chisel sharpening

[Ken S]

Interesting point, Rob.

I think we have two different situations: We have the long established Stanley and Record blades, which would have been the norm when Torgny wrote the handbook. They are thinner, and no problem to sharpen using the traditional Tormek method.

We also have the more modern premium planes, which are considerably thicker and made of a more sophisticated alloy. With these "super blades", a micro bevel would be most welcome.

Surprisingly, recent handbook editions have not mentioned this.

Ken

[Herman Trivilino]

you can go for months without regrinding the primary bevel.

This of course depends on what you're doing with the chisel. A rough carpenter is bound to be tougher on a chisel than a cabinet maker. In cases like this the carpenter may want a primary bevel at an angle a few degrees smaller than the secondary bevel so as to make resharpening on a bench stone faster. He will be making contact at only one point, not two, to take out the nicks that resulted when his apprentice used it on a nail.

[Ken S]

Good point, Herman.

In our Walter Mitty idealized lives, we would be using our chisels only for things like dovetailing drawers in Queen Anne highboys. In real life, most of my chisel work has been of a rougher variety. Since purchasing my Tormek,I rarely use micro bevels. If I ever have to revert to using my old waterstones, micro bevels will return.

I have found a chisel edge will last longer with a steeper bevel, 30°, for rough work. I ground a long partial 30° bevel on a chisel before realizing that I only needed to gring a micro bevel.

Talking with the three carpenters who worked on my house over the years, I was surprised to learn that only one of them ever sharpened his chisels, and then only rarely. They are competent carpenters, who work primarily with power tools.

Ken

[Jan]

In fact, by resting the chisel on a fine stone at the two touch points created by a hollow grind (the edge and the other end of the arc described by hollow grinding) you are in fact honing a very fine micro bevel without realising it!  So it's not the opposite benefit as you've stated, it is in fact a benefit of hollow ground tools in general (not specific to the Tormek, rather specific to any wheel based grinding system that yields a hollow grind).  The benefit is simply that a hollow ground bevel creates two touch points, that in turn means the bevel wont rock when you're moving it on a fine hand stone, in fact it positively registers on those two "high points" and that presentation angle certainty allows a uniform micro or secondary bevel to be created at the very tip of the edge. 

Rob is correct!  From the attached drawing it is clear that flat honing of a hollow grind primary bevel (25°) adds a secondary microbevel with an edge angle of 31° (in this case).

For chisel cutting performance the thickness of the microbevel is important as mentioned by Herman. 

Jan

[Herman Trivilino]

Talking with the three carpenters who worked on my house over the years, I was surprised to learn that only one of them ever sharpened his chisels, and then only rarely. They are competent carpenters, who work primarily with power tools.

Likely the only hand tool they use on a regular basis is a utility knife. They carry it in their tool belt and replace the blades when they get dull. If they have a chisel it's probably buried in a tool box and not carried in the tool belt. The primary reason they don't use it is because it is dull. And the reason it's dull is because it's so hard to sharpen by hand. Yes, a cabinet maker can sharpen his by hand, but a rough carpenter can't because the chisel gets too much rough use.

Ken, that chisel you mentioned that you sharpen at 30°. If you instead sharpen it at 20° you will find that the edge very quickly crumbles upon heavy use. If you then grind that tip to 30° on a bench stone (a secondary bevel) you're be back in business in a short time.

[Rob]

You make good points about the nature of the use chaps.  I was thinking of cabinetry rather than 1st or 2nd fix carpentry.  And funnily enough, the chippies that did my house could frequently be seen with an upturned belt sander in a vice, roughing a dinged blade back to a useful state.  Those chisels had no idea what a micro bevel looks like!!

I have also done a fair bit of house carpentry myself having built/renovated 3 houses now.  The first time I did this I was kind of apprenticed to a close friend who was at that time a professional carpenter.  He taught me to hone a micro bevel on a stone with one of the old honing guides.  We used to clamp it to the end of a saw horse and then sit with the saw horse between knees (end on) and hone away.  We only ever did that procedure just before we were about to do careful work on either window frames or other finer pieces, often related to kitchens etc.

I know carpenters that use angle grinders to sharpen their chisels!!  Makes me shudder just thinking about it :-)

[Waterstone]

In fact, by resting the chisel on a fine stone at the two touch points created by a hollow grind (the edge and the other end of the arc described by hollow grinding) you are in fact honing a very fine micro bevel without realising it! 

Good point Rob.

In fact you'll get a small honed area at the very edge that looks like a micro bevel. It's a question of definition after all. To my understanding a micro bevel or secondary bevel is steeper than the primary bevel. If the latter one is ground to 25 deg, the secondary bevel will be 30 deg. or so.

On the Tormek the things are different. If the blade was ground at 30 deg. (what is my most used primary bevel on chisels), the honing will keep this angle. The "micro bevel" will be exactly 30 degrees. Why? The 30 deg. hollow grind that was produced on the Tormek will cause the very bevel tip to be less than 30 deg. due to the hollow geometry. When it will be honed by using both "edges" of the hollow, the edge tip will be "corrected" to be exactly 30 degrees. It's easy to test it.

So while I agree with all of your comments, I doubt that this honed bevel can be called a micro bevel in the usual sense (steeper angle).

