At our first meeting of 2019, Mike Lonergan started the year off right with his presentation on sharpening chisels and plane irons.
First, he discussed some terms.
- Primary Bevel – Most chisels and plane irons have a primary and a secondary bevel on one side and a flat surface on the other side. The primary bevel can be either hollow ground or flat ground, either are acceptable. Most new chisels that come from the manufacturer are ‘flat ground’. This is also true of chisels re-sharpened on a horizontal grinding stone. Bevels sharpened on a conventional grinder with a vertical grinding stone are said to be ‘hollow ground’ since the stone leaves a slight concave curvature to the overall shape of the bevel. When sharpening the primary bevel, it is okay to switch between the two depending on the type of grinder you have available.
- Secondary Bevel – A secondary bevel is ground at the tip of the tool with a sharpening stone. It is usually just a few degrees greater that the primary bevel. Mike favors 5-degrees additional. For instance, it the primary bevel is 25-degrees, then the secondary bevel is 30-degrees.
- Backs – It is especially critical the backs of chisels be flat as these are use as a reference for working. Depending on the quality and manufacturer, chisels may need some work for flattening the back. Mike used a surface plate and sandpaper to flatten the backs.
Chisels are categorized into three groups: pairing chisels, bench chisels, and mortise chisels. Each have a different primary and secondary bevel per the chart below.
Next, Mike defines 4 levels of sharpening. Each level is more aggressive than the next
Level 1: Stropping
- Used when a chisel is sharp, but just need a minor touch-up of it’s edge. This is done periodically as the tool is being used. A piece if leather is glued onto a flat substrate, such as MDF, and loaded with an abrasive. Some use jewelers rouge or Mikes favorite, which is 1-micron diamond paste. Finally, the burr on the back of the chisel is removed by honing and stropping.
Level 2: Honing
- Used when the secondary bevel needs to be sharpened. There are many types of stones that are can be used: water stones, oil stones, dry diamond stones. Mike used the later for his demonstration. Two grades are usually necessary: Course (325 gr) and fine (1200gr). A honing guide is used to assure the angle set correctly and maintained while it is worked back and forth on the stone. For each of his honing guides, Mike created a simple jig to set back the edge of the iron from the honing guide for each of the angles on his chart. This eliminates the tedious measuring or setting by eye.
Level 3: Moderate Grinding
- Used when there is moderate damage to the edge of the chisel such as small nicks. This is done on a slow speed grinder. Mike prefers that the grinding wheel is dressed almost flat and perpendicular to the sides of the wheel.
Level 4: Major Grinding
- Used when damage to the edge is significant. This will almost require the edge be re-squared to the chisel’s parallel sides at 90 degrees. Mike uses a Sharpe felt tipped pen (substitute for machinist’s layout fluid), a miniature square, and the back of an X‑Acto knife (substitute for a machines’ marking awl) to mark where to grind the edge flat to start. Then, a new primary bevel is ground conforming to the newly established edge. Mike demonstrated this with a really abused chisel that looked like it was used for a pry bar.
Prior to sharpening, the chisel is evaluated by parring the end grain of soft wood, then placed into one of categories above. Sharpening begins with that category and proceed upward through each category until the tool is finally stropped.
- Generally, plain Irons are just like a wide chisel. Hence the same sharpening steps are used. The back needs special attention as it must firmly contact the bed of the plane and not rock. Other that that, it does not require a high polish.
- Also, when removing the burr on the back of plane irons, they don’t have to be touched off perfectly flat to the back of the iron. This is because the back of plane irons are not used for a reference, the sole of the plane is. When bedded, the tip of plane iron is hanging out in the wind, see the lower left illustration on the above chart. Mike showed us the “ruler trick”. A very thin 6-inch metal ruler is placed on one end of the sharpening stone. Then the plane iron is placed back down on the stone and over the ruler. As the burr is removed a micro back-bevel is created. This should only be done for plane irons, and never chisels.
- The tip of the chip breaker must contact the back of the plane iron when installed. There can be no gaps between the plane iron and the chip breaker, or wood chips will become trapped. Sometimes the chip breaker must be re-fetteled, similar to sharpening, the tip of the chip breaker is honed to fit flat on the back of the plane iron.
- Lastly, some plane irons benefit from a small camber to the edges, see the lower right illustration on the above chart. This prevents the corner edge of the blade from scoring the work surface. Mike showed us a trick like the “ruler trick’ to accomplish this. A thin metal ruler is placed on each side of the sharpening stone, and the one side of the plane iron rides on top of it when honing. The process is repeated for the other side. This technique kicks the angle just slightly to accomplish the camber. This is done on the secondary bevel only.