Forging and the physics of plasticity

Tai Goo

Tai Goo

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Here’s something that seems hard to find good applicable information on.
http://en.wikipedia.org/wiki/Plasticity_(physics)

If we take into account the geometry of the metal, the effects of heat, the amount of force applied, the placement of the force, chilling effect of the anvil or die, the shape or geometry of the hammer or die face, etc… it gets really complicated!

Is forging strictly intuitive or are there some helpful rules and guidelines that it will always follow?

Strictly based on my own experience, I can come up with a few basic principals, but still desire to know more about it.

Check me on this…

#1. For every action there is a reaction.
For example, when you apply force to the surface of a flat bar of metal it becomes thinner, AND wider. Whenever possible use both the action and reaction to your advantage.

#2. The path of least resistance.
The flow of the metal always follows the path of least resistance. The path of least resistance can be manipulated or altered, by heat placement, and to some degree by the geometry of the hammer face or applied force.

#3. The effects of the amount of force applied.
In other words, with a light blow the force may not penetrate the metal clean through, and mushrooming on the side of the hammer blow or force will result. With a medium blow or “efficient” applied force the metal does not mushroom but reacts the same clean through. With a heavy or hard blow, the metal swells in the middle, due to reverberation of the applied force. Not that any of these are right or wrong, but each can be used in certain instances to our advantage.

#4. Centered and symmetrical force and mass.
If the metal is centered or symmetrical the applied force can travel through the center and effect both sides equally or symmetrically, if the blow is efficient. If the metal form is asymmetrical, the flow will follow the path of least resistance.
 
Thought provoking as always Tai.... what music were you listening to when this came to you? :D

Will
formerly known as badbamaump
 
There's probably a lot we could say about the size and shape of the "dent". For one thing, the metal displaced is equal in relationship to the size of the dent. Maybe this too,...the shape and placement of the dent has an effect on the path of least resistance.

The dent is opposite to the applied force. :D LOL
 
I've got a buddy that's a physicist... it's not like I normally hang around them but he likes to drink beer in the same dank, dark hole in the walls I frequent. I'll ask him about it, it'll tick him off.

Tai, perhaps using some clay/playdoh or some other flexible medium we could see better the effects of our hammer strikes on the medium. Perhaps rigging some sort of "swing o meter" to ensure the repeatability of the hammer swing force and use a pieces of elastic medium in varying thicknesses to determine the effect of the strike? Use 2 sets of the material, one set being struck on the hard surface like a normal metal on anvil and another set that has 2 layers of contrasting colors so we could see the effect of the strike on the target material and it would allow the study of the "through" effect of the strike.

I'm just babbling because it's an interesting topic and I'm not educated enough to write some purty equations on the subject.

Will
formerly known as badbamaump
 
Hell, I just hammer it 'till it looks like a knife.

Then I take it to the grinder and remove everything else that doesn't look like a knife.

Viola! a knife. (or KSO in some cases)

Very interesting though. I have tecniques that follow that logic, (like using the anvil to help hammer in the opposite bevel) so there is truth to what you say.
 
Tai Goo said:
There's probably a lot we could say about the size and shape of the "dent". For one thing, the metal displaced is equal and in relationship to the size of the dent. Maybe this too,...the shape and placement of the dent has an effect on the path of least resistance.

The dent is opposite to the applied force. :D LOL
Click to expand...

Now you gone and done it. You've hit on the principle of denting of materials. For every reaction, there is an equal and opposite reaction. Because I deal with damaged bearings everyday as part of my occupation, I need a strong understanding of dent formation.

Let's take a look at dent formation from a couple of different views.
  1. The dent made by a meteor when it strikes a planetary body will create a hole with a high raised rim around it.
  2. A dent from throwing a baseball down into soft sand on a beach will create a dent with a high raised rim of sand around it.
  3. A piece of debris in a ball bearing when run over by the ball will create a dent with a high raised rim.
  4. When you watch a slow motion video of an object dropped into water, the first thing that happens is that the water vacates the area of the object (a dent in the water surface) and the water initially raises up around the dent in the water to create a momentary high raised rim before splashing back down.

Every force that is applied to a solid or liquid that is great enough to create a dent, will raise a high raised rim. What does that mean to a bladesmith?

In my opinion, it means that every blow of the hammer that we make creates a higher spot around the hammer dent that we must deal with. As we use softer and softer hammer blows to finally tune the shape of our blade, we are not really getting rid of the dents, but we are getting rid of the high spots.

The better job we do of limiting the amount and size of those high points, the less work we will have to do at the grinder to clean them up.

