Reply To: how to make edges perpendicular to complex shapes
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Great project. If you did a good job with your hand plane of flattening the back of your guitar body, then couldn’t you just register the body of your square against the back and check the sides, and make any small tweaks to the areas which aren’t perfectly perpendicular?
If there are large areas out of square, then what about a scribing tool first, followed by a saw. Basically the scribing tool in this case will function like a square, but instead of the flat blade of a square, it has a pencil or marking knife at a right angle to the body. You can make one — just stick a pencil or a sharpened finish nail or etc through a flat piece of wood at 90 degrees. Run the body of the scribe along the flat back of your guitar body, and the pencil or knife will mark a line along the side where you need to cut away material. Where the side is undercut or perfectly 90, it obviously won’t draw a line. You can then fix the undercut areas later by finding them with an actual square.
If the top / front of the guitar is also flattened to a high standard, the you can do that procedure from both sides, and when you perform this procedure from the top it will find all the areas which were undercut with respect to the bottom.
Good luck, I’d love to see (and hear) the final product