Guards and Bolsters
The guard is the piece of metal between the blade and the handle, guarding the user's hand from slipping up onto the blade. The bolster is similar, though the metal does not extend further than the handle in thickness, and so is a transitioning and aesthetic feature than a guard for your fingers. The only difference between the guard and bolster is the shape. For now, I'll use the terms interchangeably.
I'm a big fan of copper. I would go so far as to say it is my favorite metal; inexpensive, polishes nicely, color is unmatched, easy to work, easy to solder if need be, and predictable. The material should be dictated by the color of the handle and blade, but copper normally fits most colors (except black or white, which would be more appropriate with a silver colored guard). Copper is my normal first choice.
It's not always easy to get ahold of however, and you'll want some at least 1/8" thick. I got mine from a kind bladesmith, who sent me quite a bit a few months ago as a gift. Scrap yards, metal suppliers, even hardware stores might have it. Look around.
Brass is also a good choice. I personally haven't worked it much, though find it to be a little finnicky; tough to solder, when you melt it it burns easily, etc . I haven't found it in thick enough pieces to work into guards, though it shouldn't be too hard to go to a local rifle range, pick up all the spent shells, melt them into a blob and work from there.
Steel or iron, aside from copper, is what I use most. Easy to find in thick enough amounts, predictable, but a little tough to work. Using a plumber's torch, it's easy to oxidize to get a range of colors, from yellow to light blue to black.
Many metals can be used, as long as they don't oxidize too easily, are tough enough to hold up, are hard enough not to mark, and are pretty enough to use.
Forming the slot
The guard works by sliding up the tang to be flush with the shoulders; the bottom of the ricasso.
The first step is to make the slot in the guard, through which the tang will fit. Be warned; this is the most tedious part of making the guard, and the one requiring the most perfection, concentration, and effort. The slot must be fit perfectly, so there is no gap between it and the tang or the guard and the shoulders, and so it does not wiggle around. There will be no glue or pins holding it still, so it must be a perfect fit. Once again defining perfect; something done so well it cannot be done any better. If you go too far, throw away and start again.
So first, you may need to do a bit more grinding on the tang. It should be tapering in both thickness and width, with the thickest and widest point right up against the shoulders. The shoulders should be slightly rounded; if it's a sharp corner, it's more likely to break. The guard then will have to be slightly rounded too.
Measure the thickness and width of the tang and mark it in the center of the guard material (forming a rectangle), making sure you have room to cut out the guard later on. Some people cut out the guard first then do the slot, but it's more reliable and there's less chance of making a mistake if you do the slot first.
Drill the first hole on one end of the rectangle, making sure it's centered and doesn't move outside the boundary. Drill the next hole as close as you can to the first, and so on until you reach the other end of the future slot.
Guard by Mark Cooke
At this point I do a bit of fiddling around with the drill to connect the holes, making one long slot. More efficient is just to use a round file.
Files are some of the best tools for the bladesmith. From larger bastard files to jeweler's files, they're what you need for detail work. I picked up ten or so various jeweler's files at a yard sale for a dollar. They're pretty easy to find and very cheap. Stock up on files, it's worth it. For guard work, you'll need files for sure.
With all the holes connected, use a flat file to smooth out the slot and make it straight. If the tang has sharp corners, sharpen the corners in the slot as well. At this point you should be able to fit the tang at least partway into the slot, and so begins the fitting process.
Fitting the Tang
The tang is likely not completely symmetrical, so make sure you know how the guard is orientated when you fit it onto the tang. Push the tang in, make mental note as to where the tang makes contact with the guard, and file away at those areas. This will take a while, and keep test fitting and comparing to see where you need to work more at. It is tedious, but if you don't fit it perfectly, you need to throw it away and start on a new piece.
Eventually you'll get very close to the shoulders. The last millimeter or so, I like to hammer in place; I stick the blade point first into a board of wood, fit the guard down the tang, then use something like a wrench, with the jaws around the tang. I tap lightly and hammer the guard further down, making a very snug and tight fit. Don't overdo it here; if light hammering doesn't get it in place, do a little more filing and try again.
Shaping the Guard
With the guard fit snugly on the tang, it's time to shape it. One of the easier mistakes is to make the guard crooked in relation to the blade, so mark a line continuing the guard slot. Now, on the actual shape it's up to you; whatever you drew in your design is what you should go for. There are plain bolsters that are flush with the handle, there are S-guards, there are flatter disc guards (what I would recommend for a first knife), there are simple guards with only a guard in the front and bolster shaped in the back, etc. Trace or scribe the shape on the guard.
Whatever your shape, use your powered grinder, if you have one, to get rid of the bulk material only, then use a file for the rest. Put in a vise and use clean strokes, being precise and sticking to the marks. This part isn't hard and it goes by quickly. Just make sure everything's symmetrical and straight and you're good to go.
A simple flat guard like this is easy yet pretty. Note how it tapers more in the front rather than the back
Like I mentioned, I like copper because it's easy and predictable to work, and turns out beautifully. So at this point, there's nothing wrong with some creative carving. It's quite easy to do, just make sure you control the file, keep it steady, and go slow; one inch of file length per second in places. This doesn't take much skill, and the results are astounding. Simple designs like scale or twisted rope patterns on the edge of the guard, or simpler yet bigger carvings on the front and back as in the case below. Simple is beautiful; don't try to make it too elaborate.
An S-guard by the author, showing some filework
Finishing the guard
Most guards will be flush with the handle. Thus, you'll possibly be doing some grinding on it later to bring it flush with the wood. However it's best to sand and finish the guard here, because doing the top and bottom are very hard once the knife is assembled. I start out at about 220 grit, sanding with my fingers and getting into any corners or carvings. Sanding doesn't take too long due to the size, and if you're using something like copper, the softness, but you do have to make sure you eliminate the scratches from the last grit, as detailed in the previous article.
When at as high a grit as you can get, about 1500-2000, polish it. There are several ways to get a polish, but all require polishing compound, which is very cheap and lasts you a long time (I got mine for five bucks a few years ago). A large buffing wheel, dremel with a polishing bit, or even scrap leather rubbed with the compound work well. Not all guards require a polish of course, and if you're unable to get one, generally matching the finish on the blade is a good strategy.
An alternative finish is something called Orange Peeling, something possible on softer (nonferric) metals. All you need for this is a wire brush wheel and an angle grinder (which by the way is the first power tool you should get as a bladesmith). Pass the guard over the spinning wheel a few times and the bristles grab and move the surface in a random pattern, looking like an orange peel texture.
Folder by the author with an orange peeled bolster
Remember, the design and finish are all up to you. Choose something that will fit the feel and flow of the blade, and have fun. Once you're done with fitting to the tang, it's more like jewelry making at this point. Which, by the way, is an excellent way to practice your skills.