Thread: Hollow grind bevel - 200mm or 150mm wheel?

Brett, I think that there is something incorrect with your drawings. An 8" diameter wheel should create a shallower hollow than a 6" wheel. That would leave more thickness behind the bevel. Your calculations reflect the opposite. Consider how shallow the grind is from a 10" Tormek wheel. I can lap it out pretty rapidly. So the 6" hollow is deepest, and the bevel angle is more acute.

I believe that the surface speed of a 6" wheel would be slower than an 8" wheel. This may be an advantage with a white wheel (runs cooler), but I doubt that you would see this on CBN wheel.

Regards from cool Cornwall

Derek

Originally Posted by derekcohen

Consider how shallow the grind is from a 10" Tormek wheel. I can lap it out pretty rapidly.

Actually, I think Brett may be onto something here. I do not think he is interested in lapping out thehollow, but rather maintaining it, Not lapping out the hollow means a lot of tool steel is saved.

At my initial glnce last night I can see what Brett means about not much difference in the two over a 5mm blade and even from his drawings you can see they are not much different out to 10mm even.

I have not thought it through completely yet and I am sure that there will be a lot of diverse opinions on this one but it is certainly enough to make one think. Without seeing Bretts diagrams I would have thought the difference was much greater.

Cheers

Doug

Hi Derek

It will depend on the definition of how the angle is set for the hollow grind. Bear with me here because I think Doug may be right in that I may be on to something here.

I would have thought that to do a 25° hollow grind then the blade would be presented at 25° off the vertical (or tangent to the radius of the wheel). If this is not the definition then my theory falls apart - more on that later.

I have done another diagram with finer lines that should show the detail better.




Noo then, here is the kicker. You can see that the Orange line is a tangent to each of the three wheels, so is 90° to the radius of them (by definition of a tangent).
The blade is presented at 25° off the tangent.
You can see that as the wheel diameter gets smaller then the top of the hollow grind is closer to the front of the blade.

If you were then to freehand hone with just the ends of the hollow grind on the stone you would NOT be getting a 25° angle from any of the three grinds. You would in fact be getting (the 25° FLAT bevel is represented by the orange tangent):
200mm grind ~28°
150mm grind ~29°
75mm grind ~33°

Note that as the blade increases in thickness so too the angle from the freehand honing will increase.

So the point here is this: we can see that the smaller diameter wheels move away from the orange tangent quicker as the wheel gets smaller, resulting in thicker
metal behind the edge. The difference in metal thickness directly behind the edge would only be measured in microns, and is thus irrelevant, BUT it IS thicker, and not thinner as I have been given to understand over the years of reading.

" more on that later."
If this is not how the angle of a hollow grind is determined then it has to be a bit hit and miss (and I think this would be a most unlikely scenario, hence favouring my theory).

That is to say, that if a 25° hollow grind should yield a 25° angle from end to end of the hollow then it can be seen that the angle of presentation would need to be less than 25° for a 200mm wheel (about 22-23°), and even less as the wheels get smaller. This is all obviously very hit and miss, although I'm quite sure that Bob could provide some insanely complex formula to establish what the angle would need to be.

However, if this IS the case then a larger diameter wheel would provide thicker steel behind the edge - but the blade would have to be presented at three different angles to yield the 25° angle from end to end of the hollow.

I think this second scenario is most unlikely.

Derek you are right about the surface speed of 6" being slower. Surface speeds are a direct ratio of the diameters (or radii), so a 6" wheel will be 3/4 of the speed of 8" (6/8 or 3/4). Thus, a 75mm wheel will have a little over a third (37.5%) of the surface speed of a 200mm (75/200 = 37.5%).

What prompted this particular though bubble was this chisel:



It was supplied with what I think is a 20° hollow grind from either an eight or ten inch wheel. It was clear to me that it had not been freehand honed because the top of the hollow is untouched (and it would be impossible to get a freehand flat hone without balancing on the top edge as well).

So, it was clearly honed using a jig. I had to sharpen it and get some minor chips out after use - so that meant I had to work out what the honing angle was. I have one of those little brass wheels with all the angles cut into it, but of course it's useless on a bevel this small.

Trial and error showed that the 20° hollow grind had been honed at 30°. The shallow hollow grind yields thinner metal behind the edge, but when that is then honed at 30° that front very thin metal is removed to leave a more substantial edge.

Currently that bevel you can see is about 1/16" wide and when it was supplied it was about 1/32".

I came to thinking that this is actually a rather good way of doing things, even though it necessitates a jig and is therefore slower. I could in fact just keep on repeatedly honing it at 30° until the whole bevel is flat and that would be many many sharpenings. In other words, it preserves the life of the chisel by removing the minimum amount of metal each time it is honed, and means more honings between hollow grinding sessions. (erm, I think that's right.....).

In fact, once the bevel is fully flat I could then click up to 31½° on the Veritas Jig for a new micro bevel - still without going back to a hollow grinding session.

