What a clamp does
A clamp does one of two jobs. Holding keeps a part still while you saw, drill, rout or pare it — the clamp only has to stop it moving. Squeezing closes a glue joint and keeps it closed while the adhesive cures — the clamp has to bring the surfaces into close contact along the whole joint. The same clamp can do both, but the second job is the one that decides how many clamps you need and where they go (see the plate above).
Force and pressure
Makers rate a clamp by force: the total push, in pounds, that its screw or lever can apply. A glue joint needs pressure: force spread over the area of the joint, in pounds per square inch.
Pressure on a joint
P = F ÷ A
- P
- pressure on the joint (psi)
- F
- total force the clamps actually apply — usually well under their rating when tightened by hand (lb)
- A
- area of one of the two surfaces being glued (sq in)
Worked example: an edge joint in hardwood
Find the area
Two 3/4 in boards edge-glued along 24 in: 0.75 × 24 = 18 sq in.
Take the adhesive maker’s figure
Titebond gives 175–250 psi for hardwoods with its wood glues: 18 × 175 = 3,150 lb, up to 18 × 250 = 4,500 lb of total force.
Divide by what a clamp gives
Jorgensen rates its heavy-duty steel bar clamp at 1,000 lb, so four or five would be needed at full rating — and a clamp tightened by hand is rarely at full rating.
Read it as a ceiling, not a target
Titebond’s first criterion is pressure enough to bring the joint tightly together, and the USDA notes that small areas of flat, well-planed surfaces bond at lower pressures. A well-fitted joint needs fewer clamps than the arithmetic suggests; a badly fitted one needs a better fit, not more force.
| Adhesive or wood | Stated pressure | Source |
|---|---|---|
| Titebond wood glues, softwoods | 100–150 psi | Titebond, for its own glues |
| Titebond wood glues, medium woods | 125–175 psi | Titebond |
| Titebond wood glues, hardwoods | 175–250 psi | Titebond |
| Wood adhesives, low-density wood | Near 100 lb/in² (0.7 MPa) | USDA Wood Handbook, 2010 edition |
| Wood adhesives, highest-density wood | Up to 247 lb/in² (1.7 MPa) | USDA Wood Handbook, 2010 edition |
| Small, flat, well-planed surfaces | Lower pressures | USDA Wood Handbook, 2010 edition |
Each figure applies to the adhesives and conditions its source describes; follow the maker of the glue you use. The 2021 Wood Handbook keeps the principle — moderately high pressure for the wood’s density, not so high that the adhesive squeezes out — without the numbers.
Caution— Too much pressure starves a joint
The USDA Wood Handbook warns that pressure high enough to force most of the adhesive out of the joint, or deep into the wood, leaves too little to fill the glue line. Tighten until the joint closes along its length with a thin, even bead of squeeze-out, then stop.
Reach, opening and jaws
- Opening (capacity). The widest work the clamp closes on. Bar clamps come in lengths; Bessey’s K Body REVO parallel clamps run from 12 to 98 in, while a spring clamp may open only 1 in.
- Reach (throat depth). How far from the edge of the work the pad presses. Pony Jorgensen’s steel bar clamps reach 2-1/2 to 3 in; its Cabinet Master parallel clamp 3-7/8 in. Pressure is applied at the pad, not across the whole width of the work.
- Jaws and pads. Small steel pads concentrate force and dent wood; large plastic jaws and removable caps spread it. Put a scrap or a caul under any small pad.
- Parallelism. A clamp whose jaws stay parallel — a parallel-jaw clamp, whose faces Bessey sets perpendicular to the rail — pushes squarely. One whose sliding arm flexes or tilts can pull a joint out of line.
- Spreading. Many bar clamps reverse to push outward instead, for easing a joint apart or bracing a case from inside; IRWIN, Bessey and Jorgensen all build this in.
