How hollow is the hollow grind?

[Pastor_Zatx]

My Google-Fu is failing me today. Has anyone seen a chart that reflects how concave a knife edge is with the 8 and 10-inch wheels? I understand this will vary based on the length/width of the bevel, so let's assume we're just talking about the final beveled edge of a full flat ground blade that would normally be 2-4 mm.

Thanks.

[cbwx34]

My Google-Fu is failing me today. Has anyone seen a chart that reflects how concave a knife edge is with the 8 and 10-inch wheels? I understand this will vary based on the length/width of the bevel, so let's assume we're just talking about the final beveled edge of a full flat ground blade that would normally be 2-4 mm.

Thanks.

No chart... but p.14 of the Tormek manual has an example...



Edit to add: Questions like these always spark my curiosity, (unfortunately, not smart enough to get the answer),    but a "Circular Segment Calculator", like one HERE, might help with the solution?  (There's some smart guys on the forum, Dutchman & Jan are two... that usually guide me along).   

Ok, 1 more edit:  I found this thread/post that you might find of interest, that shows the formula used, and this thread, that talks about hollow vs. flat grinds.

[Ken S]

Hollow grinding goes back to the era when most home grinders were six inch (150mm) diameter. At that smaller diameter, the hollow is much more pronounced. This facilitates "two point" grinding.

In my opinion, one can essentially ignore the hollow of a 250mm Tormek wheel. (By visual comparison, I also find little difference between the 200 and 250mm Tormek wheels.)

(The old question of how many angels can dance on the head of a pin comes to mind.    )

Ken

[Pastor_Zatx]

Thank you both for your responses!

While I appreciate the included formulas, I do ministry, not math (for a reason)! 

The reason I ask is that I just purchased the T-4 and I'm looking at diamond and CBN wheel options. CBN wheels that do not include the CBN coating on the side of the wheel are a third of the cost of the ones that have this option. With CBN or diamond on the sides of the wheel I could sharpen a knife with an entirely flat bevel, but if the 8-inch wheel doesn't create much of hollow grind, then I could get by with the cheaper wheels.

[cbwx34]

Thank you both for your responses!

While I appreciate the included formulas, I do ministry, not math (for a reason)! 

The reason I ask is that I just purchased the T-4 and I'm looking at diamond and CBN wheel options. CBN wheels that do not include the CBN coating on the side of the wheel are a third of the cost of the ones that have this option. With CBN or diamond on the sides of the wheel I could sharpen a knife with an entirely flat bevel, but if the 8-inch wheel doesn't create much of hollow grind, then I could get by with the cheaper wheels.

Yea... don't blame you a bit on the math. 

I don't think the hollow would be noticeable on a knife.

[Ken S]

Jason,

For forty years, sharpeners have been using Tormeks to sharpen knives and other tools using only the circumference of the wheel. If I really needed to sharpen an edge flat ground for some reason, I would establish the basic edge with my Tormek, and then pull out one of my long neglected water stones. Frankly, I would not pay substantially more for flat edge grit unless I had a specific and reoccurring need for it.

(If you really need a bunch of chisels with flat backs, return the ones more than a small amount out of flat if they are new. If you are flattening old family chisels, spread the task over several days and turn on your favorite music. Start with a coarse enough grit stone.)

Ken

[Herman Trivilino]

With CBN or diamond on the sides of the wheel I could sharpen a knife with an entirely flat bevel, but if the 8-inch wheel doesn't create much of hollow grind, then I could get by with the cheaper wheels.

Have you tried to sharpen a knife on the side of the grindstone that came with your T-4? I doubt that sharpening on the side of any grindstone would not work very well.

Plus, as others have pointed out, the hollow is too small to have an effect on how the knife performs.

[Ken S]

Jason,

As I recall, when you started this topic, you were still deciding whether or not to purchase a Tormek. You now have a T4. At this point, you can forget the math and computer searches and actually sharpen a knife. Let your eyes be the judge with the amount of hollow grinding.

I did a similar experiment with my Blue Chip chisels. I sharpened one using my T7 and one using my T4. Both were sharpened usng the full bevel width as primary and only bevel. Even using a straight edge in comparing the two, it was difficult to distinguish them apart. With chisels, or knives, it is easy enough just to dial in a couple extra degrees if one feels so compelled.

Look among your knives for a David to answer the Goliath hollow ground question.

Ken