Sharpening Class, Part IV – Grinding the Bevel, and the Straight Scoop on Angles

At this point in the sharpening process the back of the tool should be flat and polished.  You shouldn’t see any errant scratches running the long way on the back of the tool – the polished surface should extend from side to side, and run off the end of the tool at the bevel.

Now it’s time to discuss our approach to the beveled face of the tool.

There are two methods for honing the bevel to get it into the same shape as our nicely polished back.  The first approach is to hone it flat – in other words, no grinding.  This technique may appeal to a woodworker with an aversion to the grinding wheel, and there’s no reason why a sharp edge can’t be obtained this way.  It may take you a bit longer to hone, because you’ll be removing more steel.  Changing the angle of the bevel in relation to the back will take much longer than at the wheel – but it can be done.   I’ll discuss the technique later in this post.

Grind Angles

Here are a couple of facts.

  • You own your tools.
  • You can put whatever bevel angle on the tools you choose.
  • The tool manufacturer didn’t know what you would use the tool for when it was fabricated.  Consequently, the grind angle on that tool may or may not be the right one for your work.
  • If you don’t like the results you get from the tool’s bevel, you can change it again.

Most of these little “pearls” come from my own experience, and from teaching sharpening classes to others.  I have found a general reluctance to grind tools, and I’ll be the first to admit it can be scary and intimidating.  I’ve had students who tell me they would rather use a dull tool – or buy a new one – rather than grind.  That fear is the reason you shouldn’t practice grinding bevels on a $50.00 Lie-Nielsen bench chisel or on a valuable antique.  Practice first on the one you’ve used to open paint cans, or buy one at a flea market for $5.00.  If you have replaced hand plane irons, save the old ones for practice.  Get the technique down first, then grab the Lie Nielsen.

There are a lot of books written about sharpening, and a long discussion about the “proper” or “correct” angle at which a tool should be ground.  You’ll often read that the proper grind angle for a tool used in softwoods is about 25 degrees, and about 30 degrees for hardwoods.  That’s all well and good, but what if you use your tools for both?  And what about those softwoods that are harder than some hardwoods, or hardwoods that are softer than some softwoods?

Here’s the reasoning behind varying grind angles.  Think about a razor blade: very acute angle on the edge; very sharp, but not suitable for working with wood.  Why?  Because the edge is too thin, and therefore it’s too frail.  Now think about a chunk of steel with an end that is cut at right angles to the sides.  Picture a piece of bar stock.  Very durable end, right?  Not too likely you’ll  chip that flat end – also not much good as a cutting tool.  The optimal angle for cutting wood is clearly somewhere in between; but at what angle do you find the optimal edge?

That’s the million-dollar question.  Here’s the one rule everyone agrees on: less steel behind the cutting edge makes the edge more delicate than more steel.  Examine a mortise chisel, and you’ll see it is ground at about 45 degrees.  Now look at a paring chisel: that might have a grind angle closer to 20 degrees.  The two tools are used for much different purposes.  That nice sharp paring chisel wouldn’t last too long chopping out a bunch of mortises.  And the big heavy bulk of the mortise chisel won’t give you the fine control to shave off .001″ or .002″ from a tenon.

The “rule” that hardwood tools get a 30̊  bevel is simply aimed at beefing up the steel behind the cutting edge to make it more durable in “harder” woods.  The opposite is true for the 25̊  bevel for softwoods.  We talk a lot more about this in class.

More than anything else, the grind you choose really is a matter of preference that is related to the work you expect the tool to do for you, and how often you care to re-hone.  An angle of 20̊  or 22̊ is fine for your paring chisel.  Most of my bench chisels – and plane irons, for that matter – are ground at about 27̊ or so.  I couldn’t tell you precisely what the angle is.  It’s clearly a compromise between the extra sharpness of a very acute angle, and the durability of a less-acute grind.  In your own shops, experiment a little bit, and don’t be afraid to make changes.  They are, after all, your tools.

Now for the Why.  There are a few reasons to hollow grind your tools, which is what grinding with a wheel is commonly called.  Both are related to simplifying your life.  The first one is simply that when it is time to change the bevel angle on a tool, grinding is the most efficient way to get the job done.  Secondly, honing by hand (which is the way a lot of honing gets done) is a lot surer process when you have the tip and heel of the bevel to register against your abrasive.  Honing a flat-ground chisel or plane iron can be done – but since you have to remove a lot more steel the process takes more time.  Most woodworkers would rather be doing almost anything else than honing a tool, so any technique that reduces honing time counts in the plus column.

