Strength of modern epoxy

Justin Schmidt

Justin Schmidt

Joined
Feb 18, 2016
Messages
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So I'm working on just my 2nd knife and I've ruined 3 or 4 knife shaped pieces of steel and more pieces of walnut scales. I don't have a drill press so I have a VERY difficult time lining the holes up on the scales properly. When watching forged in fire the other day (I know I know) a contestant said with modern epoxy you don't really need a pin. I've been wondering the same thing myself. And also think corny bolts are way overkill. So let's find out!
The test subject is a .128 piece of 80crv2 that didn't make it to heat treat. Scales are 2 pieces of 3/8 unstabilized walnut. Not great figure but good for practice.
Let me preface this by saying normally (twice. The amount of knives I've got to this point) I will drill a few .250 holes in the tang and hollow grind it to give the epoxy a place to go. I also flatten/ rough up the scales with 120g sandpaper glued to a granite surface block. Then I will wipe everything down with acetone to ensure it's all clean so the glue can make a good bond.
Well I'm not doing that on this one. I literally grabbed the steel and scales out if my scrap bin and glued them with zero prep.
I'll be using gflex epoxy, and a single .187 416ss pin.
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I put the epoxy on the scales and squish the blade in between
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Btw I got this thing of gflex from iboats.comand it was less than 20 bucks shipped and it arrived in 24hours. Fyi.

Alright now I will let it set up for 24 hours then I will do some kind of clean up and then attempt to destroy this thing.
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Plan is to try and separate the scales from the tang. If it does separate. Then I will do the test again but with properly prepared tang and scales and I will use 2 pins instead of 1.
Thanks for following!
-Justin Schmidt
 
Properly prepared I'm sure the wood will be destroyed before breaking loose from tang, even without the pin. Let's see how it goes on this test.
 
Prep is as important as the glue.

Be sure the surface is dead flat and roughed .

You should have extra holes in your tang so glue can migrate from one scale to another so the actual scales are glued together as well as to the tang.

You are using far to much pressure with those clamps and will have "squeeze out" and lose almost all your epoxy.

If the surfaces are dead flat you only need a couple of squeeze type clamps to hold the scales down. If all surfaces are flat there will be no gaps.

If you are having drilling straight holes with your hand drill, look on Youtube for a solution there are several.
 
When I use CA to temp glue a piece of wood to a piece of flat steel (so I can attach it to my surface grinder) it holds very well. To get it off I put it in the freezer until it gets cold. At that point a moderate shearing blow will pop it apart. This is where pins play a part. Pins offer some resistance to the scales pulling away from the tang but mostly what they offer is resistance to shearing forces. Modern epoxy is good stuff for sure but I wouldn't make a slab handle without pins. Hidden tang knives can maybe go without a pin if you notch the tang for a mechanical lock inside the handle. But a pin is still a good idea.
I'm interested to see your results.
 
If you wish to see just how well epoxy will hold to the metal tang with NO pins, rough tang with 50 grit range, drill several thru holes in tang, then dimple about 1/8" or so into opposite scale thru the tang holes so both scales will have these dimples located at the tang holes. This filled with epoxy will act as an epoxy pin to help on shearing. Using the unstabilized walnut scales you mention, I wouldn't e surprised to see the wood destroyed even with shearing strikes to side of scale. The final result will be some places the epoxy will turn loose from wood, other places I'd expect to see wood torn from scale and still stuck to tang. Properly done, epoxy is darn strong.

With that all said, I still use corby screws - I never had a problem with pins, but corby screws are easy to use (really help control amount of pressure to scale) and only add $3 to $4 per handle.

Ken H>
 
Will look forward to the ongoings in this 'test' Tracy Mickley from USA knifemaker did a glue/epoxy test a good while back, and several glues people poo-poo all the time did incredibly well--straight up Gorilla glue comes to mind.
 
Natlek said:
Without the pin wood will separate from steel with little effort .Wood is not problem ,adhesion to steel is problem .
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Could you maybe either score the steel or better yet drill holes through the tang so that way the epoxy will be holding it in better? If the tang has a hole and the epoxy "bridges" that hole then it would have to break that epoxy "bridge" for the blade to come out.
 
I was thinking that a shearing force would separate the glue. That's why I'm only using one pin. I want to be able to see if you can safely secure a set of handles only using a single pin.
 
