In the very first podcast of Woodwoorking ONLINE’s podcast series, the presenter demonstrates a simple technique for making grooves and dados without measuring them out. I suck at rudimentary math and my measuring skills are fairly basic as a result… as I’m also without a dado blade, I thought the it would be interesting to have a try myself.
The basic premise is fairly simple: establish the blade height, account for the width of the blade to establish the left outer cut, factor out the width of whatever materials will ride in the dado to establish the right outer cut, and then clear out the waste. Clearing the waste involves taking advantage of the width of the blade kerf; the teeth on my CMT fine cutoff blade are supposedly 2.5mm so I simply move the work piece over about 2.5mm with every successive cut. For the rest of the article I’ll discuss the other steps involved.
Establish the blade height
If you’re lapping two pieces of wood, you’ll generally want to set the blade height to half the height of the work piece. This can of course be measured but I’ve found the technique described here to be very accurate.
Start with a piece of scrap wood the same width as the wood you’ll be using. Use your eye to set the blade height to slightly less than half the width of the stock. Cut the first slot and then flip the work piece over and make a second cut from the opposite site as though you were making a tenon. The two cuts probably won’t meet up so sneak up on the centre point by raising the blade height progressively—bear in mind you only want to raise the blade by half of the width of the remaining waste as you’ll continue cutting from both sides.
When the final sliver is nipped away, you’re at the halfway mark for the stock in question. Lock down the blade height so things don’t move.
Establish the left outer cut
You’ll need to determine the width of the slot left by your saw blade for this task. Although the teeth on my saw blade are marked as 2.5mm, in practice I’ve found a piece of scrap 3mm MDF fits perfectly into the slot left behind. The work piece will initially be positioned in relation to the rip fence and the blade; the 3mm spacer provides the offset necessary for the first cut.
Roll the fence into position, drop in the spacer, and then position the work piece against the mitre gauge. Adjust the fence to line up the first cut and then lock the fence in position. If the work piece it too short, use an additional block to pad out the gap between the fence/spacer and the work piece. Make the first cut to establish the left shoulder.
Note the fence is only being used as a stop block; as usual, you shouldn’t use the mitre gauge and fence at the same time, although in this case there is less opportunity for the work piece to bind if the fence is out of alignment.
Establish the right outer cut
Look ma, no measuring! No math! Leave the fence where it is and remove the spacer. Keep the extra “pad” block in place if necessary. Position an offcut of the same width as the material that will sit in the final dado between the fence and the work piece (or pad block). This offsets the work piece the width of that material. Make the second cut to establish the right shoulder.
Clean out the waste
At this point go ahead and clear out the waste material. When you’re done, the material intended to fit into the dado should fit fairly perfectly! The one issue I ran into was a result of the teeth on my blade being angled in opposite directions; this resulted in a series of v grooves that would necessitate some additional cleanup with a shoulder plane or router. To correct this situation, the Woodworking ONLINE presenter suggested shifting the work pieced left and right over the blade; this does an okay job but I’m not sure what would happen if the work piece came in contact with the shoulder of the dado!