Blade width to thickness question

Joined
Mar 6, 2017
Messages
72
Hey I'm going to begin my 3rd knife as well as a new design. The design is a wider blade 1.5" where as my previous knives were about 1". My question is, do you think that 1/8" steel would be too thin for a blade with 1.5 inch width and a flat grind? The purpose of the knife is a small to medium general use survival type knife.

With that, and I know much is preference/purpose of knife, what would you say is a general rule(or your recommendation) for thickness of blade compared to length and/or width of blade, when doing a flat grind. Not talking kitchen or filet knives but outdoor/camping/hunting blades. Meaning, I'm sure you wouldn't want a flat grind on 1/8" steal if the bade width were 3" or say a 12" blade with 1/8" thickness, or maybe you would, I really don't know, I'm still a newb so wanted some thoughts.

Thanks
 
For what you are describing, in 1/8" thickness I would stick to a saber or scandi grind. For knives of similar proportion that I want to have a full flat grind, I usually step up to 5/32 or 3/16.

But there are no rules really.
 
1/8" is ok for that purpose,I prefer 5/32. If using 3/16 you need to distal taper the blade and taper the tang. YMMV.
 
Thanks for the replies, I was kind of thinking 1/8" might be a bit too thin, and I don't feel confident enough to do a saber grind. I have a bunch of 1/8" stock still so I think I'll go with a different design for now until I can save up for 5/32s or so. thanks again.
 
From a strength perspective, a flat ground 1.5" wide blade at 1/8" is stronger in both directions than a 1" blade of the same thickness. It is especially stronger in plane with the edge. Form an aesthetic perspective, I think it can look fine as well. I forge plenty of hidden tang utility knives around those dimensions. I don't baton them through hardwood often, but then I still suspect that with a decent steel selection and heat treatment it would be fine. I wouldn't want to use it as a prybar, but then I prefer not to use knives for serious prying tasks anyway.

I think that, in general, blades should only be as thick as required to achieve the necessary rigidity/strength for their intended task. Any extra thickness beyond that makes them cut more poorly than required. Unless this is going to be a HARD use knife, 1/8" is plenty.
 
From a strength perspective, a flat ground 1.5" wide blade at 1/8" is stronger in both directions than a 1" blade of the same thickness. It is especially stronger in plane with the edge.

This is very interesting, but I'm having a hard time understanding how that works. With my very limited understanding of geometry I would figure that lengthening a triangle would only make it weaker. I'm very intrigued, if you could expound this for me that would be great.
 
The 1x1x.125 triangle will fit inside of a 1.5x1.5x.125 triangle. So the 1.5 triangle simply has more mass. In regard to a failure like the blade breaking completely, it is stronger because of the additional mass. It's edge however, is slightly weaker than the 1" triangle, because the angle is more acute and therefore has less support. That's why I suggested a saber grind. A 1" high saber grind on a 1.5x.125 piece of stock would be stronger than either with a full flat and still have the same cutting geometry as the 1x1x.125 triangle.
 
Maybe I am not getting something that I'm missing here. A "survival" sort of knife with a blade .125" thick, with a full flat grind, and some here say that's too thin for a 1.5" tall profile? Do I have this wrong? Well, maybe I do. But in my opinion, a blade that is .125" (1/8") thick and is 1.5" tall with a full flat grind will work quite well. This would be the overall blade geometry. What then would matter most is the actual edge geometry (how that full flat grind was terminated into the actual edge geometry). The spine thickness is ok being 1/8", so is the grind chosen (ffg). What matters is the termination geometry. A dedicated slicer might have a .010" edge (or thinner), whereas a chopper might want more mass behind the edge (maybe .020" for example " ). So....my humble opinion is use 1/8" spine, use full flat grind, and then thicken the edge geometry to tailor for impact/abuse. Around .020" or so. I don't even know what alloy you intend to use. Rex121 would be vastly different than 52100 for example.
 
I don't think anyone is saying it's too thin. I like them a little thicker.

The last couple posts are just discussing the overall strength of the blade without regard to edge geometry.
 
My Father served in the Army Air Force during WWII.. he was a flight engineer so he kept the B-17 flying while they dropped bombs over Nazi Germany..———////////—_________________ So no knives used there for the most part.. Still I grew up using knives from that era of 3/32” 1/8” & 5/32” mostly and they only broke if you did something really Stoopid with them! —————————-How did our Fathers, Grandfathers & Greatgrandfathers that we’re Scouts, Marines, Airmen & Soldiers win this war, survive on a remote island etc with such wimpy thin knives? ...Let’s add in Korea & lots of it was still used in Vietnam ................. Ive never seen a single USA Military issue knife that was a Scandi grind?———-////Look at the pal 36 Knife, the Remington’s , The Theater made knives, other than the Ka-bar. Which is a tad thicker......................Most were 5/32” or less. How many stories of these knives breaking in Combat, Field use, Survival have you hear?? ———————————In the past 10-15 years, people have gone nuts with thickness in a blade steel.......To the OP.. Make’m as thin as you can till they break! Then build from there..:thumbsup:
 
In the past 10-15 years said:
This might be the issue. My only experience with knives is what is popular today and there seems to be a lot of knives that I really liked that were usually 3/16" -1/4" so I assumed that I should being doing that as well. This is why I'm grateful for this forum and you all being so helpful to someone like myself that knows very little and to get better guidance. Just for more further clarification, below is my design I plan and will be using 1/8" 1084. Thanks again and I'm excited to make another knife.
RIFj0jy.png
 
Last edited:
kuraki is right, more mass is stronger than less mass. He is also right that there is no need to make the spine thicker to get this increase. As he said, foe te same thickness, the 1.25" blade has more mass than the 1" one.


