My electric planer ran over a nail which put a 2.5mm deep chip in one blade, and a 1mm chip in the other.
To get rid of the chips, both blades had to be ground back 2.5mm and then the cutting angle had to be restored. This is the Jig I made to do the job.
Because the grinding wheel on my grinder is 150mm and the wheel does not stick out past the motor housing, a blade that is any wider than 95mm runs into the motor housing. To get a bit of extra room I set the blade up on the wheel so that when it traversed to the right, it moved into one of the bolt holes of the motor cover, giving me an extra 20mm of clearance. If your grinding wheel clears the motor housing, then it may be possible to build a much bigger jig to re-grind planer/thicknesser blades. (!!!)
The key to this design is a horizontal slide attached to the bench grinder where the grinding rest normally bolts on. I considered making a slide that would fit over the grinding rest, but I couldn't think of a way to make it secure enough.
Step 1: Materials:
75 mm x 13 mm x 130 mm (main slide backing)
68 mm x 13 mm x 160 mm (top slide backing)
65 mm x 20 mm x 50 mm (Hinge Joiner)
2 x 9 mm x 11 mm x 160 mm (main slide rails)
2 x 9 mm x 11 mm x 130 mm (upper slide rails)
1 x 35 mm x 9 mm x 160 mm (main slide)
1 x 50 mm x 9 mm x 130 mm (upper slide)
3 mm Plywood:
2 x 15 mm x 160 mm (main slide retainers)
2 x 65 mm x 25 mm (spacers)
1 x 45 mm x 45 mm x 3 mm steel angle
1 x 6mm machine bolt, 35 mm long, preferably with a countersink head
1 x 6mm wingnut
1 x 6mm washer, OD 18mm
1 x 65mm hinge
2 x 5mm countersink head machine screws, 15mm long
2 x 4mm flat head machine screws 10mm long
2 x 10g countersunk wood screws 25 mm long
6 x 12g countersunk woodscrews 15 mm long
Step 2: TOOLS:
Tape Measure or Ruler
Drill and bits
25mm chisel and mallet
150mm bench grinder
NON-ESSENTIAL (but very useful) TOOLS:
Drop saw/slide saw
Step 3: The Main Slide:
Cut the pieces to size.
Slide = 9mm x 35mm x 160mm, s
Baseboard = 13mm x 58mm x 160mm.
Side guides = 9mm x 11mm x 160mm
Plywood overlaps = 3mm x 15mm x 160mm
Partially drive 3 x 20mm x 1mm nails into each of the guides and put PVA glue on the lower side of each guide. Wrap 1 layer of paper over the slide and position it on the baseboard with the 2 guides. If everything fits, nail the guides into position, ensuring that the ends line up with the baseboard. Remove the slide and the paper, and wipe any glue out of where the slide will run. Check the slide for free movement. It will have a clearance of 2 x the thickness of a piece of paper. If you want tighter clearances, you could use 1 piece of paper, or even plastic wrap, but your slide must be very parallel.
Sand the flat faces of the slide until you can place a piece of wood over the guides and the slide will still slide smoothly, and lightly sand the corners of the slide to reduce jamming, then glue and nail the plywood retainers onto the guides. Recheck for free movement of the slide.
Step 4: The Steel Bracket:
I used a piece of 3mm x 45mm x 45mm x 60mm steel angle for the bracket. (A piece of discarded bed frame.)
-See fusion 360 drawing
It can be cut to size with a hacksaw or angle grinder, and after the mounting hole is drilled and the rough edges are filed or ground off, it should be test fitted to the bench grinder to ensure free movement.
If you intend to tap the attachment holes with a thread, mark, punch and drill 2 x 4.2mm (11/64”) mounting holes, or if you want to use nuts on the bolts, drill 2 x 5mm holes.
Line the bracket up with the main slide so that the slide will be centred when the bracket is attached to the bench grinder. (my bracket was offset 4mm from the centre of the slide due to the way that the bracket attached to the grinder.)
Clamp the bracket in position, square to the slide, and drill one hole through one of the bracket holes and through the wooden slide backing. If you will be threading the bracket, enlarge the hole in the wood to 5mm and countersink it for the bolt, then tap the hole in the bracket.
Attach the bracket to the slide backing with this bolt, re-align it so that it is square with the backing, then drill & countersink the second hole, and tap the thread on the bracket. I like to drill one hole first then the other so that there is no misalignment of holes and out-of-squareness when the second one is done.
