I make small films and we sometimes need to hurt and kill each other in a safe way for our productions (lol I couldn't find a believable prop for sale, or if it was real looking it costs a small fortune. Here's how I made my own blood squirting box cutter, you could make a similar rig for a knife or other weapon. The idea is you "draw" a line of blood on the actor with the prop acting like a fountain pen. Thanks to Nathan Rose, and AJ Reynolds for help and inspiration with this project.
Step 1: Get a Plastic Box Cutter
You can use a metal one, I just didn't want to struggle with cutting the metal. Plus the plastic ones are cheaper. Take the body apart to see what you got on the inside to work with.
Step 2: Take the Guts Out
I didn't want to mess with making this prop retractable, it'll be hard enough to make it work. If you want to film the blade coming out, get a real one and film an insert of the blade being slid out. Then when you need, switch to the prop rig, and voila! movie magic. Anyways, you wont need the spring or other junk, so rip them out.
Step 3: Make a Safe Blade
One problem with this model plastic box cutters is the blade only comes out a little to make it safer. So I just made the fake blade longer so it sticks out the right amount. I made the blade out of an old lid from some tupperware. Its stiff enough to stay rigid, yet flexible enough to ensure nobody will get hurt plus its the same thickness as a real razor blade. Next I covered it with alumnium tape used for HVAC work. You might glue aluminum foil on yours or simply paint it.
Step 4: Glue You Blade Into the Rig
I used a hot glue gun to secure my blade, you can use what ever works.
Step 5: Size Up Your Blood Squirter
I am using a "pipette" for my squirter. You can find them at hobby stores for 50 cents. I figure the rig should squirt somewhere in the middle of the blade. I taped it where I thought it would work best so I could draw an outline with a sharpie. Next disassemble the rig for cutting.
Step 6: Dremel and Hack Saw Your Void Into the Body
I had an old rotary blade on my dremel so it sort of burns into the plastic, which means the plastic sticks back together. Its easily seperated with a small hacksaw blade. You could use just a hacksaw, but a dremel speeds the process up and is less likely to break the body parts. If you do break it, you will be hot glueing the mess together so don't consider it a loss cause, just piece it back together with the glue. Keep your pipette handy to check the fit as you go. The burned pieces of plastic that clung to the body I cut off with the razor blade. On the thicker part of the body I used my wire clippers on to sever the "fins" holding the body together.
Step 7: Glue It Together
Pretty self explanatory, glue it back together. I didn't want the bulb of the pipette to be way down inside the handle and harder to operate, so I glued the piece I took off the handle down inside to keep the bulb above the surface. You might decide to take more time and recess the bulb inside the handle etc, I just needed this for one scene and didn't want to take the time.
Step 8: Test Out the Fit
Put your pipette into the rig to make sure it fits. Mine was snug enough to hold the pipette in place.
Step 9: Trim Your Pipette
I thought this might help direct the flow of the fake blood toward the victim. I cut the tip at angle and when I put it together I face the trimmed side towards the skin to be cut.
Step 10: Add Tabs to Hold the Pipette in Place
I want to be able to remove the pipette for cleaning after each use, so I loosened the hole by cutting away more material. Since it now wouldn't hold the pipette very accurately I took some pieces of the body and glued them over the opening in two places. This worked really well, I would recommend it.
Step 11: Fill Up the Pipette, Install Into the Rig
That's pretty much it, I used a hypodermic needle to fill the pipette, then I found the pipette has enough suction to draw the fake blood so I nixed the needle. All I had was some really old karo syrup with red food dye, sorry it looks so fake, lol. Also, I had a hard time taking a pic while holding my phone so the pic is at an awkward angle. I found from playing around with it that if the surface you are cutting is at a more severe angle (vertical) the blood flow downward and looks a lot more realistic. People freak out when I show them, even if they know its fake, so Im pretty sure it'll look good on camera. Hope this helps you make something, Please post pics in the comments of this instructable of anything you make based on/ or inspired by my design. Thanks!