Chopping away the waste wood between the dovetails neatly and efficiently requires good chisel technique, which starts with understanding the cutting action of the chisel.
The chisel is a wedge
Think of the chisel as a wedge with one flat side and one beveled side. When the tool is driven vertically into solid wood, the bevel is attempting to push or compress the wood in front of it. The wood, Newton tells us, pushes back against the bevel, and the chisel is driven backwards. Thus, the chisel does not travel straight down.
The way to avoid this is to effectively remove the bulk of the wood in front of the bevel. For this, there are at least two strategies. I will start with the one I generally prefer for its efficiency and reliability.
First, saw away the bulk of the waste
Using a fret or coping saw, remove the bulk of the waste, sawing horizontally about 1/16", or less, from the baseline. This has been done between the two tails on the right in the opening photo.
The photo below shows a high quality fret saw on the left and a hardware store coping saw on the right. I prefer the fret saw because its very thin blade fits easily into the kerf made by the dovetail saw and can then be turned 90° to directly start sawing horizontally. The small amount of remaining waste will offer little push back against the bevel of the chisel.
Use a holdfast to secure the workpiece over scrap plywood on the bench. I place the line of dovetails at a shallow angle to the front of the bench to give me views of the chisel angle and the inside of the joint. I do this work seated. As a precaution, I suggest start work on the inside face so the final chopping will be done from the outside, thus chiseling away from the show face.
Recall from the first installment in this series the specific chisel profile for this work. It is unlikely one of your chisels will exactly match the width to be cut, so use a slightly narrower chisel and two cuts across the width.
Here's the workpiece ready to for chopping:
For the initial chopping, hold the chisel vertical to the wood with the edge against the knifed baseline in the little "V" that you made earlier, as shown in the photo below. Tap the chisel gently! You want to avoid any push back of the chisel. To see how far you've cut in, you may want to clear the thin bit of waste by a gentle tap against the end grain.
You are establishing a deeper and sturdier vertical "wall" at the baseline. This ensures the pieces will assemble squarely. Now you can chop increasingly aggressively. For this, angle the chisel - about 2° from vertical - so it is cutting back toward the inside of the joint.
This "undercutting" serves two purposes:
1. The flat side (back) of the chisel is no longer in contact with the baseline, which is thus protected against being compressed, permitting you to chop harder and get the work done faster.
2. It ensures there is no "bump" in the interior of the joint that would prevent the pieces from mating tightly.
In the first photo below, the section on the right has not been chopped yet. In the middle, a plenty adequate vertical wall has been established to start angling the chisel. The section on the left has been undercut by angling the chisel as in the photo just below it. Note that the angled chisel is no longer contacts the baseline.
Most instructional materials neglect to explain that the work proceeds somewhat differently depending on the species of wood, and you must practice, gain experience, and use your judgment. Here I'm working with hard red oak that has end grain very resistant to compression - so the baseline "wall" is quite sturdy - but also has side grain that is hard to chop. So I balance those two factors to regulate the force of my initial chops and to decide when to start angling the chisel to undercut and chop more aggressively.
When you have chopped about half way down for all the sections, turn the wood over and chop likewise from the other side. The final break through chop may tear out some wood in the interior of the joint. In my experience, this does not cause a problem with the strength of the joint, as long as it is not excessive.
Here is the result:
An alternative method is to skip sawing away the interior waste and go directly to chopping. Because there is a bulk of wood that can push the bevel of the chisel backward, start chopping with the edge of the chisel a little bit in front of the baseline, as shown below. This protects the baseline. When you've made some headway, gently chop away the thin remaining wood to reach the baseline.
Both methods work! Experiment and see what you prefer in a particular wood.
Saw the ends
Removing the waste at the ends is best done by sawing directly to the baseline. Use a fine crosscut or hybrid tooth saw, Western or Japanese, which will give a cleaner cut across the grain than the rip tooth saw that you used for the tail cuts.
Guide the saw using the three knifed layout lines that are around each end of the workpiece, along with "V" cuts that have been chiseled into all of them. If there is any remaining waste after sawing, it will be minimal and easily pared away.
In the next installment of this dovetail joinery series, we'll check and clean up the tails and use them to layout the pins.