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Monday, 4 March 2013

Paring Chisels 101

A paring chisel is used for...

As the name suggests, a paring chisel is used for paring away small amounts of material to achieve precisely the surface that you want. A paring cut is parallel with the surface of the surrounding timber, whereas a chopping cut is perpendicular to it.

A classic example is slightly undercutting the endgrain surfaces that will form part of a joint. Undercutting a tenon shoulder for example leaves a small line around the outside edge that will 'make' with the adjacent component, the undercut guarantees that the joint will not be 'held open' by any irregularities or debris. Because endgrain is not a glue surface, the joint is no weaker, and if you do get a little bit of glue squeeze out from the tenon cheeks as the components are assembled, the undercut gives it somewhere to go that doesn't show on the finished piece.

Another traditional use for paring chisels is in conjunction with a saw. Slicing away an incline on the waste side of your knife line creates a shoulder for the saw to ride against, guiding it onto the line. One tip that I picked up from David Charlesworth (who I believe got it from Rob Cosman) is to start the saw on the top of the incline and allow it to drop naturally onto the shoulder as you make the first stroke. It's all very subtle stuff but it does seem to produce a noticeably better result than trying to start with the saw teeth in the bottom of the v groove.

Paring chisels are also excellent for shaping work, using facets in the same way that a stonemason would in order to monitor your progress. To create a cylinder from a square for example, you slice away the corners to make an even sided octagon, then keep cutting away the remaining crests keeping all of the facets straight and even as you sneak to within sanding distance of the final shape. For another example of paring using facets, see our "making a wedge for a Richard Kell No.3 honing guide" video.

Why do paring chisels have such a long blade...?

The primary reason for a paring chisel's length is not reach, but balance. Imagine walking upstairs, carrying a cup of tea, holding the saucer with both hands. Now repeat the exercise but with the cup and saucer on a tray - much more stable. The further apart your hands are, the more accurately your body can find and maintain horizontal. Indeed with Japanese paring chisels the blade length is not that much greater than a normal bench chisel, but the handle is much longer. One of the reasons I prefer the western design of paring chisel is that by having most of the length as blade rather than handle you get the additional reach and potentially longer blade life thrown in as a bonus, but these are really secondary benefits.

Why aren't all paring chisels cranked.....?

On first impression a cranked paring chisel looks like more versatile version of a straight one with the added advantage that it can go anywhere. In fact it is quite the opposite. Lets go back to the tea tray example and put one handle higher than the other, the amount of concentration required to keep the thing level is now greatly increased, one slight lapse and you've sploshed PG tips all over your custard creams. Cranked paring chisels are specialist tools with comparatively limited application. For the majority of jobs where you might use one there are usually two or three alternative ways of doing it (bullnose plane, rebate plane etc).

How do you use a paring chisel....?

It's all about minimum force and maximum control. The cutting edge should be absolutely shaving sharp, a very sharp edge cuts with less resistance and the smaller the amount of exertion required, the more control you have. Paring chisels are often ground 5 or so degrees shallower than the standard 25, sacraficing a little edge strength to achieve greater sharpness and lower penetrative resistance. They are sometimes tempered a couple of points softer than a bench chisel so that they are easier to touch up frequently.

You can further reduce exertion and increase control by using bigger muscle groups and letting your body weight work with you rather than against you. Your arms hold the chisel steady, elbows tucked in close to the body but with all of the muscles in your arms relaxed. The pommel of the handle nestles into your dominant hand, palm facing upwards. With the other hand, pinch the the blade as far forward as possible so that your forefinger is supporting the blade and resting on the timber. Stand with your feet apart, as you would to play snooker or pool, and allow the blade to slide between the fingers of your guiding hand as you make the cut, using the bigger muscles in your legs and backside to control the application of your bodyweight.

If you are paring vertically, the principle is exactly the same but you use your core (stomach) muscles to control the application of your bodyweight. The workpiece should be positioned low enough that you can get your shoulder directly over the cut.

Do paring chisels need to be lapped dead flat...?

Within reason, yes they should be within two or three thou of flat. Ideally a paring chisel should be ever so slightly concave. This makes it much easier to sharpen because you can access the underside of the cutting edge. If it starts out absolutely flat then the chances are that normal sharpening will eventually cause it to become convex. A slight convexity is undesirable but nothing to be afraid of, just work the chisel on a coarse abrasive surface that is shorter than its length (across the stone or a narrow strip of film) and concavity will return. A narrow strip of PSA backed 3M 100 micron AO microfinishing film is ideal for this task.

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