hollow vs flat grinding thoughts

[Ken S]

Elden has mentioned an interest in exploring the effects of hollow or flat grinding. I found this online article:

http://www.amgron.clara.net/page74.html

I don't think the article tells the whole story. One of the most practical solutions I have encountered was from Ernie Conover. Ernie just added three degrees to his bevel angle to compensate for the Tormek's hollow grind. Considering that A2 chisels are routinely ground to at least thirty degrees, this seems a practical solution.

Personally I think the "weakened edge" due to the Tormek's large wheel hollow grinding is more of a theoretical "problem" than a real world deterrent. Looking at ground bevels from the smaller (200mm) T4, I can visually detect a slight bevel, the T7 bevels being less even more slight. I just think the issue goes back to when the standard dry grinders in home shops were six inch wheel diameter, often reduced further from wheel wear. Even then, the hollow grind was valued because it made it easy to place a finer stone on the two high points.

I don't have all the answers on this, and welcome responding posts.

Ken

[Herman Trivilino]

The edge is not weakened. Rather, it is strengthened. Let me explain.

When the edge angle is measured using the angle master, it's measured at the very tip. As you move away from the tip the angle increases, making the tool stronger, not weaker.

The only way it would be weaker is if the angle at the tip were smaller. And that would occur if the angle were measured incorrectly.

[Ken S]

Herman, your post is most interesting, however I don't fully grasp it. Would you please explain it more fully for me?

Thanks.

Ken

[jeffs55]

I don't get it either.

[Jan]

Ken, Jeff, the question was discussed last year in Rob's thread "riddle me this".

I fully agree with Herman. The wheel grinding process leaves more steel behind the edge than a straight grinding. This statement is true if angle is measured at the very tip of the bevel.

In the scaled drawings below there is the situation for a grindstone with radius R = 125 mm, chisel blade thickness 4 mm and 25 degree bevel.



The dimensioned detail below shows the situation at the edge.



Sorry, all measures are given in mm.

The drawing is much more clear for larger blade thickness. Let me know if you would like to see the drawings for 8 or even 16 mm chisel blade thickness.
Jan

[jeffs55]

I must say bullsomething. It is clear from your drawing that there is less metal at some point behind the tip DUE to the hollow grind. If the hollow grind ever touches the leading edge of the grind then the edge becomes so acute as to be useless for wood projects but maybe superb for shaving. I saw the last attempt at explaining this and did not bite then either. Your drawing clearly illustrates the removal of metal behind the cutting edge. Less metal=less strength in general. If I hold a knife edge to the most acute angle with a hollow grind leading up to the cutting edge, you are telling me that it would be stronger than the same edge when flat ground. Measure the edge angle from anywhere you want. There is still a "hollow grind" which means the same as a void of metal. Void means there is nothing there.

[Elden]

   Take it easy there fellows.

   Jan stated that was so if measured at the tip of the bevel. I have kicked this around since reading the "riddle me this post"  and have thought that could be the only way more metal could be left due to a hollow grind.

   I believe the real question is, how does the the Angle Master truly measure an angle? I don't mean theorectically but truly. Has anyone measured an angle with the Angle Master on a hollow grind, then checked that angle with an accurate machinist type protractor measuring device? Have you taken in account there might be a slight deviation from one Angle Master to another one? How accurate is the Angle Master? How accurate is the machinist protractor? How accurately am I reading both devices?

   According to Jan's statements, I believe the machinist protractor then would have to show a higher degree number (be it fractional or greater) than the angle master does. If not, then I believe Jeff's statement would bear truth.

   I have not tried it yet myself. I do have protractor made by General that I plan to check it out with. It is not a Starrett or other high dollar machine shop measuring device, but I beliveve it to be a good quality product. My jury has not issued a verdict yet. I think we need to go by actual verified measurements, however realizing the the above mentioned possibilities of error introduction.

As it has been stated before, in essence, does it truly matter? If it does, why do I have a Tormek? I say because it works well!

[Ken S]

One of the things I value about the Tormek is its versatility. We are not wedded to any particular "correct" angle. We do have logical parameters. A bevel angle may become so acute that the tool no longer has enough supporting steel for the edge to be durable. At the other extreme, the bevel angle may be too obtuse to be practical for the desired work. Nothing says that chisels must be sharpened with twenty five degree bevels.

The factory bevel angle on Veritas chisels varies with the width of the chisel. 3/8" (10mm) and narrower arrive with thirty degree ground bevels plus two degree micro bevels. Their wider chisels come with twenty five degree bevels plus two degree micro bevels. This seems logical as the cutting force is more concentrated with narrower chisels.

Lie-Nielsen chisels with A2 steel are factory ground with thirty degree bevels at all widths. This is largely due to the structure of the A2 steel. It will hold an edge for a long time, however that edge will never be as keen as a carbon steel edge.

I believe it is valuable to have forum members like Jan who have the mathematical expertise to map out things like this and share the results with the forum. Thanks, Jan.

I also subscribe to the machinist's concept of things not being "exact", but rather "within tolerance". Tolerances may be very narrow depending on the application. The interpretation of the application should guide the craftsman. With wood chisels, I believe we have a fairly wide tolerance range.

