Laying out dovetails is probably the simplest part, but there are a few tricks that help.
I’m doing tails first. Apparently there are two camps, the pins-first folks and the tails-first folks. I’m just trying to get a handle on dovetailing, and this is what is (sorta) working for me.
To do the layout you will need a red pen, sliding bevel gauge, a marking gauge and two sets of dividers.
Start by scribing a line across all four faces using the marking gauge set to the thickness of the wood. This is your baseline, don’t cut or chisel below it. I really like the crisp line the Glen-Drake gauge makes, but the brass leaves a black smudge on the pine and I’m not sure why. I cleaned it with solvent top make sure there wasn’t any leftover polishing compound on it. Maybe I’m pressing too hard against the end grain?
We’ll use one of the dividers to mark the half-pins at each end, and the other to mark off the spacing. Jim and Tim recommended using half the thickness of the board as the width of the half-pin on the ends, so I’m going with that for now. Set one divider to about half the thickness of the stock, and make a mark that distance in from the front and back of the board. If you guessed half the thickness exactly there will only be one mark, otherwise adjust the dividers to split the difference. Now make a mark that distance in from each edge and set those dividers aside. We’re using two dividers because in the real world we would be making several identical joints, like the four corners of a drawer, and we want to have them match.
Now we need to mark out the spacing for the dovetails. You could measure the distance, divide by the number of tails and measure the difference off using a set of calipers. Or you could use the other pair of dividers to simply step the distance off. I’d recommend the later.
When I first read about this approach I had to read the description a few times, then try it myself. Essentially what we’re going to do is set the dividers by eye, step off the distance and adjust them calipers. Once we’re satisfied we’ll step off the distance, starting at the left half-pin mark and moving to the right and making a small mark each time. Then start at the right half-pin mark and step off the distance to the left, again making a mark.
How wide do you set the dividers? If you want 3 tails set them to about 1/3 third the distance. Make a practice run, stepping off the distance but not making any marks. When you get to the other end look at the amount that you’re overshooting the other half pin mark. That overshoot is going to be the size of the space between the ends of the tails. I usually have that set at about 3/16 of an inch. In other words, the space between the tips of the second set of dividers is set to the width of one tail plus the width of one pin.
If you’re happy with the size of the pin, go ahead and step off the distance from each half-pin mark to the other side, making a mark at each point.
This method is very fast, and foolproof (I can do it!).
Next, draw a straight line across the endgrain with a square.
Now set you sliding bevel for whatever slope you’re using for your dovetails. This setting, like pins-first vs. tails-first, is one of the things woodworkers like to have debates about. Typically the angle is specified as a ratio or slope like 1:6 or 1:8. This just means that for one inch in width the angle meets at 6 (or 8) inches up. You can easily draw out a series of these angles and use it to set your sliding bevel gauge.
For what it’s worth, if you want to know the degrees measurement you can take the arc tangent of the height number times 10. For example, a 1:8 ratio is arctan(8) * 10, or 14.4644133 degrees. Ratios are easier. The only reason I know this is that Lee Valley sells these groovy dovetail saddle squares that you can use to mark both the straight line across the endgrain and the angled line on the face in one shot. They sell them in 1:6, 1:8 and 14 degrees. Why? Turns out 14 degrees is a 1:7.24460662 (call it 1:7.25) slope. Still, it’s odd.
Here are my 1:6 an 1:8 Lee Valley gauges. I have no idea where my 14 degree gauge is, off on a weekend rendezvous with my metric hammer probably.
I used my sliding bevel gauge to lay these out at a 1:5 slope. Or 13.7340077 degrees…oh, never mind.
Finally, mark the waste. It’s too easy to cut in the wrong place. Or plane your thumb, but that’s another story.
At this point you’re ready to cut the tails. We talked about sawing yesterday, so let’s skip that (and chopping/paring) and mark out our pins next. This is easy. Clamp the pin board in your vise with it level with a plane or other spacer, then set the tail board on top. Line up the two sides, and align the baseline on the tail board with the inside face on the pin board. If you marked the baselines exactly the width of the stock you can just align the ends of the tails with the face of the pin board. Make really light cuts at first so that your knife doesn’t follow the grain, but follows the tail.
In case you’re wondering, I HATE that marking knife for laying out dovetails. I’m going to buy or make a spear-point marking knife. The problem is that on the left side of the tails the bevel of the blade is in the wrong place and it leaves the mark in the wrong spot.
Finally, use your pen and square to draw in vertical guidelines from the knife cuts down to the baseline. Thats about it. Marking the tails is pretty failure proof. Marking the pins from the tails is a little dicy because if you don’t have the two parts perfectly aligned they won’t fit properly in the end. There is a trick to help this, which I’ll experiment with and cover in the future. The idea is to cut a shallow rabbit on the inside of the tail board, right at the baseline, before laying out the tails. This gives you a shoulder to align the two parts.