The hollow mortise chisel mortising attachment converts your drill press into a mortising machine. If your woodworking project calls for a number of mortise and tenon joints, you need to invest in a dedicated, hollow-chisel mortiser, with which you could crank out a number of clean mortises in a hurry.
If you don't have space (or the budget) for another power tool, but have a drill press, there is another option. Almost all floor-standing drill presses can accommodate a hollow-chisel mortising attachment. This attachment will connect to your drill press in just a few minutes, and allows you to use hollow-chisel mortising bits in sizes from 1/4" to 3/4" in width.
This mortising attachments also include a stock hold-down, which connects to your drill press table. This hold-down will hold your stock (wood) firm when the square chisel is withdrawn from the stock. Using this attachment makes cutting mortises safer and quicker, so resist the urge to cut your mortises without it.
Most mortisers are equipped to handle 1/4", 3/8", 1/2" and 5/8" square chisels. Converted to metric: 6,35mm, 9,5mm, 12,7mm and 15,9mm.
Some are even capable of handling a 3/4" chisel, but keep in mind that this is a pretty healthy chunk to cut at one time and may require a lot of physical exertion. Remember that even if you're cutting a mortise that is 3/4" wide, you don't need to cut the entire width in one pass. A few cuts down one side of the mortise and then back up the other with a 1/2" chisel will be just as effective and not nearly as taxing on you as the operator.