Why *not* a six inch serrated tomato knife?

Yeah - the fixturung was very quick and dirty … just a couple hold down clamps secured into the 123 block. The “backward angle “ was just because of lack of travel on my table. When I do this for real I will need to something like what you are saying. This piece was just to try to identify the angle.

I want to stick with 3/8 because then I can use a 3/8 ceramic rod to clean up after HT. 😊
 
Hubert - sitting at a computer keyboard now, so can type a little more detail. Like I said - this was just a quick and dirty effort to try to identify what kind of angle I should be targeting to mill at. The test piece is just held directly to the 123 block by two milling clamps. I *barely* had enough room to clamp the thing ... and I could not have milled the other side because of the clamps would have been in the way. I definitely got some chatter, so your point about needing to mill with pushing the blade into whatever supports it is right on correct. also, with this setup, I could not tilt the block the other direction because of overhang by the clamps on either side....

That is why I commented I will likely need to create a fixturing plate. I can make it longer, and give more vertical (and horizontal) space for clamping .... but I am still limited by needing to clamp the 2 inch dimension of the block by the size of my vise :-( . I'll need to think about it a little and toy with a couple different plate designs...

You are right about the sensitivity of the angles .... thats because we are playing with a trigonometric function where things change dimensions really fast as the angle approaches 90 degrees..... luckily we will have some leeway to determine how much of that scallop gets exposed by how wide we make the bevel on the opposite side (hopefully that makes sense when you sketch it out.....

I think this will work, and will look pretty nice .... just need to play with a little more with how to fixture the thing....
 
It sounds like you have a plan, and I am pretty certain this will work. I look forward to seeing your progress.

You have a good bit of leeway in grinding, but there are some caveats. If you machine the divots too deeply, you may lose a bit of blade height. There is also a danger of leaving long flats between the serrations, but maybe that is what you are going for anyway. I did not spend a lot of time with this, but I did try out some parameters in a quick OpenSCAD model I wrote and it seemed quite sensitive. I have attached the model (it's plain text, just rename to scad extension) in case you or anybody else is interested.

emfdxkF.png
 

Attachments

That is a cool CAD model, and shows pretty much exactly what i have in my mind (btw, i did not say, but my stock is about 0.075" thick). I am planning on flats between serrations, which might not be optimal, but i thought i would try it...

Ive only been able to get about a half hour in the shop each day, so am not sure how fast ill make progress on this...
 
The previous picture was for 1.6mm stock. The first picture below is for 75 thou thickness, blade height 1", 3/8" cutter milled to a depth of 70 thou, 1/2" bevel with 5 thou behind the edge and the 0.42" spacing you used for when you tested it out on the mill. The second picture is for a spacing of 1/4" with the serrations milled in to a depth of 35 thou. Both use a 15 degree angle. The last picture uses the same parameters as the second one, except the size of the end mill is 1/2".

CBssdO0.png

DSZuQiV.png

zskqq5W.png
 
FredyCro said:
Sorry, yeah. Simetrical ffg or saber grind with serrations on one side. Like
Click to expand...
Lol ... actually a couple reasons are going through my head ... not contradictory i think. First, if i have the serrations on one side and do a japanese-like chisel grind on the other, on the unground side the serrations will be preserved and not partially (or wholly?) ground away, and second, when i grind that chisel grind, i can target grinding at the edge right up to the line where the opposite side meets the flat profile of the edge (well, flat as profiled, but the flat then gets ground away) ... that is a nice clean target to hit ... otherwise you are trying to grind to a centerline, which will not be cleanly visible because of the serrations.

Second, since this is a single-purpose blade, to slice relatively thin sections off of an inherently soft food ... this is exactly what knives like the yanagiba (chisel grind with ura on the opposite side) are used for. So ... after creating serrations, doing the chisel grind, grinding a Ura on the opposite side, i will sharpen the thing in the same manner a yanagiba is sharpened. Kind of a weird hybrid ... but why not try?

