Best Medium/Method for testing impact force

T

the possum

Joined
Jul 31, 2002
Messages
2,959
Hey there, fellas.
My thoughts have once again drifted toward modifying one of my knives for better chopping power. I mostly plan to do this by changing the weight distribution, but while I'm at it I will probably also change the edge geometry.

Since the edge geometry will add another variable, what have you guys found as a good way to measure impact force from chopping independent of cutting ability? I'm afraid that if I just try chopping through various materials, I will see as much difference from the better edge geometry as from the better balance.

My ideas have run the gamut from chopping into a pile of sand and measuring the depth it buries itself, all the way to whacking a scale or rigging up some kind of telescoping tubing that would dissipate the energy...

If you happen to know something simple and effective, it would be nice to really know how much difference I've made, instead of just saying, "Yeah, it works better now."
 
The easiest way to measure this roughly (without specialized equipment) is to use a known weight. The energy of a weight dropped from a given distance is just the distance times the weight.

First take your bowie and using the spine drive a set of 3.5" nails into a medium. Use a lot, say 12 hits and measure the distance penetrated, longer nails if you drive them all the way in, use the average result.

Now take a steel weight and figure how from what height you need to drop it to achieve the same penetration, do it a few times again and average the result. This then tells you the emergy of the bowie impacts.

You can of course then check the impact energy along the length of the blade as well as the effect of swings, how much energy you gain from wrist to elbow to shoulder all the way up to hips/back.

You can then calculate energy efficiency ratios by measuring say the penetration and calculating a penetration/energy ratio and compare various blades.

-Cliff
 
Thanks for chiming in.
I hadn't even thought of that. I'll see if I can come up with some way to apply this idea.... I was actually thinking about my khukri in this case. I don't know if the spine shape will lend itself to driving nails. (my bowie is double edged, so it would likewise have some trouble with this experiment) I'll have to figure out somethin' else similar. I don't think the blade flats would work the same since the sectional density (and curved blade shape)
would be so different, but it might be worth a shot.

I brought it out to the shop and got my work area set up to do some grinding, but then I realized I didn't have an adequate clamp to hold it to the bench... I'll have to borrow one from the farm. Will report back when I've got more info, but don't anyone hold yer breath. It took me several months to get this far. :)
 
With fairly simple blade shapes the spine and edge should give similar results, but with dropped profiles it is no go, you have to actually use the edge as an impact surface. The same general idea holds though just use weight drops as reference points. Ideally of course you just reference the work against another blade with a similar cross section and thus the penetration difference is just due to the impact energy, but that requires making a lot of mules. If you have access to decent digital equipment, you can also just calculate the moment of inertia and from this and some software calculate the impact kinetic energy. You can also get some rough estimates from just having someone time a number of swings and estimating the linear and rotational speed and combine this with the two inertia masses.

-Cliff
 
Cliff Stamp said:
If you have access to decent digital equipment, you can also just calculate the moment of inertia and from this and some software calculate the impact kinetic energy.
Click to expand...

I am planning to do some regular old chopping anyway just to get a more qualitative comparison. But I am interested in learning exactly how to calculate moment of inertia, since this is one of the main things I plan to change. (or perhaps it would better be described by polar moment??) Any way it can be calculated by making up a spreadsheet type program?

If it takes too much effort, I may scrap the idea of measuring entirely and just go with the qualitative judgements. I'm finally getting motivated to do some grinding, and I don't want to waste the time I could use to actually do the modifications.
 
Calculating the moment of intertia is nontrivial for extended bodies, however measuring it is trivial. Attach a piece of rope to the piviot point, set the knife oscillating as a pendulum. Let it oscillate a large number of times and calculate the average time per one oscillation, the moment of inertia then is :

I=(T/(2*pi))^2*mgd

T is the average time for one complete oscillation (seconds)
m is the mass of the object (kg)
g is 9.81 m/s^2
d is the distance from the center of mass to the pivot point (meters)

This gives the SI unit of moment of inertia which is useful if you want to do energy calculations and so forth, however if you just want a number to use for ranking, then you can leave out all the constants, and it doesn't matter how you measure distance and time and mass, just do it the same each time.

