Monday 9 July 2018
This is an easy to make little project, which produces an attractive and very useful knife. When marking dovetails, there are a number of important features I believe a knife should possess:
1. A blade thin enough to mark between fine tails
2. A blade stiff enough to cut straight and not bend under pressure
3. A tough point that does not break easily
4. The ability to resharpen the tip easily
5. A handle that registers the blade square in use
6. A blade long enough to reach into the corners of larger tails
Surprisingly I do not know of any knives which satisfy all these criteria. The usual recommendation for a scalpel or craft knife is a good choice although the blade bends in use and it has a brittle tip as well as being a bit short for larger tails.
Blue Spruce makes a pair of beautiful knives and the smaller one is great for dovetails. However, the thin V-point blade is very difficult to sharpen and the handle is round with no registration for square.
My favourite marking knife is the one made by Chris Vesper, which ticks all the boxes except that the blade is too wide for all but the largest dovetails. This also applies to the very good Japanese knives.
Faced with this lack of perfection I decided to make my own. An off-the-shelf blade was the easiest solution and I eventually found what I was looking for within the large range made by Swann Morton. The SM 01 blade has a steeper angle than the normal scalpel blades, which gives it a tough, more durable point. It is also thicker at 0.6mm, which gives the stiffness required while being thin enough to access the smallest pins.
Consideration was also given to the ease of resharpening which is not something you would normally carry out on scalpel blades.
As the only part of the blade to be used is the tip, it makes good sense to ignore the narrow bevels and just concentrate on sharpening the back.
The initial sharpening, with locked wrists and elbows, is best done on a diamond stone to prevent grooving, photo 1. This quickly re-establishes a nice sharp point without compromising the factory-ground bevels.
The back can be polished on a finer stone for ultimate sharpness. Again, the profile of this particular blade is helpful as the angled back is fully accessible and not fouled by the handle.
Ferrule and blank
The next consideration is the ferrule which is simply made from half inch brass tube. The 16-gauge wall thickness gives an outside diameter of 12.7mm and an inside diameter of 9.6mm. This suits the width of the knife shank, which is 9.5mm.
The length of the ferrule is 16mm, which is enough to take the whole knife shank. When making one or two knives this is easily cut with a hacksaw but for batches of 20 they can be more easily and accurately cut on a bandsaw, photo 2. Note the false fence in the picture preventing the cut pieces jamming against the fence.
With the blade chosen and the ferrule cut, it is time to consider the handle. Hard and dense timbers are a good choice and this is a great way to use up those tiny offcuts you cannot bear to throw away.
I cut the blanks to 16mm square by 190mm long, aiming for a final size of 14.7mm round and 150mm long.
The first cut on the lathe is at one end to take the ferrule. With the ferrule added to the blank it makes the correct fitting of the ferrule quicker, although this step is not necessary for making one or two knives.
The blank is then roughed with a gouge and finished off with a straight chisel, photo 3. The actual shape is a personal thing, but I am happy with this simple yet elegant design.
Sanding on the lathe is fast, but I always like to hand sand with the grain to finish off. Using a disc sander and a mitre fence, the ferrule is mounted on the blank and carefully flushed off square, going slowly to prevent burning, photos 4 and 5.
The next step is to cut the blade slot. It is vital that this is centred in the stock as well as being straight and I have found use of a fine dovetail saw to be the best way, photo 6. I take great care and go slowly to make sure the cut does not wander, although if disaster strikes I simply super glue in a piece of commercial veneer and accelerator and have another go, photo 7.
With the slot cut I insert the blade and use this against an engineers square on the drum sander table. This allows me to sand a thumb recess, photo 8, which will be at exactly 90 degrees. I have found a 3in-diameter drum gives the right sort of shape to the recess.
The idea of this thumb hold came from an antique glasscutter and is simple yet very effective. Remember to glue the blade the right way round to suit right- or left-handed use.
After carefully finish-sanding the recess, the finish can be applied. I like to use two coats of shellac to seal the pores to keep the colour lighter and follow this with a few thin coats of Osmo Hardwax Oil for durability and easy repair, photo 9.
The brass ferrule is polished on a buffing wheel with blue compound, photos 10 and 11, which quickly removes scratches and forms a nice shine. The body is personalised with a hardened name stamp, photo 12.
Before glue up I liberally coat both the ferrule and the top of the handle with wax, taking care not to get any on the inside.
The blade and ferrule are then glued with epoxy resin, using enough to fill up all available space, and give a solid seating.
The inevitable squeeze-out is left until hardened and then broken off the waxed surfaces. The handle is then finished off with 0000 wire wool and wax.