Knife handle failure...

[dkemppai]

Hi All,

So about 9 months ago I was tasked with making a custom carving knife. The client wanted a lower cost unit, just a full tang with sharp blade and scales, no pins, no bolsters, no guard. He did want some nicer looking scales and picked out some oak that was done by WSSI.

About two months ago the thing comes back into the shop, both scales peeling off. Closer inspection showed the scales were pulling up off the steel. The oak just pulled away from the steel. Epoxy was left on both the blade and wood. When my scales go on a knife the steel gets etched, scrubbed, and degreased before. Wood gets sanded down with a fresh sharp belt, and wiped down with acetone before gluing. I keep everything spotless clean. Only fresh epoxy, and DO NOT touch the surfaces being glued together. No oil, or contamination.

Anyway, I pop the scales off the knife. I clean the blade and bring the blade to my buddies auto body shop, where the whole tang gets sand blasted with crushed glass. Nice rough finish on the tang. It gets cleaned and wrapped up in paper and put away. Scales were bowed slightly (Obviously, the stabilized wood moved!), so I let them sit for a week or so and sanded them flat. I took a 400K RPM grinder and drilled up the mating surface with a small round burr. There were holes every 1/16th of an inch over the whole inside surface of the scales. Not a roughened surface, actual holes. Everything was cleaned again. Applied epoxy to the scales until the holes were filled, and applied epoxy to the tang. Mated everything together and used some string to apply light pressure. After the epoxy cured the extra epoxy was removed and the handle was refinished. There is absolutely no doubt in my mind this epoxy job will last forever!

Well, last night I get a call. One of the scales is lifting off the knife again.

Couple of things come to mind. First was I was not completely happy with the WSSI job on the wood. It just didn't have as much resin in it as some of the other blocks from the same batch, and nowhere near the resin in the stuff from AKS or other vendors. Over a three inch length, this wood curled up about 1/16th of an inch. Seems too much for stabilized wood. Of course, this knife doesn't have bolsters or a guard. This was intended as a low cost knife, and this 'extra' was too expensive. Hindsight makes me wonder if the extra metal would have helped strengthen the whole thing and prevented this.

Also, This guy's carving style is pretty forceful. He doesn't break blades, but puts a lot of pressure on the knife and wood. I spent some time watching him carve before we designed the blade. He knows how to handle a knife, but he does work them hard. No doubt this knife gets used harder than any of my other knives. The blade has taken the work just fine, and he loves how sharp it stays. However the scales just aren't sticking.

I'm really at a loss here. I don't know if it's the wood, the lack of bolsters, the stabilization, the hard use, or a perfect storm of all of these. Currently, I'm thinking the solution is a new blade with a much thicker tang to take the force. This current blade is about .140 thick, but it is tapered to the back of the handle, so it's possible it's flexing too much in use.

I'm just at a loss. I'm hoping you all may have some nuggets of wisdom here.


Thanks,
Dan

 

[Sampson knifeworks]

First question I would ask the customer is how they wash the knife, if it's in a dishwasher that would explain a lot right there.
I learned awhile back how much stabilized woods move around, even the top commercially done woods. I pretty much seal all stabilized woods now with tru-oil or a tung oil and have had no problems since. Corby bolts or pins help a bunch and are relatively cheap. Hope this helps.

 

[mike miller]

Go to G-10 which will not absorb water.

 

[KenH]

Welcome Dan - what part of world are you in? With .140" thick tang, I can't imagine it's the tang flexing. OR - is that .140" at rear of tang, but thinner at blade end of tang that could be flexing?

What epoxy are you using? A good slow cure I'm sure.

It sounds like you've done good surface prep.... When you say "Epoxy was left on both the blade and wood", you mean the epoxy was split? OR, there were spots where the epoxy pulled away from metal and left on wood, and other places where it pulled away from wood but stayed to metal tang? I have used hidden pins where I felt the need for extra strength but didn't wish a pin showing thru.

For sure confirm they're not putting the knife in dishwasher.

Since customer is happy with blade, I'd think about putting pins (perhaps hidden pins?) in handle and maybe even using a different wood. Just mention the stabilization seems to be allowing wood to warp when it shouldn't.... that is "IF" customer is NOT using a dishwasher.

Good luck and keep posting.

Ken H>

 

[BossDog]

Stabilized wood will absorb water like a sponge. i don't think the stabilizing is the cause. Some woods hold more acrylic than others. It depends on the wood species and density of the piece.

can you post a picture of the failed joint pieces?

if it was put in a dishwasher, that is my vote for the failure. I tested 20 plus glues. Most of them failed from repeated dishwasher tests after just a few passes.

what was used to fasten the handles? Pins, Corbys, cutlers rivets?