The clamp family
| Clamp | Capacity | Stated force | Job |
|---|---|---|---|
| F-style steel bar (Jorgensen) | From 6 in; 2-1/2–3 in reach | 600 lb; 1,000 lb heavy duty | Edge-gluing, general glue-ups |
| Parallel jaw (Bessey K Body REVO; Jorgensen Cabinet Master) | 12–98 in (Bessey); 3-3/4–3-7/8 in throat | 1,700 lb nominal (Bessey); 1,500 lb (Jorgensen) | Cases, panels, square assemblies |
| One-handed bar (IRWIN SL300; Jorgensen E-Z Hold) | 3-1/4 in throat (IRWIN) | 300 lb (IRWIN); 50–600 lb by model (Jorgensen) | Holding, dry fits, light glue-ups |
| Pipe clamp (Pony #50) | Any length of 3/4 in black pipe | Not stated | Long glue-ups on a budget |
| C-clamp (Pony light-duty; Bessey 540) | 1–8 in (Pony); to 14 in (Bessey) | 400–1,000 lb (Pony); 1,100–2,850 lb (Bessey) | Pinning work, local pressure |
| Spring clamp (Pony) | 1–4 in opening | Not stated; 1–50 lb adjustable (Adjust-A-Clamp) | Quick, light holding |
| Hand screw (Jorgensen) | 3–8 in opening; 6–12 in jaws | Not stated | Angled, tapered and delicate work |
| Band or strap (Pony; Bessey) | 10–30 ft band (Pony); 23 ft (Bessey VAS-23) | 2,200 lb band limit (Pony canvas) | Frames, polygons, round work |
| Corner clamp (Bessey WS-3) | Parts up to 4 in thick | Not stated | One 90° joint at a time |
Figures are each maker’s own for the models named, in the maker’s terms; another maker’s clamp of the same type may differ.
The long-reach clamps are compared in bar, pipe, parallel and F-clamps; the short-reach ones in spring clamps, C-clamps and hand screws; frames in corner clamps.
Spacing, cauls and where pressure goes
Pressure spreads out from each pad as it passes into the work, so between two clamps there is a zone near the edge that gets little of it. Clamps set too far apart leave gaps in the glue line there, even when each clamp is tight.
- Space clamps closer for thin or narrow parts, and near the ends of a joint.
- Use cauls — stiff, straight strips, often slightly curved so their ends press first — to carry each clamp’s pressure along the edge and to keep a panel flat.
- Alternate bars above and below a panel so it does not bow; see the bar clamp guide.
When more clamps are not better
- A poor fit. Clamps can close a gap only by bending the parts, which spring back when the clamps come off. Fix the joint instead.
- Pressure past the point of closure. Beyond a tight joint with even squeeze-out, extra force only risks starving it.
- Pressure in the wrong place. Clamps that pull an assembly out of square or bow a panel do harm however many there are; check with a square, a straightedge and the diagonals.
- Too slow an assembly. Titebond gives its Original glue 4–6 minutes of open assembly time at 70 °F: a dry run tells you whether you can place every clamp in time.
Which clamp for which job
| Operation | Clamp | Why |
|---|---|---|
| Edge-gluing a panel | Bar, pipe or parallel clamps, alternated, with cauls | Long reach; balanced pressure keeps it flat |
| Gluing up a case or cabinet | Parallel-jaw clamps | Large faces at 90° help keep the case square |
| Holding work for sawing, routing or drilling | One-handed bar clamps, C-clamps or holdfasts | Quick to set; hold without squeezing a joint |
| Gluing a small part or a veneer patch | Spring clamps or small C-clamps, with pads | Light, local pressure |
| Tapered, angled or delicate parts | Hand screws | Jaw angle set to the work; wooden jaws do not mark |
| A picture frame or polygon | A band or strap clamp | Pulls every corner at once |
| Screwing or nailing a 90° butt joint | A corner clamp | Holds one joint at 90° while it is fixed |
Before a glue-up
- The joints close by hand, or with light clamp pressure, in a dry run.
