Micro-cracking during grinding.

[bugbear]

Hi

I think you've answered your own question - the edge is no longer ground to 25 degrees in your illustration.

Regards Mick

I'm confused Mick, are you disputing that the arc of the grind produced on any grinder will not be symmetrically distributed across the grind?

No - but your diagram represents an overall angle (from edge to heel) of 25 whereas his diagram
represents a edge angle of 25.

Both diagrams shows a grinding hollow (of course!!!)

BugBear

 

[Random Orbital Bob]

I'm now so confused I can't remember my own name!! All I'm saying is that the "scoop" taken by the arc of the grindstone will be equally distributed across the scoop in other words symmetrical. That means that once the angle at the very tip has been satisfied (25 deg in our example) then metal will be less than a straight ground edge because of the scoop principle discussed above. Only at the very tip is the 25 degree angle satisfied, after which the arc begins to subtract metal according to the circumference of the stone.

 

[Spindle]

Only at the very tip is the 25 degree angle satisfied, after which the arc begins to subtract metal according to the circumference of the stone.

No

The edge angle is 25 degrees and as the grind is concave LESS metal is removed as the grinding wheel diameter is reduced, thus providing more support for the edge.

Regards Mick

 

[bugbear]

I'm now so confused I can't remember my own name!! All I'm saying is that the "scoop" taken by the arc of the grindstone will be equally distributed across the scoop in other words symmetrical. That means that once the angle at the very tip has been satisfied (25 deg in our example) then metal will be less than a straight ground edge because of the scoop principle discussed above. Only at the very tip is the 25 degree angle satisfied, after which the arc begins to subtract metal according to the circumference of the stone.

No. If the chord angle were 25, the tip angle would be a good deal lower than 25.

Edit;

In your diagram. the edge angle is around 10 degrees (red line),
and the grind (your dashed line) rises ABOVE
the edge angle (and don''t blame me for the diagram being upside down!).

A straight grind at 10 degrees would support the edge less.

BugBear

 

[Random Orbital Bob]

You're missing the purpose of the sketch. It's not meant to be to scale, I did mention its exaggerated to make the point. The point is actually simple. Its that the scoop taken out by the arc of the grinding wheel will always have its highest point at dead centre on the bevel being ground. Therefore there is necessarily less metal attached to the bevel than were the grind straight due to the scooping effect. My sketch is simply to illustrate this "half moon" effect being evenly distributed across the bevel. It wasn't meant to accurately model the angles. It WAS meant to illustrate the governing principle behind all grinder made bevels namely they have a half moon scooped out of them. That depth of that half moon is at its greatest exactly in the centre of the bevel and the arc of that half moon is a perfectly replica of the arc of the circumference of the grind wheel (whatever that happens to be).

With a straight ground bevel, no such half moon exists since it was abraded on a flat surface (a stone or belt) and therefore there is a greater quantity of metal "behind" the edge. Greater in fact by EXACTLY the amount that would have filled the half moon scoop taken out by the grinding wheel.

 

[Spindle]

Hi

Why do you persist in ignoring the fact that in your illustration the edge is no longer ground to 25 degrees? To maintain the edge angle of 25 degrees the grinding wheels adopt the positions in my illustration and thus less metal is removed.

Regards Mick

 

[Random Orbital Bob]

OK, you've forced me to do it on the Tormek to prove the point. Old chisel. Reground bevel at 25 degrees. Note the half moon of light showing through the straight edge (chisel on its side). The straight ground edge would of course mirror the path of the straight edge exactly. If it did that there would be a greater mass of metal behind the edge, greater to be precise by exactly the amount making up the mass of the missing half moon, which of course was caused by the arc of the grinding wheel.

The square edge jig from the tormek

A freshly ground 25 degree bevel

chisel on its side so you can see any light shining through the bevel which is touching the bevel at both top and bottom

closeup

closeup again clearly showing the half moon ground out by the arc of the wheel. Note the half moon is perfectly centred across the width of the bevel

Is that not conclusive??

 

[Racers]

Hi Bob

Can you measure the depth of the hollow? crush some solder or lead into it and measure it, it would be good to know how much the maximum is.

Obviously it tapers down to nothing at the cutting edge.


Pete

 

[Random Orbital Bob]

Trust me...I don't enjoy being this pedantic but if you track back in this thread I was being told I was flat wrong in a somewhat blunt manner! It's taken this many repeats of the same explanation to illustrate the point which is that a hollow ground bevel has a weaker edge (agreed its a miniscule difference and for practical purposes irrelevant) than a flat ground edge.

I don't mind a discussion and I'm wrong plenty of times believe me, but being told you're wrong over and over again when you're clearly not gets plain rude in the final analysis.

 

[Random Orbital Bob]

Ironically, the benefits of hollow ground chisels and plane irons is well documented. This wasn't about dogma for one method versus another, this was about being repeatedly told you're wrong....when you're not. Manners rather than methods.

