Thursday, June 27, 2013

Marking knife face off

I mentioned in an earlier post that my dovetails are often fairly good, sometimes a little better, sometimes a little worse and that I believed improvement is dependent on marking out the pins with greater accuracy.  A particular problem I have is that I have been unable to see knifed lines in the red oak end grain and so have to rely on pencil lines.  I don't know why I have this problem when I never read about anybody else struggling with it.  As a result, I resolved to find some way to make knife lines I can see and use.

As a first experiment, I took out my Lee Valley marking knife, two flavors of Exacto knives, one with a pointed tip and one with a rounded one, and a Swiss Army knife.  I made lines on the end grain of red oak and the result was . . . I couldn't tell the difference.  As for sawing to these lines, I found the same old problem:  I couldn't see these lines well enough to saw to them accurately.  Darkening them with a pencil didn't help because then you just have a pencil line on both sides and I didn't find that it helped me.  Then I had an idea.  If you use a knife to mark pins from tails, without moving the pin board you can use a pencil to mark a line that will be just to the side of the knife line.  I found this to be helpful because with this hint I could make out the knife line more distinctly.  This helped but I still hoped for a better approach.

As I was experimenting, I decided to go back to Ian Kirby's book on dovetails to remind myself of what he does.  He prefers a Swiss Army Knife.  Paul Sellers' views are quite similar; he uses an inexpensive Stanley knife.  My conclusion is that it probably doesn't make much difference what kind of knife you use as long as you practice with it and are comfortable.  You can learn to use a double bevel marking knife quite easily by just tilting it slightly.

I discovered something else during this process.  Maybe it is just my spasticity, but I find it extremely difficult to mark out the pins from the tails without moving the tail board at all and just slight movement makes a big difference.  I found that I needed to find some way to immobilize the pin board so I could mark distinct knife lines without introducing inaccuracy by moving the pin board.  A pin board, a tail board and a marking knife take three hands!  It isn't that difficult to find a substitute:

As this suggests, I get brainstorms, my own ideas about how to improve on what master woodworkers have been doing for hundreds of years.  :-(  Hubris aside, I wondered what would happen if I put a piece of blue masking tape across the end grain, used a knife to very lightly cut the tape along the edges of the pins, then selectively removed the waste pieces so I was left with just the pins in blue.  It doesn't take that much time and seems to work fairly well:

Add some vertical guidelines in pencil and you're good to go.  Now this I can see!  (Like many of my best ideas, I may have read this somewhere in the past, forgotten and now think it is my original idea.)

By my standards, anyway, the result off the saw is tight:

I think that the effect of these techniques would be magnified on wider work pieces with more pins and tails.  For a minute or two extra per joint, I think they are worth doing and, if the joint fits off the saw, a joint takes less time than I have been taking

One thing that getting more accurate, sharply-defined lines did was expose some error in my sawing to the line that I hadn't been able to see before with pencil lines.  It came about as I started the kerf in the pin board, because I was a little too aggressive and the saw would sometimes skate very slightly away from my thumb.  Just the weight of the saw was enough.  Accuracy here is absolutely essential.  I found that if I lift the saw to the point that barely any weight at all is applied to the work piece and take short strokes starting at a corner I can get a more accurate kerf.  Standard stuff, but it is easier to appreciate the value when you can see the effect clearly.

The combination of all of these things seems to make a real difference.  When you cut dovetails in a forgiving wood like alder, they aren't needed, but when both pieces are oak, there's less room for error.  I am certainly not going to cut any three minute dovetails this way and may not need to do all of these things forever, but they seem to be moving me down the path.

Got any tips and tricks to share?


  1. Hi Andy,
    David has a blogpost and you tube video on how to make/use a dovetail alignment board. I made one and it works surprisingly well.

  2. Hi, the Woodwhisperer has a video on inlay where he rubs a pencil over the knife line then uses a rubber eraser to remove the pencil off the surface leaving only graphite in the knife marks. It seemed like a good trick.

  3. I saw a post here from Chris Swartz that talks about using a 0.3 pencil to highlight knife lines. I have not tried it yet but the post says that a 0.3 pencil is small enough to mark inside the knife cut rather than on both sides.