Experience building a Farmer's Jack Folding Knife

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Intro  

I recently finished what has been my most challenging build to date: a double ended jack knife (two blades sharing a single spring). Specifically, a rendition of the iconic Farmer's Jack (FJ) pattern.  Compared to a basic single blade, single spring, the challenge lies in nesting the two blades together when closed, and dealing with two blades tensioned off of the same spring. 

My approach simplified the architecture slightly by relying only on offset (aka asymmetrical grinding) grinding to achieve blade clearance, as opposed to needing to crink a blade. I also opted out of a  catch-bit based design. 

Though I also introduced three new processes which significantly increased the difficulty but this would have applied to any type of build: 1) crescent nail nicks with a fly cutter; 2) mill relieved liners; and 3) jigging and dying bone.  I had to redo the blades twice learning the nail nick and redo the liners twice with the milling.

In the end, I'm very proud of what I accomplished and wanted to share some of the process while it is fresh in my mind. This construction was just right in that it pushed me to my current limit. However, it was fully achievable with my skillset and equipment. So I hope to encourage other makers who are at my similar level with some single blades under their belt to tackle a build like this. 

The audience of this article is primarily other beginner pocket knife makers, but hope this will give collectors and others some insight into what is under the hood of this and other similar knives.   I had a lot of help and encouragement from the community and part of my mission is giving back by sharing. This is not an end-to-end how to as there are other resources . 

The organization of this article is as follows: I'll start with some background on how I even ended up taking this build on, discuss some of the nuances of this patterns geometry, overview of how I simplified the approached and limited the build to offset grinding, and overview of how I approached setting the tension. We'll wrap up with some glamour shots and thoughts on what I'll do different next time.

Be forewarned, I'm an over thinker, over analyzer, and often make things more complicated than needed. I am a beginner so prove out anything I suggest yourself.

Background

In January 2023, a collector reached out and asked if I was taking orders and could make a Farmer's Jack. He shared some pictures of the pattern and showed me some customs from one of his fellow collectors including a marvel from Ken Erickson.  I took one look at it and immediately responded with a hard no. I was several years into single blade making but still struggling and couldn't even bring myself to consider a two blade single spring. But a seed was planted about that single spring two blade design. I've always like the muskrat and others.

Fast forward to March 2024, and I received another inquiry on All About Pocket Knives where I often post some completed items in the customs board.  

Would you build a farmers jack?

Examples of the pattern:

In brief discussion, they offered to send some old ones to study. At first, I had the "I'm not ready for this reaction" and politely turned it down.   Then a few days later, I reconsidered. Thankfully realizing what an incredible opportunity this was. A chance to get an actual vintage one, which are incredibly hard to find, and ability to collaborate with someone with incredible knowledge on the pattern. 

Thus I was introduced to Lyle Fry, aka wlf on AAPK and wlfryjr on Blade forums. Who also turns out to be the friend of the first person that reached out to me and the owner of the Ken Erickson FJ that he showed me. It's a small world to begin with, and an even smaller world in knifemaking and collecting, and an even smaller one in the world of FJ.

I learned from another friend that in the world of pocket knife collecting there are collectors and then there are COLLECTORS. Super collectors if you will and subject matter experts (SME). Lyle is one of these well respected SMEs but humble to the core. This includes having one of the largest collections of Farmer's Jacks around, being central to the popular GEC #38 Farmer's Jack, and being a major contributor to the two mega threads on Farmer's Jacks on AAPK and Blade Forums: "The Roosters are Crowing". 

After some initial phone conversations he put two old ones with broken blades/ scales in the mail for me. I asked over and over again if it is ok I crack them open.

The FJ Pattern observations

The Farmer's Jack is an iconic pattern representing a working knife for horticultural uses. Levine's Guide to Knives classifies it as a double ended jack knife. Like any old pattern, there are variations on nominal features that include a stout main pruning blade and a more delicate secondary blade, usually a spey. The curvature of profile probably evolved for comfort over hours of use but also ended up alluring. 

The FJ also goes by the name "Rooster Comb". Paraphrasing discussion in the forums, this term applies to a a "spud" secondary blade instead of the spey. Some call the spey blade secondary a Standard Farmer's Jack and the Spud blade a rooster comb because of the multi-curves in the spine of the spud. Below is typical example of standard. Picture source: Old Schrade Catalog

From a maker's perspective, the things I'm studying include 1) swedges (from aesthetic viewpoint and functional to improve blade clearance; 2) asymmetrical features like the spring pivot point and length of each blade's spring and of course asymmetrical grinding; 3) size and shape of each blade's tangs; 4) thickness of spring at the tang which is related to the tang walk up height; 5) use of a catchbit;  6) were any blades crinked; and 7) embellishments like fluting (small flutes vs rat-tail flutes), threading 

Lyle sent me two vintage FJs, a Henckel and a Camillus shown below in that order.  The Camillus uses a catchbit and the secondary has more of the profile of the rooster.  

