WARNING: Knife training is an inherently dangerous activity. Take these suggestions at your own risk. Mongrel Combative Arts disclaims any liability resulting from injuries sustained while training using these ideas.
Originally written by Guro William and posted on Defend.net in 2000-2001. Printed in the Martial Artist on-line magazine with permission of Guro Schultz.
As with training knives, I needed some choices -- in this case, as to the amount of head protection I used for myself and my students. Some students are unwilling to make the leap to fencing masks right off the bat. By making these modified helmets, I can start them off with something heavy-duty and then gradually wean the students to the lighter helmets (after they have developed some ability and courage). I also wanted to use the helmets for blade sparing, so I had to come up with a way to close up the gaps in the cages to prevent thrusts or slashes to the face from getting through. Basically I wanted a helmet that offered a little more protection and better visibility then a fencing mask, yet still allowed good shots to the head to be felt (so you know you did something wrong). The helmet will protect your head from heavy hits, but the reverberation from the hit can be disorienting. This helps prevent the student from developing the bad habit of taking shots to the head to get into corto range. With heavier helmets you may not even realize you have been hit.
I ended up using a standard Joffa hockey helmet with a full face goalie attachment that offers a little neck protection. (If you keep your chin down like you should, it actually does a great job.) To offer protection from stick thrusts and knives during blade fighting, I removed the cage and took some 1/4" steel mesh, cut it slightly larger than the cage shape, and then formed it to the cage using heavy duty black zip ties (white is too distracting when looking through the cage).
To make this modification, start at the center and work your way out, attaching zip ties at every juncture. Turn the zip lock to the inside of the cage and cut off the excess with wire cutters as close to the zip lock as possible. When you get to the edge of the cage, cut off any excess mesh so you are left with about 1/2" off mesh protruding from the edge all around.
Roll around the edge to the inside and fasten it with the ties. Cut off any protruding wire. The mesh should be VERY tight to the cage. Rub your hand along the outside and make sure there are no sharp edges. Like a fencing helmet, it will have a bit of a "cheese grater" effect if your opponent tries to punch or elbow to the face using exposed skin. It's a good idea to wear light gloves and elbow pads. I've been thinking that one of these days I might take the mask off and dip it in "Tool Dip" to give it a rubberized coating. The helmets I modified have held up great and so far I have only had to replace a couple of zip ties that took a number of hard hits. Two of the helmets shown here are obviously motorcycle helmets. They both have slightly different shapes. The key here is finding a cage that is as close as possible to the shape of the helmet face. I took both helmets to a hockey shop that sold used gear and held all the cages they had to the helmets to find the closest fit.
On the white motorcycle helmet, the cage is actually upside down compared to its original placement on a hockey helmet. I had to trim a little off both cages to get them to fit adequately. Now, here's where things get a little fuzzy. I don't know the proper terms for some of these parts but I will do my best to describe them:
After you have cut the cages to fit, apply the steel mesh as described above. Next, you will want to determine where to fasten the cage to the helmet. I put two fasteners on the top, bottom, and both sides. The fasteners as seen on the white motorcycle helmet are white and "d" shaped. The fastener opens so that you can slide the edge of the cage down to the inside of the "d". There is a mounting hole at the top of the cage.
Mark where the holes are and drill about a 1/4" hole, no more than 1/2" deep. Now, I'm going to get technical on you here. A screw will not hold if you try to fasten it directly to the helmet (it's just Styrofoam in there). The anchors that I used look like rubber plugs with a screw head on one end and a nut on the other. Take the screw out and thread it through the cage fastener ("d"). Thread it back into the rubber plug and then insert the plugs into the mounting holes you drilled into the helmet. As you tighten the screw, it pulls the nut up causing the rubber plug to expand inside of the hole. These hold surprisingly well. There you go. The helmets are not the prettiest things out there, but they have taken a lot of abuse and are still going strong.
As with training knives, I needed some choices -- in this case, as to the amount of head protection I used for myself and my students. Some students are unwilling to make the leap to fencing masks right off the bat. By making these modified helmets, I can start them off with something heavy-duty and then gradually wean the students to the lighter helmets (after they have developed some ability and courage). I also wanted to use the helmets for blade sparing, so I had to come up with a way to close up the gaps in the cages to prevent thrusts or slashes to the face from getting through. 
I ended up using a standard Joffa hockey helmet with a full face goalie attachment that offers a little neck protection. (If you keep your chin down like you should, it actually does a great job.) To offer protection from stick thrusts and knives during blade fighting, I removed the cage and took some 1/4" steel mesh, cut it slightly larger than the cage shape, and then formed it to the cage using heavy duty black zip ties (white is too distracting when looking through the cage). 
Roll around the edge to the inside and fasten it with the ties. Cut off any protruding wire. The mesh should be VERY tight to the cage. Rub your hand along the outside and make sure there are no sharp edges. Like a fencing helmet, it will have a bit of a "cheese grater" effect if your opponent tries to punch or elbow to the face using exposed skin. It's a good idea to wear light gloves and elbow pads. I've been thinking that one of these days I might take the mask off and dip it in "Tool Dip" to give it a rubberized coating. The helmets I modified have held up great and so far I have only had to replace a couple of zip ties that took a number of hard hits. 
On the white motorcycle helmet, the cage is actually upside down compared to its original placement on a hockey helmet. I had to trim a little off both cages to get them to fit adequately. Now, here's where things get a little fuzzy. I don't know the proper terms for some of these parts but I will do my best to describe them: 
Mark where the holes are and drill about a 1/4" hole, no more than 1/2" deep. Now, I'm going to get technical on you here. A screw will not hold if you try to fasten it directly to the helmet (it's just Styrofoam in there). The anchors that I used look like rubber plugs with a screw head on one end and a nut on the other. Take the screw out and thread it through the cage fastener ("d"). Thread it back into the rubber plug and then insert the plugs into the mounting holes you drilled into the helmet. As you tighten the screw, it pulls the nut up causing the rubber plug to expand inside of the hole. These hold surprisingly well.