Thumb Holes and Blade Strength

D

diletante

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Mar 20, 2002
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What effect, if any, does a thumb hole have on blade strength?

Please forgive me if this topic has been covered before. I tried searching the terms "hole" "blade" and "strength" in topic headings, but the search tool did not return any results. I also did not see anhy FAQs that looked like they were on points. If this topic has been covered (and I imagine it has been), please post links.

The reason I ask is I'm trying to decide whether to buy a Benchmade 710HS or 806D2. That's the subject of another topic, but I thought this question might be general enough and interesting enough to other members (at least the newer ones) to justify a separate thread.
 
I posted on the other thread, but I'll post here as well, since the topic is ,IMHO, worthy of some input.

I have often wondered as well about the presence of a hole of significant size at a point in the blade where it takes a fair degree of stress in any kind of flexing situation.

True, a knife is a cutting tool and as such the hole should have little or no effect on it's efficiency. If, however, you get into a true survival situation, where the blade is subjected to unusual stresses, the situation could be dramatically changed.

I hope this thread generates some interesting dialogue.
 
I don't know about a opening hole, or hole for a thumbstud, and am about totally ignorant when it comes to determining what creates a "stress riser", but can tell you about what happened with a knife with a thumbdisc.
I had a BM Monarch drop point (weekhawk?) with a 12C27 blade, thats edge had been worn back over the course of 5-6 years so much that it would have taken extensive reprofiling of the entire grind to get it decently thin. It was just sitting around being dull, and I couldn't bring myself to throw it away. Decided one day to break it just out of curiousity. I stuck it about 1/2" between two concrete slabs and torqued the handle. The blade took a permanent set at about 30 degrees along the last inch (very robust tip, on par with say, a CRKT M16 tanto). When the blade finally broke, it was not at the tip, but at the thumbdisc cutout. It was much harder to break than I expected, and the handle (which did flex considerably, but came right back to true), lock, and pivot were totally unaffected. Pretty tough little knife, I thought.

I'd like to hear more about stress risers, and what causes them, since it's something I don't know much about...
 
Hmmmm- interesting topic, I would guess the Spyderco type hole, because it is nicely radiused and not "sharp" functions to relieve stress much like a rounded choil. Perhaps the thumb studs are more stressful because the rim of the hole- although smaller- presses against an object (the inserted stud) instead of flexing freely. This could cause more stress than if there was nothing present at that spot/location. But thius is hypothetical, it will be interesting to read other replies, perhaps someone with physics knowledge can comment?
Martin
 
I don't have an answer either, but I have been wondering the same thing everytime that I look at the HUGE thumb-hole in my 2 Spyderco Chinooks. Maybe Sal will wander over here and give us an educated reply.:).
 
I have several thumbhole openers some BM's, some Spyderco's in 15 years only one blade snapped, a Centofante/Spyderco collaboration.
My impression was, because the blade was a flat ground, very thin blade, the hole occupied almost 50 % of the blade area by the pivot pin, and the blade was made of ATS 34, these factors combined with perhaps a poor heat treat, the blade snapped while cutting cold, hard Wisconsin cheddar cheese.
I sent the knife back to Spyderco, and apparently they agreed, even after the knife had been broke for 3 years, and I had no reciept,(Just want to thank Mike Hayes and the warranty/repair guys at Spyderco) they couldn't repair or replace the knife, but they gave me credit for $129, the cost of the knife.

Now non of my other thumbhole openers have ever had a problem, and i'm pretty hard on my knives. IMHO the knife was a little to thin and the hole occupied to much area in relation to the rest of the blade.
 
Drilling a large hole in the blade weakens it. But it hardly matters, as the handles on folding knives are not constructed to endure the kinds of lateral stresses that would break the blade anyway.
 
I would lean towards the theory that a even a large hole, if well radiased and smooth, wouldn't create a major problem at all. Any force generated would be pretty evenly distributed and I would think there is so much steel surrounding it that it would be hard pressed to create any problems. If anything, it might affect the lateral stress of the blade, IE stick knife in something and apply torque, but in normal push down/pull back cutting, shouldn't do anything. I think its preferable to a disc or thumbstud that that weakens the blade with a hole and or notch cut into the blade and the hole does not stop the knife from penetrating to the hilt if needed. Where are all the physics majors...engineers...machinists etc on this one?
 
Originally posted by Sgt 127
Where are all the physics majors...engineers...machinists etc on this one?
Click to expand...

Cliff S. I hear them calling you, how about an opinion?
I'm always interested to hear what you have to say, and usually get an education along the way.:)
 
For a folder, what little data we have suggests that the lock or pivot will gave way well before the blade breaks.

Joe
 
A lot depends on how you apply force. if you grip the handle towards the end, the handle will fly apart on most folders even before you can break off the tip. However if you grip up on the top of the handle, around the part where the blade pivots, you can break the blade without breaking the handle. This can be done even on cheap Zytel folders.

As for the hole, if done properly, it will not weaken the blade significantly, as most prying is done with the tip, and the tip is far weaker than the main body of the blade, even if it has a big hole in it. I saw an Endura lose a big section of the tip (~1"), without breaking through the hole.

The problem with thumb knobs is that they can be overtightened and sharp and thus are easily broken, there are in fact cases of Reeves folders breaking very easily because of this flaw in manufacturing.

In general a stress risor is just a place where when a load is applied, instead of being spread out, it is concentrated in one area which then becomes a weak point. For example, if you score a line across a bar of steel with a Dremel, it will significantly weaken it and thus the bend will be induce there. As the set starts it actaully causes the concentration of force to increase and thus it gets worse faster and a break sets in quickly.

What you want to look out for are sharp transitions in geometry. For example plunge grinds which are square, holes which are sharp, spines which are not rounded, tips which are very angular etc. . Note though, that this is only one element in determining strength (and impact toughness). Steel type, heat treatment, and raw geometry are also major factors and in fact will swamp out stress risors in most cases. Cleaning up sloppy grinds and smoothing contours usually increases performance by a matter of a few percent except in extreme cases.

About the worse stress risor is a triangular notch in the blade opposite to the loading point. I have seen edges when notched in this way fall under drastically lower loads than normal. This is how charpy v-notching is done, to examine the notch sensitivity of steels.

-Cliff
 
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