Tool Review: Parallel-Jaw Clamps

Woodworkers long for parallel-jaw clamps the way kids yearn for the latest video game. With steel-reinforced, resin-covered jaws 3" to 4" deep that tighten up parallel to each other, beefy steel bars, heavy-duty handles and screws, and loads of clamping strength, these clamps have gained a reputation for high performance and prices that keep hobbyists at a distance. But is that a fair characterization? We decided to sort them out in a head-to-head test. For each make, we tested 24" models and either a 48" or 50" version.

The ability of the jaws to remain parallel under pressure separates these clamps from other styles. To test this, we clamped equal-length 2x6s on edge in each set of clamps, and then measured the distance at three places between the jaws: at the bar, in the center, and at the tip. Of the 24" clamps, six remained parallel; two deflected ¹⁄₁₆ ". Among the longer 48" and 50" clamps, only one remained parallel. Most clamps deflected ¹⁄₁₆ "; one was off by ¹⁄₈ ".

So what to make of it? We also tested a few other types of clamps (one-handed bar clamps, pipe clamps, and aluminum bar clamps) and found their jaws canted more than the parallel-clamp jaws. Given the deep jaws of the parallel clamps, ¹⁄₁₆ " deflection is acceptable: That did not affect a glue-up where the full length of the jaw made contact with the assembly.

All of the clamps have a fixed jaw at one end of the bar and a sliding jaw that travels along the bar. On most models, that jaw often slides uncontrollably down the bar, crashing into the fixed jaw when held vertically. One model, though, has a finger trigger below the handle that engages the bar reliably, with no backsliding when tightening the jaw. Half of the clamps use setscrews to engage the serrations on the top and bottom of the bar while you tighten the handle. However, all four crept backward more than half the time as we applied force to the assembly.

Despite the advantages some clamps have over others in engaging the bar, ultimately they all were able to pull together everything we tried in our tests. So we staged a "strongest clamp" competition to sort out the pecking order. Using a small hydraulic ram with a dial gauge, we had 15 woodworkers of varying ages max out each clamp, and achieved from 496 to 1,071 lbs of clamping force with the tested clamps. That's more than enough force for most glue-ups—100 pounds of force proves adequate to pull together a well-machined glue joint. Rarely do you need more clamping force to secure a project, but cold-bending wide, laminated workpieces around a form, for example, requires oodles more force, and the deep jaws of a parallel clamp excel here.

Top Tool: Jet Parallel Clamps
Top Value: Woodcraft Parallel Clamps

Clamps Tested: Bessey K-Body, Gross-Stabil PC2, Harbor Freight, Jet, Jorgensen Cabinetmaster, Stanley Parallel Clamps, and Woodcraft parallel jaw