Essential Features of the Lathe

General

To learn the operation of the lathe, one must first be familiar with the names and functions of the principal parts. In studying the principal parts in detail, remember that all lathes provide the same general function even though the design may differ among manufacturers. For specific details on a given lathe, refer to the manufacturer’s technical manual for that machine.

Bed and Ways

The bed is the base for the working parts of the lathe. The main feature of the bed is the ways which are formed on the bed’s upper surface and which run the full length of the lathe. The tailstock and carriage slide on the ways in alignment with the headstock. The headstock is normally permanently bolted at one end (at the operator’s left).

(a) The ways are accurately machined parallel to the axis of the spindle and to each other. The V-ways are guides that allow the carriage and the tailstock to move over them only in their longitudinal direction. The flat way takes most of the downward thrust. The carriage slides on the outboard V-ways which, because they are parallel to the V-ways, keep the carriage in alignment with the headstock and tailstock at all times. This is an absolute necessity if accurate lathe work is to be done. Some lathe beds have two V-ways and two flat ways, while others have four V-ways.

(b) For satisfactory performance of a lathe, the ways must be kept in good condition. A common fault of careless machinists is to use the bed as an anvil for driving arbors or as a shelf for hammers, wrenches, and chucks. Never allow anything to strike the ways or damage their finished surfaces in any way. Keep them free of chips. Wipe them off daily with an oiled cloth to help preserve their polished surface.

Headstock

(a) The headstock carries the head spindle and the mechanism for driving it. In the belt-driven type headstock, the driving mechanism consists merely of a cone pulley that drives the spindle directly or through the back gears. When the spindle is driven directly, it rotates the cone pulley. When the spindle is driven through the back gears, it rotates more slowly than the cone pulley, which in this case turns freely on the spindle. Thus two speeds are available with each position of the belt on the cone; if the cone pulley has four steps, eight spindle speeds are available.

(b) The geared headstock is more complicated but more convenient to operate, because the speed is changed by changing or by shifting the gears. This headstock is similar to an automobile transmission except that it has more gear-shift combinations and, therefore, has a greater number of speed changes. A speed index plate, attached to the headstock, indicates the lever positions for the different spindle speeds. To avoid damage to the gear teeth, the lathe is always stopped before the gears are shifted.

(c) The driving pulley at the left is driven at a constant speed by a motor located under the headstock. Various combinations of gears in the headstock transmit power from the drive shaft to the spindle through an intermediate shaft. Use the speed-change levers to shift the sliding gears on the drive shaft and the intermediate shaft to line up the gears in different combinations. This produces the gear ratios needed to obtain the various spindle speeds. Note that the back gear lever has a high and low speed for each combination of the other gears.

(d) The headstock casing is filled with oil to lubricate the gears and the shifting mechanism contained within it. The parts not immersed in the oil are lubricated by either the splash produced by the revolving gears or by an oil pump. Be sure to keep the oil to the full level as indicated on the oil gage, and drain and replace the oil when it becomes dirty or gummy.

(e) The headstock spindle is the main rotating element of the lathe and is directly connected to the workpiece which revolves with it. The spindle is supported in bearings at each end of the headstock through which it projects. The section of the spindle between the bearings carries the pulleys or gears that turn the spindle. The nose of the spindle holds the driving plate, the faceplate, or a chuck. The spindle is hollow throughout its length so that bars or rods can be passed through it from the left and held in a chuck at the nose. The chuck end of the spindle is bored to a Morse taper to receive the solid center. The hollow spindle also permits the use of the draw-in collet chuck (to be discussed later in this lesson). At the other end of the spindle is the gear by which the spindle drives the feed and the screw-cutting mechanism through a gear train located on the left end of the lathe. A collar is used to adjust the end play of the spindle.

(f) The spindle is subjected to considerable torque because it drives the work against the resistance of the cutting tool, as well as driving the carriage that feeds the tool into the work. Because of the torque and pressure applied to the spindle, adequate lubrication and accurately adjusted bearings are absolutely necessary.