Other than injection molded handles, most (all?) commercial kitchen knives use cutlers rivets and no glue.

 

[J. Doyle]

You could use hidden pins if he doesn't want any visible pins or fasteners. It might help.

Another thing, I personally have ditched using acetone on anything. It leaves a little rainbow colored 'slick' of it's own. When I used to use acetone, I found that it cleaned some oils and grease really well but the acetone itself had to be cleaned with denatured alcohol. Lots of guys use acetone but I had bad experiences with it that stopped when I quit using it.

 

[C Craft]

You need to provide some more details as there is help to found here. And pics they can tell a lot as well.

I was recently told by the rep for West Systems that supplies G flex 650 and G Flex 655 the following info about there product.

Many custom knife makers use G/flex 650 (honey like liquid) and G/flex 655 (like gelled toothpaste) to assemble their knives. The only complaint in 8 years I can recall was from a knife builder (a seasoned builder that had used G/flex successfully for a couple years) who used a contaminated rag and solvent to clean the surfaces to be glued before applying epoxy. I convinced him to try it again but by using plain white paper towels to apply and wipe off solvent instead. He called back and said it worked although he struggled initially to believe that the rag could be the problem. Even recently laundered rags can cause problems because of residue from fabric softeners, from furniture polish or from auto wax residues that the detergent failed to fully remove.

I suspect that more failures may be caused by this same type of contamination, not matter the type of epoxy used!!

Give some more info and I'll bet you'll get some better answers!!!

 

[rhinoknives]

If he puts them in a dishwasher that is the culprit. Its not just the hot water. The phosphates in the detergent will frost the cutting edge within a few washings and distort most handle materials. Also from his "Carving Style" I only use Loveless bolts, Kydek for liners and G/flex Epoxy all ruffed to 80-120 grit. Hidden pins may be a weak point if he is really torching the handle when carving?

No failures yet! You didn't mention a liner? That gives the two dis similar materials of steel and handle material a common material to bond onto with a little give.

 

[Calvin Robinson]

Your problem is,'no pins'. Pin and peen or use corbys or loveless bolts. Without mechanical fasteners you can be sure the handle material will come off.

 

[TacticallySharp]

I clean all my materials with denatured alcohol using "el cheapo" brand white paper towels. I use only GFlex epoxy and all hard use knives get Corbys. The only way I will make them. The cost of not using them has already been higher than if you had. You fixed it once and now you're going to do it again. My advise is in the future set a minimum standard of what you are willing to do on a knife and stick to it. Your rep is worth more than that. If the client can't afford it just walk away from it. You're only as good as the last one you made.

I have lost custom work when someone wanted a cheap knife but, when i have made them an inexpensive knife that meets my minimum standards we have both been happy. This time tell him your going to throw in for free a couple of Hi Tech Corbys to provide a mechanical attachment, give him a proper care sheet. I include one on all my knives. My receipt which is signed by the client references the sheet as part of the guarantee statement.

 

[J. Doyle]

I think I would use some type of pins or fasteners. If the pins showing is a deal breaker, then I would use hidden pins.

I want to take a minute to explain my definition of hidden pins. I'm not talking about 1/8" or 3/16" brass rods that fit into a sloppy hole with tons of room for epoxy.

I'm talking about PRECISION press fit hardened 1/16" steel dowel pins. Like so precisely fit there is absolutely ZERO room for epoxy in the pin holes. Used this way, they are VERY secure and VERY strong. I made a knife for my brother that used torx screws to hold the scales on so we could remove them for maintenance. I used hidden dowel pins to hold everything in place properly. NO epoxy whatsoever anywhere. After taking the screws out, it is all I can do using every last bit of my hand strength to get those scales off.

Also, precision fit pins help prevent scales warping and lifting because there is absolutely no way for the scales to move because they don't have any clearance.

Done sloppily, hidden pins don't do much. Done properly, hidden pins are as good as it gets with the possible exception of corby bolts.

 

[Knifemaker.ca]

There are so many "possibles" that are unlikely. Type of epoxy - proper mixing of epoxy - poor prep of scales prior to gluing... but you have been making knives for a while and have done this one twice now - the last time with extra attention to detail. I'm guessing you have done pinless scales before as well.

If this knife is not significantly different than the others you have turned out, then the question begs 'what is different about the use / user?'

You've already identified this guy's cutting style as aggressive. Can you just explain to him that there are few people as confident in their blade handling as he is and that you recommend corbys or dogbones or whatever?

Cost is obviously an issue, if you're going to make it work, you might also consider something like a 1/8 G10 spacer to stiffen the entire handle and provide an improved substrate for bonding.

