Offset between WSKO apex angle and BGA setting is now understood

Cyrano

Cyrano

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Jun 13, 2015
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For the Work Sharp Ken Onion Blade Grinding Attachment (BGA), previous threads have examined offsets between the angle setting of the tool and the resulting apex angle.

To gain more understanding, I conducted an experiment using bar stock of varying thicknesses, on which varying BGA settings were used. I supplemented these data with additional data using knife blades of varying thicknesses.

Conclusion: The thickness of the blade stock immediately behind the edge apex determines the magnitude of the offset between the BGA setting and the resulting apex angle.

Experimental results:

The results of this experiment are shown in this chart:

i-jcD64gQ-X2.jpg


This chart shows data in 5 series, where each series represents a different thickness of blade stock.
  • Each series has been plotted using circular markers to show actual data points. Series including 3 or more data points show a trendline created by linear regression of the data. Numerical values of the regression lines are shown in the legend in the lower right area of the chart.

  • The 3 thickest series (0.125", 0.094" and 0.062") show data obtained by sharpening blank blade stock. Each of these series includes 6 data points.

  • The 2 thinnest series show data obtained by sharpening real knives.
    • The series for blade thickness 0.012" includes 3 data points obtained by sharpening Opinel folding knives.

    • The series for blade thickness 0.008" includes 1 data point obtained by sharpening a Shun kitchen knife.
The dotted green line shows the theoretical ideal case, where any BGA setting results in an apex angle exactly twice that setting.

Discussion and Interpretation:

All series having multiple data points shows extremely good linear fit to the data. This indicates the total experimental system (human, machine, materials and measurement) is well-controlled and precise.

The data point from the thinnest blade stock falls on the theoretical ideal line; there is no offset between the BGA setting and the resulting apex angle.
  • I expect this scenario to be representative of many common use cases for the typical user, where the presence of a primary bevel on the blade reduces the effective blade stock thickness to a regime where the offset is insignificant.
Every series from thicker blade stock thicknesses shows offset between the BGA setting and the resulting apex angle. As the blade stock thickness increases, the magnitude of the offset increases. Consequently, sharpening thick blades will result in large offsets. Thicker blades cannot be sharpened to angles as acute as are possible for thinner blades.
 
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This is largely a function of the flexible nature of grinding belts and resulting deflection. So no real surprise there. :)
 
FortyTwoBlades said:
This is largely a function of the flexible nature of grinding belts and resulting deflection. So no real surprise there. :)
Click to expand...

Indeed. At Blade, I shared these results with several people experienced in the industry, and they too were not surprised.

What seems to be a surprise to many is the magnitude of the effect for this particular system.
 
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What I take away from this is that you might need to do a hybrid sharpening method with the BGA if:

1. Your blade is so dull that you need to cut in new edge bevels.
2. You are (foolishly) grinding a blade blank from scratch with this machine.

It would seem like you should "eye ball" an approximate angle using the platen side of the BGA to get it close. Then, if you really want a specific angle, use the side with the adjustments and markings to dial in a specific edge angle. I would expect that this approach would mean that you start with a "pointy geometry" touching the belt, which should minimize the deflection and resulting angle issues.

Brian.
 
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