I have been meaning to do this for the larger knives which are used for heavy swings for some time. You can of course do it for various grip positions and show numerically the change it makes in "heft". It would be interesting to note you can find knives which have higher moments of inertia but actually feel lighter when you just hold them dead in hand. This as you noted there is a big difference between static and dynamic balance.

-Cliff
 
Thanks for the formula. I made up something in Excel so I can just plug in the weight in ounces and measurements in inches, and came up with some rough Moments of inertia. I was just guessing on the oscillation time from memory, so maybe when I get home I can actually measure them, as I think something ain't jiving right. The nice thing about the pendelum is that once you've established what string length is needed to make the pendelum swing in time with the blade, you can just work with the pendelum to time oscillation period.

Assuming I can actually calculate an accurate moment of inertia, how do you go from there to get impact energy? Is it complicated? :o Math has always been my worst subject...

You can of course do it for various grip positions and show numerically the change it makes in "heft".
Click to expand...

This is something I've been thinking about for a while now. I'm interested in seeing how different grip positions affect the optimal striking point (distal rotational node) and also how that relates to chopping power. I started thinking about this a while back when I saw a picture of Reggie Barker in action in Blade magazine. I noticed during the 2x4 chop, he seemed to have his hand further back on the handle, gripping the butt. Anyone who has done some chopping will usually notice you can get more overall power by gripping further back like this.

However, by just playing around doing some "waggle testing" with my bowie, things are interesting. Gripped normally close to the guard, (assuming a rotational point just behind the guard, for simplicity) the distal rotational node (and therefore optimal striking point) is right at the tip. But if I hold onto the pommel, so that the rotational node is now 2 1/2 inches further back, then the optimal striking point shifts nearly nine inches back from the tip. To me, this suggests that even if we can gain some total chopping power by gripping the pommel, our chopping efficiency is decreasing dramatically. This seems to be accurately predicted using the "area of the rectangle" method, measuring distances between the rotational and impact points from the center of mass. (just using some rough measurements from the waggle test, the areas were pretty close.)

Now, think about if the knife were already balanced so that the optimal striking point was a third back from the tip. (as it appears to be on many such knives) By gripping further back, the optimal striking point probably isn't even halfway down the blade, so we're giving up the speed we could have had with a tip strike.

This is probably something that most folks wouldn't worry about. But to a serious cutting competition participant, I bet it could make a huge difference if the knife were balanced to accommodate gripping the pommel.

Then again, I could be wrong... :)
 
Energy from inertia is trival, it is just proportional to inertia unless the inertia is so high that it actually slows you down. It takes a silly amount of weight for this to happen, if it does though then the energy falls off massively as it is

1/2 I w^2

where w is the rotational velocity, basically how fast the blade rotates. This energy is combined with

1/2 m v^2

where v is how fast you move the blade in a straight line. Depending on how you chop you can have almost no rotational effect, or it can be the majority of the impact energy if you use a lot of wrist motion.

There are other complications of course such as you have to consider the role of momentum, even though a heavy knife might be so slow that the impact energy is reduced the momentum can be so high it would be critical. Consider for example striking someone with a heavy hammer on a shield.

There was an interesting discussion on this on rec.knives a few months ago by guys who actually do it.

In regards to balance and impacts yes, the problem with rear grips is that while you get lots of power on the swing, the impact point isn't where it should be and thus if you do chopping comparisons, in the long term you start moving back and thus the overall penetration is similar but the fatigue is higher because you are swinging more weight.

It is mainly off due to the common belief that proper balance is neutral, or on the index finger which is horribly flawed for many reasons on a lot of knives which you outlined awhile ago in email. There is more to balance that how a knife sits static, and optimal points are very dependent on user strength though generalities hold in regards to positions of impact.

I plan to talk about this in more detail with a section on balance in the lareg knives, I'd like to see more from you as you get the time.

-Cliff
 
Cliff Stamp said:
Energy from inertia is trival, it is just proportional to inertia unless the inertia is so high that it actually slows you down. It takes a silly amount of weight for this to happen, if it does though then the energy falls off massively...
Click to expand...

Well, I can't maneuver my khukri nearly as fast or easily as my bowie, so I think that at least in regards to my own strength, I've crossed the threshold where it's really starting to slow me down. Thus I can't get up to maximum speed as easily, and upon impact the blade does not transfer its maximum potential power because the rotational centers are off.