Swedges

Many of the standard FJs have a long swedge at a low angle only on the inside of the spey blade.  My understanding of this asymmetrical swedge's purpose is to help with blade clearance. I did put one of these on my secondary, but it got washed out in the hand sanding. It was very difficult putting the swedge on at a shallow angle using a jig to hold he blade. the secondary blade is tiny. I always ended up with what I call a 'smiley' at the plunge which is a small curved section where more is ground away near the plunge. This is an artifact of starting the grind too often from the plunge and sliding the blade over the belt, as opposed to just pushing the knife/jig straight into the belt and straight  out I think.

The swedge on the main blade is symmetrical and seems more aesthetic.

Asymmetry in Grinds

Any double ended jack knife is going to require some type of asymmetrical, also known as offset, grind in order for the blades to clear each other when closed. An interesting subset of FJs are ones with chisel ground (one side totally flat) main blade.  The flat side seems to go to the outside. The next sections will go into detail how I approached doing offset grinds.

Spring and Tangs

The spring pivot hole is not in the center of the spring. It is closer to the main blade. The combination of variables that include length of spring (tip to pivot), size of tang, tightness of the radiuses of the tang, even difference in hardness between the tang and spring, and difference in final grits is ones of the most interesting rabbit holes in slip joints.  It is a multi-objective optimization problem where you have multiple inputs, and multiple outputs to optimize. In this case, the outputs are smoothness of pull and snappy walk and talk (galling of spring/tang is another output).

The longer the lever arm of the spring, the easier it feels to open the blade. But you can still get snappy walk and talk by tightening up the radiuses of the corner. The risk is if they are too sharp, the tang will dig into the spring.  

So here, the lever arm shorter on the main blade. The tang is also bigger which means the spring has to move further during the rotation of the blade.  I believe this yields stouter tension which makes sense if this is the heavy working blade. With lack of locking blades, you'd want it stiff.

Speaking of walk and talk. I live for snappy walk and talk. This is what keeps me coming back trying to achieve that slick pull and snappy action without having to rely on nail breaking tension. So when asked if I was going to do a cam tang or halfstop on the main blade I immediately said halfstop.

However, most of the knives in Lyles collection have cam tang on the main blade. For secondary there was a variety with most having half stop.  After having built one, I understand fully why cam tang is desired on the main blade.  The curvature of the hawkbill blades can really bite if not careful. More than a few times, I was closing the blade and it snapped slightly into half stop with the tip taking small nick out of the heel of my palm.  Next time, I will do cam tang and get my fix for walk/talk on the full open and close.

To Crink or not to Crink

Crinking is the act of physically bending the blade at the tang area to assist in blade clearance. Not to be confused with slacking  Which is the art of loosening a tight blade and causes me panic attacks as I have broken several blades slacking.  

I did not attempt a build with crinking to simplify as many things as possible. For me, crinking is one of those feared mystical capabilities that seems well beyond my skillset. When most likely, it is just like every other skillset in slip joint making: it is hard at first, then is nothing but a thang. 

I have the Shadley workshop copy on making multi-blade folders. It shows a simple crinking jig. I've seen GEC use a kick press for final centering of their blades. But I think they temper back their tangs as part of this process. 

Regardless, I decided it was enough of a stretch and complication to just stick with offset grinding. I wasn't sure when the crinking would happen, before or after heat treat. After the blade is bent, how to deal with any more processing on the tang and keeping it flat seemed challenging. I saw in the Shadley book supergluing the tangs to surface grinder mag chuck.  I only have a surface grinder attachment for my belt grinder so another problem solving step to address.

Catchbit

One of the patterns I received had a catchbit.  It didn't give it much consideration to try. My understanding is that it is essential a spacer that lets you build one of the blades longer. Without it, the blade length is limited to where the plunge is on the other blade. It also acts to take up some thickness so the need for offset grinding one of the blades is mitigated.   Picture source: Bladeforums

The downsides are 1) it's another piece to deal with; 2) it needs to be relieved to to mitigate scratches; 3) it requires controlling thickness of one of the blades to an exact target.  I have a surface grinder attachment for my belt grinder.  Getting a blade and spring to the same thickness is one thing, but getting one of them to an exact thickness is another.  This is one of the downsides of using bronze washers, the blade has to be 2x washer thickness thinner than the spring.

