I’ve told this story to many woodworkers, but I have yet to post it in a blog. One day before Dad passed, he called me to complain that he was having to fit every tenon to it’s mortise by hand. (Many woodworkers choose to work that way, but to us it was a detriment in that the process is slow, takes too much time and lengthens what is a mundane process at best.)
We formed our tenons in a two-step process at a table saw – one cut made with a board flat to the saw top (shoulder cut), and a second cut made with the board standing vertical (cheek cut). From the saw, we were ready to assemble the joint.
When he told me he was fighting the fit, I asked what he had changed. He had changed his planer knives. He had installed the three knives so there was a slope in the cut; his knives were not parallel to the planer bed. This is a problem even if the knives were out only .003” because lumber pushed through the planer held toward the right of the bed would be a different thickness from lumber run through held toward the left side of the bed. Those small differences change the thickness of a tenon completed using a two-step method. We reset his knives and he was back in the game.
If you set your planer knives and don’t wish to fuss to get the settings right-on, there is an end-around method. Cut your tenons with a dado stack then trim each tenon to fit into the mortise. If this is the method you choose, you have to cut your tenons oversized in thickness because of the nonparallel settings of your knives – you cannot dial in a perfect fit due to the lumber being different thicknesses. (That’s more shop production time lost as you fit each tenon.) If you do get lucky enough to hit a few of your tenons at the correct thickness to fit the mortises, your surface is not as smooth off a dado stack, as shown in photo below.
Where else can you lose production time when cutting your tenons with a dado stack? How about your tools? If you need to, you can work a precisely cut tenon with a rasp or shinto (that band-saw-blade looking tool in the photo below). If you have to fit your tenon, you’ll need a shoulder plane due to the amount of material you need to remove. That requires set-up and sharpening. More time.
What does all this have to do with helical cutterheads? If you install a helical head in your machine, or purchase a planer with one installed, you do not need to set the blades. The head is set parallel to the planer bed and if you get a nicked knife, you can rotate the insert and get back to work. (Read about that here.) As a result, the possibility of your planer giving you different thickness of lumber is nil. That means you can use a two-step method to accurately cut your tenons so the fit is dead-on. Therefore, a helical cutterhead makes better tenons.
For more information on woodworking joints, check out:
“Woodwork Joints” by William Fairham. It’s a revised edition from the first quarter of the 20th Century that discusses both the major and minor woodworking joints.
“Illustrated Cabinetmaking: How to Design and Construct Furniture that Works” by Bill Hylton. This is one of the most prized furniture constructing books in my collection.