EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES

[KnifeGrinders]

Edge Stability in Butcher’s and Kitchen Knives as a Function of Edge Angle and Initial Sharpness

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Structural Edge Tester (SET) is a method and device developed by Edge On Up for testing edge stability. In a nutshell, the edge is subjected to controlled rolling, the extent of which is quantified.
Edge sharpness tester used in the study: PT50A Industrial.
Laser protractor: CATRA HobbiGoni Knife-Edge Protractor.



Impact cycle  explained

The impact roller is a linear bearing slant at 10° to the horizontal base or in other words at 80° to the plane of the blade clamped vertically.
Standard impact assembly weight is 150 grams.



The impact roller is lowered at "A", then moved (rolled) over to "B" and then back to "A".
A-B-A is one cycle. 


See our Video on YouTube >>

PLAN

The plan is to use SET to test edge retention in butcher’s and mainstream kitchen knives sharpened in the "very sharp" range of 50 BESS, 100 BESS, 150 BESS and 200 BESS for edge angles 8, 10, 12, 15 and 20 degrees per side. 

The initial sharpness of 50 and 100 BESS represents the sharpest edges nearing a DE razor, while the initial sharpness of 150 and 200 BESS – just sharp knives in the range of utility blade sharpness.

Edge angles of 8 and 10 degrees per side (dps) are more typical of high carbon Japanese knives, while edge angles of 15 and 20 degrees are common in Europe (30° included) and the USA (40° included).
Mainstream kitchen knives are commonly produced with the edge angle of 30° included.
Meat processing plants usually sharpen their knives at 35-40° included.

These edge angles have become common through the sharpening tradition, and our research is to validate the tradition or challenge it. 

Another aspect of our study is to see whether there is any relation between the initial knife sharpness and edge retention. Traditionalists say that to hold the edge well the knife should be just working sharp, while enthusiasts say “the sharper the better”; this is the least researched area.

KNIVES

We started the testing with Victorinox/Wenger SWIBO professional meat processing knives of HRC 56-58 and Carbon content 0.5%, new out of the box.

To minimise change in the blade profile due to repeated sharpening, for each edge angle we used a separate new knife; all knives are identical, Victorinox Catalogue # 5.8401.14


Though steel of these knives best matches Victorinox kitchen knives X50CrMoV15, they can represent butcher’s and mainstream kitchen knives of other brands as well.

SHARPENING METHOD

 Edge angle was ground on Tormek T-8 with the help of our computer software for Tormek, and verified with a CATRA laser protractor in each case.


Sampled knife edge is 15 dps (30 degrees included)

CBN wheels 254mm in diameter were used: edge bevels were ground on CBN grit #400, and the edge set on CBN grit #1000.
The CBN wheel was chosen over the Tormek or Japanese wheels for the precision it offers, as the CBN wheel diameter never changes, while the stone wheel diameter drops with grinding due to the consumption of the abrasive, slightly increasing the grinding angle in the process.

The target edge sharpness was set by controlled-angle honing on paper wheels with a fine diamond paste within +/- 10 BESS of the target.
The honing angle was controlled with the help of our software for paper wheels.




DATA
Data numbers in the charts is the number of the impact roller cycles with the resultant sharpness.
E.g. “ x1 = 150, x2 = 300 “ means after 1 impact cycle the edge sharpness is 150 BESS, after 2 cycles 300 BESS, and so on.

For the purpose of this study, we measured the edge sharpness after every cycle for the first 5 cycles (Phase I), then after every 5 cycles to 50 cycles (Phase II), and then (i.e. from the 50th to 100th cycles) after every 10 cycles (Phase III).

This way we’ve covered the two checks that have been agreed as checkpoints for all field testers:
sharpness value after 5 cycles - Phase I checkpoint for “elastic deformation”; and
number of cycles causing "permanent rolling" - Phase II checkpoint for “plastic deformation”.

VICTORINOX/WENGER SWIBO BUTCHER’S KNIVES
Stainless steel, Hardness HRC 56-58, Carbon 0.5%

Link to raw SET data >>
Graphs I build on those numbers later on are a tad more comprehensible.

