Micro-cracking during grinding.

[bugbear]

It would probably benefit people to go back and look at Spindle's (scaled) drawing,
and see the actual extent of the difference we' ve been discussing, which was
his original (and IMHO important) point.

BugBear

 

[Corneel]

Like I said, the mathematics and that drawing are absolutely correct, but for handtools they are still not realistic. You don't use the edge like that. You hone it first, which leads to the result I wrote about a litlle earlier. In real life hollow grinding leads to a weaker edge then a straight grind. But in real life the difference is also so small that it doesn't really matter.

 

[Random Orbital Bob]

Hollow ground makes a weak edge.

that's true. So the more your 10" Tormek wheel gets ground away, the worse that problem gets

Strange comments :roll: :roll:

If an edge is ground to a specific angle how does concave grinding make it weaker? If anything it will be stronger due to the increasing thickness resulting from the concave grind.

Regards Mick

Bugbear, here is the original point made. The point is clear and unambiguous, a hollow ground tool provides for a stronger edge. There is no discussion of the miniscule practical differences between hollow and straight. We all agree on that in fact! The simple fact is Mick's contention is that hollow grinds make for a stronger edge and mine is the opposite. He argued his case for that contention, I did the opposite. No one has ever disputed that the difference is so small as to be academic. The debate was always theoretical.

 

[MIGNAL]

Well going by the diagram that Mick has provided I would say that it's neither weaker or stronger, it's the same. Effectively the hollow grind appears to be ground at a higher angle but at or near the tip it isn't. That's my (limited) understanding of it.

 

[Corneel]

Because everyone proves everything with a diagram, here is mine. A real life, hollow grind edge at 25 degrees. The red bits are the honed parts of the edge which finalise the preparation of the edge. Everyone can see that this edge is weaker then a straight grind edge where the whole edge is being honed. There is also an in between solution, a straight grind with a microbevel. And of course lets not forget the convex edge, which is also stronger, because it has more metal behind the edge where it really counts, at the very edge.



Drawing is not to scale!

Of course, the advantage of the easy honing ability of a hollow ground edge far outweight the slight disadvantage of lost strength.

 

[Jacob]

..... And of course lets not forget the convex edge, which is also stronger, because it has more metal behind the edge where it really counts, at the very edge.

I haven't forgotten it! Where your diagram shows a hollow there'd be a bulge. Gotta be good.

Drawing is not to scale!

Of course, the advantage of the easy honing ability of a hollow ground edge far outweight the slight disadvantage of lost strength.

Wos this honing problem? It's perfectly easy to hold a thing at 30º (near enough) without assistance.

 

[Corneel]

Yes it's a crudge but helpfull when removing much. The biggest Advantage is having to remove less metal. Thebig lifting is done on the grinder.

 

[bugbear]

Hollow ground makes a weak edge.

that's true. So the more your 10" Tormek wheel gets ground away, the worse that problem gets

Strange comments :roll: :roll:

If an edge is ground to a specific angle how does concave grinding make it weaker? If anything it will be stronger due to the increasing thickness resulting from the concave grind.

Regards Mick

Bugbear, here is the original point made. The point is clear and unambiguous, a hollow ground tool provides for a stronger edge. There is no discussion of the miniscule practical differences between hollow and straight. We all agree on that in fact! The simple fact is Mick's contention is that hollow grinds make for a stronger edge and mine is the opposite. He argued his case for that contention, I did the opposite. No one has ever disputed that the difference is so small as to be academic. The debate was always theoretical.

If we're just talking about grinding, the answer depends on ... the question, or at least how the question is defined.

If you define "the angle" to be at the edge, hollow grinding is stronger, since the angle increases going back. If (on the other hand) you define "the angle" to be the chord, the hollow removes more metal. The two lines on my diagram show these two options, with the black line showing a flat grind. One of the hollow grinds is stronger than the flat, one is weaker.

The question is not well enough posed to choose between them.

BugBear

 

[Random Orbital Bob]

It would probably benefit people to go back and look at Spindle's (scaled) drawing,
and see the actual extent of the difference we' ve been discussing, which was
his original (and IMHO important) point.

