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The fit of a dovetail is dependent on matching up the vertical saw cuts and the coplanarity of the baseline.
to a couple of years ago I would place a fence across the board and
use this to pare the baselines. There are a couple of jigs on my
website, one was published by Popular
Over the past few years I have come to prefer a method that creates a fence as a product of marking out. This is how I saw tenon shoulders. One day it occurred to me that it could be done with dovetail baselines as well.
This is clearly not an original idea. In discussion today I discovered that Chris Schwarz had published essentially the same tip nearly 4 years ago in his blog. My contribution has a small addition, one that possibly makes the process a little easier, but otherwise Chris definitely got there first.
I mark my dovetail baselines with a Titemark or LV wheel gauge. The line created is fine and lightly scored, which is good at this stage. Mark out the remainder of the dovetails and saw them. Leave the waste in place.
Today I was dovetailing the carcase for one of two Military Chests in ¾” thick Jarrah.
The plan is to score the baseline deeply between the pins (as in these pictures). I fine a wheel gauge does not work well in this regard. You can use a knife, which work but one runs the danger of running off the line. I prefer using a cutting gauge, such as a Kinshiro as this creates greater downforce on the knife edge.
The cutting gauge creates a deeply scored line, which is perfect for the next step, which is the undercutting the baseline with a chisel ...
With cabinets I am not a fan of the scored baseline. Consequently I choose to score deeply only between the pins. This is only necessary on the outer boards, not the inside as the latter will not be seen.
At this point I remove most of the waste with a fretsaw, leaving about 1/8” above the baseline. Too much less than this and it is more difficult to undercut the line.
Once this is completed the next step is to chisel out a fence by undercutting the scored lines.
Once all the undercutting is done it is time for chopping/paring out the waste.
With the undercut fence in place the coplanarity of the baseline is ensured. If one instead placed a chisel in a cut line, such as from the marking gauge, the triangular shape of the chisel bevel would drive the blade back beyond the line. The fence prevents the blade from moving beyond the baseline.
I prefer to work from the back of the board first, leaving the show side for last. There is no danger of damaging the show side as the waste is only removed to the midpoint (by contrast, always saw from the show side as it is easier to monitor the kerf approaching the baseline).
Now move to the show side …
You can see that we have a good, straight baseline.
When paring or chopping I keep the chisel vertical or tilted slightly for a fine undercut.
Here is one side completed. The fit looks good. The Moxon-dovetail-vise-plus-I-beam is also really useful in test fitting pieces. It is like a second set of hands.
Improving time-and-motion planning
Now before we finish there is an update. Paul Womack (BugBear) made an excellent (and now so obvious) identification that the baseline fences should be chiselled out before the waste is removed with the fretsaw. That way it is possible to fretsaw closer to the baseline, and of course there is less waste to pare away. Thanks BB.
Regards from Perth