Ken

[Jan]

I must say bullsomething. It is clear from your drawing that there is less metal at some point behind the tip DUE to the hollow grind. If the hollow grind ever touches the leading edge of the grind then the edge becomes so acute as to be useless for wood projects but maybe superb for shaving. I saw the last attempt at explaining this and did not bite then either. Your drawing clearly illustrates the removal of metal behind the cutting edge. Less metal=less strength in general. If I hold a knife edge to the most acute angle with a hollow grind leading up to the cutting edge, you are telling me that it would be stronger than the same edge when flat ground. Measure the edge angle from anywhere you want. There is still a "hollow grind" which means the same as a void of metal. Void means there is nothing there.

Jeff, I think you need some more time to digest the surprising statement. 

Understanding where we measure the edge angle is of crucial importance for our issue. My statement that "The wheel grinding process leaves more steel behind the edge than a straight grinding" is true, provided we measure the angle at the cutting edge.

You probably consider the edge angle to be the angle of the bevel at the midpoint of the chisel. Then you are right and there is less steel behind the edge due to hollow grind.

Jan

[Ken S]

I believe Jan and Herman are correct. With the speed and ease of sharpening with the Tormek, it is easy to forget than the cutting only happens at the edge. A good example of this is the clever way Lie-Nielsen has marketed extra blades for its low angle jack planes. One plane can work as low, normal, or high angle just by switching the blades. The user can even quick switch just by regrinding a micro bevel.

While the highly polished full bevel certainly looks pretty, the cutting business only happens at the edge.

Ken

[jeffs55]

OK, OK, OK I finally see the light. If an edge is hollow ground or not it is sharpest at the edge. For the sake of clarity I am going to make up some numbers. Say the edge itself, where it meets the object to be cut is .0001 inches thick. Then, anything behind the edge (behind means closest to or towards the spine) is going to be thicker and grows in thickness the further from the edge you go. The blade grows in thickness as it widens. Widens is from the edge to the spine in this case. A hollow grind allows the edge to become finer, sooner as there is less metal behind the cutting edge. Less metal still makes the blade weaker for hard work but also allows us to put a finer edge on the blade. A flat grind could produce the same edge but the overall thickness of the blade would be so compromised as to make useless at some point except for shaving. That is why razor blades are so thin and is the same concept. I think! It is difficult to shave with an axe but easy to with a razor. I sure agree that the Tormek is a great machine, it is not an "end all" but is a good start towards one. I do not regret buying either one of mine. Once again, I was wrong.

[Jan]

My jury has not issued a verdict yet. I think we need to go by actual verified measurements, however realizing the the above mentioned possibilities of error introduction.

As it has been stated before, in essence, does it truly matter? If it does, why do I have a Tormek? I say because it works well!

The hollow shape of the bevel becomes much more important when grinding really thick tools, e.g. heavy duty mortise chisels. In the drawings below there is the situation for 4, 8 and 16 mm (0.16", 0,31" and 0,63") thick tools ground with 25 degrees using grindstone of the  diameter 250 mm (10").



All measures are given in mm.



The curved wedge circumscribed by the angle 6.9 degrees is the surplus of steel, left behind the cutting edge, due to hollow grind of a 16 mm (0.63") thick tool. This surplus steel disappears when we grind the tool flat with a 25 degree edge angle.
Jan

[grepper]

Isn't this being made more difficult than need be?

Maybe I'm slow, but I don't get it. 

By definition a hollow grind is concave.  A inward, curved area "below" a straight line (flat grind) perpendicular to the grind.

Obviously, if the entire face of a chisel for example is concave, more metal is removed than if it were a flat grind.

If the face of a chisel was flat and square and only the first half is hollow ground, then yes, a flat grind would remove more metal because it would "chop off" the top square edge. But that would be a very odd chisel.

[Jan]

I believe it is valuable to have forum members like Jan who have the mathematical expertise to map out things like this and share the results with the forum. Thanks, Jan.

Thank you Ken for your kind words. 
It is a great pleasure for me to be on this forum and share my views with you. Despite of my background in (geo)physics I need your feedback and proofreading to find possible errors and mistakes in my posts.

As I mentioned earlier, the nice drawings shown here, were prepared by my brother, when I was considering how to grind my old heavy duty mortise chisel.


Jan

[Jan]

Isn't this being made more difficult than need be?

Maybe I'm slow, but I don't get it. 

By definition a hollow grind is concave.  A inward, curved area "below" a straight line (flat grind) perpendicular to the grind.
Obviously, if the entire face of a chisel for example is concave, more metal is removed than if it were a flat grind.
If the face of a chisel was flat and square and only the first half is hollow ground, then yes, a flat grind would remove more metal because it would "chop off" the top square edge. But that would be a very odd chisel.

Grepper, sorry for my inability to explain it clearly. 

Please reconsider it because it is a question of basic importance, paradigm shift.

Referring to my last drawing above, if you hollow  grind the tool with a 25 degree edge angle, measured at the very tip of the cutting edge, you will not remove steel below the straight line 25 degrees. There is no concave curved area below the 25 degree straight line.