Optimum hollow grind diameter

[scott lipscomb]

Just curious, any opinions on the optimum diameter of a hollow grind? 10”, 8”, 6”? Seems like there would be an obvious answer, but I am not coming up with one by myself…

[Chuck Hill]

I've wondered this myself. I'd think that the smaller diameter, the better. As you move to larger and larger diameters, you approach a straight line which is what hollow grinding wants to avoid. A 6" should give you more honing before you need to re-grind.

[Jim Koepke]

My thoughts are in agreement with Chuck on the nature of the hollow grind.

Another thought is the distance vs speed of the abrasive against the tool.

With a 6" wheel one revolution is a little less than 19", 8" is a bit more than 25" and 10" will be a bit more than 31". Multiply this by the rpm and it is easy to see how a bigger wheel can result in less time at the grinder.

There is also the price vs availability of the different size grinders and wheels to consider.

As Edward R. Murrow once said, "anyone who isn't confused really doesn't understand the situation."

jtk

[Richard Verwoest]

I believe the optimum diameter is the one you currently have.

Richard

[Tom M King]

I like larger. 6" leaves the edge too thin to suit me.

[Edward Weber]

Just curious, any opinions on the optimum diameter of a hollow grind? 10”, 8”, 6”? Seems like there would be an obvious answer, but I am not coming up with one by myself…

Taking into account what's already been said, Optimum for what?
What are your criteria?
I think we would need a bit more info before answering with any substance.

[James Pallas]

On chisels that require 30 to 35 degrees to support the cutting edge that may be a problem with hollow grind. Someone can do the math if they want.
Jim

[Chuck Hill]

Another thought is the distance vs speed of the abrasive against the tool.

With a 6" wheel one revolution is a little less than 19", 8" is a bit more than 25" and 10" will be a bit more than 31". Multiply this by the rpm and it is easy to see how a bigger wheel can result in less time at the grinder.

That is another interesting variable. The larger diameters will more abrasive pass over the steel per unit of time so you would expect that to produce more heat. Offsetting that is the greater wheel surface which would dissipate heat. I have no idea which effect is greater.

[Tom M King]

Larger wheels usually turn at a slower rpm.

[scott lipscomb]

Per Edwards question above, I am talking about straight plane blades and chisels. Attached is a quick sketch of the section showing a 3" radius grind and a 5" radius grind on a 3/32" thick blade. Seems like a pretty insignificant difference.
491281

[Tom M King]

Makes more difference on a chisel that is a lot thicker? What are the edge angles for a chisel?

[William Fretwell]

Larger diameters make sense for chisels with a long face. Easy to position the hollow on a chisel. Any hollow is all you need. Plane edges make alignment with the wheel more difficult but again any hollow will do.
if the hollow alignment is too difficult then you probably don’t need one anyway.

[scott lipscomb]

Meant to point out, the hollow in the drawing in my previous post is 25 degrees, where I tend to grind my chisels and plane blades.

[Chuck Hill]

I don't see a hollow, I see a flat bevel. Wrong file?

[Derek Cohen]

Just curious, any opinions on the optimum diameter of a hollow grind? 10”, 8”, 6”? Seems like there would be an obvious answer, but I am not coming up with one by myself…

I've had all three sizes. For about 7 or 8 years I used a Tormek with a 10" wheel. When I hone a blade with a hollow grind, I freehand directly on the hollow. The hollow then acts as a jig and balances the bevel on a waterstone.

The Tormek created a shallow hollow, and this would last about 3 sharpenings before the blade needed to be reground. Over time the 10" wheel became smaller, and eventually it reached 7". The deeper hollow lasted longer, understandably. However, this friable wheel was slow - owing to the slow speed it turned - and the changing size meant that setting were continuously being altered. I really like the blade holding system, however.

Eventually, I moved to an 8" half-speed bench grinder and two CBN wheels (80- and 180 grit). At the time I did this, nearly 8 years ago, no one I knew, outside of turners, were using CBN wheels. At least, no one had written about them for flat woodworking. At a result, I needed to gauge what grits to get. I chose 180 grit as the main wheel since this was the closest to the 220 grit of the Tormek.

The advantage of the 8" hollow is that it will last through about 10 sharpenings. In part, this is also due to the straight and clean edge at the bevel achieved by the combination of the ultra reliable CBN and the Tormek BGM-100 blade holder. It does not require much to create a bevel. This is a hollow grind with a micro bevel at about 10000 grit ...

http://www.inthewoodshop.com/WoodworkTechniques/UltimateGrindingSharpeningSetUp_html_71b2c2d0.jpg

So what about a 6" diameter? It will create a deeper hollow still, but also with a shallower entry. I have not heard of this being more fragile, and do see the 6" as a cheaper alternative to a 8". There are always pros and cons. The 8" wheel is expected to run cooler. Generally the wheels are wider, and the machines are more powerful and better built (than 6" machines).