Embellishments

Bolsters, flutes, threading and file work all fall into the embellishment category not being really functional. Most FJ's seem to have what are called rat tail bolsters. This is similar to a flute but much wider. I use a small chainsaw file and edge guide to cut in flutes. The challenge with rat tail bolsters is where the pin ends up. With small flutes you can usually get on close to the edge of the bolster and fully away from the pin. With a rat tail, the pin is going to be inside the wide flute.

The issue is hiding the pin. Blending a pin into a bolster is another of hose holy grails in slip joint making along with centered blade, flush in all three positions, slick pull, and snappy walk and talk.  With the pin inside the flute, I was little unsure how to do the final peening and flushing down the pin.  Again, it probably would not have been that bad, but I just decided to go with a few threads.

Deciding my Build

I decided to build a FJ with no cachbit, no crinking, just asymmetrical grinding, with half stops on both blades, with threaded bolsters and end caps. This was the most straightforward approach that I thought was achievable with my skillset and tooling. 

My Build Strategy

Having hard pattern to scribe from is ideal compared to paper. I initially started with the Henckels as the hard pattern. However, I ended up messing up the blades when putting in the crescent nail nicks. So started over. The problem was I had sent the blades off to another maker to study.

I often make drawings from catalog pictures or scans of real knives. I took the scans and made this drawing using Inkscape 2 D CAD.  One of the challenges with these CAD drawings and then printing out to paper is scaling.  There are rulers in the CAD borders, and I sometimes draw my own ruler or put a picture of a scan in there.  There are multiple scaling variables that include settings in Inkscape, and then print to full size check boxes.  I have tried printing to PDF first and then to paper.  Frankly I have not figured out how to get the final print to match 1" to 1" on the drawing.

I was partway into the second build based on the paper drawing and was comparing against the old first iteration blades. Hmm, this secondary seems a lot shorter and taller than my first one.  So when you see mine side-by-side with a typical, you'll notice it is more compact and 'squished' with shorter/taller secondary and shorter than ideal proportioned main blade. I think I had dragged the corner and instead of holding down shift while dragging to maintain X and Y proportions it squished the parts.

Approach to offset grinding

With offset of asymmetrical grinding, you essentially move the center line of your blade closer to one side.  In a typical build, the goal is to have your edge centered on the steel.  A common approach is to scribe two parallel sight lines down the center of the edge.  To determine where to put those scribe lines, you take your steel thickness, subtract desired edge thickness and divide by 2. Then grind 45 degree bevels to the site lines, then do your edge bevel.  

I was having some trouble visualizing and thinking about how to create those site lines with an offset grind.  Things going through my head included offsetting the secondary blade more so the main blade could stay thicker and stouter.  Considering whether the total of the offset of each side, if put into %, should total 100%. 

To help me think about it, I made a little calculator  spreadsheet that spits out where to scribe lines. It takes steel thickness, edge thickness, and offset (in terms of %) as the inputs, then kicks out two values referenced off one side of the blade for scribe lines, like the sketch below.   I also scribe a line on the spine as a guide for hogging off material.

It was completely arbitrary, but ended up with 30% offset on the main blade and 55% offset on the secondary blade.  There was not much reasoning other than I wanted the main blade thicker. These are just targets, I did not measure what it actually came out to be. In knifemaking, for me at least, scribe lines are just suggestions and things end up where they end up sometimes.

With this offset grinding, you need to remove a lot of steel from one side.  I was thinking of doing it in the mill. But when I tried clamping the blade in the mill vice it was crushing the kick.  If I'd one it before cutting out the blade shape, maybe a mill would have worked.

I ended up putting a file guide on the blade and holding it against a 1-2-3 block and hogging material off on the flat platen.

This seemed to work well enough.  One issue, is I hollow ground the blades. The fillet of the grind when hollow grinding did not intersect perfect with the offset grinding. If I flat ground, which is more typical of this pattern, it would have looked better.

After this hogging off step, the grinding proceeds as typical of any blade.

In hindsight, I might have used too much on the secondary blade. I ended up with plenty of clearance. I think a goal is to get them as thick as possible while still clearing. 

The secondary blade is thin and flexy. But maybe that is ok as it is for lighter work.  During the grinding the blade was so thin, it sucked down in the gap between the tool rest and my 10" wheel.

