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Many drills have a maximum torque that they can provide to make round holes. The larger the diameter, the larger the motor of the drill unit has to be. When an oversized hole (e.g. larger than 40 mm) only has to be made occasionally, using a machine with an oversized motor may not be the preferred choice. In that case, it is possible to use a mill, giving the same result and execution class as a drilling process. In standard processes, the drilling machine will pre-drill a hole in which the mill will be lowered. It will then clear the hole up to the required diameter.

These holes have 4 straight sides and a certain corner radius. The corner radius determines the maximum diameter of the mill. In order to make holes as efficient as possible, it may be needed to change the mill diameter, i.e. large diameter for removing the rectangle and a smaller diameter for the making the smaller radius.

I'm looking on ebay at the Drill Hog ones but I don't know what the difference is between the M42 colbalt, Niobium or their "Pig Steel" drill bits. I've even seen platinum ones by a different manufacturer.

Milling is a process where a milling tool cuts away the material in a rotary motion. As with drilling, this is possible with a wide array of different tools with different diameters and different hardness’s. Because the mill is moving, the rotational speed must be high in order to get a clean finish of the milled hole. The goal is to find a good optimum between the moving speed, the rotational speed and required quality.

In a slotted hole, the diameter of the radii of the two ends of the slotted hole are the same as the width of the slotted hole. Again, a hole is drilled and the mill widens the hole.

Milling is used for various purposes, but mostly for three distinct types of holes: oversized round holes that cannot be drilled, round slotted holes for e.g. connections that require flexibility during construction and rectangular holes. These different types of milled holes are explained here:

How much material the mill can remove into the material depends on the diameter of the mill and the hardness of the material. As a rule of thumb, the maximum thickness that can be removed in one go is the diameter of the milling tool. If the material is thicker than the mill can remove, the mill has to go through the material multiple times. Since there are many variables, it is good to investigate which options are available and how choices are implemented in the machine of your choice, in order to get the most efficient and productive machine. For example, the software must be able to deal with different thicknesses and hole dimensions without operator intervention. For more complex shapes, coping is a process with virtually unlimited freedom and a higher processing speed.

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