As always the first step is to set the depth of the router bit. The depth of cut is determined in the planning stage of the project and has a direct relation to the length of the shelf. This is a very important measurement.

Example: we want to build a shelf unit that has an overall width of 24 inches, the thickness of the sides is set at 1-1/2” and we have decided to make the depth of the dado at 1/2". The question is: What is the length of the shelf that fits into the dado? Answer: If the depth of the cut has been set at 1/2" and the thickness of the sides is 1-1/2” what’s left over from the dado cut is 1” for each side which means the length of the shelf needs to be 2” smaller than the 24” overall width of the shelf unit. The length of the shelf needs to 22”.

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Now for the math of the cuts we have to make with the 1” router bit. We need to make a 1-1/2” dado with the 1” diameter router bit which leaves a difference of 1/2". To compensate for this difference we need to make 1/2" thick spacer that is the length and height of the router fence.

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We have a router bit that is smaller in width than the cut to be made so we will be making two cuts on each dado. It is important to remember that the two cuts always have to be on the cutting side of the bit.

First mount the spacer on the router fence then layout the distance from the inside edge of the dado to set the fence.

Clamp into position and make the entire inside cuts at this fence position.

Keep the fence clamped in position and remove the spacer. Now make all of the final cuts.

To make more dados in other positions just repeat the above setup. Remount the spacer, reset the fence position and make the first cut. The remove the spacer and make the final cut.

We have easily made a 1-1/2” dado with a 1” straight router bit.

Be safe and always set up to make the first cut on the inside edge of the dado. Remember to check to make sure that the second cut is always made on the cutting side of the router bit.

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