طنطا جامعةالهندسة كلية

الميكانيكي والتصميم اإلنتاج هندسة قسم

Capstan Lathes &

Turnet Lathes

الوكيل رأفت محمد / الطالب عمل

إنتاج / ثالثة الفرقة

/ إشراف تحت

الشوربجي صالحد.م.

Capstan lathes are mainly used for bar work, whereas turret lathes are applicable for large work in the form of castings and forgings.

In a capstan or ram-type lathe, the hexagon turret is mounted on a slide that moves longitudinally in a stationary saddle Figure (a). During setup of the machine, the saddle is positioned along the bed to give the shortest possible stroke for the job. The advantage of the capstan lathe is that the operator has less mass to move, resulting in easier and faster handling. The disadvantage is that the hexagonal turret slide is fed forward such that the overhang is increased, resulting in the defl ection of the ram slide, especially at the extreme of its position, which produces taper and reduces accuracy.

In the turret or saddle-type lathe, the turret is mounted directly upon a movable saddle, furnished with both hand and power longitudinal feed Figure (b). This machine is designed for machining chuck work, in addition to bar work. Owing to the volume of the swarf produced, the guideways of the machine bed are fl ame-hardened and provided with covers that protect the sliding surfaces. The bed must be designed to allow free and rapid escape of swarf and coolant.

2.a.Schematic configuration of capstan lathe

2.b.Schematic configuration of turret lathe.

DIFFERENCES BETWEEN CAPSTAN AND TURRET LATHES, •Turret lathes are relatively more robust and heavy duty machines •Capstan lathes generally deal with short or long rod type blanks

held in collet, whereas turret lathes mostly work on chucking type jobs held in the quick acting chucks

•In capstan lathe, the turret travels with limited stroke length within a saddle type guide block, called auxiliary bed, which is clamped on the main bed as indicated in Fig. 2.a., whereas in turret lathe, the heavy turret being mounted on the saddle which directly slides with larger stroke length on the main bed as indicated in Fig. 2.b .

•One additional guide rod or pilot bar is provided on the headstock of the turret lathes as shown in Fig. 2.b., to ensure rigid axial travel of the

turret head •External screw threads are cut in capstan lathe, if required, using

a self opening die being mounted in one face of the turret, whereas in turret lathes external threads are generally cut, if required, by a single point or multipoint chasing tool being mounted on the front slide and

moved by a short leadscrew and a swing type half nut .

PRINCIPAL ELEMENTS OF CAPSTAN AND TURRET LATHESA ram or turret lathe has essentially the same elements as an engine

lathe, with additional elements like hexagonal turrets and front and rear cross slides. However, the controls used are more complex. The motor is more powerful to enable the machine to perform overlapped cuts. The elements of a standard turret and capstan lathe are described in the following sections.

1- HEADSTOCK AND SPINDLE ASSEMBLY

The headstock is heavier in construction than that of the engine lathe with a wider range of speeds. A typical layout from Heinemann Machine Tool Works-Schwarzwald is shown in Figure 3. Mounting of the free-running gears should be noted, in addition to the use of roller bearings with a taper bore for the spindle. The multidisk clutch drive is widely used in conjunction with constant mesh gearing. The use of these clutches provides rapid acceleration and the ability to sustain hightorque loads (Browne, 1965). In modern machines, pole-changing motors offer four speeds, which simplify the design of the gearbox and limit its size. One of the chief characteristics of the turret headstock is the provision for rapid stopping and starting, and for speed changing through speed preselectors.

Through these measures, the minimum loss of time is realized. When components are turned from bar stock fed through the hollow spindle of the machine, a collect chuck is used. The bar is generally of

round or hexagonal shape. Collect chucks may be pneumatically or hand-operated. A sectional view of the hand-operated collet chuck is shown in Figure 4 (H. W. Ward and Co. Ltd.). When the handle shown in Figure 4.a is moved to the close position, the sliding sleeve (A), Figure 4.b rotates and is therefore forced to move to the left, as the groove accommodating pads (B) are cut on a helix. Consequently, the sleeve forces the ball operating sleeve (C) to the left, which causes the right-hand (RH) ring of balls held in the ball cage (D) to move radially inward.

This closes the sliding cone sleeve (E) and hence the collet. Moving the lever in the opposite direction reverses the action and the left-hand (LH) ring of balls (D) moves the sleeve (E) to release the collet. In the shown position, the collet is closed. The machine spindle (H) and the housing (K) are bolted to the headstock. The knurled cap (F) adjusts the collet for variations of the machined bar size. By a slight modifi cation, the design can be altered such that the sliding sleeve can be actuated pneumatically to reduce the operator’s fatigue and reduce the chucking time.

3. Typical headstock and spindle assembly of a turret lathe.

4. Hand-operated collet chuck: (a) general view and (b) sectional view.

5. Cross slide and square turret tool posts.

2- CARRIAGE/CROSS-SLIDE UNIT

The cross-slide unit on which the tools are mounted for facing, forming, recessing, knurling, and cutting off is made of four principal parts, namely, the cross slide, the square turret, the carriage, and the apron Figure 5. The rear and front square turrets are mounted on the top of the cross slide. Each turret is capable of holding four tools ready for use. If additional tools are required, they are set up in sequence and can be quickly indexed and locked in correct chucking position.

The slide is provided with a positive stop to affect diametrical control of the depth of the cut. Dogs on the side of the cross slide engage these stops to regulate the cross-slide travel.

The carriage has two hand wheels for manual longitudinal and cross feed. In some machines, besides hand feed, a power feed (rapid or slow) can be engaged by a lever.

3- HEXAGONAL TURRET

The hexagonal turret is carried on a saddle and is intended for holding and bringing the tools in a forward feed movement. On the turret-type, each face is provided with four tapped holes to accommodate screws for holding fl anged holders and attachments in which tools are clamped. On capstan lathes, the turret may be circular; it has also six holes for accommodating shanked tool holders that are normally used for small works that do not need to be held in a flat face.

4- CROSS-SLIDING HEXAGONAL TURRET

Sometimes, the hexagonal turret has a cross-sliding ability to feed in four directions. This characteristic adds greatly to the versatility of the turret lathe on certain diffi cult types of work. This unit is used only on the larger size turret lathe of the saddle-type construction. The mobility of this turret makes it especially adaptable to small-lot work where multiple inner surfaces can be machined using a minimum of quickly set up cutters (Tool Engineers Handbook, 1959).

Going beyond the small production lots, a cross-sliding turret offers other advantages on certain types of work. For example, it provides the possibility of machining large-diameter work, which prohibits the use of square turret cross slides. The graduated dial for the cross motion of the hexagonal turret enhances the accuracy and makes it the same as the square turret on the cross slides.