Feel for you man. It sucks when a project seems to defeat your best efforts.

Rob!

 

[dkemppai]

Not Giving up!

Wow! Great responses! You all are awesome!

Some additional information, since I have received the knife back this morning and had a chance to talk to the guy for a while.

This knife has not been washed. It has not been exposed to water. It's a carving knife, so it sits in the sheath in the guys tool box unless he's carving with it.

There is no liner material on this knife.

Last time it came back, there was epoxy on both the wood and steel. It adhered well to both materials. This time, the epoxy appears to left on the wood, but with the extra surface carved into the wood, I'd expect that.

The blade is ~.140" in the middle. The tang is tapered down to about .060 at the rear of the handle. It's strong in the middle where it should be.

When prepping for glue up, only clean white paper towels are used. No contaminated rags used, ever. I don't even keep rags in the shop.

We're in Upper Michigan, and houses tend to get dry this time of year. He admits his house is very dry.



So, now that I have the knife in hand it's an even more curious looking problem. The right hand scale has curled up off the rear of the handle. There is a gap of about 1/16" at the rear of the handle. Three quarters of the scale is separated from steel, and I can't pull the rest of the scale off by hand. Pinched my fingers trying, that epoxy is tough! In trying to pull the rest of the scale off, you can hear wood cracking...

At this point my train of thought is it's got to be something with the wood. The wood is northern red oak, and is a particularly tough cut from a crutch block. Red oak moves A LOT with moisture. I've made quite a few knives with this cut of red oak. Typically I use a very slow process of penetrating oils and months of time to let it cure. Have not had any failures on those knives. But, most also have pins and guards.

This is the first batch of oak I've had stabilized. I'm starting to wonder if this particular wood is not compatible the process, or doesn't like being stabilized. So, I check another knife with wood from that same batch under a the microscope. Sure enough, that oak is pulling off the steel there too. Pins and guards are holding it down, but it's broken loose. Only a few thousandths, but it's loose.


So, I'm guessing: It's not an abuse problem. Wood lifting from the rear of the handle seems way too weird to be related to hard use. There are no nicks, scratches, or dents in the handle. Epoxy job is sufficient. Not being able to pry off a 1 inch patch of glue with a 3 inch lever arm would indicate it's adhered well. I know wood movement is a real possibility. The scale didn't just pop off. It's curled up and away from the handle at the rear of the knife. The fact that the handle moved 1/16" again is a good indicator something isn't right.

So, I'm guessing seasonal strain in this wood is enough to overcome the strength of the epoxy. Not exactly sure what the next step is. My first thought is to pull both scales off, hollow them out, and put some sort of reinforcement in them that is mechanically fastened to the tang. Cut down on the cross sectional area of wood, and give the wood and epoxy something to really bite into...

Still need to think about it for a while...

 

[rhinoknives]

One more time, If you are using the 5 Min Devcon epoxy that's part of the problem. Also I always use Loveless of even corby's pins. I insist on a mechanical connection on my knives. Culinary knives are subjected to wet hands, blood and food moisturize constantly.

Oak is not the best candidate for stabilizing, but I've had it done with usable results. Get G/Flex epoxy and use some liners and a mechanical connector and you should be fine.

 

[BossDog]

One more time, If you are using the 5 Min Devcon epoxy that's part of the problem. Also I always use Loveless of even corby's pins. I insist on a mechanical connection on my knives. Culinary knives are subjected to wet hands, blood and food moisturize constantly.

Oak is not the best candidate for stabilizing, but I've had it done with usable results. Get G/Flex epoxy and use some liners and a mechanical connector and you should be fine.

agreed..

 

[dkemppai]

One more time, If you are using the 5 Min Devcon epoxy that's part of the problem. Also I always use Loveless of even corby's pins. I insist on a mechanical connection on my knives. Culinary knives are subjected to wet hands, blood and food moisturize constantly. Also, can you elaborate on why you feel oak is a poor choice for stabilizing?

Oak is not the best candidate for stabilizing, but I've had it done with usable results. Get G/Flex epoxy and use some liners and a mechanical connector and you should be fine.

Hi,

Not sure why you think this has blood, food and moisture on it. It's for wood carving. Never been wet. A bit confused, sorry... No five minute epoxies, ever. Missed responding to that in previous post, sorry again. The G/Flex looks nice, have not used it. Will order some today.

Can you elaborate on why you feel oak is a poor choice for stabilizing?

Trying to figure out how to do a blind mechanical fastener for this one. Customer doesn't want visible pins. Might have to go with pins, but for now looking for alternatives.