Klaus

[Elden]

On the Tormek the things are different. If the blade was ground at 30 deg. (what is my most used primary bevel on chisels), the honing will keep this angle. The "micro bevel" will be exactly 30 degrees. Why? The 30 deg. hollow grind that was produced on the Tormek will cause the very bevel tip to be less than 30 deg. due to the hollow geometry. When it will be honed by using both "edges" of the hollow, the edge tip will be "corrected" to be exactly 30 degrees. It's easy to test it.

Klaus

Klaus (Waterstone),

   The content of your comment has been a lengthy hotbed of discussion in the past on this forum!  I imagine will continue to be debated from time to time. Notice Jan's comment above.

Addendum:
   Here are a couple of threads in regard to the effects of hollow grinding as ground by the Tormek when the angle is set by the Angle Master

http://forum.tormek.com/index.php?topic=2413.0

http://forum.tormek.com/index.php?topic=3052.0

[Waterstone]

Thanks a lot, Elden.

It was an interesting read for sure. I completely agree with the conclusion that a hollow ground edge doesn't necessarily has to be weaker than a flat ground bevel. If you compare a 25 deg. flat ground bevel with a hollow ground bevel that has 25 deg. at the very tip, the hollow ground bevel indeed is stronger, no doubt.

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. You easily can test it. If the stone doesn't have the full diameter, it's even easier to see because of the more pronounced hollow it creates. So I don't get 30 deg. at the edge tip where I want to have it.

To get the wanted angle at the edge tip I've to set the angle master at 35 deg. OR I hone the 30 deg. hollow ground bevel in the above mentioned way by using both edges of the hollow.

Klaus

[Ken S]

This topic is the forum "How many angels can dance on the head of a pin?" debate. We really should be debating it in Latin.

Joking aside, I believe the hollow ground issue is a left over from the 1970s when six inch (150mm) diameter grinders were the standard in most home workshops. (I still have mine.). This diameter produced a very obvious hollow grind. I have compared freshly ground chisels from the T4 and T7. I find it difficult to see a difference by eye. I can see that each is hollow ground, however, the amount of hollowing seems miniscule.

The numbers on the Anglemaster are not sacred. If anyone wants to use a different angle, the Tormek Board of Standards will often grant a variance.    In fact, manufacturers of premium chisels with A2 steel recommend using 30°. Part of the skill of a sophisticated sharpener should be knowing when to alter the standard bevel angle to suit the work. This should be as natural as a baker knowing how to alter the recipe for baking conditions.

We have carving, paring, firmer and mortise chisels. We have chisels made with different amounts of quality standards in alloy and heat treating. We have harder and softer woods. We have different grain directions. Some chisels are pushed by hand, some by wooden mallet or steel hammer. The variable most of us overlook is the width of the chisel. Leonard Lee discussed this in his sharpening book. ( I consider Leonard Lee's book the first and primary book in every sharpener's library.) A wider chisel spreads the stress over a greater area. Therefore narrow chisels should have a somewhat steeper bevel angle.

These factors apply regardless of whether the bevel is flat, hollow or convex.

Ernie Conover, a prominent woodworking writer and teacher from Ohio, US, recommends a useful compromise. He just adds three degrees to the bevel angle to compensate for the hollow grind. Klaus, your thirty degree setting should work just as well and probably extend the sharpness period.

Please forgive my vernacular; my Latin is rusty.

Ken

[Waterstone]

I have compared freshly ground chisels from the T4 and T7. I find it difficult to see a difference by eye. I can see that each is hollow ground, however, the amount of hollowing seems miniscule.

Ken,

Jan shows above with his excellent sketch, that the influence of the hollow isn't that miniscule. The difference between the ground angle and the one at the blade tip is about 5 degrees. I don't want to have this kind of deviation. My bench chisels work very well with 30 degrees, with 25 the edge durability will be bad. I've tested it.

You're completely right, that there are many different types of chisels and blade materials. My bench chisels for general purpose work best at 30 deg., the paring chisels are working good with 25 deg. and the mortise chisels want to have 35 deg. at least. But that's not the point to my eyes. Regardless the angle you want to have on the chisel you are grinding, you want to have it as exactly as possible. 5 deg. more or less won't do it.

Klaus

[Jan]

Klaus, the angular difference between the chord and tangent depends on blade thickness.

Jan

[Ken S]

Klaus,

I hold Jan's math and technical skills in high regard. I certainly do not doubt his conclusions. I also do not doubt your experience.

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

I would guess that a very long time ago, in the era of oilstones, someone chose it as an easy number for students and apprentices to remember. When sharpening was a laborious and frequent chore, long before thinner blades and microbevels, my money would be on the work being delegated to the apprentices.

Twenty five degree bevels have long been a part of our catechism. I vote for following our experience and changing our bevel angles to whatever works best.

Ken

[Jan]

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°.

So, flat honing of a hollow grind primary bevel (25°) adds a microbevel with an edge angle of 25° + 3° = 28°.

Jan

[Herman Trivilino]

On the Tormek the things are different. If the blade was ground at 30 deg. (what is my most used primary bevel on chisels), the honing will keep this angle.

Not if you followed the usual procedure of establishing the 30° angle with the Angle Master. The honing on a flat bench stone where both the tip and the heel of the (hollow ground) bevel touch the stone will produce a micro bevel at the tip that's somewhat more than 30°. How much more depends on the radius of the grindstone used to produce the hollow grind and on the thickness of the chisel.