One every 150 to 200mm along the joint, alternating above and below the panel so the pressure does not bow it. A metre long panel therefore wants six or seven. Dry fit and count before mixing glue, since glue open time is measured in minutes.
- How tight should clamps be for gluing?
- Enough to close the joint and squeeze out a thin even bead, and no more. Over clamping forces glue out and leaves the joint starved, so a brutally clamped joint can be weaker than a moderately clamped one. A flood of squeeze out means too much glue, not enough pressure.
- What is the difference between a clamp and a cramp?
- Nothing, other than British woodworking tradition using cramp for the large bar types like sash cramps. The tools are identical, and most manufacturers now use clamp for everything, so the two words appear interchangeably in the same catalogue.
- Why do my clamps leave black marks on oak?
- Tannin in the oak reacting with iron from the clamp bar in the presence of wet glue or moisture. It stains deep enough that sanding often will not remove it. Masking tape along the bar, or paper between bar and work, prevents it completely.
- What clamps should I buy first?
- Four F clamps around 300mm for general holding, and a pair of long sash cramps or parallel clamps if you are gluing panels. Add quick release clamps for one handed work and a bench vice. Buy in pairs, since clamps work in opposition and an odd one rarely gets used.
- What is a caul used for?
- A stiff batten laid across the work so one clamp's pressure spreads along its whole length. It turns four clamps into even pressure across a wide panel, and a slightly curved caul presses the middle of a joint that clamps at the edges cannot reach.
- How do I keep an assembly square while clamping?
- Measure both diagonals rather than trusting a square in one corner. Equal diagonals means square. If they differ, angle one clamp slightly along the longer diagonal to pull it back, and do it while the glue is still open, since afterwards nothing can be corrected.
- Are parallel clamps worth the money?
- For panel glue ups, yes. The jaws stay parallel under load instead of tilting, so the pressure is square to the joint and the panel does not want to bow or slip. For general holding an F clamp at a third of the price does the same job.
