How to flatten an uneven workbench top
Start by preparing the bench
Now, make and attach the flattening jig With the jig shown in this article, you can slide a router over your benchtop to remove all the high areas and leave a flat surface. The router slides back and forth across the top in a router carrier that slides along a perfectly level and straight carrier guide on each side of the benchtop. These pieces are mounted to the bench with carrier-guide supports.
To mount the carrier guides and carrier-guide supports, follow the Setting Up the Carrier Guide drawing below. (You may find it necessary to remove one or more vises.) To make the carrier guides, joint one edge of a 2x4 and rip the opposite edge to give you a straight board with parallel edges.
After mounting the carrier guides, use a level to make sure that they are level along their length and level with each other. If the benchtop is twisted you may find it necessary to shim the carrier guides with thin pieces of wood or paperboard as shown in the Section View below. Use only as many shims as necessary.
Now, use another jointed board as long as the carrier guides to check them for straightness. If the carrier guides bowed during mounting, you will need to correct the situation with shims.
These leveling and shimming steps can prove time-consuming, but stick with them -- you'll be rewarded with a flat bench in the end. Finally, construct a router carrier according to the drawing on the top of the next page.
It's time to rev up the router and start flattening To adjust your router's straight bit for the correct cutting depth, use a straightedge and tape measure as shown in photo C to find the lowest spot on the bench. Then, place the router carrier onto the carrier supports so its slot centers over the low spot. Mount a 1"-to 1 1/2"-diameter straight bit in your router, place the router into the router carrier (directly above the low spot), and set the bit for a paper-thin cut. You must securely tighten the router bit into its collet, and the router motor housing to the base, to eliminate any possibility of the cutting depth changing during the following steps.
Next, use four clamps to secure the router carrier to the carrier guides at either end of the bench. Make a full cut across the width of the bench by first running the router away from you and along the left hardwood side of the router carrier. Then, make a return cut toward you along the right side of the router carrier. Without moving the router carrier, repeat these router passes once or twice so the bit cuts to its full depth. As you move the router, remember to place minimal downward pressure on it to avoid bowing the router carrier.
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