Accurately glueing up large panels can be a real challenge. As you increase the clamping pressure, the panels have a tendency to slide out of vertical alignment or even worse buckle. 4-way panel clamps are the answer to both problems but commercially made clamps are really expensive and cumbersome to use. I made 16 of these 4-way clamps out of standard 2X4's for approximately $5 each.
The special thing about these homemade clamps is the inner faces have a slight convex curve that evenly exerts pressure in all four directions. As screw pressure is applied the curves flatten with a scissor action ensuring everything in held in perfect alignment. Vertical pressure also eliminates the potential of your project buckling in the centre as you increase the clamping pressure.
I routinely glue up 30" x 11' panels for building wooden paddle board skins that are only 1/4" inch in thickness. Perfect alignment allows me to start with thinner boards and virtually eliminates the need for tedious surface planing after the glue has dried. Since these clamps excel at this task, I am confident you will love them for glueing everything from table tops to cutting boards.
Step 1: Building the Cauls...
The curved clamping cauls are made out of standard 2 x 4 lumber. You could use expensive wood for making these cauls but it isn't necessary. If you have ever cut a piece of wood too short, you know it is very hard to stretch any species of wood. Select 2 X 4's that have straight grain and as few knots as possible. The clamps pictured are 36" long so it was easy to cut between the knots. Size your cauls to the projects you want to clamp. My cauls are 36" long as the widest paddle boards I build is 32" wide.
1.Cut your wood to the desired length.
2.Rip the 2 x 4's down to approximately 2.75" x 1.5". (You can make 1/4" thick spacers out of the cut off portion. These spacers both protect the clamps and allow you to clamp projects less than 1/2" thick.)
3.It is nice to start with straight lumber. You can either start with perfectly straight wood, joint your boards, or hand plane the board straight. As a last resort you can simply mark which way the lumber naturally curves and put the apposing curves inward. (These clamps were straighten with a jointer before adding the curves.)
4.Drill a series of holes at 2" intervals 1" in from the non-working face. (See picture) I set up a fence at my drill press and made a series of pencil marks every 2" to locate the holes. These holes are used as gross adjustment holes for different sized panels and they should match the apposing caul. If you don't have a drill press you might want to clamp both sides together and drill both holes at once. This will ensure the holes are aligned properly.
Step 2: Adding the Curve..
The curve can be made in many ways but the easiest way is to mark the centre and use a hand plane to add the subtle curve. (The curve is so subtle that using a trammel point is not a good idea.) By taking 1 pass at each location you will end up with a subtle curve. The centre effectively gets no material removed. 3" from centre gets 1 pass of the plane. 6" from centre gets 2 passes from the the plane. 9" from centre gets 3 passes.........and so on.
1. Mark the centre of you board and make marks every 3" working from the centre to the ends.(Picture 1) It is best to mark the side of the caul and not the surface you are planning.
2. Take one pass with a hand plane working away from the centre towards the ends. One pass from 3", one pass from 6" etc... Make sure you are curving the side furthest from the holes you drilled in the last step. Flip your board and do the other half again working from the centre. You want to end up with 1 curved surface that is highest in the centre.
3. You can either curve both inner faces (2 cauls) or have 1 flat side and 1 curved caul. My clamps have 1 flat side that goes on the bottom and 1 curved caul that apposes it.
4.Lay the two boards face-to-face and put a clamp on each end and see how easily you can pull the cauls together. The amount of force you feel on the clamp is essentially the force that will be applied vertically. If you are happy with the clamping force you are done. If you feel you need more clamping pressure repeat steps 2 & 3 until you are happy with the pressure. There is no magic number as it depends on how agressive your hand plane is set and the material you plan on clamping. My clamps have about 1/16th of and inch of rocker per side.
5. Run a palm sander over the curve to make the curve fair.
6. The clamps pictured have been finished with paste wax and have a layer of packing tape protecting the working faces from glue.
Step 3: The Working End...
The working end of the clamp is made out of threaded rod and standard steel bar stock. The only tools required are a saw, drill, file & 7/16th tap to add the treads to the square nut. A drill press, sawzall & bench grinder make this a simple task.