An 8 dps edge (16 ° included) collapses on the test line – see the microscope image.



I had to estimate sharpness by alternative methods described in our Sharpness Chart:
Around 50 BESS – splits a hair but won’t cross push-cut Tally-Ho cigarette rolling paper;
Around 100 BESS – won’t split or cut a hanging hair, but longitudinally push-cuts Tally-Ho cigarette rolling paper and violin hair sign is positive;
Around 150 BESS – won’t longitudinally push-cut Tally-Ho cigarette rolling paper and the violin hair sign is negative, but shaves forearm;
Around 200 BESS – won’t shave forearm, though force-scrapes off the hair, and slices a sales docket.



While measuring the 8dps blades on the Edge Sharpness Tester, the test line dents edges sharpened to 50, 100, 150 and 200 BESS, rendering further SET tests both impossible and meaningless.
Just for the protocol, the score in the dents is 800-1500 BESS.

A 10dps edge (20 ° included) is also dented by the test line, though to a lesser degree than the 8dps, so that the dents can barely be seen naked-eyed but nevertheless rendering further SET tests meaningless for practical purposes – the next microscope image shows 2 dents left by attempts to measure sharpness of an 80-100 BESS edge, followed by an image of a shallow dent left by the test line on a 10dps edge sharpened to 180-200 BESS.





The score in the dents is within 200-600 BESS, which we interpreted as promising of a better edge stability in edges sharpened at a higher angle, and further experiments proved this.

12 dps edge (24 ° included) is stable
The following microscope image shows the 12dps edge of initial sharpness 50 BESS after 20 SET measurements taken at the same spot – the black mark is where the sharpness was repeatedly measured, and as you see this point is undistinguishable from the rest of the edge.



To make sure the test line itself does not effect the edge as it was the case of 8dps and 10dps edges, after 100 impact cycles we took an additional sharpness measurement a few mm away from the mark, on an edge segment that had not been measured for sharpness but was still in the impact area, and the sharpness score was virtually the same as in the point used for measurements.

At 12 dps knife edge response to the test load remotely resembles that of a slow wear, whereas at 10dps and lower it is an immediate deformation.
I say “remotely resembles” because a rolled edge and edge blunted by abrasive wear behave differently: the bent springy edge shows fluctuating sharpness scores up and down as the edge deteriorates, while abrasive wear shows more linear blunting.

EDGE RETENTION

The following graphs show edge retention as a function of initial sharpness.

12 dps (24° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
50 BESS - 368
100 BESS - 399
150 BESS - 425
200 BESS – 435
The sharper is the edge the better is retention.




15 dps (30° included)
Initial Sharpness - Average Sharpness Score over 100 Cycles
50 BESS - 411
100 BESS - 438
150 BESS - 465
200 BESS – 494
The sharper is the edge the better is retention.


500 BESS is where the knife turns blunt.
At 15 dps, knives sharpened under 100 BESS (i.e. near razor sharp) stay sharp twice as long as knives with the initial sharpness of 150-200 BESS (i.e. utility blade sharpness) – 80 impact cycles vs 40.

20 dps (40° included)
Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 455
150 BESS - 541
200 BESS - 545
The sharper is the edge the better is retention.


500 BESS is where the knife turns blunt.
At 20 dps, knives sharpened to 100 BESS (i.e. next to razor sharp) stay sharp twice as long as knives with the initial sharpness of 150-200 BESS (i.e. utility blade sharpness) – 40 impact cycles vs 20.

***
By the far-famous Cliff Stamp’s concept and his experimental data, an edge of 12 dps should outperform those of 15 and 20 dps. Cliff Stamp advocates that knives should have the thinnest edge possible for a given blade steel and task.
If our SET tester data are in line with Cliff Stamp’s concept it would prove the SET can be an alternative to cutting tests that Cliff used to prove his concept.

