Saturday, July 19, 2014

Four-Stroke Tenoning Exercise


I did this exercise on pine, walnut, African mahogany, cherry, and oak. The African mahogany was the fussiest due to reversing grain (which is different from South American Mahogany, right Shannon?).

No, this isn't the suck-squeeze-bang-blow of a four-stroke engine. This is more like four strokes of a pool cue.

I've run into three schools of thought on how to make tenons (this also applies to lap joints):
  • Should fit right off the saw.
  • Get it as close as you can with the saw and pare or plane to adjust.
  • Deliberately saw it fat by 1/16" and pare to the line with a chisel.
While the first is certainly the fastest, it requires a lot of practice to get there, so isn't suitable for a novice.

The second is much more common, since it can be done no matter what your skill level. This is how I started out. The problem is that the adjustment stage can be extremely time consuming. It also quickly leads to a losing battle of correction and over-correction until a good fit is no longer achievable.

The third method is what I've come round to. With only a little practice it can be fast and efficient. Looking back through various books, I see it mentioned in passing in one or two sentences. There wasn't enough detail to follow it in a practical manner.

Phil Lowe's article on chisel techniques in the October, 2011 issue of Fine Woodworking put me on the right track. I followed this making the tenons for my Queen Anne foot stool project and was happy with the results.

What really made me a believer was actually watching Phil do it during his SAPFM Seymour night stand demonstrations. He was fast and efficient, and his joints fit together beautifully. Yes, his 40 years of experience had something to do with that, but it was a convincing demonstration of the technique.

Specifically, after marking shoulders and cheeks with marking knife and marking gauge, Phil saws both shoulders and cheeks fat by 1/16", then pares them each down exactly to his lines with a chisel. So the sawing is coarser work, while the paring is very fine, precise work. Breaking it up into these two stages makes it fast and efficient.

I've begun incorporating this method when teaching others how to cut tenons. The problem I've found is that most people have difficulty handling the chisel properly. It's just a very unfamiliar implement, with no analog in daily life.

I've developed this exercise to address that. It's significantly improved my own tenoning skills and will give you new respect for the mighty mighty chisel.

The Exercise

Here I'm going to focus in excruciating detail on just the tenon of a full mortise and tenon joint; as with the sawing exercise for dovetails, we'll concentrate on just a single aspect of the joinery. Mortising is largely a separate skill, but the good news is that a variation of this technique applies.

The type of chisel required here is a paring chisel, in the range of 1/2" to 3/4" wide. These are harder to find than standard bench chisels, but there are several modern brands available. Paring chisels are used for lighter, finer work using hand pressure or only light taps of the mallet. Consequently they are thinner, sometimes even a little flexible, with a lighter tang and handle.

Most importantly, they are sharpened to a lower cutting angle: somewhere in the range of 20 to 25 degrees, as opposed to the typical 30 degrees for bench chisels. This allows a finer edge, but it's more delicate, so can't take heavy pounding. Naturally, this needs to be razor sharp.

Don't have a paring chisel? You can reshape a bench chisel to the finer angle. The metal in cheap, lower-quality bench chisels may require more frequent sharpening when used this way.

Spoiler alert: this method helps for mortises as well! My pile of practice joints in a taste of things to come.

In this exercise you'll work on just two surfaces, one cheek and one shoulder of a tenon. You'll work these repeatedly on a practice piece. Then cut the last bit off, flip the piece over, and repeat the process, now on the opposite cheek and shoulder.

Why flip the piece over? For the typical flat-sawn stock, if you look at the end grain, you'll see that the growth rings are convex with respect to one face, and concave with respect to the other. These show you the bark side and heart side of the board, respectively.


The rings visible in freshly-shot end grain. They're concave with respect to the left face, and convex with respect to the right face, making that the bark side of the tree.

When you pare with a chisel across these rings, the convex or concave orientation affects how the wood responds. This can cause the chisel to dive down into the grain and cut too deep at some points, or blow out the grain at the far end of the cut. Flipping the piece over and practicing on first one grain orientation and then the other helps you learn how to control each situation.

Use a length of pine about 2" wide. I know some people grumble that anyone can look good working pine, but I would say that's not true. Beginners should start with a wood that's not going to fight them so much, then move on to more challenging woods once they have some skills down.

Draw a line down one face of the piece just to mark your starting side. When you flip the piece to repeat with the other side, work on the side without the line. Alternate this way over a few practice sessions until you've used up the whole piece.

Every step here is critical, so concentrate on doing each one well. This exercise is fairly quick, allowing a high cycle rate. In half an hour, you can do it 3 or 4 times. That fast repetition helps you develop skill not only with the chisel, but also with the marking knife, marking gauge, and saw.


The tools and materials required, from top left: square, marking knife, crosscut backsaw, freshly sharpened paring chisel, marking gauge, pencil, bench hooks, and length of pine about 2" wide.

Marking

You need a sharp pencil point and sharp marking gauge pin. Marking consists of knifing a shoulder line across the face of the piece, then scribing a line along the sides and across the end grain with the marking gauge. Then darken the scribe line with the pencil so it's clearly visible.

