In this case, a dedicated, hollow-chisel mortiser can be a godsend. A mortiser is little more than a modified drill press that holds a square, hollow chisel with a drill bit in the center.
Features to Consider:
Look for a model with a travel of at least 4-1/2". Any less travel and you may have a difficult time cutting through-mortises on some boards.
The mortiser should use a smooth but strong steel rack-and-pinion system with a single arm to drop the chisel and bit into the stock. Pushing the chisel into hardwood can take a bit of muscle, and the action should be strong enough to handle considerable torque from the operator. Additionally, the chisel should complete the entire length of the travel in less than a 90-degree rotation of the arm.
Using the Mortiser:
Also, remember that the bottom of your mortises will not be nearly as clean as the sides, since the drill bit isn't square on the bottom. This is rarely an issue, but something to keep in mind if the bottom of the hole is ever going to be visible.
Before using your mortiser, always check to see that the chisels are square to the fence, and that your stock is securely against the fence. Otherwise, your mortises will not be square to the face of the board held against the fence.
Also, before cutting your mortises in your stock, make test cuts in scrap boards of the same thickness. This will allow you to set the depth of cut accurately and make any mistakes on scrap so you don't damage your project.
Caring for Your Mortiser:
Second, keep your unit as clean as possible. Sawdust can wreak havoc on the rack-and-pinion and drill chuck.
Finally, over time, the teeth on the rack-and-pinion can wear down a bit. Check your tool's manual for instructions on adjusting for normal wear and tear on the mortiser's travel mechanism.