Regards from Perth

Derek

[Tom Bussey]

There are are other things that effect the hollow grind. For instance the angle at which the tool is presented to the wheel. Unless you are free hand sharpening. If a 25 degree cutting angle is wanted ( secondary bevel) then the primary bevel needs to be less than 25 degrees. A 20 degree primary bevel will allow more sharpening's than a 23 degree primary bevel, before the tool needs to be returned to the grinder. And the same holds true for a 30 degree angle. In all honesty, it doesn't matter what the primary angle is as long is, as long as it is less than the secondary angle is. Lets use 30 degrees. if the cutting edge is honed at 30 degrees and and the length of the shiny part is 1/16 inch long then the length of the side opposite the angle is 1/32 of an inch. And it doesn't matter if the hypotonus is 1/8 or 1/4 the strength of the cutting edge at 1/16 is the same as the first 1/16 of an angle at 1/4, because the they are the same.

My dove tail chisels have a primary angle of 15-17 degrees. My secondary or cutting angle is either 25 for soft woods or 30 degrees for hard woods. it doesn't take much effort to change the angle. The steeper primary angle allows the chisel to cut deeper into the wood with out allowing as much force to build up causing the chisel back into the marking gauge line.

However if one is using a chisel to chop out the Dato in 4x4 post then it has been somewhat cut out with a circular saw, then a cheap hardware store Chisel that came with a flat ground, would my first choice. . Application, application application.

In other words, if you have a 6 inch grinder, use it. You won't really gain anything by switching diameter sizes. Worrying about the circumference of an arc on a cutting tool is not of high priority when building anything.

[Jim Koepke]

My secondary or cutting angle is either 25 for soft woods or 30 degrees for hard woods. it doesn't take much effort to change the angle.

Even less effort if one has a set for low angle bevels and another set for high angle bevels. My chisels get honed as used. A flat bevel is maintained. They are seldom in need of grinding.

jtk

[James Pallas]

I use 20* for paring and 25* for regular bench work. I do have some chisels at higher angles for very tough work and mortise chisels of course. I think that Derek has mentioned that he doesn’t hollow grind Japanese chisels. I think he said that that is to help with chipping of the hard steel. I would guess that steels like A2 would react the same if thinned behind the cutting edge. I grind only when I have damaged and edge and have a lot to remove. I than just sharpen even to a flat bevel and more if I haven’t damaged the edge.
Jim

[Edward Weber]

Like some others, I don't hollow grind. My chisels rarely get near the sander (flat grind) once the bevel has been established.
I prefer to have specific angles for specific tasks. 20 for paring, 25 general purpose, 30 for chisels that get repeatedly struck.

[Derek Cohen]

I use 20* for paring and 25* for regular bench work. I do have some chisels at higher angles for very tough work and mortise chisels of course. I think that Derek has mentioned that he doesn’t hollow grind Japanese chisels. I think he said that that is to help with chipping of the hard steel. I would guess that steels like A2 would react the same if thinned behind the cutting edge. I grind only when I have damaged and edge and have a lot to remove. I than just sharpen even to a flat bevel and more if I haven’t damaged the edge.
Jim

Jim, I do not hollow grind Japanese chisels as I prefer to hone them with a flat bevel, which is traditional. The soft iron backing is the equivalent to a hollow grind.

I hollow grind all other blades, then hone directly on the hollow. No secondary bevels. Paring chisels at 25 degrees and all bench chisels and BD plane blades at 30-32 degrees.

Regards from Perth

Derek

[Tom M King]

I hollow grind because the grinding wheel is round. No edge gets ground unless it has been damaged. The grind just gets the bevel back out of the way from the angle I want the cutting edge. For simplicity, and to cut down on amount of grinding required, we don't use micro-bevels. The cutting edge bevel gradually takes over the bevel face until it has to visit the grinder again.

I don't think any plane blade has seen a grinder in years. Mostly just chisels that get used for all sorts of things on the old houses I work on.

[Mike Brady]

It would perhaps be of interest to know if there is a significant decrease in the lifetime of chisels subjected to wheel grinding. That spinning wheel can remove steel quickly, particularly if you don't achieve that perfect grind on the first pass. Sharpening on stationary stones is relative slow, if you have to re-establish the primary bevel. Granted, I might be revealing a bias in raising this point.

[Jim Koepke]

It would perhaps be of interest to know if there is a significant decrease in the lifetime of chisels subjected to wheel grinding. That spinning wheel can remove steel quickly, particularly if you don't achieve that perfect grind on the first pass. Sharpening on stationary stones is relative slow, if you have to re-establish the primary bevel. Granted, I might be revealing a bias in raising this point.

It probably depends on who is doing the grinding. In my experience there have been some who felt putting a blade to the grinder was all that was needed to sharpen it. These people are the ones who would tend to grind a blade into oblivion over time.

My tendency is to only take a blade to the electric powered sharpening system is if the bevel needs to be reestablished. My turning tools are more likely to be subject to powered sharpening than my chisels and plane blades.

jtk

[Derek Cohen]

It would perhaps be of interest to know if there is a significant decrease in the lifetime of chisels subjected to wheel grinding. That spinning wheel can remove steel quickly, particularly if you don't achieve that perfect grind on the first pass. Sharpening on stationary stones is relative slow, if you have to re-establish the primary bevel. Granted, I might be revealing a bias in raising this point.