The knife line serves two purposes. First, it cuts the outer layers of wood precisely for a clean, straight shoulder. Second, it creates a recess to set the edge of the chisel into, registering it precisely.

Similarly, the scribe line along each side provides a recess to drop the edge of the chisel into. Along the end grain, it serves as a visual reference for lining up and guiding the chisel.


Scribble two sides of the pencil tip on a scrap to form a chisel point that will fit in the marking gauge line.


Sharpen the marking gauge pin to produce a finer line and reduce dragging. Here I'm using a medium diamond stick.


Scribe a shoulder line across the face of the piece against the square. Use light pressure and repeated strokes to get a clean line about 1/32" deep.

The marking gauge is another one of those tools that people struggle with, because it's just a very unfamiliar implement. To use it, come in from each corner, don't run it off the corner. This ensures better contact of the gauge face to keep it registered properly.

Use firm pressure to press the face against the wood as you move it. Use light pressure on the pin, rolling the gauge so that the pin trails lightly along the wood. Don't try to mark a deep line at once. Use repeated passes to deepen it.

Often the grain will catch the pin and try to pull the gauge off track. The light pin pressure and firm face pressure help to counteract that; you can also tip the gauge the other way and move it in the opposite direction along the grain.

Set the gauge so it will mark a line about 1/16" or so from the face when referenced off the back of the piece. This 1/16" is the thickness of the material that you'll be paring off; it represents the last 1/16" of the tenon cheek to be removed when you're making a real tenon.


On the end grain, scribe up from the lower corner... 


...and down from the upper corner. One the side, scribe back from the end corner.


Fit the chisel edge of the pencil in the scribe line and mark the sides and end.

Sawing

This is the simplest step. Saw a rough shoulder line about 1/16" away from the knifed line in the waste. I say rough because this saw line doesn't need to be precise or fine. Just don't saw any deeper than the gauge line.


Crosscut a line to the depth of the scribed line. Use the tip of your thumb to guide the saw to the right position.


The resulting cut, 1/16" away from the knife line in the waste.

Cheek Paring

Now we come to the four strokes. Does it have to be exactly four? No, you may find some pieces take 6 or 8 because of the thickness, the grain characteristics, or the hardness of the wood. You may find some pieces are so easy to deal with they just need 2.

But the first two strokes are not precise. They allow you to thin down the paring and observe how the grain is behaving. The final two strokes are where you commit to your most careful work.

This is where the pool cue analogy comes in. You want to line up your shot, then take it, smoothly and with control. Or rather, line up your cut, then take it.

If hand pressure isn't sufficient, bump the end of the chisel handle forward with your palm. This should provide just enough impulse to start cutting, yet not enough to completely blow through the cut out the other side. You may need several successive bumps. None of the woods I tried needed more than this.


First stroke: line it up. Note the edge of the chisel is set about halfway through the total thickness to be removed. Sight down the length of the back of the chisel and line it up with the dark scribe line across the end grain.


First stroke: take it! Lean your body into it slightly as you apply hand pressure forward at the handle. Note how the chisel is gripped against the wood by the thumb, and the finger pinches the back of the wood, safely out of the cutting path. This keeps the chisel flat against the wood and applies braking control so that you can stop roughly halfway across.


Second stroke: line it up again at half the thickness, coming in from the other side.


Second stroke: take it! Ideally it should meet up with the first stroke. Pay attention to what the grain does for these two strokes so you know how to deal with it on the final strokes. Again, note the grip of the fingers and thumb holding the chisel flat against the wood.

This is where it all comes together. These last two strokes are where you need to bring all your skill and control to bear. This is a precision step. You'll set the chisel into the scribe lines on the side to place it at exactly the right position, then watch carefully as it splits the end grain scribe line, correcting the steering minutely as you go.


Third stroke: drop the chisel directly into the scribe line to position it exactly, line it up carefully with the end grain scribe line, then commit and take it! You know what to expect after the previous strokes.


Fourth stroke: drop the chisel into the scribe line, and you know the rest.


The resulting cheek face.

Checking And Correcting The Cheek

While you can do the whole job in four good strokes, things don't always go smoothly. The chisel may dive into the cut, or be deflected out. The wood may fracture and tear. Different woods will behave differently.

But hopefully the four strokes will leave a mostly flat surface, with just a few bumps or divots. There's nothing you can do about cuts that have removed too much, so it's better to err on the side of leaving high spots. They can be pared down flat with the rest.

To check the surface, run you fingers or thumb back and forth over it. They're very sensitive and can detect very small variations. To see where the high or low spots are, lay the flat back of your chisel across the face to see if it rocks or is out of parallel (this is like a winding stick).


With the back of the chisel laid across the surface, I can see a small high spot that acts as a pivot. Small as it is, it may affect the fit and alignment of a real tenon in its mortise.

To make the high spots visible, rub your pencil on the chisel back, then lay it across the surface and rub it back and forth.


The evil high spot is revealed! Plus another in the lower right.

Removing these spots is a delicate operation. Any extra material you remove beyond what's necessary will diminish the fit of the tenon.

