Monday 9 July 2018
David Barron devised this magnetic guide to achieve perfect dovetails much more swiftly
My magnetic dovetail guide works by capturing the saw blade with a magnet and guiding it at a set angle through the work. The cut is dead straight, it is at a consistent angle and most importantly of all it is dead square. If the cut is not square you cannot hope to mark one part of the joint from the other and achieve a good fit.
Like most woodworkers I cut the tails first, although this guide can be used for either method. With the pins marked, they can be cut with equal accuracy by reversing the guide and positioning the cut to the waste side of the line.
The purists of hand-cut dovetails may baulk at the idea of de-skilling the process and I doubt that The Barnsley Workshop would ever use this method. However, once you have put the little guide back in the drawer, no one will know it was used. Your dovetails will be cleaner, sharper and will have taken you a lot less time to do.
Choosing the right saw is important, and I favour the Sun Child duzuki from Thanet Tool Supplies which cuts cleanly and very quickly. Japanese saws will work well as long as you choose the right tooth configuration. Crosscut teeth are very slow and ripcut teeth are too coarse. What you are looking for is called a modified rip with about 18tpi. Naturally the depth of cut is controlled by the depth of the blade up to the back, so you need to bear this in mind.
The Sun Child will cut to a depth of 30mm, fine for most cabinetmaking, although this can be increased by shortening the back and using the end teeth to complete the cut.
Western-style saws can be used but the nice dovetail saws on the market do not have enough blade depth.
Using magnets to guide a saw is not a new idea and a number of versions are commercially available. Having used them with varying degrees of success, I decided to make my design as accurate and user friendly as possible.
Instead of using a clamping mechanism I simply glued on fine sandpaper which prevents movement while allowing fine adjustments to be easily made. The low-friction pad stands proud of the body to reduce the risk of unwanted damage. As this guide is just for dovetailing and not for multipurpose use, I scaled it down to suit the specific task. This makes it easy to handle as well as allowing the maximum depth of cut.
As I only ever use a 10° angle, around 1 in 6, for my own work, I made the 1 in 6 size, but if you decide to make your own then you can have as many different angles as you like.
I use goncalo alves for my guide as it is hard and stable and can be bought kiln dried as two-and-a-half or 3in turning blanks. Ideally choose older stock which by happy chance is likely to be cheaper.
I let the wood acclimatise for 6-12 months in my dehumidified workshop before using it. By choosing grain that is curved across the corners you can avoid one of the legs being of brittle short grain. This is another advantage of turning blanks as you can pick and choose your pieces.