Monday, May 7, 2007
The round mortise and tenon is one of the simplest woodworking joints to make. There is probably no more widely used woodworking skill than drilling a hole. But to get accurate and repeatable results just using a drill and drillbit takes a bit of care. The picture above shows the tools that I use to fit my mortise and tenons. I am going to start by focusing on the drill bits. I don't do any of my drilling with a bit and brace, I know, I'm no purist. When I started making chairs, I didn't own a brace and a good set of bits was not in my budget. As a cabinetmaker, I became very familiar with a cordless drill, and that's where I've stayed. I grind my own drill bits using high speed twist bits that I fashion into long spur brad points. They allow me to enter a cut at almost any angle (vital in chairmaking) and leave a beautifully clean hole and exit cut. Why grind my own? I have bought just about every bit on the market and found them to be lacking and costly. However, the biggest problem that I have with them is that they are difficult to sharpen, meaning that from the first time that I use them, I have to accept some lessening of their cutting quality. A drill bit should be judged like any tool in the shop, it is a cutting tool that can be sharpened and modified to perform at its maximum. The method that I use to grind bits is simple and forgiving (I'll be covering it soon).
There are a couple of basic ideas about drilling (with a power drill) that I find useful. The same bit drilling into different woods will yield different sized holes. If you drill a block of hardwood to test your tenons for a pine seat, you'll find them swimming in their mortises. It is vital to test at least one of the tenons in the actual wood that it will be mating with to understand the way it should fit in a test block. Perhaps a supersnug fit in the test block will be just right in the pine.
Whenever I drill, I use very light pressure and high speed, my bits are very aggressive. Always start with the highest speed to get a clean entry hole and let the drill advance at a speed that clears the chips cleanly. Going too slow can burnish the hole. I am careful to hand fit each tenon to it's mortise, one at a time. It is one of those times that I slow down to take extra care. I think of it as a time to pay tribute to the time that I've already invested in the parts that I am joining. I love the look of a clean joint. It may sound ridiculous, but getting a few different species of wood and making some practice holes with your drill bits can be enlightening. Are the holes clean and accurate? Did you blow out the exit? Are they so dull that they smoke? Do the chips come out in beautiful curls or dust? Remember, the round morise and tenon is a terrible glue joint, and no glue can make up for a burnished, oversized hole.