Good epoxy is surprisingly strong. the few times Ive had to take handles off a knife Ive had to either destroy the wood completely or heat it til the glue melted. it wont just pop off even without a pin..
Something overlooked besides hold strength, is hold life. how long the epoxy holds. The two longest I know of are acra glas(50 years) and g/flex(30+ years) according to manufacturers
 
That's great advice bill. And I completely understand why most guys use mechanical fasteners and I have no problem with that. I just wanna see if push comes to shove or for aesthetic reasons I could get away with one pin. And I have a hard time spender 2 bucks for ONE corby bolt when I can get a foot of round stock for less than half of that.
 
I have made knives with one pin and have had no problem. They may be statistically less strong but only in test with presses pushing the handle scales away from the tang or trying to cut it off with an axe would it be a problem.

You also need to remember that the handle is in the users hands when being used for hard use, the only time a handle may possibly come off. Its not the handle being used for hitting etc. The only occasional issue is when the handle hits whatever is being chopped.

Of course in knives we want to make everything stronger how we can but if you are really worried about getting crazy strength (which as I was trying to say isn't necessary for knives in even hard use), don't use wood and use carbon fiber or G-10 and pay the money for bolts.
 
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I've never made a knife with wood scales, but I was an adhesives / coatings chemist, and my superhero power is gluing to dissimilar materials so here is my 2 cents (agreeing with nearly all the comments so far). Epoxy is awesome when used correctly, especially more elastic ones like G Flex, because the wood will move tangentially when the steel will not, and this is important for longevity of the joint (why you don't want to over clamp the joint). It will likely hold for a long time, but its the constant months of expansion / contraction due to thermal and moisture forces that will cause it to eventually fail, even with the ingenious ideas of multiple holes bonding though, etc because once loose, then the epoxy "pins" will break.
My favorite adhesive for wood to steel is not epoxy, though. By far superior is simple PL Premium polyurethane construction adhesive because it stays more elastic than epoxy, soaks into wood like epoxy, yet has insane bond to steel. I have never seen joint failure on high torque loads using PL for wood to steel. Its similar to Gorilla glue, as many know, but has higher cohesion due to less foaming, and is cheaper (7 bucks for 10oz tube - goes in a caulk gun). I made half my boat with it (other half epoxy).

The trick with any adhesive (particularly epoxies and urethanes) is to heat the wood prior to application - it off gasses air and moisture, then the heat lowers the viscosity allowing deeper soak in, then as the wood cools it literally sucks adhesive deeper into its pores as it returns to ambient pressure. Then the heat kicks off the epoxy faster. All around underutilized great tip if you have a heat gun.
 
Kentucky said:
Something overlooked besides hold strength, is hold life. how long the epoxy holds. The two longest I know of are acra glas(50 years) and g/flex(30+ years) according to manufacturers
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I've heard that from another maker, that epoxy has a limited life, and some have even said it's designed to fail. Epoxy is going to last a lifetime and more without a chemical failure. Just think of the boats that are around. My 40 ft sailboat was built in 1964 and I'll assure you the hull was just as strong when I sold her in 2009 as the day she was launched. I've posted emails and reports from at least 3 different manufactures of epoxy (West System who makes gflex was one), who'd never even heard that epoxy had a limited "hold life" - other than fail due to UV light or being exposed to excess heat. Seems like the tech engineer from West Systems said Lloyd's of London rated their epoxy for 100 yrs in boat building - don't quote me on that, but it was a LONG time.

Ken H>
 
Kentucky said:
Good epoxy is surprisingly strong. the few times Ive had to take handles off a knife Ive had to either destroy the wood completely or heat it til the glue melted. it wont just pop off even without a pin..
Something overlooked besides hold strength, is hold life. how long the epoxy holds. The two longest I know of are acra glas(50 years) and g/flex(30+ years) according to manufacturers
Click to expand...
This one I make before 25-27 years , I don t remember exactly .Carbon steel and walnut . It s tool in my auto service all that years , used for everything........contact with oil , brake fluid, antifrize, gasoline , diesel , water ....... .I can t see even sign of delamination on handle .Araldite is used for handle .... and most likely will outlive me :)
PS . I clean little spine ... to see it better .
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If I was purchasing a Hard Use Camp or Bushcraft Knife I would be looking real close at the handle construction. If I were looking at a Custom High Dollar Safe Queen I would be less concerned about handle fasteners. I have abused one of my Large Camp choppers and have knocked the scales loose due to batoning and they are attached with hollow tube fasteners only no epoxy that are swedged tight. I'm still skeptical about long term use with just epoxy.
 
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