(Ok, ok, one could argue why not create a short (six inch) yanagiba, and reserve it for tomato use. My response, i guess, would be mostly "dont confuse the issue with facts" 😊)
 
I am asking because I think thinnest behind the edge would cut the best (eg for tomatoes) and it wouldnt wedge and steer the cut like one sided grind. It might be less of an issue with serrations anyway. Its just what I ve seen with those half serration half normal blades and i think even full serration spydercos have double sided grind eith serration on one side only.
 
FredyCro said:
I am asking because I think thinnest behind the edge would cut the best (eg for tomatoes) and it wouldnt wedge and steer the cut like one sided grind. It might be less of an issue with serrations anyway. Its just what I ve seen with those half serration half normal blades and i think even full serration spydercos have double sided grind eith serration on one side only.
Click to expand...
Honestly - I really do not know the answer. I do know that the commercial serrated knives I have are all double bevel - but who is to say that this is a performance thing or a "marketing" thing (like .... most westerners would have no idea at all what to do with a single bevel blade). What I can tell you is that the japanese single bevel blades (when used correctly for left versus right handed users - i.e. bevel side out away from the piece of food being sliced), when used on soft foods (raw fish being the prime example) do not "steer" the cut at all .... in fact they are really good at producing consistent and flat slices.

But who knows in this application??? Like I said, this is really an experiment. I've been talking with Storm, and with his help I'm working on three different approaches to the basic issue of a dedicated tomato knife - this serrated single bevel version is one of those...

Oh, your comment about "thinnest behind the edge" - now that I think about it I am not really even sure how to apply that concept to a single bevel blade with a Ura. These blades do not have a "primary and secondary" bevel like we think about them - the bevel is dead flat all the way to the edge, and the back (Ura) side is likewise ground flat right to the edge. Those things are SHARP, and when cutting thin slices on soft food they just do not wedge the food (some I have seen recommend putting a slight secondary bevel at the edge (for edge strength) where you could maybe talk about TBE - but not all make such recommendations - and the more reputable japanese sharpeners tend to not make that recommendation
 
Got you. Regarding TBE I meant the thickness behind the inside of serrations would be thinner with a double sided grind, but also the teeth. Maybe prone more to chipping, but if used on tomatoes this shouldnt be a problem.
 
FredyCro said:
Got you. Regarding TBE I meant the thickness behind the inside of serrations would be thinner with a double sided grind, but also the teeth. Maybe prone more to chipping, but if used on tomatoes this shouldnt be a problem.
Click to expand...
ah. understood.
 
Ok .... with my ability to upload images directly now not working, I am going to need to resort to using Imgur.

I got my fixture plate built. I dont have the best connection to the 123 block, but hopefully will be enough. Here is the blade mounted and held onto the face of the fixture plate (though when I actually machine the serrations, it will be on the other side (left side) of the blade...
4cNmcRkm.jpg


setting the angle relative to the endmill vertical is a manual process - but using a jig set to define the angle seems to work within a couple degrees...
pPKQ6yIm.jpg


here is a side view with the mill just barely not touching the blade , showing the clamps and fixture plate, with the blade held between the plate and clamps.
An62ERhm.jpg


Dont have time tonight to machine the thing .... but will get it flipped so the correct side is next to the mill - hopefully tomorrow I can get the thing machined....
 
Update - I got time to finish milling those serrations. I thought I would post photos, along with what I consider "lessons learned". Here is the piece coming off of the mill:
2Xleg2Rm.jpg


here it is sitting on the table after a little cleaning up on the grinder to remove rough edges...
g7UgWmim.jpg