Here an overview of the parts for the working end:
4 - linkage plates made from 1/8" x 3/4" bar stock.
1 - Screw - The screw is made out of a 6" length of 7/16th threaded rod.
1 - Square Nut - 1.25" x 1.5" x 1/2" piece of bar stock with a threaded a hole through the centre and two 1/4" holes for the linkage plates.
1 - Clamping Foot - The clamping foot is made from 1.5" piece of 1/2" x 1/2" bar stock. There is a hole drilled part way into the stock with treaded rod locked in place with a split pin.
1 - Speed Handles - Speed handles are simply a 1/4" steel rod bent at 90 degrees in a vice. A 7/16th nut serves as a hub with a 1/4" hole drilled through one side face.
4 - 2.5" long 1/4" 20 bolts with nylon lock nuts.
Step 4: The Non-Working End...
The non-working end of the clamp is made from 1 block and 2 linkage plates per side.
Parts list for the non-working end:
4 - linkage plates made from 1/8" x 3/4" bar stock.
1- Stop Block - 2.5" x 1.5" x 1/2" piece of bar stock with a hole through the centre
2 - Hardware - 2.5" 1/4" 20 bolts with nylon lock nuts.
2 - washers as a spacer for the linkage plates.
1 - 3.5" 1/4" 20 bolt with the treads removed and edge chamfered as a pin.
Step 5: Linkage Plates...
The 8 linkage plates are made out of 4" lengths of .75 x .125 steel bar stock.
1. Cut stock to length.
2. Drill a .25" hole .375" from the edges.
Note: The easiest way to make the holes perfectly spaced is to use a indexing pin. Attach a piece of scrap to your drill press table. Drill the first hole. Slide your stock 3.25" and use a handheld drill to drill a hole through the first .25" hole into your table. Insert .25" bolt in the hole in the top while you drill the second hole.
3. Chamfer the edges.
Step 6: Stop Block...
The stop block is made from 1.25" x .5" bar stock. The length was chosen so that it sticks out past the linkage plates when clamping thin panels.
1. Cut a 2.5" length of stock.
2. Drill a .25" hole through the centre of the .5" edge.
3. Chamfer edges.
Step 7: Square Nut...
The Square Nut is made from a 1.5" length of 1.25" x .5" bar stock.
1. Scribe lines corner to corner to locate the centre of a flat side.
2. Drill and tap for 7/16 coarse thread.
3. Drill two .25" holes in the .5" face for the linkage plates.
4. Chamfer edges.
Step 8: Clamping Shoe...
The clamping shoe is made from a 1.5" length of .5" x .5" bar stock.
1. Drill a small hole all the way through for a split pin. This pin locks the shoe to the screw and allows it to rotate. Drill about 1/8" down from the top of your stock intersecting the edge of the larger hole (as pictured) you are drilling in step 2. If you drill these holes in the other order you will probably break the smaller drill bit.
2. Drill a 7/16th hole 3/4 of the way through the stock at right angles to the small hole form step 1.
Step 9: Treaded Rod...
The screw is made from a 6" length of 7/16th threaded rod. The 7/16th screw diameter works well with the 1/2" thick material. The speed handle is simply a 7/16th nut cross drilled for a length of 1/4" rod.
1. Mount the screw in a drill press and rotate your stock. Hold a file to the rotating stock to add a chamfer that roughly matches you drill bit angle.
2. Insert the chamfered screw into the clamping foot and drill into threaded rod enough to mark the location of the notch.
3. Rotate the stock in the drill press again and make a notch around the circumfrence of the screw. This notch allows the screw to turn freely inside the clamping foot.
Step 10: Assembly..
The clamps are assembled with standard 2.5" 1/4" 20 hardware with nylon locknuts. A 1/4" pin is used to quickly open and close the clamps. The 1/4" pin is made out of a 3.5" 1/4 20 bolt with the treads cut off and the edges chamfered. It simply slides in the required hole and clamping pressure holds it in place. Store the pin in one of the unused holes when not in use.
I prefer these homemade clamps to commercially available 4-way clamps I have used. Many of the commercial clamps require assembly during use and are not as quick to tighten. Since I use 16 of these clamps at once the speed handles are really appreciated when glue is setting.