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Tune Up an Old Chisel

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Turn a Beater into a Razor-Sharp Tool

 

by Tom Caspar

 

 

 

One of my favorite tools is a legendary Stanley No. 50 chisel. Made in the 1920s, it had seen hard times. Restoring it was a labor of love, and well worth the effort. Its steel holds a long-lasting, super-sharp edge. No doubt you’ve got some beat-up chisels in your toolbox that could be revived, too.

I’ll take you through the complete process of restoring a chisel that’s in tough shape. These steps are equally useful for a new tool, fresh from the box. Please notice that I put equal emphasis on the chisel’s bevel and back. Both must be in perfect condition, for every sharp edge has two sides. Let’s begin with the back.

 

 

Evaluate the Back

 

Inspect the back by sanding with fine paper (Photo 1). Put 220-grit pressure-sensitive-adhesive (PSA) sandpaper on a flat surface, such as a granite surface plate, 1/4-in.-thick piece of glass, cast-iron tablesaw wing or jointer bed. Sand the back a few times using diagonal strokes. Sanding reveals low spots. With an old tool, you’ll probably find rust pits, large hollows or a dip at the leading end. 

 

 

Photo 1: An old chisel usually needs lots of help. Lightly sanding the back reveals hollow spots, rust pits and a rounded-over or low leading end. This chisel’s bevel is also chipped and uneven.

 

Flatten the Back

 

My chisel’s back looked so bad that I began flattening with 60-grit paper (Photo 2). If the inspection sanding indicates few low spots, begin with a finer grit. The point is to avoid making unnecessarily deep scratches. Machinists call this process lapping. For the coarse work, I use premium-grade sanding belts stretched tightly on a shop-made jig. They can be reused many times, unlike PSA paper. Lapping a back in poor condition may require many strokes, which is hard on your hands, so I often wear rubber-coated gardener’s gloves and take frequent breaks. 

 

 

 

Photo 2: Flatten the back on sandpaper using heavy pressure and diagonal strokes. I prefer to work on a 6-in. x 48-in. sanding belt. It’s easy to reuse and lasts a long time. The belt is stretched taut on a shop-made jig (see “The Lapping Jig, below”).

 

 

The Lapping Jig

 

Opposed wedges tighten a sanding belt placed over this jig. Strike the wedges with a hammer to stretch the paper taut. This jig works for a belt of any size, though I prefer 6-in. x 48-in. belts for their huge surface area. Make the jig from three layers of 3/4-in. MDF glued together. To round the ends, make two 45-degree crosscuts first, and then sand in between them. 

 

 

 

Continue sanding until you reach the bottom of the low spots. How far up the back must you go? Two to three inches are minimum, but I usually lap the whole back. (A totally flat back enables me to use guide blocks when I pare mortises, tenons and dovetails.) If 1/4 in. or less of the back’s leading end is lower than the worst rust pits and hollows (and that’s not unusual), don’t worry about it (Photo 3). Let it go. It’s too much work to lap the entire back down to this level. Instead, you’ll grind off the leading end later.

 

 

Photo 3: Sand until the back is level. You’ll know you’re done when all the rust pits and low spots are gone and the back is completely covered with scratches. If the leading end is low and unscratched, don’t worry about it. You’ll grind this off later.

 

 

Smooth the Back 

 

Smoothing the back requires going through a series of finer grits (Photo 4). With each one, you must remove all traces of the scratches made by the previous grit. How can you tell when that happens? I change direction with each grit. This makes coarser scratches easy to distinguish from finer ones. On the 60-grit paper, for example, I held the chisel pointed right. On the 80-grit paper, I pointed it left. It doesn’t pay to skip grits. If you start with 60-grit, continue with 80-, 100- and 120-grit papers. 

 

 

 

Photo 4: Start smoothing the back with finer grits. Hold the chisel in an opposing diagonal direction on each grit. Keep sanding until all the scratches from the previous grit are gone. It’s easy to distinguish new scratches from old ones because they run in opposite directions. Go up to a 120-grit belt.

 

 

Precision is critical as you continue to refine the back. After a 120-grit belt, I switch to 150- and 220-grit PSA paper (Photo 5). (Fine sanding belts won’t work because their backing has too much give. This rounds over a chisel’s sides.) Mount the PSA paper on an absolutely flat surface. I prefer a granite surface plate because, unlike glass, granite is virtually unbreakable. The granite also can be stored with sandpaper stuck to it, which you can’t do when using your tablesaw or jointer bed as a flat reference surface. An inexpensive granite surface plate costs $18.

 

 

 

Photo 5: Switch to 150-grit pressure-sensitive-adhesive (PSA) sandpaper and a flatter surface, such as a granite surface plate. Repeat the process with 220-grit paper. The back isn’t fully polished yet, but it’s time to take a break and go to the grinder.

 

 

Grind a New Bevel

 

Grind a blunt edge if the bevel requires major reshaping (Photo 6). This is the best strategy when the back’s leading end is low or if the bevel is heavily nicked or out of square. The blunt edge should be square to the chisel’s sides. Draw a pencil line across the back to guide your grinding. 

