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Questions about guards


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If this is in the wrong place please feel free to move it!

 

I was looking on the net for information about installing guards. So I ran across this video. If you watch the video in its entirety, it is very informative.

 

However my first question came at the beginning of the video! About minute 120 - 128 he shows the blade he is about to solder the guard onto.  

 

So the first question the notch for the guard to lock into. I was always told that a notch like that is great place for a stress point to show up! So is that notch a problem??

 

 

 

Edited by C Craft
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I hope others will chime in, especially if I'm giving bad info, but I had 2 thoughts (well 3, but the thought about that honkin' big knife he showed first isn't pertinent to this conversation...). 1).  If the corners are rounded, then that reduces the stress risers significantly.  2) Because this is a skinner, there shouldn't be a huge amount of stressse like a chopper would have.

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on that knife, the slot is like 1/10th the width of the knife so probably not a problem. 

 

proper blade tapering will reduce stress on that part of the blade which is the most likely area to break, bend, or flex with no taper. if your tang bends at the handle you are very likely to ruin the fit of the blade/handle/guard even if you straighten it, material will get deformed somewhere. 

 

you shouldnt need much of a slot at all, he could have done a 1/16" deep slot and i bet that would be just as good. since the solder has so much contact on the sides and there seems to be no need for a slot at all on the spine of the blade i think the slot is mostly for alignment. 

 

 

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Look at how thick and wide the blade is at that point.  I don't think that if you hammered into a stump and stood on that it would break it.  Second, I have made a 6 inch blade with a .250x .250 stick tang with sharp corners where it meets the ricasso.  I tested it by bolting into a 3 foot long handle and taking 2 handed swings at the end grain of a log.  I have unable to break it.   That is partly a good HT, or maybe even mostly a good HT.  I managed to bend one and break one, but I broke where the bolt pinched it, not a the shoulder.  I have never seen a knife break at that spot.  I'm sure that it happens, but I have never seen one.

G

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I have always thought that the "square corners form a stress riser" idea, is a bit of an urban legend. If the truth be told, it's nigh impossible to make a truly square corner with hand tools or a belt grinder. The specific application in the video has so much meat above that notch, it would eliminate any stress point that the notch may form.

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7 minutes ago, Joshua States said:

I have always thought that the "square corners form a stress riser" idea, is a bit of an urban legend. If the truth be told, it's nigh impossible to make a truly square corner with hand tools or a belt grinder. The specific application in the video has so much meat above that notch, it would eliminate any stress point that the notch may form.

your urban legend has research to back it up making it not an urban legend per say but that being said it might be over stated in its importance for say 75% of the blades made the only time i have witnessed a stress riser break in a knife was on a commercial knife being battened in wood with a very square tang to blade transition behind the guard and on examining the tang was found to have some of the worst grain growth i have seen from something that wasn't purposefully over heated to grow the grain

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I guess that's my point. There are probably dozens of "facts" that are "well known" to be true in knife making circles that are based on shoddy testing and questionable research. Like the stress riser question here, the examples are likely isolated to specific conditions, and have grown to an acceptance far and above what what actually occurring.

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Thanks guys! I have purposely stayed away from the discussion because I wanted to see what everyone had to say.

 

I have cut a similar notch to lock in a guard. Using a cut off wheel to start the notch and a round file to finish out the cut. Much like an upside down U. After all it just needs to be deep enough to give the guard that locking point! When soldered in the solder will fill the inside corners between the metal and the upside down U and it can't be seen after soldering! I felt by doing the upside down U, there was less of a chance of making a stress point!

 

I have also pinned the guard but again that method has its pitfalls! You almost have to make the pins from the same material as the guard was cut from. A slight difference in the make-up of the guard material and the pins, and the pins stand out like a sore thumb!!

 

So I guess I was looking for more ideas on doing a guard. Always looking for the end of the rainbow, so to speak!! how a mind works.jpg

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On 1/11/2021 at 3:08 PM, C Craft said:

A slight difference in the make-up of the guard material and the pins, and the pins stand out like a sore thumb!!

So use that to your advantage. Use a completely different material for the pin. Perhaps one that is a different color. For example, a brass guard and nickel-silver pins. Just be very careful about pin placement and suddenly you have a design element rather than a design flaw.

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On 1/10/2021 at 9:36 PM, Joshua States said:

I have always thought that the "square corners form a stress riser" idea, is a bit of an urban legend. If the truth be told, it's nigh impossible to make a truly square corner with hand tools or a belt grinder. The specific application in the video has so much meat above that notch, it would eliminate any stress point that the notch may form.

 Stress concentrations are a very real engineering concern, but as you say, it is nearly impossible to make a perfectly sharp inside corner.  For knifemakers, the question becomes, "How much of a radius is necessary for a knife to do its job?"

