Popular Woodworking 2009-04 № 175, страница 53

Wheel diameter

the final jig.

the flat portion is held against the tool rest while it is tightened. This sets the tool rest relative to the wheel to produce the desired hollow-ground bevel angle.

To begin construction you need the diameter of your grinding wheel. If your wheel is new, you can assume it's the diameter on the packaging; if it's been substantially used, you should remove it and measure its actual diameter. The jig's angular accuracy depends on having a correct (it to the wheel's diameter.

Next, obtain a piece of ^l/2" MDF with the dimensions shown in the drawing at right. Drawa line one inch below the top edge. From the midpoint of this line, use a protractor to draw a second line at the desired bevel angle plus 90° as shown.

With a compass set to the wheel's radius, place the compass at the midpoint of the 1" line and strike an arc on your second line. Now place the compass at the intersection of the arc and second line and draw an arc intersecting the original line as shown in the layout illustration at right. This completes the layout.

Carefully ripthejigalongthe 1" line, leav-ingthc linejust visible. Band saw the arc close to the layout, leaving some material, then carefully sand to the arc. Test the arc on your grinding wheel to check for a snug fit as you sand it. You may need to trim the length of the jig to fit your grinder's wheel guard. You can make small adjustments to the arc's fit by holding it against the grinding wheel and turning the wheel by hand. Finally, mark the bevel angle on the jig. You are now ready to set your tool rest as shown in the opening photograph.

Honing

Once the blade's bevel is hollow ground, you will need to go through the honing grits to finish the edge. While practice will make you adept at freehand honing, many find an inexpensive honingguide will do a better job. There are many honingguides available, and some come with angle-setting jigs. But what you really want is your honing guide set for the bevel angle ground on your blade. A set-tingjig may not match this exactly. There are several factors such as blade thickness and taperangle of a chisel that introduce small variations into the exact bevel angle set by the tool rest jig, so the best approach is to set your honing guide to match the bevel angle that your jig produces. This is easily accomplished as shown in the photo at right.

Use a straightedge applied to the edges of

the hollow grind and adjust the guide's roller to match that angle. Once this is done, record the blade position in the guide by butting the guide against the edge of a piece of plastic laminate and scribing the blade edge location with a sharp knife. This distance won't change as the grinding jig always produces the same bevel angle. You now only need to place a ground blade edge on the scribe line to set the honingguide position.

Conclusions

A jig to accurately set the tool rest on your grinder will eliminate the guesswork of repeatedly tweaking the bevel angle. There are several factors that come into play when calculating a bevel angle, such as blade thickness and taper. For typical chisels, these combine to an error of less that one degree, so they were neglected insetting the bevel angle for

the jig. For a plane blade that has no taper, you should add one degree to your desired bevel angle when laying out the jig to compensate for these factors.

It should go without saying, but to get top results, you need to use a soft grinding wheel such as an #80-grit aluminum oxide white or blue wheel on your grinder. Then true it square and flat with a diamond wheel dresser. Dressing removes glazed surfaces, making the wheel cut faster and cooler. In like manner, be sure your honing stones are flat. Waterstones sharpen fast, but they also wear fast. To get the best results, true them with an appropriate flattening stone. These steps will save you time in the long run and give you razor edges. PW

Bruce retired as an electrical engineering teacher from MIT in 2000; he now enjoys more time in his shop.

Adjust honing guide. The wheel and both edges of the hollow grind should all touch a straightedge to hone the desired angle.

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