 

[gasman]

Pete you can work this out using Pythagoras. If you measure the length of the bevelled edge of the chisel (i.e. the length which has been ground on the tormek (call this b), and assuming we are talking about a 250mm tormek wheel (radius 125mm), then the gap x you are wanting to measure is given by the formula x = 125 - sq root [(125 x 125) - (b/2 x b/2)]. So if the length b on a chisel is say 12mm, then the gap would only be 0.1mm putting that value into the formula. If it was a 200mm Tormek wheel, then the gap would be 0.13mm!!
Not sure of that is helpful but that is the distance
Regards Mark

 

[Phil Pascoe]

That the edge of a hollow ground tool is slightly weaker than a flat ground one is academic anyway - are you likely to put a shallow hollow grind on your finest chisel or plane iron and attack a piece of palletwood? No? thought not. Chances are, if you need an edge that sharp you'll be near finishing something, not starting it so any perceived weakness is irrelevant. In any case, after two or three honings there's little difference anyway.

 

[Spindle]

I don't mind a discussion and I'm wrong plenty of times believe me, but being told you're wrong over and over again when you're clearly not gets plain rude in the final analysis.

Hi Bob

Sorry if you find my manner blunt but the hard fact is that the edge as you are demonstrating is not ground to 25 degrees or if it is then the additional material removed by the wheel is less than that which would be removed by a belt.

One last try:

Image 1 shows an edge ground on a belt sander at 25 degrees

Image 2 shows an edge ground to 25 degrees on a 250mm wheel - the edge is reinforced.

Image 3, (your method), shows that although the tool is presented to the wheel at 25 degrees the edge is no longer ground at 25 degrees, this is leading to your misconception that the edge is weakened. In point of fact the 21.21 degree edge obtained is also being reinforced.

Regards Mick

 

[bugbear]

I've also done a last attempt:

The black line is the "target" angle.

The two red grind lines have the same curve.

The lower grind line creates an edge at the "target" angle.

The upper grind line creates an edge a good deal lower.

For both grind lines the lowest angle is at the edge.

BugBear

 

[MIGNAL]

It's all a bit academic. I don't know anyone who goes from a 6 inch Grind wheel to putting it straight to use. The edge/tip of the tool gets sharpened/honed to a flat. Might only be 0.5 mm in width but it's there.

 

[Random Orbital Bob]

Don't worry chaps I've given up you'll be happy to hear as life is too short. Your contention is that a hollow ground blade is stronger than a straight ground blade at the edge. Mine is the opposite ie that a hollow ground blade is weaker. In all cases we're talking about a miniscule difference, I understand that for practical purposes it matters not a jot and that was never what this little feud was about. My last tack on this will be to suggest you google straight grind vs hollow grind and read any of the threads you find at random long enough to find that pretty much all external opinion on this accepts the principle that a hollow ground tool has a weaker edge than a straight ground one for a given angle. That's why the advice for mortising chisels and other choppers is to straight grind while paring is fine with hollow. The other advantage of hollow grind is of course that when honing on a flat stone, the 2 points of contact make it more simple to locate the correct bevel angle without the tool rocking off true and there's less effort. But every opinion piece I've read supports my contention ie hollow grind = weaker edge. Here is just one opinion at random, this one from the chap who moderates the Tormek forum incidentally

The only advantage to a hollow ground edge is in honing on bench stones. When you drop a hollow ground edge down onto a bench stone, it is quite easy to feel the contact of the heel and the edge if you rock it back and forth. Also, with two small contact points honing is quick.

The downside is just as you've said...the hollow weakens the edge. The more pronounced the hollow, the weaker the edge.

Anyway...I shall say no more.

 

[Corneel]

Everyones mathematics are perfectly allright of course, but still, Bob is more right then the rest. The problem with the sketch from bugbear and spindle is that they get a 25 degree edge, but they can't hone it. Nobody will use the edge straight from the grinder. After grinding an edge with the true edge exactly at 25 degrees, you have a chord line around 27 degrees or so. Then you hone it and what have you got? A 27 degree edge, which is weaker then a flat grinded 27 degree edge.

But in reality it doesn't matter. The edge is still plenty strong enough.

 

[Spindle]

Hi

People are now moving the goal posts to suit themselves, the original statement was "a hollow grind makes a weak edge". This is demonstrably incorrect if the same edge angle is retained.

Plenty of people, turners, use tools straight from the grinder or wet stone and in these cases the reinforcement of the edge can be seen below in the scale drawing of a parting tool sharpened to a 50 degree edge on a belt sander and 250mm wheel.

I think I'll leave it at that

Regards Mick

 

[Corneel]

Ok, sorry, I thought this was the handtool forum. Honing the edge is standard practice in handtools.

(You see, it's not a shame to say sorry when you aren't 100% correct).

 

[Spindle]

Hi

Maybe I've been a bit bullish but the thread is about grinding and not honing - sorry if I've offended anyone.

Regards Mick