Thursday, April 29, 2010

Learning to Plane - Part III

When Should You Regrind a Bevel?

You only need to regrind after you have honed your secondary bevel, say, six times, if it is taking a long time to get a good polish, the secondary bevel has grown large or if you've hit a nail or dropped the blade and nicked it.  Chris Schwarz grinds on sandpaper, starting with 80 grit and moving to 120 grit.  Grind until the secondary bevel is small, unless it has been damaged.  In the latter case, grind it off entirely.

If you need to regrind on a grinder, grind the point flat, because the point is the most likely place for the blade to burn.  Alternatively, you can quench the blade while grinding.  If you burn a blade blue, hone it and use it anyway.  It will need honing sooner than if it had not been burned, but otherwise it will be OK.

When using a grinder, Chris draws a square line with a magic marker--red shows up best.  Then he grinds the bevel flat using an 80 grit wheel.  He prefers the gray wheels over the white ones because the gray breaks down easier and stays cooler.

How do you tell if your blade is dull?  When you can see a bright line across the end of the blade.  If it's sharp, you can't see the end at all.

Cambering Blades

Some planes will have straight blades.  Others, especially those designed to flatten boards, should have a curve to them.  Jack planes, when set up as fore planes, should have a curve with an eight inch radius.  You would create this camber on the grinder.  Jointer or try planes can be straight or curved.  Chris prefers them curved.  They get a much smaller curve, about .006 inch from the center of the curve to the edge of the blade.  This would work out to a radius of about 37.5 feet.  Bench planes get a smaller curve yet, about .002 or .003 inch.  In case the case of jointer and bench planes, you grind the blade flat, then hone it curved.  The goal is to have enough curve to the blade to avoid tracks and still take a good shaving.

Chris's method is to use a honing guide that allows some rocking motion side to side.  He holds the blade square against the 1000 grit stone, putting heavier pressure on one outside edge, then pulling the blade 10 times.  Then he shifts the heavy pressure to the other side and pulls 10 times.  After that, he moves the pressure to the midpoint between one side and the center and pulls the blade 5 times, repeating that motion on the other side.  Finally, he puts the pressure in the center of the blade and gives it two pulls.  This sequence is then repeated on the 4000 and 8000 grit stones.  There is no need to repeat the ruler trick on the back of the blade.  Stropping is about like using the 8000 grit stone.  Stropping the blade is unnecessary after honing and may even lead to rounding over the bevel.

You also want to trim the corners of the blade.  This only needs to be done the first time you set up the blade and is done by pushing a fine file against the corner of the blade, using a rounding motion.  This helps prevent blade tracks on your work.

Thanks to Jeff Fleisher for these photos.  Next time I'll cover setting up planes.


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