Approach to setting spring tension

One way to set the tension is to just use the same holes from a hard pattern.  Sometimes this works for me, but I like to set the tension manually. Plus, I didn't have the pattern in my possession when at this step.  

For single blades, I use the method Luke Swenson does, where you drill the center pin, and scribe a hole in the un-tensioned position with the blade in place against the spring. Remove the blade, and rotate the butt of the spring up until the hole in the spring has 1/3 overlap with the scribed circle drawn.  Then clamp the spring and drill.

This doesn't really work in this case because there are two blades sharing the same spring. But the concept still holds of offsetting the holes for the blades so when all the pins in the blade and spring are under tension.

I essentially did the same approach but did it with the blade holes.  I clamped everything up with no tension and drilled the spring hole.  Then scribed circles under the blade pivot holes. Then removed the spring, then individually offset each blade up and scribed new circles. I should have just clamped the blade down and use the hole of the blade as the guide. less chance of error.  

It was arbitrary how much I offset each hole. But I tried to offset the secondary blade more because of the of the previous discussion on spring lengths and tang size.  In hindsight, I should have created more tension. Like  a hair cut, you can always remove tension by removing material from the spring, but it is much harder to get it back.

I put the knife together by putting the blades in with their pivot pins, sandwich the liner and spring and use a tapered pin into the spring hole.  It is tricked when the spring is in their free trying to get everything aligned.

Putting together and taking apart a slipjoint, especially at the beginning when tension is supposed to be higher than the final is a procedure I need to think through more. There are several 'detensioning' devices, methods for making this easier and mitigating wear and tear on the holes in the liner.  

End result | Changes for next time

As promised, here are some final shots.  For the more work in progress, check my instagram

• OAL: 3 3/4" 
• Steel: 1095 Carbon
• Grind: Hollow 10" wheel
• Liners: 410 SS
• Bolsters: 416 SS threaded
• Shield: 416 SS
  
• Mill relieved liners
• Pinned Shield
• Scales: Self Jigged Cow bone, dyed with leather dye

Whew! Getting to the finish line with any slipjoint is an accomplishment. This one took some perseverance with the combination of learning curves overlapping. Between cutting nail nicks to jigging bone to double ended pattern is was a combination of big steps forward. 

Things I will do differently, once recovered, include 1) paying more attention to the design/profile.  I'd make the blades longer, going for slightly more traditional look; 2) offset the blade pivot holes more to start of with more spring tension initially; 3) not do as much offset grinding to try and squeeze as much material in there; 4) try to get the swedge on the secondary to be more prominent. It sanded out during the hand finishing; 5) possibly try flat grinding.  I've invested so much time to get to hollow grinding, and feel more proficient at it than flat now.  I am reluctant though to go back to flat; 6) I do want to figure out the taper on the fronts of bolsters. This has eluded me; 7) rat tail flute; and 8) cam tang on main blade.

I would get some narrower stones for my Hapstone sharpener also. This would help in sharpening that hawksbill.

Conclusion

If you have made it this far, thanks for sticking with me. In summary, the double ended jack knife, when simplified to offset grinding only is doable for makers of moderate skill level. All the concepts of tang geometry, griding, spring tension hold just modified.  The Farmer Jack it self is an intriguing pattern with rich history and passionate collector base.  The knifemaking community and ecosystem continues to be an amazing thing and I appreciate the help and discussion from several makers including Knife Rehab for planting the seed of this knife in my head early on,  Reuben James, Zach Hoskins, Jason Cattadoris of Tulip Knife Company, Mike Rothman, and Ken Erikson. And finally, thanks to Lyle for the push, constant cheerleading, and wealth of information.

Very Respectfully,

Ashwin

On deck next, some Dog Legs based on New York Knife Hammer Brand pattern # 2469. Picture source: Old New York Knife Catalog

Useful Resources:

[1] Various Contributors, "Farmer's Jack." All About Pocket Knives Forum,  2011-2024, source: https://www.allaboutpocketknives.com/knife_forum/viewtopic.php?t=26653

[2]  Various Contributors, "The " Roosters " are crowing for the " Farmers." Blade Forums, 2014-2024, source: https://www.bladeforums.com/threads/the-roosters-are-crowing-for-the-farmers.1142705/

[3] B. Levine, Levine's Guide to Knives and Their Values, 4'th Edition, Krause Pubns Inc, 1997.

[4] E. Shadley, T. Davis, How to Make Multi-Blade Folding Knives, Krause Pubns Inc, 1997.