Some questions about chisel sharpening

[Adam Johan Bergren]

I have a few basic questions that I have not seen addressed in satisfactory detail as of yet regarding sharpening various chisels:

1. For general use bench chisels, is re-grinding to a 30 degree angle a good idea? I know that this is subjective, but as I understand it, the main difference between 25 and 30 degrees is that at 25 degrees, it is easier to get a keen edge, but it will break down faster when doing fairly heavy chopping work. Is this view correct? Is it that much harder to hone a chisel at 30 degrees? I am finding that I do a good blend between parring and chopping with my bench chisels, so 30 degrees seemed like a good choice, but I'm not sure.

2. For mortise chisels, would a hollow grind be okay, or should the bevel be totally flat? Also, would a 35 degree bevel be best here? The Hirsch mortise chisels I recently bought are ground at 25 degrees or so, and this seems like a bad idea!

3. If I wanted a flat-grind on a mortise chisel, can this be accomplished using a bench grinder? I do not currently have any grinder yet, but I need something soon: I want to get the right equipment for what I want/need to do.

4. Is a bench grinder a good idea, or is there something better? I am looking for something that can change bevel angles for new tools and clean up gouges in edges, but I also am aware of the much higher cost equipment that runs slower, cooler, etc. Are these worth the price? I have my eyes on a 6" bench grinder with variable speed down to 2000 RPM. With the proper wheel, it seems like this should grind tools without burning (with care and water). Is this view correct?

Well, that is all I can think of right now.

Thanks in advance!

[Casey Gooding]

Lots of questions. I'll give them a shot.
1. 30 degrees is a good angle for domestic hardwoods. The more shallow the bevel, the easier to pare with. The higher the angle, the more durable the edge. Another option I've heard of but haven't tried is a 25 degree bevel with a 35 degree secondary bevel.

2. Some hollow grind on a mortise chisel is OK. You would need to use a bigger wheel for a more shallow grind. I would simply add a 30 or 35 degree secondary bevel.

3. You cannot get a flat grind with a bench grinder. It will leave hollow grind. To get a flat grind, you would have to use something like a Worksharp or grind on stones or sandpaper, which is much slower.

4. I like a bench grinder. Variable speed is good. Six or eight inch wheel is personal preference. Use a light touch and cool frequently, and you'll be fine.

[Frank Drew]

Adam,

1. I think your understanding is correct; what might be an appropriately acute angle for a paring or carving chisel might be too fragile for a bench chisel. And I don't see why it would be harder to hone a chisel at any particular angle, but maybe I'm not understanding something here. I think over time you'll arrive at bevel angles for various tools that suit your working style. No one I know, though, goes at the task of sharpening with a tool and die maker's sense of exactitude -- close is close enough.

2 & 3. I prefer the strength of a flat grind on a mortise chisel. You can rough out a flat grind on a rotary grinder, sort of, but there will be lots of little facets that you'll have to hone out on a stone, or, you could get one of those grinders with the stone mounted horizontally and that you work on the flat side of the stone. I think a grinder is pretty much essential for turning tools but not so much for bench edge tools except for the very occasional reestablishing a bevel or grinding out a nick.

4. I think the bigger grinder the better, because the hollow will be shallower from a bigger wheel. Slow speed is nice but not, IMO, a deal breaker; you can just use a lighter touch with faster machine. (In other words, if someone offers you a great deal on a heavy-duty 8" Baldor 3750 rpm grinder, take it.)

[Seems Casey said pretty much everything I was going to say :D.]

[Adam Johan Bergren]

Adam,

And I don't see why it would be harder to hone a chisel at any particular angle, but maybe I'm not understanding something here.



My understanding was that when two edges meet at a sharp angle, it is basically automatically "sharper." I guess if you had an edge at 90 degrees, it could still be perfectly honed, but it would be hard to cut anything. Thus, 35 degrees would take more skill to get as sharp as 25 degrees since you have a less acute angle.

I guess it would be determined by how the chisel was being used (i.e., parring would be better at 25). I will leave it as yet another question:

5. Is there an inherent sharpness scale for different angles (i.e., 80 degrees can never be as sharp as 25 degrees, etc.)?

[Danny Thompson]

3, 4. A belt sander can be used to grind a flat primary bevel.

http://www.inthewoodshop.com/WoodworkTechniques/index.html

[David Keller NC]

My understanding was that when two edges meet at a sharp angle, it is basically automatically "sharper." I guess if you had an edge at 90 degrees, it could still be perfectly honed, but it would be hard to cut anything. Thus, 35 degrees would take more skill to get as sharp as 25 degrees since you have a less acute angle.

The first part of your post is true, but not the last part. Even if the very edge of the cutting blade is absolutely perfectly honed, a higher cutting angle will still act as though it is less sharp. In other words, 35 degrees will never pare as easily as 25 degrees, no matter what your skill at honing is.

There is, by the way, and enormous difference between longeivity of the edge of a chisel between 25 degress and 30 degrees, particularly in a brittle steel like A2. Between 30 and 35 degrees, not so much. Nevertheless, I still hone my bench chisels that are going to be struck with a mallet to 35 degrees, and with both a set of crappy plastic-handled Buck chisels and a set of Lie Nielsen bench chisels, the edges do not chip even with hard mallet strikes. However, I've a set of Blue Spruce paring chisels that are honed to a 25 degree angle - these in use feel a great deal sharper than the Buck or Lie-Nielsons, but they are never struck with a mallet. My guess is that the edge would crumble and chip pretty much immediately were I to do so.


I guess it would be determined by how the chisel was being used (i.e., parring would be better at 25). I will leave it as yet another question:

5. Is there an inherent sharpness scale for different angles (i.e., 80 degrees can never be as sharp as 25 degrees, etc.)?

Generally speaking, you're correct - the apparent sharpness of an 80 degree edge will never be that of a 25 degree edge, all other things being equal.

Regarding a grinder - I've used both a high-speed dry grinder and a slow-speed Tormek. What I'll say about it is that the Tormek is far more expensive, unless you purchase a new dry grinder with a Baldour motor. However, it is not possible to soften the edge of a chisel, plane blade, carving tool, turning tool, etc... on the Tormek. In return, you give up a great deal of speed.

If you go the dry grinder route, I would highly suggest buying a very cheap set of plastic-handled chisels from the Home Despot or Sears and practice with these on your grinder before taking on an expensive set of Blue Spruce, Lie-Nielsens, Ashley-Ilses, etc... If you blue the steel on the cheap set, you've probably only lost about $4 per chisel. While you can recover an edge with temper by grinding past the area that's gotten too hot, that could take a substantial length off of your chisel.

[Cliff Rohrabacher]

I watched a video of Roy hand cutting dovetails in a little chest. He had one chisel onto which he'd ground a very sharp angle. the thing looked like 15 degrees he pushed with his palm it into wood like cutting butter.

Take a look: http://flash.unctv.org/woodwrightss/2800/wws_2802.html
at around 19:10 he whips out this itty bitty chisel. Take a gander at the angle of the grind on that thing.
So I stopped it dead on 19:39 where you can get a real crisp look at the chisel handle.
That thin' look like it aint neer seen no whackers. This is just a pushin' chisel. He gets behind it and gives a push and it slips into dat dere pine or popular slick 's ya please.