To see what our SET tester shows in this regard, we’ve averaged sharpness data across all initial sharpness for each edge angle– this way we abstract from the initial sharpness and focus on the edge angle.
For example, average sharpness after 10 Cycles was calculated for 12 degree edge as: (338 + 385 + 384 + 419)/4, where 338 is the score for the initial sharpness of 50 BESS, 385 is the score for the initial sharpness of 100 BESS, 384 is the score for the initial sharpness of 150 BESS, and 419 is the score for the initial sharpness of 200 BESS.
Averaging this way not only better reveals the tendency, but also increases trust in the data we’ve obtained, because the sets of data for each initial sharpness can be treated as a retention test repeated on 4 different knives to confirm reproducibility of the results; in confronting reality people cut with varying initial sharpness.

Link to averaged SET data >>

The following chart and graph show edge retention as a function of edge angle.

EDGE ANGLE – AVERAGE SHARPNESS
12 dps (24 ° included) - 407
15 dps (30 ° included) - 452
20 dps (40 ° included) - 514




These averaged data tell us the best retention has the edge sharpened at 12 dps, and we already know that at this edge angle the best performing is the initial sharpness of 50 BESS i.e. razor sharp.
Knives sharpened at 20 dps turn blunt (500 BESS) by the 20th impact cycle, knives at 15 dps outlast twice as long, and knives at 12 dps over 3 times longer.
The edge angle has clearly a more definitive effect on the knife performance than the initial edge sharpness, unless this sharpness is <= 100 BESS.

Our SET data are in line with Cliff Stamp’s cutting tests and concept. 

COMPARISON TO EDGE RETENTION AT A MEAT PLANT



SWIBO knives similar to those used in this research were used in a separate research on edge retention at a meat plant.
Overall 8 boning operators used four SWIBO knives for two days: 4 operators on the day 1, and another 4 on the day 2; the edge sharpness was measured every 1.5 hours throughout the work shifts.
These knives were sharpened at 40 degrees with the initial sharpness about 100 BESS.
The meat plant averaged data are shown below

Sharpness through the work shift (BESS)
Initial sharpness - 115 BESS
In 1.5 hours - 308 BESS
In 3 hours - 316 BESS
In 4.5 hours - 324 BESS

Compared to the knives sharpened at 40 degrees with the initial sharpness of 100 BESS in our SET research the meat plant edge retention numbers fall within the range of the first 5 impact cycles.

CONCLUSIONS

The testing regimen had been designed right, because sharpness of all knives neared or exceeded 500 BESS i.e was rendered blunt by the 100th impact cycle.

The SET method has proved a valid and better alternative to cutting tests for edge retention, providing the researcher with precise data suitable for quantitative and statistical analysis.

“The sharper is the edge the better is retention” appears to be a rule for all edge angles.

The optimal edge angle for butcher’s and mainstream kitchen knives is 12 dps (24 ° included), sharpened to 50-100 BESS (i.e. nearing a DE razor in sharpness), though the edge angle is clearly a stronger determinant in the edge retention compared to the initial sharpness.
At this angle the edge and apex are both strong enough to resist deformation.
Knives sharpened at 12 dps stay sharp 3 times longer than knives sharpened at 20 dps, and almost twice as long as 15 dps knives.
On the contrary, lower than 12 dps edges are too weak and easily deform under the load.

As it has been mentioned, SWIBO knives steel best matches Victorinox kitchen knives X50CrMoV15 (HRC 56-58, Carbon contents 0.5%).
For higher end kitchen knives (e.g. Global HRC 56-58 Carbon 0.7%) we expect the optimal edge angle to be under XX dps, while for those with Carbon contents <= 0.45% (e.g. Scanpan x45CrMoV15 HRC 56-57 Carbon 0.45%) to be higher. These two lines of knives are being tested as I type this.

We’ve proved that the SET method can be used to determine the most robust edge angle for knives used at meat processing plants.
Sharpening at the best angle for a given knife brand ensures the best edge retention, sharpness and longer life span of the knives, saving plants tens of thousands of dollars a year.
We think of slightly lessening the impact to better match the SET to the meat plant numbers obtained in live studies; we should be able to do this by lessening the angle at which the impact roller meets  the edge; additional tests are required to determine the right roller angle.
You may call us day dreamers, but we hope to equal one impact cycle to one hour of cutting at the meat plant conveyer.

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