Matching factory serrations

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Mr.Wizard

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Feb 28, 2015
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Exploring methods to sharpen serrations I came to a conclusion which I haven't seen discussed: many factory serrations may not be cylindrical even when they appear to be.

Consider the bread knife serrations in fig.1 below. Each serration appears to roughly be a circular arc. This shape however is not fit using a cylindrical rod as the intersection of a cylinder and an oblique plane (either face of the knife) is an ellipse, not a circle. (Fig.2) The lower the angle (in knife edge terms) the greater the eccentricity. If a cylinder of large enough radius is used and the cut is shallow, limiting the intersection to a fraction of its diameter, a circular arc may be approximated (fig. 3) but deeper circular serrations cannot be produced. If instead a rod, wheel edge, or stone is itself given an oblate elliptical profile, this shape will be "stretched" across the plane of intersection, producing a circular arc when the angle exactly compensates. (Fig. 4)

The formula for the "stretching" ratio is (1 + cot(a)^2)^(1/2) where a is the edge angle in radians. For example a 30° edge will create a 2:1 transform, while a 40° edge will give 1.56:1.

Figure 1
serrated-knfe-serration-size-9226-630.jpg



Figure 2
TorusUnWrapped.png



Figure 3
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Figure 4
fiQKXuN.png
 
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Tapered rod, that's the beauty of it compared to uniform circumference cylinders. It does mean you have to work in a sweep or "U" shape to match the existing.

With a conical taper, a single tool can do any serration that the tip can fit inside as long as it isn't an angular cut - some are shaped as though they were cut on a corner instead of on a wheel and you either need an abrasive that matches the angle or overgrind it to a rounded profile.

Benchmade serrations tend to be deeper than they are wide, some of the older Schrade ones were more broad across - it becomes impossible to exactly match them, so you need a strategy that can accommodate different profiles.

Matching the scallop curve is then a secondary challenge to getting the "shoulder" of all the scallops to line up reasonably well. Is very easy to grind one a little deeper than its neighbor etc.

Beyond that, some of the diamond files might come it handy but I haven't had to go that route yet.
 
Interesting. Serrations cut well and longer than a plain edge, but it's difficult to maintain uniform sharpness and profiles within the serrations.

I suspect that even with a tapered rod, you'd be reprofiling the serrations unless you used very, very short strokes. The longer the stroke, the more the serration would be sharpened by a wider rod.

Maybe a spinning tapered rod would work, but you'd probably wear a groove in the rod.

I also don't know how much cutting is done by the deepest part of the serration. The tips would seem to do most of the cutting and be subject to most of the dulling.
 
Twindog said:
Interesting. Serrations cut well and longer than a plain edge, but it's difficult to maintain uniform sharpness and profiles within the serrations.

I suspect that even with a tapered rod, you'd be reprofiling the serrations unless you used very, very short strokes. The longer the stroke, the more the serration would be sharpened by a wider rod.

Maybe a spinning tapered rod would work, but you'd probably wear a groove in the rod.

I also don't know how much cutting is done by the deepest part of the serration. The tips would seem to do most of the cutting and be subject to most of the dulling.
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By and large they dull just either side of the high points. If you only sharpened them where they're really dull you'd make them more U shaped, so all the work on the rest of it is mostly just to keep the scallop shaped well cosmetically.

Per the other thread, this is why I oftentimes will make the edge slightly less acute and leave the very bottom of the scallop grind alone. If you have to do the whole thing and keep it even, it is possible but takes a lot of extra QC. A jig is mandatory IMHO.
 
HeavyHanded said:
By and large they dull just either side of the high points. If you only sharpened them where they're really dull you'd make them more U shaped, so all the work on the rest of it is mostly just to keep the scallop shaped well cosmetically.

Per the other thread, this is why I oftentimes will make the edge slightly less acute and leave the very bottom of the scallop grind alone. If you have to do the whole thing and keep it even, it is possible but takes a lot of extra QC. A jig is mandatory IMHO.
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Serrations are a chisel grind. How do you get rid of the burr on the flat side of serrations without scratching the blade on that flat side?
Do you create a microburr on the flat side?
 
I do not believe that a tapered file can solve this problem in a precise way if our target geometry is a semicircle. One could manually follow the arc of the serration using an undersized portion of the taper—the "U" sweep—but this provides no mechanical control. If we try to use the long axis of the taper, which can be approximated by a series of cylinders of varying radii, we get something like fig. 5 below, wherein the sides of the semicircle are never contacted by any of the ellipses. This is exaggerated by targeting a complete semicircle but the problem manifests with shorter arcs as well. If we want precisely cut circular serrations I think the oblate profile I described earlier will be necessary, absent CNC or a more complex jig that controls the "U" sweep itself.