Lessons learned:
1) steel does dull endmills (at least in my hands) much faster than aluminum or wood. so - start with a sharp endmill
2) the extent ("height") of the serration down the side of the blade is really sensitive to depth of cut. Trigonometry tells us that - but it became *really* obvious while I will cutting. Originally I envisioned cutting each serration by cutting through until I "just" hit the far side of the blade. This did not really work well, and I ended up judging the depth of the cut by eyeballing where the end of each serration ended up relative to the other previously cut serrations. I think it would be better to, before doing the milling, scribe a line at a fixed distance from the edge, then milling each serration until the end (bottom?) of the serration hits the line. This is much like one would scribe the height of the shinogi before grinding. Probably, one would need to pull out the trigonometry calculator, and given the thickness of the blade and the angle of the serration (in this case 75 degrees), and calculate where the end of the serration would be - then scribe the line there.
3) pay *close* attention to the indexing of the distance between serrations. If you look closely, you will see that one of the things was cut just a little closer to the serration on its left than on all of the other ones (in this case, 6 full turns of the traverse wheel versus 7 (which is what was intended). I was being careful, but I guess just got distracted on that one :-( .

Time to clean it up and send it off for HT...
 
I'm glad it worked, I was a little concerned about the jig with only one bolt connecting it to the 1-2-3 block. Did you get any vibrations at all?

1. I just removed 1.5 lb of material from a big block of mild steel with a 3/8" end mill and did not notice it getting dull at all. FWIW, the end mill was carbide and I used a kool mist system, maybe that made a difference.
2. Your thread has motivated me to make a bread knife, so I've been looking at how commercial examples are ground. The two I've looked at so far have slightly curved edges and the serrations end at a consistent distance from the edge, but the serrations appear to be parallel to each other. I guess they use the geometry to their advantage and just grind to a different depth to hit a consistent length from the edge.
3. Good reason to get a DRO.

When you grind the other side, do you plan to leave the edges of the serrations square or are you going to round them?
 
Hubert S. said:
I'm glad it worked, I was a little concerned about the jig with only one bolt connecting it to the 1-2-3 block. Did you get any vibrations at all?

1. I just removed 1.5 lb of material from a big block of mild steel with a 3/8" end mill and did not notice it getting dull at all. FWIW, the end mill was carbide and I used a kool mist system, maybe that made a difference.
2. Your thread has motivated me to make a bread knife, so I've been looking at how commercial examples are ground. The two I've looked at so far have slightly curved edges and the serrations end at a consistent distance from the edge, but the serrations appear to be parallel to each other. I guess they use the geometry to their advantage and just grind to a different depth to hit a consistent length from the edge.
3. Good reason to get a DRO.

When you grind the other side, do you plan to leave the edges of the serrations square or are you going to round them?
Click to expand...
I did get vibrations ... but i think this is more a limitation of my mill or vise - as the 123 block itself was vibrating. Not a lot, but enough to be noticeable. Ijust tried to take smaller bites and go slower (vibrations probably did not help the bit dulling either.

Agreed with the single bolt connection to tne 123 block. Again limitation of my vise, as it does not open enough to clamp the long edge of the block. Its not really a cheap vise, i just did not envision doing this kind of thing with it 😊

Im planning on keeping the edges flat - and see how that works.

Fwiw, i am not sure whether my single bevel approach will work with a bread knife, as usually the single bevel blades are used with really flexible foods, right?
 
Cushing H. said:
Fwiw, i am not sure whether my single bevel approach will work with a bread knife, as usually the single bevel blades are used with really flexible foods, right?
Click to expand...
I'm planning to do a regular bevel and grind in the serrations after HT. It will be a little while, though, I want to finish my SGA and make a slipjoint first.
 
Btw ... im not sure a dro would help. Something funny happens, i believe because of the convex curve of the edge, to result in a changing lateral position of the table at the end of each cut. I kept an eye onthat as i went, and my final indexes varied in a graduated wayby five or so mils along the length of the blade. Could have been bad tramming on my part, but like i said, it seemed to correlate with the convexity of the edge..
 
I'm not sure I follow. I thought the edge was straight? If the edge was curved, the y-position should change for every cut by a little bit to follow the curve. I might be misunderstanding you altogether. When you say lateral, do you mean the x-axis that is aligned with the blade edge? Did you lock that axis while cutting? If not, it might just be backlash.
 
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