Getting the Grind

I use a hand-cranked grinder, a slow speed (1725 rpm) electric bench grinder and a Tormek wet grinder to hollow-grind my tools.  The Tormek turns what started as a 10″ wheel, now probably just under 9″.  The bench grinder turns both an 8″ and a 6″ wheel. I use the 8″ wheel for turning tools, because the hollow is not as deep – leaving more steel behind the edge, and a more durable tool.  The 6″ wheel gives me a nice grind on plane irons and chisels, a little deeper than the 8″, and therefore lasting through one or two more honings.   The rotation speed and composition of the wheel are important for a number of reasons.  The primary reason is heat buildup.  Overheating tool steel destroys the temper, and untempered steel will not hold a cutting edge.  The only way to fix that, short of heat-treating the entire tool, is to carefully grind away the blued steel.   Just what you need, right?  More grinding.  The hand-cranked grinder is nice and slow, plenty safe to use, and keeps me in touch with the process.

For bench tools (chisels and plane irons) I use a 100 grit wheel rather than some of the coarser grits because the scratches are  shallower.  The trade-off is that grinding takes a bit longer, but I figure to make that time up when I hone.  A white wheel is somewhat less prone to heat build-up than the typical grey wheel that comes with many grinders – but make no mistake, a white wheel can blue your tool in short order.  You need to pay attention if you’re dry grinding, or you need to spend considerably more money for a wet grinder.

Let me say up front that the art of grinding is one that doesn’t translate well from the shop (or studio) to a blog, a book, or a magazine article.  So if a lot of the following isn’t entirely clear, don’t fret.  I cover grinding technique thoroughly in my classes; I suspect others do as well.

Once you determine the desired bevel angle, set your tool rest accordingly.  If you use a rest similar to the Veritas® model, you may choose to use it with or without the optional jig.  You can learn to grind free-hand (using a tool rest but no jig) just as easily as learning to hone without the assistance of a jig.  It takes more practice to be sure, but you’ll save a lot of time not fumbling around trying to get the tool set precisely set in the jig.

There are a few keys to successful grinding.  First, know what part of the wheel is actually taking away metal.  If you have trued the stone recently (leaving it a nice, pristine white) you’ll see the white abrasive turning grey or black in the areas it is removing steel from the tool.  This might be one little part of the wheel, or the entire rotating surface.  You can also tell where the cutting is happening from the location of sparks coming over the top of the tool.

Second, aim for a bevel that is made up of a single concave facet.  If you are seeing multiple facets in the bevel, something in your grip is changing, and you’re varying the angle at which the tool passes over the wheel.  Aim for consistent, even and smooth movement across the wheel.  As you check your work, keep your hand position (grip) on the tool consistent.

Third, you don’t need a lot of pressure to grind.  Truthfully, you need very little, if any pressure. Move the tool gently into the rotation of the wheel and let the wheel do the work.  Pressing the steel down on the wheel is a fast way to burn it, drawing out the temper.  As you move the steel from side to side, keep your fingers close enough to the edge being ground to feel for any heat buildup.  If the tool is getting uncomfortably warm, stop grinding and quench the tool in some water or set it down to cool off.

Fourth, you’re usually aiming for an edge that is 90̊  to the sides of the tool.  This is more important on chisels than on plane irons; in fact, plane irons are occasionally ground with a slight camber, and can be adjusted for a true cut when back in the hand plane.

If you are grinding without a jig, hand position plays a key role in getting all of these points coordinated.  Using a jig takes more time (getting the adjustment just right) but eliminates some of the other variables.

There are a few schools of thought about how far down the bevel to grind before you stop.  One view is to grind all the way to the end of the bevel, creating a wire edge on the back.  That way you know you’re done, and you have exposed fresh steel all the way to the edge.  The argument against that technique is twofold.  Part one is a concern with burning the steel, discussed above.  The second concern deals with keeping the edge square to the sides.  Concentrate on keeping the tool square to the grinding wheel, and examine your progress often.  Make adjustments to your grip if necessary, but stop before you grind the edge all the way to one corner or the other.  If you started with a square chisel or iron and you stop before the grind reaches the far edge of the bevel, you’ll finish with a square edge.

Next: Honing the Edge

_______

Jeff Zens owns and operates Custom Built Furniture in Salem, Oregon. He is a frequent woodworking instructor and writer.

Advertisements
Categories: Woodworking | Tags: , , , , , , | Leave a comment

Post navigation

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a website or blog at WordPress.com

%d bloggers like this: