chisel sharpening

[Herman Trivilino]

How did the 25° become such a sacred cow? How did it become annointed for both chisels and plane blades?

I recall experimenting with angles between 20° and 30°. I can tell you for sure that 25° is a good compromise in my experience. 30° is too blunt for work with just the hands using no mallet, 20° is too acute for mallet work as the edge breaks off.

If I have to chisel out a mortise for a door hinge I do some of the chisel work with a mallet and some by hand. But I'm working with pine or the like. If it were oak I'd probably want two chisels, one at 20° for the finer hand work and one at 30° for the rougher work with a mallet.

[Herman Trivilino]

So the actual angular difference between the tangent and the chord is only 3.1° which is the half of the difference between the heel angle and the edge angle.

You know, when I first saw that it seemed too large a difference and I intended to look over the math and make a drawing to scale to confirm it.Just never got around to it.

[Herman Trivilino]

However the Tormek angle master doesn't compensate the hollow when adjusting an angle to my experience. If it's adjusted at 30 deg. I get an angle of exactly 30 deg. measured from the edge tip to the 2nd end of the hollow.

If you have a measuring method precise enough to reveal the difference, you will notice it. Of course, the precision of the Angle Master is a limiting factor here, too.

Try it with "blade" that's much thicker. For example, a one-inch wide strip of quarter-inch plywood. Scribe a line to simulate the hollow grind and cut it out. Measure the angle with the Angle Master and also with a protractor or speed square. Study Jan's sketch.

[Jan]

There is a simple desk top exercise which can help to understand the angles discussed here. Print an enlarged copy of my sketch and use a plastic school protractor to measure the angles. 

Herman, thank you for your explanatory comments, appreciated. 

Jan

[Ken S]

I have a hobby interest in machine shop measuring. (This study was instrumental in the development of the kenjig.) A circle contains 360 degrees. Each degree contains 60 minutes. Each minute contains 60 seconds. There are other ways to measure angles, however, they are outside of this conversation.

Like any measuring instrument, the Anglemaster has its measurement tolerance. Any measuring tool with one degree marks as its smallest is probably accurate to plus or minus one degree at best. This is a fact, not a criticism. This tolerance is more than accurate enough for our purpose. After all, many a master Viennese cabinetmaker sharpened his chisels by hand, or had his apprentice do so, and produced very fine furniture.

For more precise measurement, precision blocks are used, for both angles or flat measurement. A hardened and ground steel parallel or angle block may have an accuracy of plus or minus .0001" or only a few minutes or arc. These are adequate for shop floor work. For inspection or laboratory work, accuracy must be considerably tighter, sometimes in millionths of an inch or seconds of arc.

The Tormek TTS-100 uses the principles if exact measurement. Where is excels is in repeatability. The distance of the two holes used to determine the Distance between the universal support and the grinding wheel remain constant. (The two wheels compensate for stone wear.) The three protrusion slots keep the Protrusion of the tool constant. This level of consistency allows us to sharpen with a minimal amount of steel removal.

The kenjig and other similar devices borrow these principles from the TTS-100 and reduce the added error of repeat measuring. In fact, I use the TTS-100 for chisel and plane blade set up. I use a piece of label tape in one of the protrusion slots with marks at several degree positions. I used the Anglemaster for the initial set up, but have rarely used it since. Set up this way is fast and consistent.

Herman makes a good point about bevel angles. Today most woodworkers have few chisels compared to the past. This is unfortunate. We live with compromise bevels instead of having both twenty and thirty degree bevel chisels. As Scotty said in a Star Trek movie, "The right tool for the right job."

Ken

[Herman Trivilino]

There is a simple desk top exercise which can help to understand the angles discussed here. Print an enlarged copy of my sketch and use a plastic school protractor to measure the angles. 

Yes, but that doesn't involve the Angle Master so it can't convince that the angle ß measured by the Angle Master is indeed the same as the angle you've labeled 25° in your drawing.

Herman, thank you for your explanatory comments, appreciated. 

It is you, Jan, who has brought precision measurements to the forum.

[Herman Trivilino]

Like any measuring instrument, the Anglemaster has its measurement tolerance. Any measuring tool with one degree marks as its smallest is probably accurate to plus or minus one degree at best.

The accuracy of the Angle Master is limited by the user's ability to properly align it with the grindstone and the back of the chisel blade. Of course, this also limits the precision. I would estimate its error at ±2° for a typical carpenter's chisel.

This tolerance is more than accurate enough for our purpose.

I agree.

After all, many a master Viennese cabinetmaker sharpened his chisels by hand, or had his apprentice do so, and produced very fine furniture.

It would be interesting to see how precise and accurate they were at this skill. An archaeology project!

[Ken S]

 Herman,

Many years ago I saw Duncan Phyfe's tool chest on display in Williamsburg. It was behind glass. I would have enjoyed examining his bevel edges. I suspect many of them would not pass our rigorous inspection, although he did make acceptable furniture.

Ken

[Jan]

In the topic "hollow vs flat grinding thoughts" you can find the following statement:

The angle master WM-200 is very good and are measuring the tip of the edge. It is compensated for the hollow grind. If you use a regular "angle setter" or a protractor you will fine that the degree will indicate that its not the same as the anglemaster. A protractor are not compensated for a hollow grind but for a flat surface.

I am wondering how to understand that the AngleMaster is compensated for hollow grind.

In the first picture the flat lower edge of the angle setter touches hollow grind bevel. In this situation some compensation is imaginable, because the chord angle can be slightly lowered to get the intended edge (tangent) angle.

But in the second picture the flat lower edge of the angle setter touches a flat back of a plane iron and there is no need for a compensation as it was in the previous case.

What is your opinion? 

Jan

[Ken S]

Herman and Jan,

I agree with what you are saying, even the plus or minus two degrees with the Anglemaster. My concern is that many of our members and guests may read this topic and wrongly conclude that the Anglemaster is an inaccurate tool. It is not; it is well within the accuracy needs for its function.

Leftover from my darkroom, I have an Eastman laboratory thermometer. It is mercury and guaranteed to be accurate within a quarter degree Fahrenheit from 66 to 75°. It was a laboratory  essential. For deciding which coat to put on the children before school, it is essentially worthless. Sharpening a chisel is not a job requiring the skill of a tool and die maker.

The Anglemaster is not a fixed tool. It can be easily adjusted to suit the user's needs. It is leagues ahead of the Anglemaster-100, which it replaced (and is still standard equipment on some of the clones.) I have stated before that the Anglemaster is much easier to use in very good light. This recommendation has been in the handbook for many years.

Ken

[Herman Trivilino]

Guys, my drawing posted above is correct but my interpretation of it was not. Sorry for that! 

The angle 31.2° is the heel angle of the chisel blade and not the chord angle. So the actual angular difference between the tangent and the chord is only 3.1° which is the half of the difference between the heel angle and the edge angle (31.2° - 25°) / 2 = 3.1°.

I've gone over this, Jan, and tried to replicate what you've done. I can't seem to get it to work out. I don't know how you got the 31.2° and I don't understand why you think you have to divide the difference by two. It seems to me that the difference is what we're interested in. I understand the tangent angle, I understand the chord angle. I don't understand the heel angle.

[Herman Trivilino]

I am wondering how to understand that the AngleMaster is compensated for hollow grind.

I wouldn't say it that way. I would say that there is no compensation. Rather, I'd say that the Angle Master measures the angle at the tip, and the fact that there's a hollow grind means that if you measure the angle with a protractor you won't get the same value as you do when you measure with the Angle Master.

In the first picture the flat lower edge of the angle setter touches hollow grind bevel. In this situation some compensation is imaginable, because the chord angle can be slightly lowered to get the intended edge (tangent) angle.

But in the second picture the flat lower edge of the angle setter touches a flat back of a plane iron and there is no need for a compensation as it was in the previous case.

The only reason the it touches a hollow grind bevel in the first picture is because it's touching a knife whose upper side was sharpened with a hollow grind. The illustration is intended to show the difference between a single bevel tool and a double bevel knife. The issue of the difference between a hollow grind and a flat grind is being entirely ignored here. It is, after all, negligible for something as thin as a knife.

[Herman Trivilino]

I agree with what you are saying, even the plus or minus two degrees with the Anglemaster. My concern is that many of our members and guests may read this topic and wrongly conclude that the Anglemaster is an inaccurate tool. It is not; it is well within the accuracy needs for its function.

Hopefully, Ken, everyone will realize that every measuring instrument is an inaccurate tool because there is no such thing as perfect accuracy.

I do agree that its precision is well within what's needed. In fact, most of these discussions we have about angle measurements are largely academic and have no practical significance. For example, the difference between a hollow grind and flat grind is insignificant for a 250-mm diameter and most tools. When I tried to duplicate Jan's drawing I drew it using a 1:1 scale and measured a difference of 2° for a 6-mm wide tool and a 250-mm diameter grindstone. As mentioned before, I doubt the Angle Master can be used with this precision. And even if it can, I doubt that anyone is careful enough to achieve it. We just don't care about a 2° difference for anything that we do!

[Jan]

The angle 31.2° is the heel angle of the chisel blade and not the chord angle. So the actual angular difference between the tangent and the chord is only 3.1° which is the half of the difference between the heel angle and the edge angle (31.2° - 25°) / 2 = 3.1°.

I've gone over this, Jan, and tried to replicate what you've done. I can't seem to get it to work out. I don't know how you got the 31.2° and I don't understand why you think you have to divide the difference by two. It seems to me that the difference is what we're interested in. I understand the tangent angle, I understand the chord angle. I don't understand the heel angle.

Herman, the heel angle is the angle of the bevel farthest from the cutting edge. It is given by a tangent to the stone at the heel. The heel angle is always larger than the cutting edge angle.

The reason why I am taking only half of the difference between the heel angle and the edge angle is given by the inscribed angle theorem. Its applicability was numerically proven in my AutoCad drawings.
(http://www.mathopenref.com/arccentralangletheorem.html or https://en.wikipedia.org/wiki/Inscribed_angle#Theorem)

Jan

[Jan]

I am wondering how to understand that the AngleMaster is compensated for hollow grind.

I wouldn't say it that way. I would say that there is no compensation. Rather, I'd say that the Angle Master measures the angle at the tip, and the fact that there's a hollow grind means that if you measure the angle with a protractor you won't get the same value as you do when you measure with the Angle Master.

Herman, thank you very much for your responses. 

I totally agree with you! The AngleMaster is not compensated for hollow grind. The AngleMaster is compensated for changes in stone diameter. 

Jan