 

I'm a sparkey so I didn't take that many mechanical engineering classes, and the last one I had was darn near 30 years ago.  Take the following with a truck-load of salt.

 

The stress concentration factor is a function of the geometry of the joint.  The fillet required on a large notch needs to have a larger radius than on a small notch.  A 0.030" (0.75mm) radius on  a 1/8" knife guard slot might be fine, but it would be inadequate on a 4-foot window frame.  There are charts that show the stress concentration as a ratio of the radius to size of the notch. It's also a function of the type of junction.  If I remember right, the first 30% gives you the most benefit.

 

Edited by Brian Dougherty
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I made sharp corner shoulders on this one and I'm not worried at all about it because the tang is wide and thick near the shoulders. I remember Alan mentioning not to overly emphasize on this "issue", pointing out how katanas are made. 

IMG_20200424_100730.jpg

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Perhaps Jerrod Miller our resident metallurgist has some thoughts on this subject! I am interested greatly in stress areas created by inside corners. It may be one of those urban legends but it is something I have heard about since I started knife making! 

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Yeah well, I have also heard about quenching while facing true north since I began knifemaking. I have also heard it is possible to fully harden mild steel with some weird concoction called "super quench" since I began knife making. The list goes on.

@Brian Doughertyhas the skinny. It's all physics and ratios. Even in that photo Joël Mercier posted, I can see the slight radius at the bottom of the should/tang notch.

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19 minutes ago, Joshua States said:

Yeah well, I have also heard about quenching while facing true north since I began knifemaking. I have also heard it is possible to fully harden mild steel with some weird concoction called "super quench" since I began knife making. The list goes on.

@Brian Doughertyhas the skinny. It's all physics and ratios. Even in that photo Joël Mercier posted, I can see the slight radius at the bottom of the should/tang notch.

Explains a lot I have been facing True South when doing my quench! whistling mind my own bussiness.gif

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if there's a radius, it's the one on the edges of a 4" mill bastard file, probably no more than a couple of mils.

 

Anyway, I guess what I was trying to say is if your design is well made(for example: the tang on a katana which is nearly as strong as the blade itself), this sharp corner detail becomes much less important. 

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I agree mostly with what Brain stated.  I would also add that the material plays into it a bit as well.  Brittle materials are more susceptible than ductile ones because the crack propagation would be more pronounced.  The key is that a stress concentrator makes a section act like a much smaller section.  If you have a 1" wide piece of steel and put a notch that is 1/8" deep in it, it will not act like a 7/8" piece.  It will act like a thinner piece.  The sharper the "point" of the notch, the more exaggerated the reduction.  So then it all comes down to applied forces and design.  If you have enough steel in place to be able to withstand prying with all your might at the end of the handle, then a little notch will never be a problem with cutting vegetables or skinning an elk.  Prying open a box with every bit of strength you have?  Maybe you'll have a problem then.  

 

I think I have mentioned in another thread that I had a fencing (long) sword break in my hands once (about a year ago now).  I'm a big guy and I the guys I fence enjoy a good bit of intense fencing, too, so there are some pretty good stresses put on our swords.  But nothing like absolute full force, you can't keep fencing if you break your opponents.  That being said, it was very disconcerting when I parried a cut and was left with just a grip and pommel in my hands and blade on the floor as he cut around and smacked me in the head.  He felt bad, but shouldn't have, as it was all done well and his hit landed about the same time as my blade.  Now, obviously I did a bit of failure analysis on the blade.  I am sure that the problem stems from a too-sharp corner put in at the base of the blade.  There were no signs of fatigue failure, or stress corrosion cracking.  I mean, pretty cut and dry impact failure.  And it wasn't an especially hard cut my opponent threw.  I spoke with the manufacturer (intentionally not named here) and they saw nothing wrong with that sharp corner.  Among a couple other things mentioned (like that the grip would support that shoulder - no it won't), I will certainly never buy a blade from them again.  Sadly I do not have a picture handy of the on-end view.  20200306_082536.jpg

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Long blade, lots of leverage, thin cross section. I'll wager that all adds up.

Edited by Joshua States
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Thanks for chiming in Jerrod. I am beginning to understand that there are other factors beside the notch itself!

 

It is kind of like welding a piece of the rod to to a section of flat bar!! I made a connector for a mower like I said, by welding a piece of rod to piece of flat bar. I broke it twice but not at the weld but right along side of the weld. In other words the weld didn't fail but the heat of the weld caused a stress factor to the metal along side the weld. By controlling the heat of the weld the third time the rod/flat bar was still in use 3  yrs after the last time I welded it!

 

3 hours ago, Jerrod Miller said:

The key is that a stress concentrator makes a section act like a much smaller section. 

 I get what you are saying! Thanks again Jerrod1

Edited by C Craft
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