I am making a wooden box out of Gabon Ebony solid - 4-1/2" x 12" x 1-3/4" high. I am making the box by cutting the "top" and "bottom" off of a solid piece of stock and then hollowing the interior with a jigsaw leaving a wall thickness of approximately 1/2". The wall is rough from the jigsaw and not very true and so I trim it with an over-arm type router with a table that moves on an x, y and z axis.
What bit and speed should I use to trim the box interior? Also if I want to mill out the center of the box in lieu of cutting it out with the jig saw, what bit and speed would should I use? The ebony is very brash and I have experienced a lot of failure trying to use a spiral up cut bit. Help Please
- David W.
Our Expert
Without seeing the results of the cut, I suspect that you're experiencing chatter as a result of the exceedingly hard, brittle ebony and the relatively thin walls of the box. The hardness, or density, of a wood is directly related to it's specific gravity. For example, red oak, a common native hardwood known for it's hardness, has a specific gravity of approximately .65. In comparison, ebony has a specific gravity of 1.12. The extremely high degree of hardness alone makes this beautiful wood difficult to machine. Also, as they spin and cut, even the best quality router bits can cause the workpiece to vibrate, or "chatter" if certain conditions exist in the setup.For example, a very thin workpiece, a heavy cut, extremely dense stock, and/or a dull bit can all cause chatter and a rough finished surface.
Here are a few suggestions for improving the quality of the cut:
-Add mass. Even though you're working a specie of wood with inherent mass, the walls of the box, at 1/2", are thin and unsupported. I suspect that the clamp in the overarm router that you're now using only supports the box near the base. As a result, the thin walls vibrate when they come in contact with the spinning bit.
If possible, shape a thicker piece of stock and cut it to final size after the routing process is complete. For example, to make a narrow molding I shape a wide board and rip the molding profile free after shaping. Because of the high cost of ebony this may not possible or practical an so you might try constructing a jig which supports the box on all four sides. By securing the workpiece in a large, heavy jig, you, in effect, are adding mass to the work which can dramatically improve the quality of the cut.
-Reduce the size of the cut. For example, when flush trimming I remove just enough stock with the router to smooth away the saw marks; no more than 1/16" and sometimes less. Because of the dense stock and the thin wall of the box I would reduce the cut to a bare minimum.
-Try a large, down-cut spiral bit. As the name implies, up-cut bits lift the chips up and out of the cut which makes them a good choice for cutting mortises. However, on the brittle stock of the box you're making this can create a rough top surface. A down-cut bit will leave the top surface smoother but you'll need to stop periodically and clean the shavings from the box interior. And a large diameter bit, such as Amana no.46209, will be less prone to deflection and chatter than a smaller bit.
-Use a very sharp bit. Although this seems obvious, keep in mind that the harder the wood, the sharper the tool needs to be. A blade or bit that has a small amount of edge wear from hours of use may work fine on less dense hardwoods such as cherry or oak. But ebony requires a very sharp edge for a smooth cut. It may be time to replace the bit.
-Use a high router RPM. Use a speed of at least 18,000 RPM and feed the bit slowly. Watch for signs of overheating.
