5) Lathe Accessories and attachments
There are many accessories and attachments that are used on a lathe to increase the variety of operations that can be performed. Work-holding devices hold the work securely in the proper position for machining. Work-supporting devices are used along with work-holding devices to secure longer work-pieces during machining operations.
9.5.1 WORK-HOLDING DEVICES
Three-Jaw Universal Chuck. The three-jaw universal chuck has three jaws that move in unison toward or away from the center of the chuck
by a scroll mechanism. This simultaneous motion causes the jaws to locate the work on center to within a few thousandths o
f an inch. The universal chuck is used for holding round or hexagonally shaped workpieces. The
universal chuck is not considered highly accurate over a wide range of diameters.
Four-Jaws Independent Chucks. The four-jaw chuck has four jaws, each of which can be adjusted independently by a screw and half-nut when turned by the chuck wrench .Round ,square, or irregularly shaped workpieces can be held either on or off center .The jaws on the independent chuck can be reversed for holding larger diameter work or for gripping on the inside of hollow workpieces. The four-jaw independent chuck holds a workpiece more rigidly than the universal because it has one additional jaw. The independent chuck can be used to hold parts to a high degree of accuracy and concentricity because each jaw can be adjusted individually.
Collet Chucks. The collet chuck is one of the most accurate work-holding devices.The draw-in collet chuck uses spring collet that are available with round,square,rectangular, or hexagonal openings. Each spring collet has been accurately made to hold smooth workpieces that vary only a few thousandths of an
inch from a nominal size;therefore hot-rolled steel and castings should not be used with collets.
Figure 9 shows how the draw-in collet attachment is mounted in the headstock of a lathe. As the handwheel is turned in a clockwise motion, the hollow draw bar turns. Located near the spindle nose is the collet, which fastens to the draw bar with threads. As the draw is tightened, the tapered part of the collet is forced against a tapered adapter, causing the collet to tighten on the workpiece. Collets give a high degree of repeated accuracy and concentricity.
The rubber flex or so-called Jacobs collet chuck is another type of collet chuck used on lathes. Its collet will hold only round workpiece. However, each has a range approximately 1/8 in. larger than its normal size. The rubber flex collet chuck, unlike the spring collet, does not utilize a draw bar to tighten the collet. When the handwheel on the chuck is turned clockwise, it force the collet against a tapper, causing it to tighten around the workpiece. To further ensure that the workpiece is secure, a couple of impact rotations are necessary in clockwise direction. This procedure is reversed to release a workpiece.
Faceplates. The faceplates is generally is used when a workpiece is too large,has an odd shape, or cannot be mounted in a chuck. Faceplates have slots so that bolts and clamps can be used to secure the workpiece.
9.5.2 WORK-SUPPORTING DEVICES
Lathe center. The Lathe centers are commonly used to support a workpiece between the headstock and tailstock. The center that is mounted in the headstock
spindle is called a live center and revolves with the workpiece. The tailstock center is called a dead center and does not usually rotate. Some tailstock centers are manufactured with antifriction bearings, and the tip rotates along with the work while the body and taper that fit into the tailstock remain stationary. This type of center ,as is the headstock center. Tailstock live center permit heavier machining operations to be performed and are used where higher rpms are required. All center have a 60?tapered hole drilled prior to mounting it between centers. Centers are used in conjunction with driving plates and lathe dogs.
Another type if tailstock center commonly used is the bell or pipe center. It also revolves with the workpiece and is used to support the end of longer, hollow cylindrical shape such as tubing or pipe.
Steady Rest. The steady rest is mounted directly to the ways on a lathe and supports the outer end of longer work. This setup permits machining operations to be performed on or near the end of the workpiece. Steady rests can be positioned anywhere along the ways as needed. They also can be used to support the center of very long workpiece that have been mounted between centers. On very large lathes more than one steady rest may be used. Steady rests require that the
surface of the material be round and smooth where the jaws are in contact.
Follow Rest. Follow rests are bolted to the carriage assembly and travel with the cutting tool. They prevent long slender workpiece from springing up and away from the cutting tool during turning and external threading operations. Generally, the jaws of the follow rest contact the finished part of the workpiece.
Mandrel. Mandrels are hardened and ground pieces of cyclindrical steel that have a slight taper from one end to the other. Workpiece with a smooth hole are placed on the small diameter of the mandrel and moved by an arbor press to the larger end until tight. Next, the workpiece and mandrel are mounted between
centers on the lathe. Work is generally supported on a mandrel when external operations must be concentric with the hole of the part. Gear blanks and pulleys are examples of such parts that are generally machined on a mandrel.
Arbors. For special setups, or when a mandrel of the correct size is not available, the machinist often machines an arbor. This arbor, sometimes called a spud, mounts the work for additional machining operation on the outer diameter. The shank of the arbor, diameter A, is generally held with a three - or four-jaw chuck. The smaller diameter B is machined to a size that will give a close fit with the hole in the workpiece. The length of diameter B is slightly shorter than the total length of the work, thus allowing the parts to be tightened against the shoulder with a capscrew and washer.
9.5.3 Toolposts and Toolholder
Cutting tool are held in a variety of different types of toolholders.
The left-hand offset toolholder is used when operation are required near the chuck and for cutting from the right to the left or form the tailstock to the headstock.
The straight toolholder has no offset and is used for machining the workpiece in either direction. This toolholder is generally used for most operations.
Quick-change Tool-holding Assemblies. The quick-change toolholder is used when multiple machining operations on many workpices are needed. Quick-change toolholders are the most popular type used in industry and are more rigid than the rocker-arm toolposts. Preset cutting tools (tool bits) are mounted in blocks that can be quickly changed and accurately located. The clamping lever located at the top secures the cutting tool rigidly in place.
Turret Toolholders. Another toolholder commonly used for multiple operation is the turret type. It has slots for mounting four different types of toolbits. Each toolbit is rotated to the proper position and locked into place by the locking handle on top of the holder.
Open-side Toolholder. The open-side toolholder is used for single cutting tools and is more rigid than the toolpost type. It is generally used in conjunction with carbide toolbits for heavy cuts.
9.5.4 Taper Attachment
A taper attachment is a convenient way to machine tapers. It can be used to cut both internal and external tapers and is mounted directly behind the carriage assembly. The taper attachment allows for long tapers but is limited to tappers angle up to 20 included angle or approximately 4 in. tapper per foot.
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