In manufacturing, an undercut is a special type of recessed surface. In turning it refers to a recess in a diameter. In machining it refers to a recess in a corner. In molding it refers to a feature that cannot be molded using only a single pull mold. In printed circuit board construction it refers to the portion of the wafer that is etched away under the photoresist.
Contents |
On turned parts an undercut is also known as a neck. They are often used at the end of the threaded portion of a shaft or screw to provide clearance for the cutting tool. For proper usage the undercut should be at least 1.5 threads long and the diameter should be at least 0.015 in (0.38 mm) smaller than the minor diameter of the thread.[1] They are also often used on shafts that have diameter changes so that a mating part can seat against the shoulder. If an undercut is not provided there is always a small radius left behind even if a sharp corner is intended. These types of undercuts are called out on technical drawings by stating the width and either the depth or the diameter of the bottom of the neck.[2]
Undercuts on molded parts are features that prevent the part from being directly ejected from the injection molding machine. They are categorized into internal and external undercuts, where external undercuts are on the exterior of the part and interior undercuts are on the inside of the part. Undercuts can still be molded, but require a side action or side pull.[3] This is an extra part of the mold that moves separately from the two halves. These can add 15 to 30% to the cost of the mold and also increase the cost of the molded part.[3][4]
If the size of the undercut is small enough and the material is flexible enough a side action is not always required. In these cases the undercut is stripped or snapped out of the mold. When this is done usually a stripping plate or ring is used instead of stripper pins so that the part is not damaged. This technique can be used on internal and external undercuts.[3]
In machining the corners may be undercut to remove the radius that is usually left by the milling cutter. Examples of this use are linear bearings for square shafts (i.e. racks) and machined hexalobular sockets.
Undercuts from etching are somewhat different than the undercuts explained above, because it is a side effect, not an intentional feature. Undercuts from etching can occur from two common causes. The first is over etching, which means the etchant was applied too long. The second is due to an isotropic etchant, which means the etchant etches in all directions equally. To overcome this problem an anisotropic etchant is used.[5]