the amount of stock for machining purposes
The manufacturing process begins with rough material such as a casting, forging or wrought steel. This rough material is modified by machining, or the material may be formed, bent or extruded until it meets the requirements of the print. There are two main factors considered when choosing a manufacturing process: precision and cost. The more precise processes have higher costs.
In earlier lessons, you learned of some of the manufacturing processes: drilling, flame cutting, punching, reaming, and chamfering. Now let’s look at some other common manufacturing processes.
The removal of stock with a rotating tool called a cutter. As the part is fed into the cutter, material is removed.
Cuts material from a part to produce a cylindrical shape. The part is rotated as the cutting tool is fed into it.
Produces a very smooth finish without removing a lot of material. Grinding wheels are available in different degrees of coarseness depending on how smooth the surface must be.
Each metal working process produces a texture on the surface of the part. This texture can be rough or smooth depending on the process. Usually prints contain specific information about how rough (smooth) the features on parts may be. The tolerance for roughness (smoothness) is called Surface Texture. Surfaces on manufactured parts look smooth, but if you looked closely at them through a magnifying glass, you would see they are not as smooth as they appear to the eye.
The surface texture on parts is controlled on the print by the specification 1E2122 Surface Texture. This specification appears in the title block. There are three types of surface texture symbols spelled out in 1E2122.
Letters or numbers are specified above the short leg of the symbol. When a surface texture letter is used with the symbol, the surface of the part must be compared to a corresponding surface on a Surface Texture Comparator.
The comparator has replicas for four grades of surface texture designated by letters F , H , K and N . Letter F is smoothest. Letter N is the roughest.
Each surface texture grade has three variations. The top and center rows are usually used for milled surfaces: the bottom row is for turned surfaces. When a surface texture letter is shown on a print, the machined surface produced must be equal to or smoother than the specified surface on the replica. You compare the surface texture of the party with the specified replica on the comparator by sight and feel.
The surface texture of surfaces can also be designated by a number shown with the surface texture symbol.
The numerical value specified is the maximum roughness average acceptable. Roughness average is measured with a stylus-type instrument calibrated to read roughness average in micrometers.
The instrument, a profilometer, contains an arm and needle which operates similar to a phonograph. By moving the needle across the controlled surface of the part, the roughness can be read on the calibrated dial of the instrument. The roughness is read in micrometers. On a print, a commonly specified value might be 1.6. This is equivalent to 0.0000016 (one point six millionths) of a meter.
When machining required is specified, the surface texture symbol has a horizontal bar added, and may have a number on the left side of the symbol. This number indicates a basic machine allowance. The drawing to the right illustrates a typical machining required surface texture symbol.
The machine allowance is the additional material provided on a rough part to allow for machining. The tolerance for machine allowance is a profile tolerance covered in a later lesson.