 

 

 

Photo 6: Grind off a leading end that’s low, nicked or uneven. Create a blunt edge by positioning the tool rest approximately 90 degrees to the wheel. A blunt edge is much less prone to overheating than a thin edge. That’s important when you’re reshaping an entire bevel.

 

 

Continue to grind until you’ve removed all the low spots. Adjust the tool rest and grind a 25-degree bevel (Photo 7). Go right up to the leading end. The bevel doesn’t have to be perfectly straight, but a straight end is easier to hone than a crooked one. 

 

 

 

 

Photo 7: Grind a new 25-degree bevel. Grind all the way up to the leading end. Make sure the end is square within a few degrees. To prevent overheating, frequently dip the chisel in water as you approach the leading end.

 

 

Polish the Back

 

Continue lapping the back by polishing it on your sharpening stones (Photo 8). Your goal is to achieve a mirror surface, but you can’t get there in one step. I use three waterstones: medium (800 or 1,000 grit), fine (1,200 or 2,000 grit) and super-fine (4,000, 6,000 or 8,000 grit). Begin with a medium stone, but first make sure it’s flat . A medium stone won’t create visible scratches. Instead, you’ll get a very dull shine. This should extend all the way across and 1-1/2 in. to 2 in. up the chisel’s back.

 

 

Photo 8: Polish the back on a medium stone until all the 220-grit scratches are gone. A medium stone creates a dull grey finish. You only have to work the first two inches or so, not the entire back.

 

 

Hone the Bevel

 

Begin honing the bevel on the medium stone (Photo 9). I prefer the Veritas Mk. II honing guide because it’s easy to set up and handles a wide variety of chisels. Place the chisel in the guide at the correct projection to hone a 30-degree bevel. This is 5 degrees steeper than the ground bevel, so you’ll only be sharpening the leading edge.

 

 

Photo 9: Hone the edge at 30 degrees to create a new, narrow bevel. I use a honing guide to ensure that each stroke follows precisely at the same angle. 

 

 

Creating two bevels saves time and effort. Hone until you feel a wire edge along the chisel’s back (Photo 10). This small metallic ridge must extend all the way across, from corner to corner.

 

 

 

Photo 10:  Feel for a wire edge. This small, raised ridge of metal on the back’s leading end indicates it’s time to stop honing. Be sure to check the corners. The wire edge must go all the way across.

 

 

A wire edge is the best indication that the honed bevel and the back meet, creating a sharp edge. Remove the wire edge on a fine stone (Photo 11). Polish the back until you can no longer feel a ridge. (After your tool has been restored, you should only remove its wire edge on your super-fine stone.) Keep polishing the back until it’s evenly shiny. 

 

 

 

Photo 11: Polish the back on a fine stone. Push down on the back with one finger to ensure the back stays flat to the stone. When the back is uniformly polished and the wire edge is gone, turn the chisel over. Hone on the same stone until you feel a new wire edge. 

 

 

Hone the bevel on a super-fine stone (Photo 12). With the Mk. II honing guide, you can increase the bevel angle by 2 degrees to save time honing on the super-fine stone. This creates a narrow microbevel. A microbevel isn’t necessary on a freshly ground chisel, but after a number of sharpenings, the 30-degree bevel will grow quite wide. At this point, honing a microbevel on the super-fine stone makes sharpening more efficient. Most times, you won’t be able to feel a wire edge develop while you’re using a super-fine stone. The best strategy is to hone six strokes or so, flip the chisel and polish the back six strokes. Repeat this process three or four times.Inspect the edge before you remove your chisel from the honing guide. Catch the reflection from a light or window. You should see a bright line extending to the leading end from tip to tip. If you see a dull line at the leading end, you haven’t honed enough on the super-fine stone.

 

 

 

Photo 12: Hone the edge and polish the back on a super-fine stone. A wire edge created by this stone is difficult to detect, so go back and forth between the bevel and the back a few times. Both surfaces will have a mirror polish—the key to an ultrasharp tool.

 

 

If everything looks OK, remove the chisel from the guide and test it on the barrel of a pen (Photo 13). You should be able to hold the chisel at a very low angle and make a curl.

 

 

Photo 13: Test your edge on a plastic pen barrel. If you can push the chisel at a very low angle and create a long curl, the chisel is good to go. The ultimate test for a sharp chisel is paring end grain. After I lapped and honed my tool on an 8,000-grit stone, it passed with flying colors.

 

 

Now that’s sharp! 

 

 

 

 

This story originally appeared in American Woodworker November 2006, issue #125.  

 

Source information may have changed since the original publication date.

 

 

 

 

Sources

 

Grizzly Industrial, (800) 523-4777, www.grizzly.com  Granite surface plate, 2 in. x 9 in. x 12 in., #G9649, $18.

 

Klingspor, (800) 228-0000, www.woodworkingshop,com Premium sanding belts, available in many sizes and grits, $2 to $10 ea. 

 

Lee Valley, (800) 871-8158, www.leevalley.com 150-grit PSA sandpaper, 30-ft. roll, #68Z72.04, $10. 220-grit PSA sandpaper,

30-ft. roll, #68Z72.06, $10. Veritas Mk. II honing guide, #05M09.01, $52.50.

 

 

 

 

 

 

 

 

November, issue #125

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