• This method is appropriate for parts with large length-to-diameter ratios. • At the headstock center, a device called a dog is attached to the outside of the work and is used to drive the rotation from the spindle. • The tailstock center has a cone-shaped point which is inserted into a tapered hole in the end of the work.
The tailstock center is either a ‘‘live’’ center or a ‘‘dead’’ center. • A live center rotates in a bearing in the tailstock, so that there is no relative rotation between the work and the live center. The live center can be used at higher speeds. • dead center is fixed to the tailstock, so that it does not rotate; instead, the workpiece rotates about it. Because of friction and the heat buildup that results, this setup is normally used at lower rotational speeds.
2. The chuck is available in several designs, with three or four jaws to grasp the cylindrical work part on its outside diameter. A self-centering chuck has a mechanism to move the jaws in or out simultaneously, thus centering the work at the spindle axis. Other chucks allow independent operation of each jaw. Chucks can be used with or without a tailstock center. • For parts with low length-to-diameter ratios, holding the part in the chuck in a cantilever fashion is usually sufficient to withstand the cutting forces. • For long work bars, the tailstock center is needed for support.
3. A collet consists of a tubular bushing with longitudinal slits running over half its length and equally spaced around its circumference, The inside diameter of the collet is used to hold cylindrical work such as barstock. Owing to the slits, one end of the collet can be squeezed to reduce its diameter and provide a secure grasping pressure against the work. • Because there is a limit to the reduction obtainable in a collet of any given diameter, these work holding devices must be made in various sizes to match the particular workpart size in the operation. 4. A face plate , is a workholding device that fastens to the lathe spindle and is used to grasp parts with irregular shapes. • Because of their irregular shape, these parts cannot be held by other workholding methods. The faceplate is therefore equipped with the custom-designed clamps for the particular geometry of the part.
3. Drilling and related operations Drilling is a machining operation used to create a round hole in a work part. • This contrasts with boring, which can only be used to enlarge an existing hole. Drilling is usually performed with a rotating cylindrical tool that has two cutting edges on its working end. The tool is called a drill or drill bit . The most common drill bit is the twist drill . Drilling is customarily performed on a drill press , although other machine tools also perform this operation.
Cutting conditions in drilling The cutting speed in a drilling operation is the surface speed at the outside diameter of the drill .
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- Fall '19