To many woodworkers, grinding a bevel on their cutting tools is a task filled with fear and disappointment. The dangers are many: poorly shaped bevels, burned steel, edges that are no longer square to the sides of the tool being ground. In this post, I will offer some suggestions to help correct the most common problems.
Problem: Bevels with Multiple Facets
One of the most common problems faced by woodworkers who are just learning to grind tools is ending up with many facets in the bevel. This situation is most often the result of changing the grip on the tool, changing body position during grinding, or a combination of these factors. The situation can be complicated by the relationship between the grinder, the tool rest, and the wheel mounted on the grinder. Here is what happens. During the grinding process, if the wheel is dressed flat across its face, it is cutting all the way across its width, frequently ¾” to 1”. Because so much of the wheel is in contact with the steel, the slightest change in the angle at which the tool is presented to the wheel results in a new facet. These miniscule changes can be caused by shifting your balance, stopping the grind to observe your progress, changing hand positions, and a dozen other causes. Rather than try to control all of the variables (hand position, stance, etc.) I have found it much easier to limit the amount of wheel surface area in contact with the steel. I do this by creating a very slight crown on the wheel. To crown the wheel I use a diamond wheel dressing stick (mine is made by Norton) and remove slightly more material from the sides than from the center of the wheel. The very slight crown provides me with a lot of benefit. First, I know exactly what part of the wheel is in contact with the tool. I can see the steel being removed; some of it remains on the wheel in the form of a dark grey streak. When this streak gets too pronounced I need to re-dress the wheel, because the buildup of steel in the abrasive will slow the grinding process and permit additional heat to build up. The wheel is becoming clogged in this area. Second, the grinding is a lot slower, and that leads to much more control over the process. For many of my sharpening students, this one suggestion is the single most effective change to their sharpening routine, and it clears up a lot of problems.
Problem: Overheating the Steel
The grinding operation generates a lot of friction, which, of course, can potentially build up a lot of heat in the steel. Managing heat is an important part of grinding tools safely and effectively. There are several steps you can take to mitigate grinding’s thermal impact on tools. Crowning the grinding wheel effectively limits heat buildup, because you are reducing the abrasive surface area in contact with the steel. Proper hand position is also critical; if your fingers are located close to the edge, you will feel the tool heating up in plenty of time to slow or stop grinding well before the metal turns colors.
Understanding how heat moves is also important to preventing damage. Heat moves away from its origin. Some is radiated into the cooler air around the tool, but a lot of it moves within the steel through conduction. More mass (that is, thicker steel) can “cope” with more heat than thinner steel. What this means to a tool at the grinding wheel is this: as the grinding operation moves closer and closer to the thin edge at the end of the bevel, there is less steel available to dissipate the heat. If the heat has nowhere to go, it builds up, and the steel eventually reaches a temperature where physical changes occur. These changes manifest themselves visually through a change in color, and physically through a loss of hardness. When the steel turns blue, it has lost harness, and that area must be removed by additional, more careful grinding.
This situation becomes even more critical as the point of contact between the tool and the wheel approaches the corner of the edge. In these two areas, there is the least amount of steel available to conduct heat, resulting in a much faster heat buildup and increasing the chances of damaging the steel. Here are steps you can take to stop grinder-related heat damage:
- Use a white (aluminum oxide) wheel, or other “friable” wheel. Friable means that as the abrasive particles at the wheel’s surface dull, they will break off and expose new, sharp abrasive particles. Sharper abrasives are more effective, reduce friction, and consequently limit the amount of heat buildup.
- Use a slower grinding speed. If you don’t already own a grinder, or if you are making one yourself, look for one that turns at 1,725 rpm instead of the more-common 3,450 rpm motors. Yes, this will slow down the process, but slower is better as you’re getting started.
- Use NO pressure. Let the wheel do the work. Advance the steel to the wheel, but don’t push. Let the wheel do its job of steel removal while you concentrate on a smooth and easy back-and-forth across the tool rest.
- Keep the thumb of your weak hand on the steel, near the wheel. You will feel when heat builds up. When you do, stop grinding, or dip the tool in some water to cool it off.
- Pay close attention, and be very careful, as the freshly-ground bevel approaches those thin corners near the tip. Do not let the tool linger on the wheel in these delicate areas.
Problem: Out-of-Square Grinding
This is a very common problem for woodworkers just learning to grind tools. Fortunately, the solution is quite easy, and it’s foolproof. Simply put, once you have a square end on the tool, never grind all the way to the edge. Get as close as you dare, but always leave the slightest glimmer of shiny steel from the previous honing. If the previous edge was square, the new one will be, too. The down-side to this technique is that your first honing will take just a little longer, because you will have just a bit more steel to remove as you hone, But the time this takes will easily offset the additional time you spend at the grinder, correcting for an out-of-square edge.
What to do if the tool isn’t square when you start? Adjust your tool rest so the edge of the tool is presented to the wheel at 90 degrees (rather than your usual grind angle of 25-30 degrees) and carefully grind the tip square to the edges. Use an accurate combination square to check. When you’re there, stop, readjust the tool rest to your desired grind angle, and proceed. You’ll have a blunt tip to the bevel, and as you grind, that blunt edge will slowly disappear. Go slowly, and check your progress frequently. Save just a little bit of the bluntness and hone it away. When you’re done, the tool will be sharp AND square.
Practice Makes Perfect
Just like every other woodworking skill, no one is born knowing how to grind. It is a skill that can be learned, and once you learn, sharpening becomes so much easier. No messing with jigs or tool holders; when you need a fresh bevel, you step up to the grinder, take care of business, hone, and go back to work. All it takes is a little practice, and maybe a lesson from someone who can observe your technique and offer suggestions. Practice is clearly called for – but don’t practice grinding away an expensive chisel or plane iron. Buy an old junker at a flea market or garage sale for a dollar, and get your technique down using that tool; save the Hock irons and Lie-Nielsen chisels for after the practice session is complete.
There are probably other problems people have run into at the grinder, but these are the three big ones. If you have your own stories, leave them here. I’ll try to address any problems with simple solutions. If there is a way to mess up a grind, I’ve done it – but I’ve also figured out how to correct most every error.