BugBear

This was what I was responding to. My point being, it wasn't his original point. The original point was simply refuting that hollow grinds cause a weaker edge and in fact asserting the exact opposite. No qualification about this and that, no diagrams, no "important" points about the practical differences, just a contradiction of what is widely accepted as the status quo....which is...that hollow grinds create a weaker edge.

 

[MIGNAL]

Well the important bit is, or rather close to, the very edge. That's how I would look at the angle. In fact when I was using the hand crank I'm pretty sure that I wasn't using the chord. I wasn't altogether fully aware of the geometry but I'm also pretty sure I had this vague notion that I should use a higher 'chord angle' to compensate. At the time it just seemed to make sense. That does not make for a weaker edge though.

 

[Cheshirechappie]

Blimey - I didn't expect all this response! It's a good sign that the hollow/flat bevel discussion at least ended up cordial and courteous, though.

Nice find Chappie. But I don't think that is exactly what Mike was refering to in that other thread. 0.05 to 0.5 mm deep cracks aren't exactly "micro". These are quite catastrophic defects that should be avoided at all costs, and would result in very visible nicks in the edge when the tool is used.

Anyway, light pressure, coarse, friable wheels and easy on the temperature is still good advice of course. And grinding through the edge is really not neccessary. Quit grinding a hair from the edge and do the rest on the benchstones.

Corneel - you're quite right that micro-cracking doesn't answer Mike's point in the other thread; it was something I came across whilst researching tool steels, metallurgy and grinding generally, and thought that whilst it isn't really something you NEED to know in order to grind an edge, it's useful background information about what can happen during grinding. That's all really. I just thought I'd post it for general interest, because I hadn't come across it before, and thought others might not, either.

-----

On the hollow grinding using small wheels, I found with the little 4" hand crank that it's best not to engage the whole bevel on the wheel periphery, but aim for something nearer a flat grind by moving the tool around. You can't do that easily if you're jigged up (such as with a Tormek used with jigs) but you can if you freehand. You'll never get a perfectly flat bevel, but you can remove enough from the bevel to allow you to hone a 30 degree secondary.

If you think about a surface grinder, they use a round wheel to generate flat surfaces, by guiding the workpiece past the wheel. On a small-wheel offhand grinder, you can do the same thing (though clearly not to the same level of precision). With a bit of care and monitoring of progress, you can easily get a good enough primary bevel without much hollowing just by moving the tool up and down a bit as well as side to side. I do this on the Tormek as well these days - saves time setting the tool in a jig. You have to monitor squareness of wider tools like plane blades, but that's very easy if you just keep a 3" engineer's square to hand.

That solution wouldn't suit everybody, of course - a bit 'free' for some. But it can work - well, it works for me, anyway.

 

[bugbear]

Interestingly, given the extremely small sagitta/hollow that has been calculated for the relatively small wheel
of a Tormek, I note that old texts emphasize that the tool should be held steady on the
(relatively large) wheel, so that a hollow grind resultss.

Per the maths in this thread, the hollow on a big sandstone ol' wheel ain't much!

BugBear

 

[Random Orbital Bob]

the hollow created by a Tormek with the wheel at its maximum diameter (ie 250mm) is negligible for practical purposes, in the thou range anyway. Remember though our little debate last night wasn't about the practical use, more the theoretical difference. On reflection, the different perspectives on the definition of the question raise quite an interesting point (no pun intended). Which is when people say bevel angle, what do they actually mean?

You see all the debate yesterday centred on whether or not the chord or the tip was the datum for the angle. The Tormek's own angle setting jig ( http://www.tormek.com/en/accessories/wm200/index.php) when set to 25 degrees produces a 25 degree chord, not tip angle. (at least I think it does)

When folks say, they're doing a 25 degree grind, do you mean the tip angle or the chord? That's a nice little teaser isn't it

 

[wizard]

When i buy a plane i spend a few minutes sharpening it on my linisher with jig no heat like a grinding wheel. If there is going to be more than a few strokes while using it i leave it on the shelf and get the power planer.

 

[bugbear]

When folks say, they're doing a 25 degree grind, do you mean the tip angle or the chord? That's a nice little teaser isn't it

I use a flat bevel, so the answer is easy

BugBear