Twindog, if the serrations are sufficiently refined (high grit) the burr should become weak enough to strop off easily, but I found achieving that level of refinement difficult when I last attempted it some years ago; that in part was motivation for this present examination.


Figure 5
07zQB0q.png
 
Twindog said:
Serrations are a chisel grind. How do you get rid of the burr on the flat side of serrations without scratching the blade on that flat side?
Do you create a microburr on the flat side?
Click to expand...
There. Couple of ways to de burr on the flat side , a Cotton Muslim wheel loaded with Green Chrome works great, if you don’t have a buffer. You can strop on leather or Cardboard
 
Twindog said:
Serrations are a chisel grind. How do you get rid of the burr on the flat side of serrations without scratching the blade on that flat side?
Do you create a microburr on the flat side?
Click to expand...


Yes, a very slight microbevel. Just enough to avoid scratching up the primary grind. Then at a steeper angle from the working side with a few very light passes into the edge. Strop with a bunch of card stock held on its end, loaded with compound.
 
Mr.Wizard said:
I do not believe that a tapered file can solve this problem in a precise way if our target geometry is a semicircle. One could manually follow the arc of the serration using an undersized portion of the taper—the "U" sweep—but this provides no mechanical control. If we try to use the long axis of the taper, which can be approximated by a series of cylinders of varying radii, we get something like fig. 5 below, wherein the sides of the semicircle are never contacted by any of the ellipses. This is exaggerated by targeting a complete semicircle but the problem manifests with shorter arcs as well. If we want precisely cut circular serrations I think the oblate profile I described earlier will be necessary, absent CNC or a more complex jig that controls the "U" sweep itself.

Twindog, if the serrations are sufficiently refined (high grit) the burr should become weak enough to strop off easily, but I found achieving that level of refinement difficult when I last attempted it some years ago; that in part was motivation for this present examination.


Figure 5
07zQB0q.png
Click to expand...

That is where you have to work in a side to side U pattern, with the widest portion of the taper that will fit either wedging into the scallop, or butting up against one side.

It is virtually impossible to find exactly the right sized diameter cylinder for every serration.

As a side note, are a lot of the IMGUR image links working intermittently now? Many of mine are off and on, and a lot of other folks I see having the same issue.
 
Mr.Wizard said:
HeavyHanded HeavyHanded I wasn't aware of IMGUR problems. My images are visible to me. Is there another free host I should use instead?
Click to expand...

Don't bother, I just realized this is happening at my end when I use the web from my house.
I reconfigured my router so it runs through a site-protected server and IMGUR and REDDIT are on the no-go list do to the mixed content. It was the easiest way I could give the kids free run of all the tech in the house without micromanaging every browser, app etc. The stuff is visible on my phone but only if I don't use WiFi. :oops:
 
REK Knives REK Knives Great work! You have my respect. Were these done with your cBN wheels? Would you please post photos of the edge profile on the wheels you used, and estimate the angle of the serrations you applied? I realize getting a good wheel profile photo will likely be a bit difficult but it will really help my understanding of this issue.
 
Mr.Wizard said:
REK Knives REK Knives Great work! You have my respect. Were these done with your cBN wheels? Would you please post photos of the edge profile on the wheels you used, and estimate the angle of the serrations you applied? I realize getting a good wheel profile photo will likely be a bit difficult but it will really help my understanding of this issue.
Click to expand...

Yes they were, but you can easily round some paper wheels and apply grit for the same effect. I have 1/4" and 1/8" CBN wheels. I didn't check the angle, I just get close to the bevel angle to where it looks right.

Here's some pics.

i-m8hjrpD-X2.jpg


Then I remove the burr on the paper wheel w/ some compound

i-kHgNXkw-X2.jpg



HeavyHanded said:
Sweet!

How well does that work for matching existing serration patterns?
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It works excellently, the only ones I'm not really able to match (but can get close) are microtech serrations, veff serrations (although I can, it just takes more time) and some cold steel serrations (micro serrations).
 
REK Knives REK Knives If you will humor me again would you measure the point-to-point distance on one of the serrations cut with the 1/4" wheel, of the type shown on the CPM REX 45 blade?
 
Mr.Wizard said:
REK Knives REK Knives If you will humor me again would you measure the point-to-point distance on one of the serrations cut with the 1/4" wheel, of the type shown on the CPM REX 45 blade?
Click to expand...
I don't have it available any longer it's packed and waiting on the mail lady :) but it's around 3/16 I believe
 
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