The bracket should be test fitted onto the bench grinder with the slide on it. It is critical to ensure that the slide is perpendicular to the grinding wheel, and parallel with the axle of the grinder. Sometimes the bracket supports on bench grinders are not straight and you may need to bend the grinder bracket until the slide is square in all directions.
Check for free movement of the slide over the countersunk bolts.
Step 5: The Top Slide:
The top slide is made in the same way as the main slide, except that it doesn't have plywood retainers because the adjustment bolt holds it in position.
Because this retaining bolt has a nut under where the slide moves, I opted for a square flat nut to minimise the cutout.
A countersunk bolt would be ideal for this application, but I didn't have one, so I went with a roundheaded one, and dealt with the
clearance problems. (See photos)
The actual slide has a slot 55mm x 7mm cut in the centre to allow it to be adjusted to get the correct angle of attack on the grinding wheel. It is held in position with a wing nut.
To cut the slot, after it is carefully marked on the wood, drill a 6mm (15/64”) or even a 5.5mm (7/32”) hole at each end, and at intervals along the centre of the slot, and using a coping saw or jigsaw, join up the holes, then file the slots to size.
Carefully mark the hole for the retaining bolt, and check it against the slot in the slide. It should be centred in the slot. If so, remove the slide, drill the hole, install the bolt and the square nut.
Using the nut as a template, cut around the outside of the nut with a sharp knife. Remove the bolt and nut, and chisel out a hole deep enough for the nut to sit slightly under the face of the wood, so that it clears the slide.
Re-install the bolt and square nut and do any adjustments needed to make the slide move freely.
To ensure positive and repeatable positioning of the planer blade, cut a 3mm x 20mm rebate in the end of the slide to support the blade and position it correctly. (see photos)
Once the rebate is cut, the mounting holes for the blade can be drilled. My blade needed 4mm screws, so I marked them by using the blade as a template, drilled 4mm holes inserted short bolts with washers.
Step 6: The Hinged Joiner:
This is a piece of wood 20mm x 50mm x
65mm with a 60mm hinge attached to one end for the upper slide body, and 2 x 25 mm x 3mm x 70mm plywood spacers glued and pinned to the other end. This end is screwed to the centre of the main slider. (see photos)
NOTE: Use a good quality hinge. Cheap hinges have too much play in them, and can create incorrect grinding angles.
It's best to attach the hinge to the joiner and then to the upper slider body.
In my case, because of the round headed bolt, before I attached the hinge, I had to file a groove into the end of the joiner so that the bolt head would clear the joiner. I wanted the joiner to be able to sit flush with the upper slider. (see photo)
Once the hinge is squarely attached to the joiner and the upper slide body, pilot holes can be drilled in the joiner through the plywood spacers.
The main slide should be carefully marked for the plywood spacers. The slide should be clamped to the plywood spacers, and one of the pilot holes can be drilled through the slide.
This hole is then enlarged and countersunk for the screw and one screw can be used to join the main slide to the plywood spacers on the joiner.
The hinge should be checked for squareness before the other screws are inserted.
I had to grind off the points of the screws, as they came through the base and would have stopped the slide from working.
Step 7: TESTING TIME!
Before I did anything, I used a slide
bevel to measure the bevel on these blades. It was 40o when measured with a protractor. I locked the slide bevel so I could use it to check the grind as it progressed. This was very helpful, because a couple of times I had to re-adjust the upper slide to correct the grinding angle.
When I saw how badly the blade had been chipped by the nail, I realised that the whole edge of the blade would have to be ground back to eliminate the chip. (2.5mm.) To do this, I coated both the blades with purple marking dye on the flat sides, and scribed a line parallel to and 2.5 mm away from the cutting edge on both blades, so that they would be the same width and properly balanced.
This was before I had built the jig, and I ground the edges back to the scribed lines using the original grinding rest. I would probably use the jig to do this now, as it is much easier to hold the blade in the jig, but the difficulty is keeping the blade cool.
I don't have a coolant feed to this grinder, so I have to stop grinding after every few passes and sponge the blade down with a wet cloth (rather than dipping the blade continually in water).
Coolant probably wouldn't be very kind to a wooden jig.
In operation, the jig worked well and I was able to grind both blades to where it will be quick work to hone them on a stone. I didn't want to create a sharp edge using the grinder because the wheel is not fine enough and I want to slightly flatten the (currently) concave bevel.
One good thing is that the upper slide itself can be removed with the blade still on it, and used as a handle when honing the blade.
See photos and video.