Cliff Stamp said:
where w is the rotational velocity, basically how fast the blade rotates....
where v is how fast you move the blade in a straight line.

There are other complications of course....
Click to expand...

Slow down, man! I don't know how I could measure the blade's velocity in any fashion, much less deal with even more complications! :D
It's starting to sound like it would be easier to just calculate the moment of inertia for a benchmark number, and then do qualitative comparisons from there. I dunno yet.

Will keep you all informed...
 
Yes I noticed the same thing with my larger khukuris, the 22" Ang Khola, that is a massive blade. The velocities are just inversely proportional to the time of a swing, or easier to calculate the number of times you can swing a knife in a given time since you are traveling through a known distance each time.

The hard part is figuring out how much is linear and how much is rotational, because you can alter swing styles and effect the distribution percentages significantly. If you have access to a digital camera and can record a few swings I can analyze it.

-Cliff
 
The only action video I have readily available on my hard drive is one of me lopping the head off a deer with my bowie. I actually thought the same thing last night and started looking at it in slow motion, but since the the times involved are sooo small, I'm not sure how to get an accurate measurement. The bowie is in and out of the picture again (a swing distance of perhaps 4 or 5 feet) in like .03 to .07 seconds or so. This was mostly a "whole arm" swing with little rotation, but then I also made second cut with wrist motion to sever the small flap of skin that was still holding it. (Usually one good swing will sever the neck cleanly, but this deer was already frozen quite a bit.)

If you'd like to look at the video anyway, I'd be happy to send it, though I must warn you it is 3.5 MB. (don't know if you have dialup...) It's in .mov format, so you can view it with Quicktime, but it will not work in Windows Media Player or Movie Maker. If you (or anyone else reading this) know of any freeware that I could edit this movie format with, I'd be much obliged to know about it.
 
Forget about this thread, Cliff? :D

I'll send it your way, but I'm really not sure how useful it will be. It would probably be better to take a new clip specifically demonstrating what we're after.

I already reground the khuk about a month ago. Took off a lot of steel from the fullers. I was surprised, but the distal rotational node barely moved at all, and the balance point only shifted back a little bit. However, it responds to a pommel much more easily now. Just haven't had/made time to get back out in the shop to finish it up. I was hoping to save the wood handle & just modify it, but it looks like they used epoxy instead of laha (sp?), so I'll have to just rip it off & make a new one.
 
Possum, here's a simpler method, derived from a cross-country sword and stick power test I was involved in (looking at how delivered power varied from people from different groups, areas, levels of training, etc. It takes less math but more arts & crafts. :)

Short version: play tee-ball. Set up a stand at the height you can confidently swing at horizontally, probably right around halfway between your solar plexus and shoulders. (Experiment - the height from which a blow sends an impact target the furthest is the one you want, because that's your real baseline.) But as long as you establish and then use a consistent height and target, it really doesn't matter.

Smack the target, measure how far away it lands, record it.

Now we were using blunts (steel swords for full-contact armored sparring) and wasters (wooden swords [sticks] for full-contact light armored sparring). So penetration wasn't an issue - we used a bowling ball as a target. You potentially have a problem with penetration (and damage to your knife) that we didn't.

I would suggest

a] do the test after you've regound your profile, bevel, etc - the weight and balance changing stuff - but before you sharpen. In that case I imagine you could use a bowling ball or something like it.

b] Or make a target that is dense and cut resistant enough that your knife will not cut significantly into it, but not so hard it damages your knife. Maybe a big ball/cube of brass, lead, or very dense plastic, like UNHW?
 
Thanks for the idea, Nick.

Unfortunately, since I first started this thread, I've already reground the blade, so it's kinda too late to get a "before and after" look at things.

I decided to go the easy route and just use qualitative measurements as well, but I did record things like the pendelum length, balance point, and such first. I have a big block of pine in the shop, so I just took a couple good whacks at it before I started grinding. I've lightened the fullers, but did not alter the edge geometry yet. So when I'm done changing the balance properties, I'll chop the pine some more and just see if they're penetrating to a comparable depth.

I've already changed the handling characteristics of the knife immensely. As in, night and day difference. If I can show that I've done this at least without negatively affecting the chopping power noticeably, I'll be happy.
 
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