Hi Brett

I feel like responding "angles, smangles, who cares"

it's now after 9:30 in my time zone and I want to head off to bed.
But, these are what I understand to be the benefits or aspects of a "hollow grind"

• establishing the initial bevel is faster, compared to using a flat stone
• in the "old days" the hollow grind might have been done on a wheel that very much larger than 8 or 10 inches.
  I remember my grandfather had a sandstone (well at the time I thought it was sandstone) grinding wheel that was probably larger than 24 inches -- it was mounted on a frame and turned with a handle. I've seen pictures of knife factories in France where the grinders are lying over water driven grinding wheels approaching 6 feet in diameter.
• as the grinding wheel gets larger, the hollow bevel approaches a straight line
• a hollow grind allows you to balance the chisel / plane iron on the toe and heel of the bevel to hone a micro bevel -- which means you can hone (and repeat the honing angle) without using a guide.
• what is important is the CHORD formed by the wheel, tool thickness and approach angle -- not the tangent angle per see

in terms of where this thread started ...

Brett, you worry (over-analyse) too much.
That chisel arived with a micro bevel -- replicating that micro bevel is NOT IMPORTANT -- of course you want to minimize the amount of material you need to remove to resharpen the tool, which you could do by steepening the bevel somewhat.

Depending on what I need to do tomorrow, I'll see if I can draw a few new avoirepeatdiagram or two

Originally Posted by ian

I feel like responding "angles, smangles, who cares". Yeah, I agree that is of no great importance.

• what is important is the CHORD formed by the wheel, tool thickness and approach angle -- not the tangent angle per see. Yes! That's the word.

Brett, you worry (over-analyse) too much. I've been bored - lemme alone!
That chisel arived with a micro bevel -- replicating that micro bevel is NOT IMPORTANT. Again, yes, but interesting to look at the technique involved, or speculate at least.

Guilty as charged on the over-analysis(but it's a lifestyle thing). In this case it does have great importance as I'm trying to decide on 6" or 8" grinder, and 6" sure looks the goods now - cheaper to purchase, cheaper to run, slower surface speed, and naff all difference in the bevel.

also have a look at this from Brent Beach Grinding using Grinding Wheels

Brent is sort of a guru in the sharpening / tool steel world -- Derek knows more about him than I do, but IIRC he has done a lot of experimental work around different steels, sharpening and edge retention

Interesting Ian. Down the bottom of that page:

Hollow Grind Revisited

Peter Michaux has done an extensive analysis of hollow ground bevel shapes. He argues, correctly, that a hollow ground bevel of equal edge angle is stiffer than a flat ground bevel. Consider a plane iron 0.09" thick ground at 27 degree mid angle on a 3" radius wheel. From the calculator, the angles are: edge 25.11, mid 27, heel 28.89 degrees.
To get the same edge angle using a belt sander, all three angles would be 25 degrees. The tool, away from the edge near the bevel heel, would not be quite as thick. The argument is that the thicker tool is stronger. That is true. However, the difference in strength does not matter since both grindings are plenty strong.
If you hollow grind at 27 mid angle, then you can hone at 29 degrees using my jig and then continue with the other micro bevels. A slightly stronger tool, same honed microbevels.

End of Brent Beach quote


As I said before, it's irrelevant as the differences in them would be measured in microns.

In my mind, I've always pictured the angle of a ground blade to be: the angle you get when you freehand hone the blade - with both upper and lower edge of the grind touching. (I.e. The angle of the chord not the angle of the tangent).
Not saying I'm right, just what I picture.
This being the final hone angle which, being the bit that cuts, is what you care about.
That in turn means, the bigger the radius of grinder - the stronger the edge but the resulting hollow will get flatter sooner - slowing down honing.

Hi Brett

My thoughts run along the same lines as hiroller. As you move theollow into the centre of the primary bevel, so the angles change relative to the circumference of the wheel. As I noted earlier, the hollow from a Tormek is easily demonstrated to be shallow. Conversely, the angle behind a bevel hollow ground on a 6" wheel will be more acute.

I am ot sure if this illustration from an article of mine on grinding will help (an iPad is not the easiest computer to use).



Here, the blade rests on the tool rest, and the grinding angle template is placed on that - i.e. the thickness of the blade is relevant in setting up the desired hollow grind angle on the primary bevel. Another way to do this would be to use a primary bevel of the desired angle, and ensure that the wheel scratches the bevel at the centre. In other words, the hollow is always centred on the primry bevel. That is different to what you have drawn.

On a train to London

Derek

Brett, if you change your diagram such that the middle of the orange line touches the grinder first it will represent how I would set up the grinder. Less chance of burning the blade. Some like to grind it a fraction short leaving a little meat to hone off.

Brett,

The major shortcoming of a 150mm wheel is, as indicated above, a more rounded hollow. As per your diagrams, it's not REALLY a big enough difference to worry about.

THAT BEING SAID...

When you dress a 150mm wheel repeatedly, it loses its diameter and that hollow becomes even more exaggerated. This is a big part of the reason (but not the entire reason) why people tend to go with 200mm grinder wheels. You get more life out of them.

HAVING SAID THAT...

You're getting a CBN wheel, so the issue of wheel shrinkage is non-existant.

If I had it to do over again, I would probably buy a 200mm slow speed grinder and immediately put a CBN wheel on it. The shallower (albeit only slightly) hollow leaves more metal for edge support, which is important for turning tools, especially here in Australia with the super hard woods. However, if a 150mm machine is better suited for you and your purposes in your workshop, then the difference between the two is, as you say, not a huge deal, so I suggest going with the 150mm. I sometimes find myself wishing I had a 200mm machine, but not even close to the point of even considering ditching the one I have currently.

Hope that helps,
Luke