sharpening with a grinding wheel
bill tindall
>This note is a companion to the earlier note on diamond sharpening. I was encouraged to submit it by someone who I goaded into trying this technique. They were pleased with the result and they thought a post might help others overcome the fear of having-at a delicate plane iron with a grinder. I must say that my concerns were lessened by routinely grinding edges of lathe tools.
A dull tool has a rounded edge. The bevel must be ground back to remove all the rounding. We will call this step "initial grinding". When initial grinding is complete a burr pushes up and away from the ground bevel, which indicates the edge can now be refined by honing. This burr is referred to as the "wire edge". It is easy to detect this wire edge(burr) by rubbing across the flat side of the tool. The removal of this relatively large amount of metal, large relative to the honing step, is the slow step in sharpening.
The ways to accomplish initial grinding are numerous- coarse sandpaper, coarse stones, and a variety of powered sharpening devices. The disadvantage of the hand powered approaches is they are slow even with the coarsest abrasive available, and they leave a flat bevel. With a flat bevel, all subsequent steps must either grind away at the whole bevel or the bevel angle must somehow be increased slightly. To accomplish these steps while maintaining consistent presentation of the tool to the abrasive requires daunting skill if done by hand, or it requires jigging the tool. Jigging requires added time, the cost of the jig and the shortcomings of using a jig on short stones.
Of the powered means of initial sharpening the humble bench grinder is by far the cheapest, and most shops have one anyway for lawn mower blades. Most woodworkers have not given a bench grinder a second thought for initial grinding having first thought about the peril of burning tool edges. However, one can buy cool running wheels and slow speed grinders. Is it possible to accomplish initial grinding, that is grinding the bevel to produce a straight wire edge, with a bench grinder? Yes, and easily so. It mostly takes the courage to try it.
The goal is to be able to grind a straight tool edge without burning. Essential for success is a solid and spacious tool rest, and a cool running wheel. The rests from Oneway are perfect, the Lee Valley rest is flimsy and made of "sticky" aluminium (steel sticks to aluminium), and the rests that came with a grinder useless. A rest can readily be made from wood. For wheel recommendations see my article in the Articles Section. Acceptable wheels are available from Radiac, Lee Valley, Tools for Working Wood and machine shop supply houses.
The burning issue is minimized by proper wheel selection but a cool wheel is a fast cutting wheel. Control to achieve a straight edge then becomes the issue. I can reliably grind a 2" plane iron to form a straight burr, with no burning, using a 3600 rpm grinder and an 80 or 100 grit wheel. But, this is not the ideal starting place for the inexperienced and it is far from preferred by me either. A better arrangement would be a wheel that turned much slower, like only 500 rpm or less. At 500 rpm a 100 grit wheel accomplishes initial grinding in seconds so control is the focus for grinder selection, not speed.
A Tormek grinder is the ideal tool for initial grinding but at a substantial and unnecessary price. One can learn to do this grinding with a cheap "slow' speed (1700 rpm) grinder and there are other options. I choose to use a 3-phase grinder slowed through the use of a phase inverter that allows me to slow the wheel speed to a crawl. I already had the inverter to control a 3 phase lathe motor so its cost was not a factor. Because the desired wheel speed is but a few hundred rpm a suitable grinder could be assembled from an arbor (Lee Valley), a small motor and a suitable tool rest. Even an old fashioned hand powered grinder would suffice with some adaptation.
The optimum technique could be described as "touch and go". I use a slightly crowned wheel, 1 " wide. I no longer think the wheel width is very important because crowning the wheel is desirable. In a sweeping motion the tool is brought across the wheel, left to right or right to left. The leading tool edge touches the high spot of the crown to commence grinding. Very, very light pressure is used with rapid tool movement, something like a second to complete the pass. Let me emphasize again-light pressure. Only the tiniest amount of metal is removed with each pass. I examine the edge in bright light after each pass or at least every few passes.
It is easy to see progress and correct the point of tool engagement with the wheel to compensate for under/over ground portions of the edge. Suppose for example the left edge of the tool edge is underground and we are using touch and go from left to right. On the next pass, don't touch the wheel until the right edge is past the wheel's crown. Nothing will be removed from the right edge while the center and left edge will be ground. By varying the very slight pressure used to hold the tool to the wheel, a tapered grind can be accomplished when necessary to straighten an edge. It all sounds complicated until you actually do it. The hollow of the tool bevel nestles against the curvature of the wheel. With a light touch it is easy to feel that all is well with the presentation of tool to wheel. Never-the less- practicing on some expendable iron is recommended. Oh, did I say use a light touch? These cool running wheels cut with blazing speed.
One of my editors(critics) missed the point that in this grinding the entire bevel is being ground, all of the bevel hollow right to the edge. This grinding renews the edge as well as maintains the hollow grind and bevel angle. There are but two operations-grind, hone, get dull, grind, hone, get dull, repeat.....
When the burr forms and the edge looks straight (I check with a small square against a light source) it's done. If the bevel angle has already been properly established this whole operation could take less than a minute, while grinding a new bevel angle might take several minutes. (It is beyond the scope of this note to recommend bevel angles but about 30 degrees is a place to start if you have to guess.) Changing bevel angle requires considerable metal removal and more heat is generated. I feel the edge in this operation and dip the tool in water occasionally if it appears to be getting hot.
This straight hollow ground bevel is now ready for rapid honing as described in the previous diamond post. Most people will find it very easy to hand hone this hollow bevel because it indexes so confidently on the plate. Other honing techniques could also be used. After a couple of strokes on the diamond honing plate it is easy to confirm the edge is straight from the bright polished line made by the honing grit. If the edge is way off then this polished line can be used as a guide to correct the problem back on the grinder.
I am a person of average hand skill (I can't cut flauntable dovetails) yet this means of initial sharpening was not difficult for me to master. The reward is blazing speed and low cost compared to many alternatives.