As to seeing so many blades at 3/16" and even 1/4", that is a thing that became popular a while back, but has no real need. I think some of the folks who wrote books and magazine articles helped start this.
Unless exposed to severe abuse, batoning, or using the blade for rescue type work ( prying and hacking things), there is no need for a spine that thick. Take a caliper to a good sporting shop and measure the spines on some of the very best hunting and utility knives. Many are down near .060", and few are more than .125".

I just did a HT for a fellow who is new to knifemaking. He is a wonderful machinist, and does not even own a grinder. Says he doesn't need one because he can mill to a smooth finish (lets see how that works out :) ). He made a recurve chopper in 10" blade with a .400" spine. I tried to explain that making it thick would make it cut less well, and was twice the thickness needed for a chopper of any normal use. He will have to figure it out himself.
 
Last edited:
This is very interesting, but I'm having a hard time understanding how that works. With my very limited understanding of geometry I would figure that lengthening a triangle would only make it weaker. I'm very intrigued, if you could expound this for me that would be great.

It is really only "weaker" in two ways. Its strength to weight ratio is slightly lower perpindicular to the blade flat. This isn't relevant because that ratio is enormous, so slight reductions are not noticible. The second is that the edge is slightly less well supported in the same axis. Where this would matter would be using the edge to pry via rolling/lifting the knife after sliding the edge beneath something. Once again, this is practically irrelevant for most use cases, further, it could be easily solved by convexing the grind slightly with a thicker width behind the blade. ultimately, with any knife if you are prying/lifting something that requires enough force for this to be relevant, you will destroy that portion of the edge long before you actually suffer a failure farther back on the blade's bevel.

Ultimately, in every practical way, the knife will be stronger, not weaker for extra blade width. And assuming similar blade geometry, it should cut better.

I am totally with Laurence here, many knives are overbuilt often to the detriment of their ability to cut. If you want a combo knife/can opener/pry bar/hammer, by all means leave the spine thick, but if the knife's job is to cut, it only has to be thick enough to prevent excess blade deflection in your inteded use case. And thin will be fine for most anything. Unless you just want to add mass.
 
He made a recurve chopper in 10" blade with a .040" spine. I tried to explain that making it thick would make it cut less well, and was twice the thickness needed for a chopper of any normal use. He will have to figure it out himself.

Is this a typo or am I misunderstanding? 5/128" seems like a pretty thin spine for a chopper to me. Did you mean .040 behind the edge? Or a .4" spine? Not saying .040" wouldnt work, but I would think it would be very likely to suffer serious deflection in agressive chopping tasks, and would likely be low on mass for getting a good swing in.
 
I like your Design and 1/8" will make a fine Knife.... Also I like that you have a Full Flat Grind which will cut & separate just fine! !


Another Factor is when hiking out hunting etc. You have to lug around all of your gear and I have a feeling that a lot of these sharpen Pry Bars at 1/4" and larger in thickness may just get left at home for those long Treks & Hunts for a lighter knife!;)
 
Thanks guys, the wife is going out of town this weekend, so I'll be able to get started this weekend. Now if I could only convince her of my needed drill press, metal cutting band saw, larger forge, more steel, more supplies...oh the list is long.

What's funny though is I got into this because the knife I really wanted was $200. and being the tightwad I am said to self, "self I don't have money to buy a $200 dollar knife nor would the wife approve of such a purchase, so..I'll just make one myself". I think it cost me more than $200 to get my first knife made. haha.
 
Thanks guys, the wife is going out of town this weekend, so I'll be able to get started this weekend. Now if I could only convince her of my needed drill press, metal cutting band saw, larger forge, more steel, more supplies...oh the list is long.

What's funny though is I got into this because the knife I really wanted was $200. and being the tightwad I am said to self, "self I don't have money to buy a $200 dollar knife nor would the wife approve of such a purchase, so..I'll just make one myself". I think it cost me more than $200 to get my first knife made. haha.
Harbor Freight sells a small Drill press for like $60 and watch the lot sales & Coupons——-///////They work just fine for our purposes. I even drill Hardened steel with mine & a Carbide Drill bit!.......,,,,,, The first time I saw them was at the Late Bob Loveless’s shop! He had three of them lined up, bolted down and set up for each need.
 
Skip the band saw for now, get a cold cut (dry cut) asw instead. They are cheaper, blast through cuts, have a much smaller footprint, and unless you are doing stock removal, you wont really miss the bandsaw (and even then, the bandsaw is only really nice for stock removal when it functions well in the vertical.
 
Back
Top