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Tailstock

(a) The primary purpose of the tailstock is to hold the dead center to support one end of the work being machined between centers. However, it can also be used to hold live centers, tapered shank drills, reamers, and drill chucks. The tailstock moves on the ways along the length of the bed to accommodate work of varying lengths. It can be clamped in the desired position by the tailstock clamping nut.

(b) The dead center is held in a tapered hole (bored to a Morse taper) in the tailstock spindle. The spindle is moved back and forth in the tailstock barrel for longitudinal adjustment. The handwheel is turned which turns the spindle-adjusting screw in a tapped hole in the spindle. The spindle is kept from revolving by a key that fits a spline, or keyway, cut along the bottom of the spindle.

(c) The tailstock body is made in two parts. The bottom, or base, is fitted to the ways; the top can move laterally on its base. The lateral movement can be closely adjusted by setscrews. Zero marks inscribed on the base and top indicate the center position and provide a way to measure setover for taper turning.

(d) Before inserting a dead center, a drill, or a reamer into the spindle, carefully clean the tapered shank and wipe out the tapered hole of the spindle. After a drill or reamer is placed into the tapered hole of the spindle, make sure that the tool will not turn or revolve. If the tool is allowed to revolve, it will score the tapered hole and destroy its accuracy. The spindle of the tailstock is engraved with graduations which help in determining the depth of a cut when a piece is drilled or reamed.

Carriage

(a) The carriage carries the crossfeed slide and the compound rest which in turn carries the cutting tool in the toolpost. The carriage slides on the ways along the bed.

(b) The wings of the H-shaped saddle contain the bearing surfaces which are fitted to the Vways of the bed. The cross piece is machined to form a dovetail for the crossfeed slide. The crossfeed slide is closely fitted to the dovetail and has a tapered gib which fits between the carriage dovetail and the matching dovetail of the crossfeed slide. The gib permits small adjustments to remove any looseness between the two parts. The slide is securely bolted to the crossfeed nut which moves back and forth when the crossfeed screw is turned by the handle. The micrometer dial on the crossfeed handle is graduated to permit accurate feed. Depending on the manufacturer of the lathe, the dial may be graduated so that each division represents a 1 to 1 ratio. The compound rest is mounted on top of the crossfeed slide.

(c) The carriage has T-slots or tapped holes for clamping work for boring or milling operations. When the lathe is used in this manner, the carriage movement feeds the work to the cutting tool which is revolved by the headstock spindle.

(d) The carriage can be locked in any position on the bed by tightening the carriage clamp screw. The clamp screw is to be used only when doing work for which longitudinal feed is not required, such as facing or cuttingoff stock. Normally, the carriage clamp is kept in the released position. The carriage is always moved by hand to make sure that it is free before the automatic feed is applied.

Apron

The apron is attached to the front of the carriage. It contains the mechanism that controls the movement of the carriage for longitudinal feed and thread cutting. It controls the lateral movement of the cross-slide. One should thoroughly understand the construction and operation of the apron before attempting to operate the lathe. In general, a lathe apron contains the following mechanical parts: (a) A longitudinal feed handwheel for moving the carriage by hand along the bed. This handwheel turns a pinion that meshes with a rack gear that is secured to the lathe bed.

(b) Gear trains driven by the feed rod. These gear trains transmit power from the feed rod to move the carriage along the ways and to move the cross-slide across the ways, thus providing powered longitudinal feed and crossfeed.

(c) Friction clutches operated by knobs on the apron are used to engage or disengage the power-feed mechanism. (Some lathes have a separate clutch for longitudinal feed and crossfeed; others have a single clutch for both.)

(d) A selective feed lever or knob is provided for engaging the longitudinal feed or crossfeed as desired.

(e) Half-nuts are used to engage and disengage the lead screw when the lathe is used to cut threads. They are opened or closed by a lever that is located on the right side of the apron. The half-nuts fit the thread of the lead screw which turns then like a bolt in a nut when they are clamped over it. The carriage is then moved by the thread of the lead screw instead of by the gears of the apron feed mechanisms. (The half-nuts are engaged only when the lathe is used to cut threads, at which time the feed mechanism must be disengaged. An interlocking device, that prevents the half-nuts and the feed mechanism from engaging at the same time, is usually provided as a safety feature.)

