Anybody know how to grind a rhombic puukko? Geometry?

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coldsteelburns

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Aug 2, 2010
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Hey guys, It's been a long time since I've really had any interest in any "scandi" ground or bushcraft type blades, but lately I am really loving the "simple" elegance of the rhombic puukko, soo I want to make one! :D . However, the geometry on them seems to be relatively complicated. (BTW, I am doing this one via stock removal btw, since my frrge area is a bit inaccessible at the moment and because I already have one profiled out lol ) Here is an example of a Rhombic "ground" Puukko. (I think they're normally reffered to as a "Tommi Puukko") They have a somewhat diamond cross section but with more "meat" left on the spine.

andrus.jpg


My question is, does anybody here know the process and order of grinding in the bevels on the edge side and the spine side? I've tried searching all over with no luck. I've also tried seaching for pictures of the spines on rhomibc puukko(s), as I've never owned or held one, and can't find any. The reason I was looking for spine pics is because I'm wondering how the spine side bevel terminates where it meets the tip, as well as how much thickness is left on the spine itself.

Also, I can tell it's not as simple as a straight up "scandi" grind that runs parallel to the edge, since the bevel is higher (thinner edge geometry) near the handle and the becomes lower (more stout edge geometry) towards and at the tip, which is another aspect I'm not sure about how to go about grinding.

As mentioned above, do any of you know how the spine side bevel terminates where it meets the tip area? If one is looking at it from the spine, is there a slight thickening right where the two bevels intersect, or does it flow into the tip area and remain the same thickness? I keep imagining going to grind in the top bevel, just to end up with a spine bevel that terminates at the tip, thus making something that resembles a double edged blade. Is the spine grind just as simple as grinding a spine bevel (leaving the spine thicker than a false edge or course) and then stopping about a half inch from the tip?

Grind spine first, or edge first, I do not know? I would deffinitely use a jig if I was doin a "pure" scandi, since I never do them and I'm not the best grinder ;) ... However, since the geometry seems to change from heel to tip, I can't see that helping... BUT! This guy does use a "single angle holding" jig on his belt sander, starting @ 3:12 in the video posted below. And it looks like he uses it for both spine and edge bevels.. :confused: And FWIW, it's part 2 of a 2 part video.

I appreciate any and all help as always. :)

[video=youtube;CQfiIr8lyoA]https://www.youtube.com/watch?v=CQfiIr8lyoA[/video]


~Paul
My YT Channel Lsubslimed
 
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Hi there, it turns out the original Tommi knife was developed by a relative of mine (although I don't know that side of the family). These knives vary in geometry quite a bit per manufacturer and use. The type requirements are simply that there is no guard or fullers, and a "narrowed" spine that's straight in profile (originally slightly up-turned). A well-made model (especially if it's long) will have distal taper that results in the bevels becoming shorter toward the tip even as the angle stays constant. It can be very thin near the tip - these knives are known for their lightness. Or a short, stout blade may be made with no distal taper whatsoever, and the back will display a pronounced widening where the grind line runs out but it's still called a Tommipuukko. Here's a photo from one of the manufacturers in Finland that shows this: Kainuun Puukko
 
I've made a few blades with this cross-section, it is in many ways very similar to the geometry of a katana. The primary bevel stays the same over the length of the edge, the bevel getting shorter near the point is due to the distal taper of the blade, not a change in the angle of the bevel.

The first few I made, I ended up chasing the ridge where the bevels meet, struggling to get it the same on both sides and where it was supposed to be... those first few ended up much thinner than originally planned due to this. I think the key to grinding this cross-section (and it works equally well for double-edged blades) is to maintain a flat along the ridge until the final grinding and polishing steps. As one proceeds through the grinding steps the flat gets progressively narrower, and in the final stages it disappears entirely. Done right, you end up with a good crisp ridge right where you wanted it to be.

Concerning the spine-side bevel termination... If you have two bevels, one terminating in an edge, the other not, they will naturally do this when they come to a point.... the unsharpened spine bevel will terminate before the sharpened spine bevel. Try grinding this shape on a scrap piece of wood and you will see what I mean. To complicate things, on some puukkos (maybe most) this is made more deliberate, with the spine bevel stopping well before the point... the one you posted the photo of is a good example of this, as is a traditional katana. This gives the point extra material, strengthening it.
 
The ones that I really like are where the center line goes to the tip like with a dagger. That would require some pretty carefully controlled tapering and would require the same skill and care as a dagger, just with the top bevel being a bit short. . I have had such a project on the bench for a while, but haven't got around to trying it. if I go by how I grind the clip of a bwoie today, which is to grind the clip at almost a uniform width to the tip and then find in the main vbevle and taper, I would think that the more traditional puukko top bevel would be the result, much like when you forge and grind an Japanese style blade.
 
The grind is basically the same as a tanto. In Japanese it would be called shinogi-tsukuri.