Should you remove material around low spots to level the whole surface? NO! That's just making the whole tenon thinner and looser in the joint. Just live with the low spots. If you really have to, it is possible to shave the whole thing down, then glue on a new cheek piece and redo it.


To pare off the high spot, angle the chisel so it's taking a skewed cut across the grain, and use your thumb to push it sideways. Meanwhile, thumb and finger pressure keep the chisel firmly registered against the flat of the rest of the surface.

Shoulder Paring

This final step makes the shoulder as precise as the face. Remember that knife line you made in the beginning? This is where you use it.

You'll set the chisel edge in the line and push forward to cut. Because this is end grain, rather than using the full width of the chisel to cut, use just the corner. That concentrates the pressure of the cut on just a small amount of material.

As you progress across the shoulder repeatedly cutting in with the corner, use the remainder of the chisel width to keep it registered flat on the shoulder. Between the dual registration actions of the knife line and the cut portion of the shoulder, you should get a good clean shoulder.

You also know that the very edge of the shoulder is straight and clean, because that was formed by the knife cut. Any deficiencies in your chisel work will be back from this line, hidden in the joint.

Make sure the chisel is cutting straight in, perpendicular to the cheek. If it angles up, the shoulder won't seat all the way in the mortise. Some people like to deliberately undercut the shoulder (chisel angled down just a bit) to avoid this.

If the corner of your chisel starts to dull, use the other corner and work from the other end of the shoulder.


Start paring from one end of the shoulder, using just the corner of the chisel. Set it in the knife line, push in to cut. Repeat this across the entire shoulder.


Halfway across.


Pare across the flat surface to cut the last bit of cruft from the shoulder with the chisel corner.


The completed exercise.

Repeat

Now repeat the exercise on the same face. Set the marking gauge another 1/16" in closer to the back of the piece and scribe around. Darken it with the pencil. Knife a new shoulder line 1/16" down from the existing shoulder. Saw down the existing shoulder to the depth of the scribe line.


Set the marking gauge 1/16" closer to the back of the piece and scribe registering off the back.


The next cycle knifed, scribed, and sawn.

Repeat the cheek and shoulder paring, then do it all again. And again.

Eventually you'll have extended the length of the tenon so that it's too large for the width of your chisel. To deal with that, make another saw cut across the center of the cheek, and do the four-stroke paring across each section individually.

When making real joints, that's how you make tenons that are longer than your chisel width, just divide the surface up into as many sections as it takes. This of course multiplies the stroke count, but once you get this down each set of strokes is fast.


The cheek face is now too large for the chisel to do it all at once, so I sawed another crosscut to divide it into two sections.

Alternative Hand Positions

Depending on how the wood is behaving and how much control you have, these are some other ways to do the work.


Alternate first stroke, without pinching the chisel flat to the wood.


Similarly with the second stroke, where you can rest your hand more on the workbench.


If the shoulder paring is resisting too much, start in with a skewed cut...


...then pivot the corner into the wood.

One Last Time

OK, I have just enough wood to do this one more time. It's useful to see just how far you can go and explore the limits of the material and the method. Once the pine gets this thin, it starts to flex and even cutting straight across the chisel can dive in and completely through.


Knifed, scribed, and sawn for the last cycle.


The paring chisel can take amazingly fine tiny end grain parings to level out the deep shoulder.

Now cut that piece off, flip the workpiece over, and repeat it all from the other side, with the growth rings oriented the other way.


The precisely-worked practice piece.

It doesn't take long to make great improvements in your chisel control this way. I verified that when I inflicted, um, tried it with one of my students. He had never used a chisel before and was having the same difficulties I saw other people have. Using this method he was able to produce a piece like the one in the photo above.

When applied to real joinery, this method (and a slight variation for mortises) produces slip fit joints to rival tenons cut with dado stack and mortises cut with mortising machines. To see that, continue on to  Four-Stroke Mortise And Tenon.

2 comments:

  1. Wow. Thanks for investing the time and effort to make this post, it really is useful to someone, say doing an exam next week where a tenon join is likely to come up.

    As well as this, thanks for making it clear that there is more than one way to make a join. This may seem obvious but in the local system here the guilds hold sway in trainin and there is one was to do everything and if you don't 'get' that way then you are just considered defective somehow. Strange, but true.

    I will now go back over this post later in the day and make copious sketches and notes in my journal so I've got the information with me in the test room.

    Thanks again for making this more achievable...

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    Replies
    1. You're welcome Andy, good luck with your exam! I hope the "other" way doesn't lead you too far away from the acceptable method.

      That's an attitude I find difficult, that the only "right" way is the way someone learned it. On the other hand, if there's an established system that works, that's good as well, as long as you don't get punished for being open to other methods.

      In the US the term is "There's more than one way to skin a cat". As long as it gets the job done and the result is indistinguishable from the other methods, we tend to accept it. Maybe there are some differences in cost or efficiency or resources consumed, but as long as the differences aren't significant, they don't matter.

      If nothing else, hopefully exploring other methods gives you another tool to understand the fine details of the "right" method!

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