Capstan and Turret Lathe
A capstan and turret lathe is used to manufacture any number of identical workpieces in the less time. These lathes are the development of engine lathes. For the first time in 1860, Pratt and Whitney developed the capstan lathe.
The capstan and turret lathe consists of:
- All geared headstock
- A saddle provided on which a 4-station tool-post is mounted to hold 4 different tools.
- A tool-post fitted at the back of the carriage. It holds a parting tool in an inverted position. The toolpost mounted on the cross slide is indexed by hand.
- This machine has no tailstock, but in its place, a hexagonal turret is mounted on a slide which rests upon the bed.
All 6 faces of the turret can hold 6 or more number of different tools. The turret can be automatically indexed and each tool brought in line with the lathe axis in a regular sequence. The workpieces are held in collets or in chucks.
The longitudinal and crossfeed movement of the turret saddle and cross slide are regulated by adjustable stops.
These stops enable different tools set at different stations to move by a fixed amount for performing different operations on repetitive workpieces without measuring the length or diameter of the machined surface in each case.
These special characteristics of a capstan and turret lathe enable it to perform a series of operations such as:
- Thread cutting.
- And many other operations in a regular sequence to produce a large number of identical pieces in a less amount of time.
1. The Capstan or Ram Type Lathe
The capstan or ram type lathe is shown in the figure. This machine carries the hexagonal turret on ram or a short slide.
The ram slides longitudinally on a saddle positioned and clamped on lathe bedways. This type of machine is lighter in construction and is suitable for machining bar is smaller diameter.
The tools are mounted on the square turret and 6 faces of the hexagonal turret.
The feeding movement is obtained when the ram moves from left to the right. And when the ram is moved backwards the turret indexes automatically and the tool mounted on the next face comes into operation.
2. The Turret or Saddle Type Lathe
The turret lathe is another type of lathe machine. It is used for repetitive production of same duplicate parts, which by the nature of their cutting process are usually replaceable. The hexagonal turret as shown in the figure. It is mounted directly on a saddle and the whole unit moves back and forth on the bed-ways to apply feed.
This type of turret lathe machine is heavier in construction. It is particularly adapted for larger diameter bar work and chucking work.
The machine can take in longer workpieces than that in a capstan lathe.
3. Principle Parts of Capstan And Turret Lathes
The turret lathe has essentially the same parts as the engine lathe except for the turret and complex mechanism incorporated in it for making it suitable for mass production work.
The figure shows the different parts of the turret lathe.
Following are the main parts of a capstan and turret lathe,
- Cross slide and saddle.
- The turret saddle and auxiliary slide.
the bed is a long box like casting provided with accurate guideways upon which are mounted the carriage and turret saddle. The bed is designed to ensure strength, rigidity and permanency of alignment under heavy duty services.
The headstock is a made up of large casting. It is located at the left-hand end of the bed. The different types of headstocks in capstan and turret lathe are as follows:
- Step cone pulley driven headstock.
- Direct electric motor driven headstock.
- All geared headstock.
- Preoptive or preselective headstock.
3.2.1 Step Cone Pulley Driven Headstock
This is the simplest type of headstock and is fitted with small capstan lathes where the lathe is engaged in machining small and almost constant diameter of workpieces. Only three or four steps of the pulley can cater to the needs of the machine.
The machine requires special countershaft unlike that of an engine lathe, where starting, stopping and reversing of the machine spindle can be affected by simply pressing a foot pedal.
3.2.2 Electric Motor Driven Headstock
In an electric motor driven headstock, both spindles of the machine, and the armature shaft of the motor are one and the same.
Any speed difference or reversal is achieved by directly controlling the motor. Three or four are available and the machine is suitable for smaller diameter of workpieces rotated at high speeds.
3.2.3 All Geared Headstock
On the larger lathes, the headstocks are geared and the different mechanism is employed for speed changing by actuating levers. The speed changing has done by without stopping the machine.
3.2.4 Preoptive or Preselective Headstock
It is an all geared headstock with provisions for rapid stopping, starting and speed charging for different operations and for pushing a button or pulling a lever.
For different operations and for turning different diameter, the speed of the spindle must change. The required speed for the next operation is selected beforehand and the speed changing lever is placed at the selected position.
After the first operation is complete, a button or a lever is simply actuated and the spindle starts rotating at the selected speed required for the second operation without stopping the machine. This novel mechanism is affected by the friction clutches.
4. Cross-slide and saddle:
In small capstan lathes, hand operated cross slide are used which are clamped on the lathe bed at the required position. The larger lathes and heavy duty turret lathes are equipped with usually two designs of the carriage.
- Conventional type carriage
- Side hung type carriage
4.1 The conventional type carriage
The conventional type carriage bridges the gap between the front and rear bed-ways and is equipped with four station type tool post at the front, and one rear tool post at the back of the cross slide.
4.2 The side-hung type carriage
The side-hung type carriage is generally fitted with heavy duty turret lathes where the saddle rides on the top and bottom guideways on the front of the lathe bed. The design facilitates swinging of larger diameter of the workpiece without being interfered by the cross slide. The saddle and the cross slide may be fed longitudinally or crosswise by hand or power.
The longitudinal movement of each tool may be regulated by using stop-bars or shafts set against the stop fitted on the bed and carriage. These stops are set so that each tool will feed into the work to the desired length for the purpose of duplicating the job without checking the machining length for different operations each time.
These stop the first trip out the feed and then serve as a dead-stop for small head operated movement of the tool to complete the cut. The stop bars are indexed by hand to synchronise with the indexing of the tool.
The tools are mounted on the tool post and correct heights are adjusted by using rocking or parking piece.
5. The turret saddle and auxiliary slide:
In a capstan lathe, the turret saddle bridges the gap between two bed-ways, and the top face is accurately machined to provide a bearing surface for the auxiliary slide. The saddle is adjusted on lathe bed-ways and clamped at the desired position. The hexagonal turret is mounted on the auxiliary slide.
In a turret is directly mounted on the top of the saddle and any movement of the turret is affected by the movement of the saddle. The movement of the turret may be effected by hand or power. The turret is a hexagonally shaped tool holder intended for holding six or more tools.
Each face of the turret is accurately machined. Through the centre od, each face accurately bored holes are provided for accommodating shanks of different tool holders.
The centre line of each hole coincides with the axis of the lathe when aligned with the headstock spindle. In addition to these holes, there are four tapped holes on each face of the turret for securing different tool holding attachments. At the centre of the turret on the top of it, there is a clamping lever which locks the turret on the saddle.
Six stop bars mounted on the saddle which restricts the movement of each tool mounted on each face of the turret to be fed to a predetermined amount for duplicating workpiece.
After one operation is completed, as the turret is brought back away from the spindle nose, the turret indexes automatically by a mechanism incorporated on the bed and in turret saddle, so that the tool mounted on the next face is aligned with the work.