Fillet Knife KITH!

[Ray Rogers]

It's about time for the next KITH!! We normally try to design these exercises so that they encourage you to try
something you may not have done before and this one is intended to do exactly that! In this KITH the goal will be to
build a fillet knife that is flexible. Not all fillet knives are flexible nor do they all need to be but for this KITH flexibility will be the goal. You can make the knife any size that you wish and use any materials that you want as long as the blade is flexible. I suggest that a blade length of 7" is probably the minimum you'd want to try and that most of you will want to use carbon steel although stainless is possible if you can handle it.

There are basically two ways to make your blade flexible: one is by the heat treatment and the other is by its geometry. You can choose to give up some edge holding in exchange for flexibility (this is what most inexpensive commercial fillet knives do) and that is fine for this KITH if that is your choice. The other way is by geometry or a combination of geometry and heat treatment. The one thing I would NOT suggest is that you try to do a differential heat treatment (or a hamon) on this knife because fillet knife blades are too narrow to make this practical. If you are very careful you might be able to use a torch to draw the spine just a little but even that is iffy. If your blade is stainless then nearly all the flex will have to come from the geometry.

The attached picture will illustrate what I mean by geometry. Starting with steel that is 3/32" (.093) or so you can do a distal taper as shown in Fig A. At least as important as that taper is the profile of the blade, indicated in Fig B. Depending on the steel you choose and the heat treatment you use the profile will vary from Fig B but something similar will be need to get the right flex.

What is the right flex? If you look at Fig C and Fig D you can see that the tip of C has bent faster than the rest of that blade. This is what can happened if the last few inches of the blade get too thin too fast. A blade like this can cut the meat in a wavy line as the too flexible tip 'floats'. Ideally, your goal would be more like Fig D.

You should make a cover or a sheath for your fillet knife. I usually make a kydex cover which is simply a sheath without a belt loop which offers protection for the knife (and the user!) when it is in the tackle box. You can make a sheath or a cover using kydex or leather as you prefer.

There is no need to make this a fancy knife but you can if you want to. I have attached pictures of a couple I have made for reference but there are many others on the internet. One of mine is fairly fancy with a stabilized wood handle, the other has a simple and practical Neoprene handle.

This KITH will run until Halloween so there is plenty of time for you to perfect your blades. Anyone who wishes is invited to participate all I ask is that you post a response below if you want to join in. There is no deadline for joining, you can wait until the day before Halloween if you want to, but the sooner you can let us know the better . Have fun!

[jimmontg]

My filet knives have some flex Ray, but not that much. Of course I haven't tried to bend one that much.

[jimmontg]

Ray I also have another issue.
I did the distal taper on my filet knives, but I used CPM S30V for my last batch, (0.90 thick). I had to replace one because the deckhand was in the habit of just standing there flexing the knife over and over and after about 2-3 months of this the blade work-hardened and broke just about 1 inch above the ricasso. I replaced the blade with an extra I had. (Did my attitude no favors though)

I had Peters HT do my blades to my spec of 59-60. How low will I have to spec the HT on the KITH? Or should I use something different like AEB-L and stay away from the high Vanadium series?

I never made a very flexible O1 blade, from filet knives from my experience as a commercial fisherman I have always gone for abrasion resistance over flexibility for carbon steel knives. I have never made a filet knife out of anything other than Nicholson Black Diamond files, O1, D2 and CPM S30V. I have never made a knife that won't hold up to cutting through bones and heavy scaled skin.

The reason I ever started to make my own knives (Nicholson) is because I was fed up with having to keep sharpening my knife after 3 to 6 fish. Big fish bones wear out regular filet knives rather quickly.
You filet 3 tons of fish and you may understand where I'm coming from. My freshwater filets have more flexibility because they are thinner, but no less hard. The work hardening aspect is my personal issue l guess.

[Ray Rogers]

Jim,

As I mentioned in the write up for the KITH the idea here is to get people to try things they haven't done before, in this case, heat treating and geometry for flexibility. This might not be the best idea for commercial fishing use. You don't have to HT to a much lower hardness to add quite a bit of flex and you'll still be much harder than the inexpensive fillet knives you find in stores. Geometry will do the rest.

I only used 440C when I wanted a stainless fillet knife. S30V is my all around favorite stainless but I [I]feel[I] like (having never tried it) it might not lend itself as well to this task. Also, remember that there is no requirement to use stainless for this KITH .....

[elginrunner]

This will be my first Kith... I will endeavour to make a good one!!

[KevBooth]

I知 still new to the game but I love these things. Im afraid my knife ended up at the bottom of the heap last time, but my skills have increased.
I知 in

[dtec1]

not sure yet gota see how things go over the next month or 2 but we do got a lot of time so most likely ill be in.....should be a fun project tho i only tried to make fillet knife once and well i finished it but it didn't flex any where near what i wanted to so i would like to give it another shot....ill jump in when i can

[KevBooth]

I知 mainly a stainless guy, I like AEB-L a lot just because where I hunt is all brackish water and I知 not the best at maintaining my stuff. That said, what痴 a good carbon steel, for this endeavor? If we池e going to try new stuff may as well throw a new steel in there too, right??

[Ray Rogers]

You can use the AEB-L if you prefer. For carbon, O1 would be a great choice and it is available in any thickness you might want. Other carbon steels like 1084 or 1095 would also be good but they are usually sold in 1/8" or thicker sizes. Most any carbon steel you can heat treat will do if you can find it in the size you want ....

[jimmontg]

I think I'm gonna make the blade out of AEB-L as it is capable of high hardness, but still flexible. I do not know if the high Vanadium steels lend themselves to the flexibility that you've spec'ed out Ray. Dave of "dtec" fame said he will HT for me per my instructions so l may try the S30V. I don't have any left except one .070 thick blade and it does seem to be quite flexible at 59-60 hardness. I think I'll give it a few tests.

[KevBooth]

Question about the order of operations for a knife like this. Normally I profile the blade then grind the bevels. Obviously I would need to add the taper operation for a knife with the appropriate profile. What would be the best order to do that in? I知 not forging, so all stock removal for me.
Thanks in advance.

[Ray Rogers]

I also do stock removal, all my fillet knives were done that way. In every case I profile the blade, heat treat it, and then grind it. I start the grind by doing the distal taper but I leave a little extra on until later. After the taper, the bevels are ground. After that, test flexibility and grind or taper or both as needed....

[KevBooth]

Given the slimy and slippery nature of the job most performed by a fillet knife, what are some of the better options for handle material?
Also, what is a good all around working blade length? For say, like a lower 48, regular guy? There is so much variation in the fish work I e having a hard time deciding on a good working blade length.
Thanks

[Ray Rogers]

You can use most anything for the handle if you make the shape ergonomic enough. I've used stabilized burl woods this way.

Or, you can use a 'practical' material like Neoprene or cork. I used Neoprene quite a bit by pinning it to the tang with 1/4" Micarta rod.

As for the length, I'd say at least 6" and probably not more than 9" will do just about anything you can pull out of fresh water. You're free to make this knife any size you like but 6" - 9" is enough to get the flex that we're shooting for with this KITH...

[mwhuston]

Here is my first attempt at a fillet knife. 80CRV2 with brown micarta. I'm interested in your opinion/insight on the flex.