I'm done rambling on now. I don't know if this means much, but maybe it will provoke some though as to what our hammer is actually doing. Each hammer blow that is perfectly square to our material will not only push material down and to all sides, but it even raises metal up around the hammer blow.

I find that intriguing to think about!
 
scott if you send me your damaged bearings ill forge down the rings around the dents for you. just make shure there bearings are big so that it could make a knife wate i mean get rid of the rings. :D
 
If you ever make the drive through the Portland area, feel free to take a 75 pound ring home with you! I have more steel than I'll probably ever use in a lifetime.
 
He isn't joking, I was over to his house for food and beer amd knife talk after Blade West and he gave me a bearing race of 52100 that is huge. I will have to cut some out soon and hammer out a SIT knife (Scott Ickies Timkin)
 
I like the clay idea.

Here's a little exercise we call all try:
Take a piece of clay, pinch it, squeeze it, twist it, poke it, pound on it and smear it around for 5 minutes. Then, sit back and try to wrap our noodles around everything that just happened. :D
 
scottickes said:
Now you gone and done it. You've hit on the principle of denting of materials. For every reaction, there is an equal and opposite reaction. Because I deal with damaged bearings everyday as part of my occupation, I need a strong understanding of dent formation.

Let's take a look at dent formation from a couple of different views.
  1. The dent made by a meteor when it strikes a planetary body will create a hole with a high raised rim around it.
  2. A dent from throwing a baseball down into soft sand on a beach will create a dent with a high raised rim of sand around it.
  3. A piece of debris in a ball bearing when run over by the ball will create a dent with a high raised rim.
  4. When you watch a slow motion video of an object dropped into water, the first thing that happens is that the water vacates the area of the object (a dent in the water surface) and the water initially raises up around the dent in the water to create a momentary high raised rim before splashing back down.

Every force that is applied to a solid or liquid that is great enough to create a dent, will raise a high raised rim. What does that mean to a bladesmith?

In my opinion, it means that every blow of the hammer that we make creates a higher spot around the hammer dent that we must deal with. As we use softer and softer hammer blows to finally tune the shape of our blade, we are not really getting rid of the dents, but we are getting rid of the high spots.

The better job we do of limiting the amount and size of those high points, the less work we will have to do at the grinder to clean them up.

I'm done rambling on now. I don't know if this means much, but maybe it will provoke some though as to what our hammer is actually doing. Each hammer blow that is perfectly square to our material will not only push material down and to all sides, but it even raises metal up around the hammer blow.

I find that intriguing to think about!
Click to expand...

What is the term and causation of the raised rim? Does it have anything to do with reverberation or rebound?
 
The rim is just diplaced material , it has to go somewhere. In miniature look at the indentation from a hardness test. How much rim you get depends on the plasticity and the dimensions and the dimensions of the hammer.
 
mete said:
The rim is just diplaced material , it has to go somewhere. In miniature look at the indentation from a hardness test. How much rim you get depends on the plasticity and the dimensions and the dimensions of the hammer.
Click to expand...

O.K., but why does it go up instead of out? Does this have something to do with the resistance of the surrounding material, and reverberation of force within the body or mass?
 
One thing that has always fascinated me about plasticity, is that "plastic" mediums have to capability of being "smeared". Particles of the material actually "slide" and shift positions within the whole, without letting go. Isn't this what we are seeing in burr formation and when we burnish the surface of the metal?
 
When you stomp on a cow pie, it goes out and all over, basic forging technique. If you kick said cow pie it goes mostly in the direction of your kick. Like using a cross pein, in whatever direction you use it the metal moves to either side, it's nothing more complicated than basic forging technique, no need to complicate it more than it is:confused:.
 
Isn't this raised rim just following the path of least resistance?

In most cases it seems like the raised rim reaction would not be working to our advantage, so couldn't we reduce or minimize the raised rim by lowering the resistance of the surrounding metal by using heat, and reducing the force of the hammer blow?

I know this topic of plasticity seems kind of funny, which is probably why folks don’t talk about it much,... but isn’t forging basically taking advantage of the plasticity of metals? So, to understand the principals of forging we need to have a basic understanding of plasticity. Does that make sense?
 
Sam Salvati said:
When you stomp on a cow pie, it goes out and all over, basic forging technique. If you kick said cow pie it goes mostly in the direction of your kick. Like using a cross pein, in whatever direction you use it the metal moves to either side, it's nothing more complicated than basic forging technique, no need to complicate it more than it is:confused:.
Click to expand...

... the angle or direction of the applied force and the effect it has on the path of least resistance. Good point actually. :)

... but there's more to it than that.
 
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