Optimal direction to cut a blade out of a sheet (or bar). Grain direction question.

Nathan the Machinist

Nathan the Machinist

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Steel that has been rolled develops a grain direction. Kinda like wood, though not really. Perhaps more like a sheet of paper, which is easier to tear lengthwise than width.

I came across this years ago as a hack machinist when I repaired a lever operated valve with a piece of steel and made the shaft across the grain. Broke within the day. The replacement, made with the grain, was still in production years later. Grain direction probably had a lot to do with this.

So anyway, I'm wondering what is the best way to orient the grain in a blade? I have always run the grain in the direction of the blade. If I imagine that blade in wood, with the grain running it's length, the blade and tip would be strong, but the edge would be easy to fold and break.

So what I'm wondering is, in a hard small blade not subject to rough use, but instead taken to a fine thin edge, would it be better to run the grain across the blade so it is perpendicular to the edge?

Does this question make any sense?

I know the heat treat probably makes this question moot in most steels because the martensite grains form after HT, so the question is really more about high carbide steels.

So, what effect does direction play in a non isotropic steel?
 
Food for thought.

If the bar-stock you are using, has been "PROPERLY" normalized, say three times, most traces for grain elongation induced by rolling should be erased. For most steels which normalization is not recommended, several heat/quench cycles will perform the same grain refinement. This being said, I would personally try to keep the grain running lengthwise to the blade. One other consideration, I always cut or forge my blades with the butt toward the end of the bar stock. The chances of trash in the bar are greater toward the ends so I keep the tips pointed the other way.

Hope this helps.

Jim Arbuckle
 
Anisotropy doesn't always change when normalizing .It depends on the alloy and how clean it is. Take a look transverse impact strength of CPM steels versus non-CPM steels ! Run the grain the length of the blade .Bending limits for sheet metal are often different depending on direction. Tool and die work is done annealed then it's hardened .It's important to know the dimensional changes in the three directions. One exception is A-10 which doesn't change in any direction !!
 
I have always cut blanks with the grain running along the blade length. Some time back though I remember speaking with someone (can't recall who) that suggested a cross grain leads to a 'toothier' edge. We were discussing rope cutting and he felt it lasted way longer with cross grain.

No idea if it works.

Rob!
 
Rob! said:
I have always cut blanks with the grain running along the blade length. Some time back though I remember speaking with someone (can't recall who) that suggested a cross grain leads to a 'toothier' edge. We were discussing rope cutting and he felt it lasted way longer with cross grain.

No idea if it works.

Rob!
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That kind of makes sense, I don't suppose anybody does it this way?
 
Well now. I did actually make a lot of straight knives with grain 90 degress to the edge. These were in Ats-34 and 440-C All were hardened in a vacuum chamber with the freeze treating process professionally done. The blades that had the grain running 90 degrees or close to that to the edge all had an edge holding ability of close to 20% more than those where the grain ran parallel to the edge. I never heard of there being any blades broken with this configuration. I had picked up this fact on the grain direction from a comment from Corbette Sigman who made a huge bunch of hunters this way. Frank
 
So would this be true

Maximum toughness and finest edge run the grain with the edge.

Maximum slicing ability and edge holding ability run the grain perpendicular to the edge.
 
great question! im right there with you, iv been wondering about if it becomes a moot point after ht. one of the first things i was taught about forging a blade was to hot cut the point on a knife so it doesnt bunch up the grain in the end weakening it. these are all interesting points, im glad its come up.

is there a quick answer about what CPM steel is? i havent heard that before.

im tempted to make a few test blades to get a non scientific feel for the difference.

-Lou
 
Keith_H said:
I always cut it length wise because thats how the barstock is.
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Yeah, but...

I just got three sticks of D2 at 1" wide for some 7 1/2" total length skinning knives. I'll get 12 blanks and 3 good sized drops.

I could have got one 8" short stick and sawed out 17 blanks going across the grain and got no drops.

Being a skinning knife, it may have benefited from the cross grain. I just never thought about it.
 
Thats a most excellent question Nathan. I would previously have just automatically said longitudinal grain, but your point about rolling edge is a very good one.

Assuming a worst case scenario that there was still a large amount of elongated grain structure after all Heat treats, and using your analogy of woodgrain:

If longitudinal is ideal for blade integrity but not ideal for edge; perpendicular is supposedly ideal for edge integrity but not ideal for the blade strength... Somewhere in between there must be an ideal angle which gives you a good balance of benefits. Therefore if I was making a knife out of wood (or if I was concerned about elongated grains in a knife), I would opt to have the grain direction at about 30* to 45* to the length of the blade, and angled downward as you move from the handle end to the point of the blade.

I mostly would not really worry about this.

Lang
 
langchop said:
If longitudinal is ideal for blade integrity but not ideal for edge; perpendicular is supposedly ideal for edge integrity but not ideal for the blade strength... Somewhere in between there must be an ideal angle which gives you a good balance of benefits. Therefore if I was making a knife out of wood (or if I was concerned about elongated grains in a knife), I would opt to have the grain direction at about 30* to 45* to the length of the blade, and angled downward as you move from the handle end to the point of the blade.

I mostly would not really worry about this.

Lang
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I was thinking in kinda the same direction. If you angle it a bit you gain a few things:

The ability to bias the grain to be perpendicular with the belly of the knife and

The ability to cut an 8" knife out of a 6" wide stick.




There is a third option. Cutting the blade out of them end of a bar. You could profile the blade shape into the end of a bar and cut blades off like slicing a loaf of bread. I wonder if that would be a good thing or a bad thing. Using the piece of wood analogy it would be bad. But it isn't really like wood. The martensite shouldn't have any directionality, just the carbides. So...


knife_bread.jpg
 
Nathan the Machinist said:
The ability to cut an 8" knife out of a 6" wide stick.

There is a third option. Cutting the blade out of them end of a bar. You could profile the blade shape into the end of a bar and cut blades off like slicing a loaf of bread. I wonder if that would be a good thing or a bad thing. Using the piece of wood analogy it would be bad. But it isn't really like wood. The martensite shouldn't have any directionality, just the carbides. So...
Click to expand...

Valid points. But like we all know... The wood analogy really is overkill, and most grain elongation effects will be eradicated in HT and arent really worth worrying about. Fortunately its not really practical in most instances to 'slice' blade blanks, because I dont think this would offer any advantages anyway.

I have to ask tho Nathan... What is that pic? barstock machined by CNC ready for hacksawing into blanks?

Lang
 
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