SUPERHATCH is a former AutoCAD Express Tool that is now included in the core AutoCAD product. You can create a hatch pattern from most anything without having to learn how to write a hatch pattern definition file.
This AutoCAD tutorial is by Autodesk’s own Cliff Young of the AutoCAD Test Development Team.
Step 1. Draw a rectangle and divide it into quadrants.
Draw polylines across the rectangle to simulate a wood grain.
Be sure that the number of endpoints at the top edge is the same as the number of endpoints at the bottom edge, and the number of endpoints at the left edge is the same as the number of endpoints at the right edge. (This is important, because you will be connecting these endpoints later).
Step 3. Make 3 copies
2 copies of everything (rectangle, quadrant lines and wood grain).
1 copy of just the rectangle and quadrant lines.
Step 4. Trim the wood grain and copy the quadrants.
Inside the first rectangle, trim or delete all the wood grain from the upper-right and lower-left quadrants.
Inside the second rectangle, trim or delete all the wood grain from the upper-left and lower-right quadrants.
Copy the trimmed wood grain quadrants to their opposite quadrant in the new rectangle (E.G. upper-right to lower-left, etc.)
Step 5. Clean up the wood grain.
Make any other changes you want to any of the interior vertices of the wood grain. The only rule is to never move any of the outer-edge endpoints.
Move the interior endpoints so that they are coincident. Do not move any of the outer-edge endpoints - these need to remain where they are so that the pattern will tile seamlessly.
Step 6. Join all the polylines and convert to splines.