Saturday, 17 September 2011

Drawknives , the blunt, the handleless, and the pitted. Make them cut like new

 I have used draw-knives for over 20 years now and I can still get passionate about them, as a tool for shaping wood. Both with rough and fine work, they are hard to beat. Of course you need another tool, the shaving horse to use them efficiently. Here is a link, to the Bodgers forum with lots of pictures of shaving horses.
I have a collection of draw knives I need to sell. Some of them need a lot of work including new handles, and some of them just need a good sharpening, so I thought I would take a few photos and share with you.

I start off by flattening the back of the knife. The back, in my definition, is the flat side. I always keep this side absolutely flat and never put any sort of bevel on it. I know that some authors and sharpening practitioners advocated a back bevel, but I see this as making the knife less useful. Why? Because if the back is flat then it operates as plane, against which the edge is guided. If you have a bevel on this side then the knife needs to be held at an angle against the wood, and it is then easier to erroneously dig in or pull out, when pulling the knife. A slight change in the angle the knife is held, then the more likely the blade is to produce an uneven surface. Anyway, the knife has a bevel on the other side with which to make concave cuts.

When the back has been flattened and any pits have been ground out, I then joint the edge. Jointing is running a stone at a 90 degree angle to to back of the blade. In other words, I am making the edge blunt. This takes out any chips and hollows and makes the blade straight or of the desired curve or crown.
I only joint when renovating tools, or when the edge has been damaged. If I get an odd imperfection in the edge I often leave it, and it will sharpen out over time.

The next step is to take lots of metal off the bevel. I want about a 20 to 25 degree angle for the primary bevel. The most useful machine for this is a bench grinder. A belt sander works well as well, but I have a dedicated wood one, and do not like using it for metal.
Bench grinders are great, but they do need to be used with an understanding of how they work. It is very easy to overheat the steel blade and make the edge soft. A bench grinder is aggressive, and it is easy to damage a blade very quickly.  This quality of the bench grinder is very useful in regrinding primary bevels, something that would take hours to do with,even, a very coarse bench stone. Start by using one on larger tools such as an axe.
You may well ask why do I not use the belt on this bench grinder. Simple: it is so under powered that it almost useless. This is a bottom of the line cheapest machine out there, and you get what you pay for. Put any sizeable lump of metal on the belt and only a small amount of pressure will stop the belt.

I have recently made a simple jig for the bench grinder to help guide the draw-knife. I used a few offcuts of wood and a few screws.


Note that I made the jig table at an angle, this is so the draw-knife handles do not get in the way of the grinder housing. I also use a screw to set the angle the knife is ground at, this screw can easily be moved to achieve a greater or lesser angle. This set up is pretty primitive and when I have time I may make a better jig. Before making this jig I just run the draw-knife along the metal table, and I do it freehand. Using a jig this way is a bit more accurate.
I have the jig at a slope because on this grinder it helps keeps the handle of the draw-knife away from touching the body of the grinder.

Remember to do a few passes and check to see how warm the metal is. It is best to do say 4 or 5 passes and then just get on with something else while the metal cools down. There is a lot to be said about not dipping in water to cool the metal down. Some people say that this can cause tiny fractures in the edge, invisible to the naked eye, which can lead to tiny chips.
Back to the bench stone: this one is a modern Norton oil stone and one of the coarsest  they do. I now rub the draw-knife over the stone to produce the primary bevel on the knife. I am not one to get into measuring all the time, as I have been sharpening for so many years that body and brain just get to know what is what, a tacit knowledge that only comes from doing. So do as I say and not as I do, and get a protractor that can measure the angles of your blades, make up wooden blocks so you can compare the angle of the knife you are holding. The last thing you want to do is to put a more obtuse bevel on your tool.
The main thing about sharpening is holding the tool at a consistent angle, and patience, and having a loupe or small magnifying device to study the edge with.
Continue with honing on finer stones and  strop if you want to. Yes I said strop, I strop most of my cutting tools, because it is my belief that the finer the edge of the tool has been polished to the longer it will stay sharper for. A quick strop can bring the edge back to life again and saves time using the hones.
On a side note, if you are using a draw-knife for a lot of concave cutting then a more obtuse bevel works better, or is this just an excuse for acquiring more tools?