My first boffer
This is how to make the simplest possible boffer without it being dangerous. It is made with one or two layers of Pipe Insulation covering its full circumference and an open-cell foam tip. It can be constructed completely from materials available at a Home Depot. PVC is tough, cheap, relatively easy to cut, really easy to find, and has a certain liking for duct tape, so this tutorial will assume you're using a PVC core.
What Weapon Class Is Best For You?
I made a personality quiz for my boffer group to help any new (or old but bored) members find a weapon class that best suits them. I invite anyone on here to take it as well, as it is decently accurate.
What do you want?
First, decide how long the weapon will be, including the length of the handle. My rule of thumb is to use 1/2-inch pipe for one-handed weapons and 3/4" pipe for two-handed weapons, while pole arms get 1" pipe. Double-ended two-handed weapons such as staves are usually best off with 3/4" pipe. Extra-wide pipe will come out a bit heavy and too stiff, which is painful. Pipe that's too thin may flop and whip around, which can hurt a lot too, and is unrealistic.
I'd suggest a core somewhere around the length of your leg or a bit more to make a longish one-handed sword. For a two-handed weapon, any length measuring from the floor to between the bottom of your ribcage and the top of your head will do, but any core much over four feet starts to get painful and may need some difficult padding. Different lengths in that range produce different play styles. A one-handed sword or a bastard sword (two-handed grip but just small enough to be used with one hand too) are good projects to start with.
Cap your core
After the core is cut, it is a good idea to cap its ends. Here are somethings that can do the job: Purpose-made caps for pipe and the like Coins Milk jug caps (may want some trimming but they work great for 3/4" pipe) Gatorade caps (fits on 1" pipe) Gorilla Tape, or other tapes, but stiffness is important
Anything that doesn't stay on its own can be held with duct tape or high-quality glues like Dap or Gorilla Glue. End caps rarely make a difference in the safety of a boffer because something else has to go very wrong for them to be exposed, but when they do make a difference, it is often the difference between a bruise and blood.
Padding your blade
With the core all set, it's time to start padding. Slip an appropriately-sized length of foam pipe insulation over the section of the core you want to be the blade. It should be noted that PVC pipe's width is usually measured by its inner diameter, and so is foam pipe insulation. If you use a hollow core, take care to buy insulation meant to go around the outer diameter of your core, not the inner diameter. Most PVC pipe's outer diameter is 1/8" wider than its inner diameter, so 3/4" pipe would need insulation with an inner diameter of 7/8". It is generally considered to be a good idea to secure this first layer of foam to the core with duct tape, although it isn't completely necessary.
Alternately, you can take a length of pool noodle and slide it over the core and attach. Most pool noodles have a hole running through them that fits 1/2" pipe, while some larger ones have a hole that fits 3/4" pipe. A pool noodle is an easy single-layer solution to padding, although there is some disagreement as to whether the foam survives impacts as well as the pipe insulation does.
Do you want a second layer? Probably.
With the first layer of foam on, a few people may consider it adequately padded, especially if the foam's wall thickness is 1/2" or greater and/or the core's width is 1/2" or less. Since the most common wall thickness for foam pipe insulation is 3/8", one layer usually isn't enough, unless you used a pool noodle. Personally, I would refuse to fight someone with less than 5/8" of foam on his boffer unless it was a tiny little thing. Too many bashed knuckles and groins. -Frank
Adding a second layer
The second layer of foam is often made from two lengths of the same-size insulation slit open and used together to create a larger tube. Alternately, there is nothing wrong with finding the total diameter of the core and first layer together and getting another size of foam that will fit smoothly over them. When using the two-piece method, it is usually necessary to shave some off a bit from the edges of each piece to get the right circumference. Fit them together over the first layer, and you're ready to start taping!
When the thickness of the foam is satisfactory, secure and cover the length of the blade with duct tape. Strips going up and down the full length of the blade are usually smoother, less painful, and stronger than a spiral wrap. Have around two inches extra at the base of the blade and use that extra length to attach it to the core at the handle. Assuming a 3/4" pipe with two layers of 3/8" wall thickness foam, covering can be done with three long strips going up one side and down the other. Doing that also covers the tip of the core nicely, but it can be hard to get the placement right if you're new to duct tape. If you don't want to shoot for the three-strip method, just take strips that go the full length of the blade (and then two inches down the handle) but not over the other side and lay them onto it until you've covered the whole circumference. Remember that a little overlap in the strips won't make it much heavier. As you go, stick those extra ends at the base of the blade onto the handle. I've reiterated this three times, because if your blade is not secure it could fall off in the middle of combat. Once that's done, make sure you have some tape capping over the end. It would be a bad thing if the core could push out.
To make the weapon safe for stabbing and greatly reduce the pain from strikes with the very tip, get a piece of open-cell foam about two to three inches long and as wide as the boffer, and attach it to the end of the blade. Cover with tape and then poke some holes in the tape enclosing it. If the air in the foam has nowhere to go when it is compressed, it may rupture the tape or defeat the purpose of having something very squishy at the end. Do take care not to compress the foam much when wrapping it. The only time a boffer has broken my skin was a hard swing down my face with a thrusting tip that was way too densely packed. -Frank
If you really don't plan on jabbing with your weapon, just make sure you have SOMETHING soft over the tip of the core. But even so, a thrusting tip is a nice safety touch and adds two to three inches to the blade that weigh nearly nothing.
Now your blade is good to go! But not the rest of the boffer. Build a pommel to your liking or your group's specifications. You can use a lot of the same techniques you used in making the blade. Most groups disallow pommel strikes but still require a pommel that can't fit into an eye socket. Whatever you do, make sure that you have some foam covering over the end of the core! Accidentally or intentionally hitting with an unprotected core tip is a great way to hurt your friends and convince authorities that boffing is dangerous.
Now both ends are suitable for combat. To give it a cleaner look, save your knuckles a little grief, and help secure the blade and pommel, wrap your handle. Duct tape and electrical tape work, and so does almost any sport grip tape. Hockey tape has great grip but may leave your hands feeling sticky. Spiral wraps or any technique that makes the grip less smooth will improve the traction your hands can get. If you feel like messing around with glue, a wide variety of craft materials are open for use too. Remember to wrap tightly over any tape ends used to secure your blade or pommel. Don't want them coming loose.