Tracy, got any catalog parts for a blind pin or fasteners to look at? Something with a lot of bite would be great.


More info. Took a screwdriver and pried the scale off. It left epoxy and wood on the tang. Also, checked with a straight edge, and the tang is bent by the still attached scale! Not sure how to release the epoxy on the still attached scale. Tried a chisel/knife edge, broke the chisel. Tried flexing the whole thing in the vise, bends a bunch, but epoxy won't let go. Will try some heat next, most epoxies give in to temperature...


Other suggestions greatly appreciated!

Thanks,
Dan

 

[dkemppai]

Update.

With a heat gun brought the tang temp up to a couple hundred degrees F. The heat on the tang softened the epoxy. The remaining scale pulled away from the tang once everything was hot. The tang straightened out perfectly again. No more bend.

So, The wood is pulling hard enough on the tang to bend a 1 inch wide tapered tang 1/16" of an inch. The wood is only .350" thick! I'm impressed!

-Dan

 

[rhinoknives]

Oak, is already a very hard wood most of the time. The Oak I see around here certainly is. Stabilization is done for the most part for woods like Buckeye first, that's not usable unless its well stabilized then for the medium woods like Maple. Then there are woods like Oak, Walnut and others that will take little of the stabilizing agent due to the oils and acids present in the woods naturally.

I got the impression that this was a food carving knife. Never saw a blade? Just a handle.

 

[C Craft]

Update.

With a heat gun brought the tang temp up to a couple hundred degrees F. The heat on the tang softened the epoxy. The remaining scale pulled away from the tang once everything was hot. The tang straightened out perfectly again. No more bend.

So, The wood is pulling hard enough on the tang to bend a 1 inch wide tapered tang 1/16" of an inch. The wood is only .350" thick! I'm impressed!

-Dan

I am wondering if that is your culprit????? I am wondering if the lateral flex of the blade is putting enough strain on the handle to start the scale breaking loose! If the blade is flexing enough to break the handle loose, you won't be able to overcome that with out a fastener that goes thru the handle and is exposed on the topside of the Oak scales.
Have you ever watched him use this blade?? That may be your best clue as too why the scales are popping loose.

I know you said this was for carving but what type of blade is this???

Can you show a picture of the whole knife? Is it like this set of carving knives???

Oak being a hardwood it is very dense, the epoxy doesn't really have an opportunity to soak into the grain to help the bond to the steel but, the drilling of holes in the back side of the scale should have over come that!

 

[dkemppai]

I am wondering if that is your culprit????? I am wondering if the lateral flex of the blade is putting enough strain on the handle to start the scale breaking loose! If the blade is flexing enough to break the handle loose, you won't be able to overcome that with out a fastener that goes thru the handle and is exposed on the topside of the Oak scales.
Have you ever watched him use this blade?? That may be your best clue as too why the scales are popping loose.

I know you said this was for carving but what type of blade is this???

Can you show a picture of the whole knife?

Oak being a hardwood it is very dense, the epoxy doesn't really have an opportunity to soak into the grain to help the bond to the steel but, the drilling of holes in the back side of the scale should have over come that!

Hi,

I think you are right, it's strain from something. Stresses should be highest at the front edge of the handle near the middle of the knife, where the bending load is the highest. However, the scales started peeling off at the rear of the handle, not near the blade where you would expect.

Since the wood starts peeling where bending stresses should be minimal, my guess is the strain is coming from the handle, not the blade. It could be due to the dry house causing the wood to shrink. If the wood shrinks, it would put tremendous mechanical load on everything. It appears to be pulling hard enough to bend the tang.

I've watched this guy carve. His style uses a lot of draw cutting, pulling the knife towards himself. He also push cuts with his thumb on the back of the blade. Both with pretty good force, but he doesn't pry or or twist with the knife. He 'hogs' a lot of material off, but does so without straining the knife more than necessary. He really does handle the knife well.

He likes puukkos. We came up with a similarly shaped knife. Because of his style of carving a long flat grind was chosen. We widened the blade out to give some strength back due to the grind. For strength we chose a full tang over traditional the traditional Finnish tang design. The tang was tapered to help balance the knife out. The knife is about 8 inches long.

I agree the extra holes in the handle should have taken care of it! My fear right now is that this particular piece of wood moves a lot with moisture. No matter what, that movement will still be there. Something has to give. If it's mechanically fastened really tight, the wood might bust up....

Here's some shots of the knife. There are two shots taken from the microscope of the epoxy left on the tang and the scale. You can see wood left in the epoxy in on the tang.

Thanks,
Dan

P.S. To All: Thanks for the ideas and input. It helps a lot to hear your thoughts.