Suppose I had a very sharp sword or some other kind of edged weapon; the limitation is it must be a simple edged weapon, no engine or machine, not attached to anything except my own hands. Could I swing it ay my own neck with sufficient force to decapitate myself?
(Don’t need answer fast.)
No. It would take quite a bit of practice to swing it in a way that would cut off comebody else’s head. Have you ever tried to cut the head off a chicken? Or even a drumstick from a broiled one? If your knife doesn’t hit the sweet spot right into the joint betrween the bones, it’s hard to get through.
You’re talking about swinging a heavy instrument with sufficient force against something very close to your shoulders, hard enough to penetrate with a single swipe a human neck bone, or score a lucky hit right between two bones, not even counting all the industrial-strength cartilage in there.
Here’s a test. Stand right up against a vertical support bean, and hit the beam with a baseball bat, neck high. See how much force you can generate.
Actually I’m going for technically possible, as long as you’re allowed to spin the weapon many times not just a single swing.
Imagine you have a heavy, semi-circular blade on a long rope. You spin your body round and round in a circle (like the hammer throw event) until the blade is moving with enough force to decapitate someone.
Now you’ve got to direct that force at your own neck; which is at the center of the circle. That’s the hard part. I guess if you have a nearby lampost with some skill you could wrap the rope around to do the job.
You might need a few practices to get it right and the first few times you might hilariously take your scalp off or open your chest up.
If that’s outside the terms of the OP, then I still reckon it’s physically possible but I’m not aware of an existing device that would let you generate momentum in this fashion then direct it at yourself.
This seems like one of those things where the Mythbusters would do one test and deem it impossible, but in all likelihood, under the right circumstances, it’s probably is actually possible. The right circumstances being some combination of freak coincidence and/or someone with a lot of strength. So, think, a body builder, swinging a really sharp sword around (not trying to kill himself) and it slips out of his sweaty hand and shifts positions and comes down towards him and hits him just right.
I have to assume, in all the time that humans have had edged weapons, someone, somewhere has managed to take their own head off. There’s been too many people with too many swords for it not to have happened, even under the constraints of the OP.
Another option would be to set the sword across something and fall on it. Let the weight of your body do the work. But that might be cheating.
You can build a guillotine:
It can probably be done, but it’s difficult to practice something like that. You’d probably give up after the first unsuccessful attempt, most likely because you’d die with your head still mostly attached.
How about making a very heavy blade, bench-pressing it above you, and then dropping it?
Or a blade with a long, extended handle on either end. You hold it up to your neck and run between two pillars that are just far enough apart for you to pass, but narrow enough for the extended parts of the handles to hit the pillar and stop. The momentum of your body does the rest.
So I’m thinking a large (running) bandsaw; if you threw your neck (and the rest of your body) towards it, your momentum would carry your neck right through the sawblade.
Plausible?
If the bench press does not count, then the blade on a rope can’t use the lamp post. If the soon to be dead guy has to stand and can only use what his two hands can lift and it has to actually cut off his head, then it might be done but would take someone with such ability, strength and determination that he most likely be one that would never be a suicide.
I vote:
Somewhere in the world at sometime that could have done it on purpose.
I also do not think this form o suicide has ever been done.
What if you hold an axe blade against your neck, then fall forward?
Maybe if you stand on a chair?
Maybe. There are historical cases where people may have done it.
Cutting Off One’s Head: Self-Decapitations of the Past
From the linked blog:
Also:
No more detail is given on who this determined butcher was and my Google searches are turning up naught.
Scarlett67 linked to this Wikipedia article in this thread (Could you chop off your own head? - 4/2010) where the Boston Beaneaters catcher (Beaneaters!) Marty Bergen came close to decapitating himself with a straight razor.
I swear I have read accounts of people doing it that were verified, but I can’t find any now…
I’m feeling a little Yoga Berraish on this thread: as in haven’t you asked this question before?
It definitely seems like déjà vu all over again to me.
Ah, here we go.
Ah, so this thread is like a decapitated quasi-zombie?
What about this guy? 
(Although whether the story can be verified remains to be seen)
How about a large, triangular shaped blade, with the width at the hilt wider than your neck? Such that you could drive the point of it under your throat, and with a lot of force (perhaps throwing your body weight against it while it was propped against the wall), force the blade into your neck. By the time your head reached the hilts, your head would be off?
Could you do it with a sharp-edged boomerang?
ETA: The consensus on Yahoo Answers is that you cannot (except for one dude who says it’s happened to him several times), but you could injure yourself quite severely.
Get your draw knife (Drawknife - Wikipedia) very very sharp. Hold it behind your head and slice quickly forward. This way you’re bringing your arms down in a more or less natural chopping motion. Also you’re hitting the bone first so you might have a tiny chance of making it through the hardest part before you collapse.
Or use a lightsabre.
Now that I read the older thread on this topic, I see someone else also suggested the draw knife method. I say it’s worth a shot.
To be fair to rickjay, first he asked if “you” (the reader) could chop off your own head if you wanted to. Now he’s asking if he could if he wanted to. Totally different question!