Introduction: Sharpening Jig for Forstner Drill Bits
Save money! Resharpen Dull Forstner Bits
1/2-inch plywood base, 6-1/4 x 9-inches
(3) 3/4-inch x 6-1/4 x 9-inch plywood spacers
3/4-inch thick x 1-1/4 x 2-3/4-inch hardwood clamp (Plywood will do.) Shave a little off the edge where the diamond card will go, so that the clamp is nearly level on top of stack.
1 x 3-inch diamond sharpening card, 360-grit (CHEERBRIGHT 3pcs 180#,260#,360# Whetstone from Amazon ASIN: B0796PP7N2)
5/8-inch thick x 6 x 1-inch plywood end support (Thickness may vary, depending on brand of Forstner bit being sharpened.)
(2) 9/16 x 1 x 3/4-inch hardwood clamps (Plywood will do.) As with other clamp, shave a little off thickness leaving an overhang of about 1/8-inch that will press down on top of end support.
(2) 1/4-inch plywood x 1 x 5/8" guides
(2) 3/16-inch thick x 3-1/2 x 1/4-inch hardwood guides (Plywood will do.)
(2) #6 or #8 x 2-1/2-inch, Phillips-head woodscrews
(2) 1/4-20 bolts x 2-1/2 long (Heads will be cut off.)
(2) 1/4-20 hex nuts
(2) 1/4-inch flat washers
(2) 1/4-20 wingnuts
(2) #8-32 x 2-inch flat-head machine screws
(2) #8 flat washers
(2) #8-32 wingnuts
(4) #4 x 1/2-inch round-head woodscrews
Wood glue (Elmer's, Titebond, etc.) White (school) glue will do.
Small amount (drops) of mineral oil
Step 1: Basic Construction
- Tools needed: Wood saw (Table saw is best) for dimensioning components, electric drill, screwdriver bit (Phillips) for long wood screws,1/4-inch (twist) drill bit, ice pick for starting #4 wood screws, 1/8-inch drill bit and small screwdriver for same screws, hacksaw, countersink that will fit in drill.
- Gather materials. Overall dimensions are not important. Project is intentionally bulky, so it will not slide around in use.
- (Refer to Photos.) Add three thicknesses of 3/4-inch plywood ("spacers"), to base, using glue and two long wood screws.
- Cut heads off two 1/4-20 bolts. Cut-off ends will be ragged.
- Drill two 1/4-inch holes, about 1-3/8-inches apart, through top clamp that will hold diamond card. Holes should be off-center at about 3/8-to-1/2-inch back from the edge where the card will be. Place top clamp on stack of spacers, and mark its holes on top of those spacers. The part of the top clamp that contacts the card can be notched a little using a table saw so that it will sit properly on top of the spacers while tightened. Otherwise, a shim can be used under the opposite edge of the top clamp to compensate for the thickness of the diamond card.
- Drill two 1/4-inch holes about 3/4-inch deep in the stack of spacers using the top clamp as a guide. Add some wood glue to each of those holes, and insert the ragged ends of the cut-off bolts
- Place end support on the base without fastening it yet.
- Lock diamond card with top clamp, and press flat blade of a Forstner bit (visible in Photo #3) against it until the shank of the bit rises.
- Turn the bit a few degrees clockwise on card, so that full cutting surface is in contact with the abrasive.
- Slide end support under the shank, center it left-to-right, and determine whether end support is too high or not high enough. If the latter, use cardboard shims; if too thick, cut a thinner end support. Mark the best location for end support on the base (end of bit directly over center-line).
- Glue 1/4-inch plywood guides on opposite sides and opposite ends of end support, then drill 1/8-inch holes in base for #8-32 machine screws. Mark and drill matching holes on 9/16 x 1 x 3/4-inch clamps. Turn jig over, and countersink 3/16-inch holes so that machine screw heads will not drag on workbench. Push machine screws through base from bottom and up through 9/16 x 1 x 3/4-inch clamps. Lock clamps in place with flat washers and wingnuts.
Step 2: Testing
- Lock diamond card in place on sharpener. Apply a few drops of mineral oil.
- Use a black permanent marker on cutting surface of Forstner bit that is to be sharpened.
- Position bit so that surface to be sharpened is facing down against diamond card, and far end of the bit is resting on the end support below. Push and pull the bit in a "forward/backward" sliding motion, for two strokes. Remove the bit, and inspect the sharpened surface. If only the leading edge is shiny, the end support is too high. If only the leading edge is still black, the end support is too low. Make a new end support that gives the correct results.
Step 3: Finish-up and Use
- When end support is exactly correct, add 3/16-inch thick x 3-1/2 x 1/4-inch guides to the sides of the end support (on the top surface, or using the "wrap-around" method pictured) to keep the bit-end centered on it during the sharpening operation. Use the 1/8-inch bit to drill through the guides about 1/2-inch from the ends. Mark the holes through the guides onto the surface of the end support with the ice pick. Fasten the guides in place with the #4 wood screws. (Screws are recommended (not glue) in case the end support eventually wears and has to be replaced.)
- When the jig is fully assembled, check it again with a bit that is a different size. It is possible that some bits will require a different end support if their shanks are not identical to the first.
- In my case, the ideal height from the surface of the diamond card down to the surface of the end support came out to be exactly 42mm. That height works for all my Forstner bits from 1/2-inch to about 1-1/2-inches. Bits that are smaller than 1/2-inch are difficult to control, and larger ones have cutters that are too big for my diamond card.
- In practical use, it is recommended that the sharpening of a Forstner bit be accomplished with the fewest strokes necessary (10-12), and the same number on each of the two surfaces.
I hope this sharpening jig turns out to be useful for you...And if it saves the cost of one sharpening or one replacement bit, I hope you'll feel that it was worth the effort!
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