So, you need to cut the surface of an object so that it will butt up against the surface of another. Easier said than done when the surface you’re butting up against has an irregular shape. To accomplish this task you need to know how to properly scribe the irregular shape onto the surface you’re cutting.
There are three steps to scribing in order to get a proper fit:
Step one: Butt the piece you are scribing against the irregular shape like it’s going into its final position. What I mean by this is if the object needs to be plumb or level then you need to shim it in place before drawing your scribe marks.
Step two: Use a compass (for drawing not for getting directions) to scribe the irregular surface onto the piece to be cut. First, inspect the two pieces as they are butt together and determine where the widest separation between them is. Your compass should be set wider than that separation. Next, hold the compass level and parallel to your irregular surface with the metal tip gently resting against it. The pencil side of the compass should be perpendicular to, and touching the piece to be cut. Now, gradually follow the irregular shape with the compass so that it transfers onto the piece you will be cutting. The important part is to keep the compass level as you move along the irregular shape. If not, your scribe marks will be incorrect.
Step three: Cutting your scribed piece may sound like the easy part but there are some tricks that will help give you the ultimate fit. Don’t make your first cut directly on the line. Give yourself some wiggle room by cutting beside the line. Check the initial fit and re-mark for a final cut. You should also do what is called “back cutting” your initial cut. You back cut by angling your cutting tool to 85 degrees instead of 90. The result is a beveled cut with the front side (one that will be seen) longer than the back. How this helps is twofold. One, it will be easier to make any final adjustments with less material to be removed and two, less surface contact will increase likelihood of a perfect fit.