Clamping the Sash Together
Clamping Up the Joints and Keeping the Sash Square
The next step is to lay the sash flat on the table, and use the bar clamp (or three bar clamps) to pull the joints closed. The bar clamps should have rubber pads, or else be backed up with the wood blocks to prevent excessive crushing of the wood fibers of the stiles.
If you have three bar clamps, place one directly over each rail. If using one bar clamp, place it over the bottom rail and pull the joint closed about 1/4" to 1/2". Then move the clamp over the center rail and close the joint slightly. Now move the clamp over the top rail and close the joint slightly. Continue to move the clamp over one rail at a time, closing each joint incrementally. Too much closing of any one joint will place excessive stress on the stiles, which will cause the stiles to deflect out of a straight line.
If using three clamps, simply tighten them incrementally to avoid excessive stile deflection.
If the sash becomes obviously out-of-square during the clamping operation, pick up the entire sash with clamps attached and while holding it in a vertical position, tilt the sash slightly so that the corner with an acute angle is resting on the floor. Then pick up the sash and inch or two, and bounce the corner on the floor until the sash resumes a rectangular shape. Then place the sash back on the table and proceed with the clamping operation.
When all six joints are open about 1/4", the ends of the tenons will bottom out on the clamps and further clamping pressure will not close the joint. At this point it is necessary to insert the slotted wood blocks between the clamp heads and the sash, aligning the slots with the mortises in the stile. This will allow the tenons to protrude into tha slots as the joints are fully closed. It is helpful to have an assistant, so that each person can handle one end of the clamp at the same time while inserting the blocks.
After inserting the blocks between the clamp and the sash, turn the sash over on the table so that the clamps are under the sash. Continue to close the joints evenly, and when the first end of each rail reaches a gap of about 1/16", pay close attention to the gap, especially the bottom rail. If one end of the gap is wider than the other (see picture), loosen the clamp slightly and move the clamp heads in the direction of the wide end of the gap. Move one end of the clamp toward the top of the sash, and the opposite end of the clamp toward the bottom. Usually about 1/2" to 1" is sufficient. Now as you continue to tighten the clamp, the new angular pressure line will cause the clamp will pull the joint square. Continue to monitor the gap as it closes to make sure that it pulls together evenly.
When all of the joints are completely closed, remove the clamps and place a long stick that is pointed on one end across the digonals of the sash to see if they are the same. Place the pointed end of the stick in an inside corner of the sash, and put a mark on the stick on the stick at the opposite inside corner. Repeat this operation for the other diagonal. If the two marks correspond, the sash is perfectly square. If there is a distance of more than about 1/16" between the two marks, the sash should be racked as outlined above until it is square.
The sash joints are now ready to be pinned.
Click on images below to view at larger size.