- Every clamp is opened to size and within reach, with pads or cauls ready.
- You know the glue’s open and assembly times, and the dry run fits inside them.
- Clamps are spaced for the parts’ width, and alternated on panels.
- A square, a straightedge and a tape for the diagonals are at hand.
- Something catches squeeze-out: tape, wax paper or a damp rag.
Clamps also hold work at the bench alongside its vises and holdfasts — see the workbench guide. Choosing a first set, and how many, is in how to choose woodworking clamps.
Sources
Clamp capacities and forces are each maker’s own figures, stated in its own terms (nominal, maximum, band limit); they are not directly comparable. Clamping pressures are from Titebond for its wood glues and from the USDA Wood Handbook; the 2010 edition gives a numerical range that the 2021 edition does not repeat. Clamping practice is established bench practice explained in BenchRoot’s own words.
- 1.
Titebond Original Wood Glue. Franklin International (Titebond).
Used for: Required clamping pressure: enough to bring joints tightly together — generally 100–150 psi for softwoods, 125–175 psi for medium woods and 175–250 psi for hardwoods; 4–6 minutes open assembly time and 10–15 minutes total assembly time at 70 °F and 50% RH.
- 2.
Charles R. Frihart and Christopher G. Hunt. Wood Handbook, Chapter 10: Adhesives with Wood Materials — Bond Formation and Performance. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-190, 2010.
Used for: Pressures near 0.7 MPa (100 lb/in²) suiting low-density wood and up to 1.7 MPa (247 lb/in²) for the highest-density woods; small areas of flat, well-planed surfaces bonding at lower pressures; overpenetration and squeeze-out starving a joint when pressure is too high.
- 3.
Charles R. Frihart and Christopher G. Hunt. Chapter 10: Wood Adhesives: Bond Formation and Performance. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Pressure applied to ensure close contact and help the adhesive wet the wood; too high a pressure, squeezing the adhesive out, to be avoided; the strongest joints made with moderately high pressure for the wood density; uneven pressure on poorly mated surfaces giving thick glue lines.
- 4.
Product Catalog, Spring 2024. Pony Jorgensen.
Used for: Steel bar clamps (600 lb standard, 1,000 lb heavy duty; 2-1/2 to 3 in reach); E-Z Hold one-handed bar clamps (50 to 600 lb); the Cabinet Master parallel clamp (1,500 lb, 3-7/8 in throat); the #50 pipe clamp fixture for 3/4 in black pipe; light-duty C-clamps (1 to 8 in opening, 400 to 1,000 lb); spring clamps (1 to 4 in opening) and the Adjust-A-Clamp (1 to 50 lb); Jorgensen hand screws (6 to 12 in jaws, 3 to 8 in opening); band clamps (10 to 30 ft bands, 2,200 lb band limit).
- 5.
K Body REVOlution parallel bar clamps (KRE). BESSEY Tools North America.
Used for: A nominal 1,700 lb clamping force; 3-3/4 in throat depth; 12 to 98 in capacities; clamping surfaces perpendicular to the rail; conversion to spreading without tools; removable pressure caps.
- 6.
C-Clamps. BESSEY Tools North America.
Used for: 540 series ductile iron C-clamps, recommended for lighter duty including woodworking: 0–2-1/2 to 0–14 in capacity, 1,100 to 2,850 lb.
- 7.
SL300 One Handed Bar Clamps / Spreaders. IRWIN.
Used for: 300 lb of sustained clamping force; a 3-1/4 in throat; removable pads; conversion to a spreader.
- 8.
Clamping angles and miters; Strap clamps. BESSEY Tools North America.
Used for: The WS-3 angle clamp holding and aligning material at 90°, for work up to 4 in thick; a strap clamp that pulls from both ends so pressure is equal on all corners.
About this guide
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