Here is the installment of the "Making a Perch" series that covers drilling the undercarraige mortises. I've built on some previous techniques that I've adopted to make it even simpler. I use one angle for all of the mortises into the legs. If you didn't get a chance to see the series on my new method for drilling out the undercarraige, you might want to check it out. Here is the first part of the series entitled "A New Way". The other postings are available in the June 2008 archive to the right, just click on the arrows to access the next in the series.
Now I've eliminated the need to use string and a straight edge to locate the axis of the stretcher and simply raise the mortises to the same height off of the table and then use the table as my axis reference. I literally woke up in the middle of the night with this one, and it still sounded good in the morning so I gave it a shot!
Once the mortise locations are parallel to the table top, I use a board with an average angle to mark an axis on the legs (I don't even know the number of degrees, I simply set a bevel next to each leg and tweaked it until it seemed reasonably close to all of them and then cut the board to match).
The marked axis will diverge from the actual axis of each leg, but this doesn't matter. The marked axis is held parallel to the benchtop in V blocks and then drilled using the same board and angle as a guide. It may seem complex at first, but soon you'll see that the method eliminates the confusion and slop of multiple readings, numbers and settings and uses only solid, exact references, the benchtop and the board.
To state it in steps:
1. Level the mortises to the benchtop
2. Mark the legs with the angled board
3. Mount the leg in V blocks and level the mark on the leg to the benchtop
4. Drill using the angled board
If you are new to this idea, or the drilling of undercarraige joints, you might mistake this method for using a "close enough" approach. It's actually just the opposite, if the process is followed, it is the most exact way I know to drill these holes.
Here is the video, I hope that it looks as simple as it is!
and here is a link to the video on Youtube.com so that you can view it in high quality
I had to think about this for a few minutes to get it, but it's starting to make sense. Intuitively it is hard to ignore the axis of the leg and go only by the tape line. I will try it and see, might be a few weeks. BTW, why is there duct tape on your v-blocks?
ReplyDeleteJohn,
ReplyDeleteI know it may seem a bit odd or unnatural, but once you've tried it, I think you'll see it. I think you put it well when you said "ignoring the leg axis". Using the angled board to mark a new axis on the leg ensures that the board is the "exact" drilling angle. I'm curious as to your results, let me know how it goes. As for the duct tape, I keep a set of padded V blocks for use on work that will remain unpainted, to prevent smudges or marring.