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(f) The manufacturers of lathe aprons differ somewhat in their construction and in the location of the controlling levers and knobs. However, they are all designed to perform the same function. The principal difference is in the gear trains for driving the automatic feeds. For example, in some aprons there are two separate gear trains with separate operating levers for longitudinal feed and crossfeed. In others, both feeds are driven from the same driving gear on the feed rod through a common clutch; they have a selective lever for connecting the drive to either the longitudinal feed or the crossfeed.

Feed Rod

(a) The feed rod transmits power to the apron to drive the longitudinal feed and crossfeed mechanisms. The feed rod is driven by the spindle through a train of gears, and the ratio of its speed to that of the spindle can be varied by changing gears to produce various rates of feed. The rotating feed rod drives the gears in the apron. These gears in turn drive the longitudinal feed and crossfeed mechanisms through friction clutches, as previously explained in (6)(c) on page 14.

(b) Lathes which do not have a separate feed rod have a spline in the lead screw to serve the same purpose. The apron belongs to a lathe of this type and is driven by the spline in the lead screw. If a separate feed rod was used, it would drive the feed worm in the same manner. The spline permits the worm, which is keyed to it, to slide freely along its length to conform with the movement of the carriage apron.

Lead Screw

(a) The lead screw is used for thread cutting. Along its length are accurately cut Acme threads which engage the threads of the half-nuts in the apron when the half-nuts are clamped over it. When the lead screw turns inside the closed half-nuts, the carriage moves along the ways a distance equal to the lead of the thread in each revolution of the lead screw. Since the lead screw is connected to the spindle through a gear train (to be discussed in paragraph (9)(a) below), the lead screw rotates with the spindle. Whenever the half-nuts are engaged, the longitudinal movement of the carriage is directly controlled by the spindle rotation. The cutting tool is moved a definite distance along the work for each revolution of the spindle.

(b) The ratio of the threads per inch of the thread being cut and the thread of the lead screw is the same as the ratio of the speeds of the spindle and the lead screw. For example: If the lead screw and spindle turn at the same speed, the number of threads per inch being cut is the same as the number of threads per inch of the lead screw. If the spindle turns twice as fast as the lead screw, the number of threads being cut is twice the number of threads per inch of the lead screw.

(c) Any number of threads can be cut by merely changing the gears in the connecting gear train to obtain the desired ratio of the spindle and the lead screw speeds.

Quick-Change Gear Mechanism

(a) To do away with the inconvenience and loss of time involved in removing and replacing change gears, most modern lathes have a selfcontained change gear mechanism, commonly called a “quick-change gear box.” There are a number of types used on different types of lathes, but they are all similar in principle.

(b) The quick-change gear box mechanism consists of a cone-shaped group of change gears. One can instantly connect any single gear in the gear train by a sliding tumbler gear controlled by a lever. The cone of gears is keyed to a shaft which drives the lead screw (or feed rod) directly or through an intermediate shaft. Each gear in the cluster has a different number of teeth and hence produces a different ratio when connected in the train. Sliding gears also produce other changes in the gear train to increase the number of different ratios one can get with the cone of change gears. All changes are made by shifting the appropriate levers or knobs. An index plate or chart mounted on the gear box indicates the position in which to place the levers to obtain the necessary gear ratio to cut the threads or produce the feed desired.

(c) The splined shaft turns with gear G, which is driven by the spindle through the main gear train mounted on the end of the lathe. Shaft F in turn drives shaft H through the tumbler gear T, which can be engaged with any one of the cluster of eight different size gears on shaft H by means of the lever C. Shaft H drives shaft J through a double-clutch gear, which takes the drive through one of three gears, depending on the position of lever B (right, center or left). Shaft J drives the lead screw through gear L.

(d) Either the lead screw or the feed rod can he connected to the final driveshaft of the gear box by engaging the appropriate gears.