There appears to be a misunderstanding about hollow grinding here, and probably more generally. My chisels and plane blades do not become shorter because of the hollow grinding. At most, when I grind to the edge of the blade, it will raise a very fine wire - about the same off a 1000 grit stone. One of the reasons I recommend the Tormek BGM-100 tool rest is that it facilitates this. Ditto a CBN wheel (rather than a white wheel), since the settings do not need to change.

Let me be clear: When one grinds a hollow, the steel removed comes from the hollow, and not the end of the blade. If you do not grind to the edge, all you are doing is removing extra steel that is superfluous to need. The only time you shorten the blade is if you steepen the primary bevel. And this would shorten any blade regardless of the means of grinding or sharpening.

Regards from Perth

Derek

[James Pallas]

There appears to be a misunderstanding about hollow grinding here, and probably more generally. My chisels and plane blades do not become shorter because of the hollow grinding. At most, when I grind to the edge of the blade, it will raise a very fine wire - about the same off a 1000 grit stone. One of the reasons I recommend the Tormek BGM-100 tool rest is that it facilitates this. Ditto a CBN wheel (rather than a white wheel), since the settings do not need to change.

Let me be clear: When one grinds a hollow, the steel removed comes from the hollow, and not the end of the blade. If you do not grind to the edge, all you are doing is removing extra steel that is superfluous to need. The only time you shorten the blade is if you steepen the primary bevel. And this would shorten any blade regardless of the means of grinding or sharpening.

Regards from Perth

Derek
Forever chisel. I simply can’t believe it!
Jim

[Jim Koepke]

Let me be clear: When one grinds a hollow, the steel removed comes from the hollow, and not the end of the blade. If you do not grind to the edge, all you are doing is removing extra steel that is superfluous to need.

Agreed, my comment was about some people who do not control their excavations at the grinding wheel.

It is imaginable one might have to grind to the edge to remove a nick. Though a bad nick hasn't happened to me since not seeing a broken staple left in a board.

So many people do their sharpening rituals is so many ways. At times it is almost impossible to convince a person to try something different.

At one time a convex bevel seemed to be the latest discovery. It wouldn't surprise me to discover there was a lot of grinding after that didn't work out so well.

jtk

[Kevin Jenness]

[QUOTE=Derek Cohen;3228906 The only time you shorten the blade is if you steepen the primary bevel. [/QUOTE]

That's not quite correct. The blade gets shorter every time it is sharpened, but only by a slight amount if done correctly. I have been using the same set of bench chisels for over 30 years, using the grinder in a similar manner as you. It is easy to overgrind and shorten tool life with various powered sharpening devices used carelessly.

I don't think the hollow grind diameter is significant for most work.

[Derek Cohen]

Kevin, of course a blade gets shorter when sharpened. Even is it is by a smidgeon of a mm. That is a given. I think that is what you are saying.

What I am saying is that a blade does not get shorter because it is hollow ground. Material is removed from the hollow.

A careless worker can do rough work whether using a grinder or a honing guide. That has nothing to do with the hollow grind, per se.

Regards from Perth

Derek

[Warren Mickley]

The optimum is a flat full length bevel.

A chisel is not just a cutting edge. The chisel also works as a wedge, pushing material to the side. The bevel opens up the cut and helps keep the tool from getting stuck.

We also often ride the bevel for both paring work and chopping work. In these cases a flat bevel with no secondary bevel performs the best.

With constant use, a chisel that is never put to the grinder will lose noticeable length over the years. I have chisels that I have used for more than fifty years without ever being on a grinder. I find it helpful to rehandle such chisels to bring them back to an optimum length.

[Prashun Patel]

I used a 6" and an 8" for a couple years and noticed no appreciable difference. I started on a 6". I now use an 8" only because there were more CBN wheel options a couple years ago; not sure if that's still the case (wheel diameter is IMHO several orders of magnitude less important than wheel type).

As far as balance goes, I can feel both well. To be honest, you can develop a feel for free-hand honing a flat bevel too.

I'd let the availabilty of CBN wheels in the chosen diameter govern my choice if choosing again.

[Thomas Crawford]

I think part of the confusion here is that if you are a hobbiest it is going to take a long time to work through a hollow ground chisel to the point where you actually need to hollow grind it again. Maybe this is Derek's point - you are generally just removing material short of the end to make it easier to set the primary bevel. I don't believe its any better or worse just less material you have to deal with on the water stones or sandpaper, etc. This is the method if you have an undamaged blade.

What I think a lot of people do is hollow grind AND fix a nick or some other issue with the end of the chisel or plane blade at the same time. I don't find that to be the best idea, as I think you are running the risk of heating up the steel too much. I tend to go much straighter into the grinder to fix the end and then follow up with hollow grind, etc. if needed. Not sure if that is the best way to do it but I like to avoid the grinder as much as possible if I can.