On my Facebook page I posted a blurb about adding dovetail rails to the back of slab style cabinet doors. These slab doors were so wide that they would inevitably start to cup and warp as the wood moves. To stop this from happening I decide to put two dovetail rails across the back of the door.
Shortly after posting this I received some inquiries about how to do this effectively.
This particular technique is used to strengthen a piece against the opposing forces– in this case the doors cupping. For the rail to work as intended the rail wood needs to be of the same species of wood as the doors, or a harder more dense wood. Otherwise the rail will bend under the stress of the door wood moving.
Cutting the dovetail rails should be done after the door is glued up and planed to a rough thickness. The door may already be starting to cup at this point so you need to act fast. It’s best to plan to plane to rough thickness, cut and insert the dovetail rails without any interruptions. Always cut the rail slot first and then trim the dovetail rail to fit as needed.
General layout is about 25% in from the top and bottom of the door. The depth of the dovetail cut is about half the thickness of the door. By removing the material in the back of the door for the rail slot you are also removing some of the stress in the door. The angle you cut the slot should be between 5-10 degrees. This gives enough angle to provide the holding power needed for the rail. Any more than this and you run the risk of the tail points being to acute and prone to breaking.
Cutting the slot can be done on the table saw, with a dovetail or straight router bit, or a combination of both. Once the slot is cut it’s now time to trim the rail. We need to understand there will be irregularities in the slot, as perfect as we may try to get it. These come from the door starting to cup, uneven pressure on the door or the tool as we machine it, the scalloping nature of a router bit cut or the run-out on a table saw blade. Keeping this in mind there will be tight and loose spots as we hammer in the dovetail rail. It will be difficult to test and check the tightness of this rail as you trim it because you shouldn’t be able fit it very far with out a hammer, but, do your best. The hammering will mash up one end of the rail so it’s best to make it longer than needed and trim after.
Once you’ve properly machined your rails it’s time for the install. Rubbing a little wax on the sides of the rail will help to it move into position. Do not glue these rails in, gluing them will ultimately damage the door. The rail should need to be hammered into place if you’ve fit them properly. There may be some trial and error during this process; if the rail is too tight it won’t move and the wood will break under the hammer. If the first rail fails, simply clean out the slot and adjust the rail accordingly. You should need to apply enough force to move it but not break the wood and still have a tight joint. In the end you want a dovetail rail which is seated firmly in place on its own, no glue required.
Dovetail rails are a great way to add strength and stability to solid slabs of wood.