Grind the main bevels up from the edge to above the desired center ridge (shinogi).
Carefully mark the desired ridge location on the blade sides.
Start at the spine and "walk" the upper swedge ( mune) down to the desired centerline. The spine can be a false edge or have a small flat.
Lapping the flats on a surface block with sandpaper taped down can make the ridge lines sharp, even from side to side, and straight.
The bevels must carry up onto the tang area.( There is no ricasso)
Once the bevels are shaped, file the shoulders in the tang so it will seat snug against the guard. Taper the tang as needed to assure easy insertion and fitting to the guard/bolster.
Carefully fit the guard/bolster and soft solder the blade on.
 
I tend to grind the primary bevels, thengrind in the spine bevels heel-to-tip on a contact wheel. It allows you to blend the transitions a bit more, and I find it easier to see what's going on when the grinding marks are perpendicular to the primary bevel. Just how I do it though.
 
To expand on what Stacy said, the "good news" is that you don't have to transition from the rhomboid grind of the blade to a flat grind on the tang at the transition. Also, at some point in the process, you can bring lengthwise on your platen because their is no plunge cut. . The bad news is that you have to file a rhomboid hole in the guard. Back to good news. Scandi knives don't typically have the very wide tang of a Japanese blade, so this not a huge rhomboid hole and th tang doesn't have to carry that shape all the way to the end. It just has to taper.. :D
 
Here is how I do this. First I grind the edge and then I grind the spine,at the same angle,until the spine grind "meet" primary bevel. Is visible on second picture.
If you do that precise,you'll get nice distal taper blade,like on third picture.





 
That is the way I was saying to do it. make the four flat bevels and then cut the shoulders. The hole won't need to be a full diamond that way.
 
Wow, I couldn't have asked for better responses! :thumbup:

Stacy, It's funny you mention shingo zukiru (tsukuri?) because I now remember that I used to think that these rhomboid puukko looked a lot like traditional Japanese style blades. I just happened to conveniently forget I made that connection before, otherwise I would've just been searching for japanese blade geometry threads lol. But I'm sure glad I asked here, there's a lot of helpful info in this thread that hopfully others will learn from. Thanks :)

jdm61, I actually just happened to come across a thread of yours (after making a google for "shinogi zukuri puukko") on the bushcraft usa forum that you started in Feb about the is exact same topic, some good info there as well. After seeing the work you're capable off, I really think you're letting the diamond slot in the bolster get to your head man! All ya gotta do it just file check, file, check ;) :D lol I'm gonna use some 1/2" wrought iron I have for the bolster on the one that I'm in the process of making now... as long as the blade doesn't end up trashed. :foot: When (if) I get to that step, I can take & post some pics of how I'll go about it here if you'd like, and if I happen get a good fit, maybe it'll give ya the boost ya need to finish yours, (considering I've yet to attempt filing any type of fitting for a hidden tang) :D

I just happened to "finally" get myself a 9" disc grinder, vfd/reversible from Jose Navarro @Pheer Grinders, and man it is already making life a heck of a lot easier!! I LOVE that thing!! :tears_of_joy: . So anyway, here are the steps and their order I am goin to take on the one I've already begun (it's Aldos oversized 1/8" 1095 btw) :

--- (1) I already have the main edge bevels ground on the blade, but the "plunges" only go about 1/4" behind the shoulders, with the rest of the tang remain flat, for now that is. (2) Next I'll heat treat it, make sure it's all straight and then true up the flats and tang on the disc. (3) Then I'll use my handy dandy B. Bump file guide the do the shoulders. (4) After that I'll grind the top bevels and bring all the "plunges" further back to about an inch or so from the shoulders, (6) and finally I'll taper the tang on my disc grinder. Then of course I'll hand sand and start the handle etc.. I'll probably use some edm stones on the bevels to get them nice and flat, but mainly to achieve a nice and crisp shinogi.

If anyone see's a reason those steps won't work, please feel free to correct me!

I'll "try" to post some pics here as I go along with this project... But first I have to hurry up and finish a "re-handle" job on a cheap stainless kitchen knife for a family frined. BTW Stacy, I finally decided to go with Corbys for that project (as opposed to peened SS pins) I was able to (very luckily) buy 6 perfectly sized, "counterbore w/pilot" drill bits (3 for the small 1/8" head corbys, and 3 for the 3/16" head corbys) from another knifemaker on facebook for an amazing deal only $28! Pretty much what it's costs for just 1! At least when I looking for them at the usual knifemaking supply sources :eek:

Anyway, thanks again for the repsonses fellas, All of them have been very helpful and it's always much appreciated! :encouragement:

~Paul
My YT Channel Lsubslimed
 
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Sounds like a plan!!! As far as using stones to get things flat, one of the Dutch guys told me that he actually uses a big medium grit DMT diamond stone/plate for that purpose.
 
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