Sharpening blade and chisel blades is not difficult, but like many things woodworking, having some knowledge goes a long way to a successful sharpening job. A long time ago when I was trying to find someone who could teach me "sharpening" I found there were basically 2 types of people. Those who did what I call and "industrial sharpen" who sharpened their blades until they did a good job, there there were the "fanatic sharpeners" who had every blade in their shop absolutely razer sharp at all times and went to huge lengths with many different grits and polishes to get an outstanding edge. They just love sharpening.
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I tried both techniques and ended up in the "industrial" group, which works perfectly for me, my blades are super sharp and work great but I don't spend huge amounts of time trying to make them even sharper.
The basis of sharpening blades, both chisel and plane, is to end up with an angle of about 25 degrees (in most cases) and an edge that is super sharp and ... easy to keep sharp and the tool works the way it should.
Before you begin to sharpen any blade, you need to be able to assess the tool to see if it's even worth sharpening. Some of the blades that come with older, vintage planes are not so good, and investing in a new blade can make a world of difference on how the plane works. If a plane blade is one of the thinner one, it's likely warped a bit and getting these back to flat is a lot of work, and even when they are, they often are too then. If the tip is rusty, or has significant chipping, your plane will likley benefit having a new blade added.
The first thing to look at is the bottom or sole of the plane to see if it warped or bowed. If the sole of the plane is not flat it will be difficult to get it to work properly, if at all. The first thing you need to do with a plane is flatten the sole of a plane, and this is done on a flat surface by sanding with variously finer grits until flat.
Next you need to look at the plane blade or the chisel blade. Just like the sole of the plane, the back of the chisel or plane blade needs to be flat, not concave or convex, but flat, especially neat the sharp end of the blade. Even a 2 or 3 thousandths of an inch bow, is a lot of work to bring that blade back to flat - just warning you. The next thing you want to look at is the very tip the blade is it square to the side of the blade. Some plane blades will have a slight hump in the middle and that is good, it means the edges sides of the blade are less likely to gouge the wood, but square across is good too (because you can sharpen the blades slighlty on the sides to create a bit of bow effect).
If the blade or chisel looks good, then it's time to sharpen it and first question that arises, is what grit to I use. I get this question all the time, and the coarseness of grit depends on the condition of your blade. If it's really rough, maybe even came with some rust, on it, after you polish the rust off, you may want to start with something as low as a 220 or 320 grit to get the bevel correct and the back of the blade flat.
I always use wet/dry sand paper on a glass surface, but any flat surface will work. The reason I like wet/dry is I use a bit of water on the sand paper to help keep the grit from sticking in the pores of the sand paper and clogging up, which allows the sandpaper to last longer and perform better. Either works, using it dry or wet, I just prefer the wet method.
Once you have the entire cutting edge, or at least the lower half of it, the part the comes into contact with the wood, at your preferred angle, having ground it down with a coarser grit, now is the time to start using increasingly finer and finer grits. Each one of the finer grits will take less and less work to get a nice fine polish on your blade. You will know when you are finished because the beveled side of your blade should have a near mirror-like finish on it.
At this point, the only thing left is to flip the blade over and use the same fine grit you used on the front, to now polish the back lower side of the blade to take off the tiny bit of metal curl that inevitably develops as the bevel part is sharpened and polished.
At this point your blade or chisel should be razor sharp and ready to put to work.
For touch ups, that may last you years depending on usage, just use the same grit as what you used in the final polish ... use the felt pen to make sure your angle is good and you are still polishing the very tip of the blade, and once again, when you are finished touching up the sharpness on the bevel, make sure you take the curl of the back of the blade too.
These are the basics of sharpening and if these are all you do .. you will still end up with very sharp tools to use as long as you are engaged in woodworking.
Copyright Colin Knecht