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(e) Twenty-four different gear ratios are provided by the quick-change gear box. The lower lever has eight positions, each of which places a different gear in the gear train and hence produces eight different gear ratios. The three positions of the upper level produce three different gear ratios for each of the 8 changes obtained with the lower lever, thus making 24 combinations in the box alone. This range can be doubled by using the sliding compound gear which provides a high- and low-gear ratio in the main gear train. This gives two ratios for every combination obtainable in the box, 48 combinations in all.

Compound Rest

The compound rest provides a rigid adjustable mounting for the cutting tool. The compound rest assembly has the following principal parts:

(a) The compound rest swivel can be swung around to any desired angle and clamped in position. It is graduated over an arc of 900 on each side of its center position for ease in setting it to the desired angle. This feature is used in machining short, steep tapers such as the angle on beveled gears, valve disks, and lathe centers.

(b) The compound rest top or top slide, is mounted on the swivel section on a dovetailed side. It is moved along the slide by the compound rest feed screw turning in the nut, operated by the handle, in a manner similar to the crossfeed. This provides for feeding at any angle (determined by the angular setting of the swivel section), while the cross-slide feed provides only for feeding at right angles to the axis of the lathe. The graduated collar on the compound rest feed screw reads in thousandths of an inch for fine adjustment in regulating the depth of cut.

Toolposts

(a) Three popular types of toolposts are the standard, castle, and the quick change. The sole purpose of the toolpost is to provide a rigid support for the toolholder.

(b) The standard toolpost is mounted in the T-slot of the compound rest top. A forged tool or a toolholder is inserted in the toolpost and rests on the toolpost wedge and the toolpost ring. By tightening the setscrew, with the tool placed in the desired position, the whole unit can be clamped in place.

Cutting Toolholders

(a) General. Common cutter bits are generally made from standard sizes of bar stock to fit into a forged cutting toolholder at an approximate 150 positive rake angle. The toolholder is fastened to the toolpost of the lathe. Special tools such as the knurling tool and the thread cutting toolholder are furnished with their own special forged toolholder and, therefore, may be fastened directly to the toolpost of the lathe. Carbidetipped cutter bits are generally unsuitable for mounting in forged toolholders. They are fastened directly to the lathe toolpost or mounted in an open side toolpost to provide rigid support for the bit.

(b) Straight-Shank Cutting Toolholder. The straight-shank cutting toolholder may be used to support roundnose turning cutter bits, right-hand and left-hand turning cutter bits, and thread cutter bits. The holder is made of forged steel and contains a hardened steel setscrew for locking the cutter bit in place.

(c) Right-Hand and Left-Hand Offset Cutting Toolholder. The right-hand and left-hand offset cutting toolholders are designed to support right-hand and left-hand facing cutter bits which require that the bit be supported at an angle to the workpiece axis. The holder has a setscrew for locking the cutter bit in place.

(d) Straight Parting Cutting Toolholder. The straight parting cutting toolholder is a forged steel holder shaped to hold flat, thinsectioned parting tools which are used to cut and separate pieces of stock on the lathe.

(e) Right-Band and Left-Hand Offset Parting Cutting Toolholder. The right-hand and left-hand offset parting cutting toolholders are similar to the straight parting cutting toolholder but are designed to hold the parting cutter bit at an angle to the holder shank. The offset toolholder is generally used when the workpiece is to be parted, because the stationary parts of the lathe may interfere with the holder if the straight parting-cutting toolholder is used. In either case, the compound rest of the lathe must be adjusted so that the parting cutter bit enters the workpiece at the correct angle (perpendicular to the workpiece).

(f) Boring Bar Cutting Toolholder. A lathe boring bar cutting toolholder comes in several commercial types. It consists of three parts: the holder, the interchangeable end cap, and the boring tool bar. The boring tool bar is a rod with one end threaded to accept an end cap. Three end caps are supplied; each end cap is slotted at different angles to accept a cutter bit. The standard angles are 30°, 45°, and 90°. Plain boring toolholders without caps are often made to accept cutter bits at each end, one having a 90° slot, and the other having a 45° slot. The holder is made of forged steel. It has a shank similar to that of the other cutting toolholders. The holder is secured to the toolpost by the lathe toolpost screw. The boring bar is adjustable in the holder and can be locked in any desired position.