Capstan and Turret Lathe:
A capstan and turret lathe are a production lathe. It is used to manufacture any number of identical pieces in the minimum time.
These lathes were first developed in the United States of America by Pratt and Whitney in 1960.
Capstan lathe is one of the types of semi-automatic lathe.
In semi-automatic lathes machining operations are done automatically.
Functions other than machining like loading and unloading of a job, the positioning of tools coolant operations are done manually.
The turret head is mounted on the ram fitted with turret slides longitudinally on the saddle.
Turret head has a hexagonal block having six faces with a bore for mounting six or more than six tools at a time.
The threaded hole on these faces is used to hold the tools.
In the case of a Capstan Lathe, the hexagonal turret is mounted on a short slide or ram which again fitted with a saddle.
The saddle can be move accordingly throughout the bed ways and can be fixed to the bed if necessary.
It is specially used for bar type jobs.
But in the case of Turret Lathe, the hexagonal turret directly mounted on the saddle. The saddle can be move through the bed ways.
Turret lathe is generally used for chucking type work.
Schematic Diagram of a Capstan Lathe:
Schematic Diagram of a Turret Lathe:
Working of Capstan and Turret Lathe
All 6 faces of the turret can hold 6 or a 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 cross feed movement of the turret saddle and cross slide is controlled 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.
The Capstan or Ram Type Lathe
The ram slides longitudinally on a saddle positioned and clamped on lathe bed ways. 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 right. And when the ram is moved backward the turret indexes automatically. And the tool mounted on the next face comes into operation.
The Turret or Saddle Type Lathe
The turret lathe is another type of lathe machine. It is used for repetitive production of the 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.
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.
Following are the main parts of a capstan and turret lathe,
- Cross slide and saddle.
- The turret saddle and auxiliary slide.
Let’s Discuss one by one
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 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.
- Pre-optive or pre-selective headstock.
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 pressing a foot pedal.
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.
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.
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 friction clutches.
3) Cross-Slide and Saddle
In small capstan lathes, hand-operated cross slide is 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
The Conventional Type Carriage
The conventional type carriage bridges the gap between the front and rear bed-ways. And it is equipped with four station type tool post at the front, and one rear tool post at the back of the cross slide.
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 the swinging of a 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. It is 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 a rocking or parking piece.
4) 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 limits 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.
Bar Feeding Mechanism in Capstan and Turret Lathe:
In the bar feeding mechanism, the bar is pushed after the chuck is released without stopping the Lathe Machine.
We use this mechanism for minimizing the setting time.
The bar is passed through the pedestal bushing, bar holding chuck, headstock spindle, and the collet chuck.
The collet chuck is screwed on the headstock spindle and holding the feed bar and also helps the bar to rotates as per spindle speed.
Bar holding chuck rotates within the sliding block with the rotation of the feeding bar.
Also, you can see a rope and a deadweight in this mechanism.
One side of the rope is attached with the sliding block with the help of pin and another side of rope passes through 2 different pulleys and then connecting with a deadweight at its end.
So now when the collet chuck released by the lever the dead weight tends to move in the downward direction, due to this it exerts thrust on the bar holding chuck and feed the bar until it touches the workshop.
As we already have seen that Capstan Lathe is best for bar types jobs that’s why we are generally seeing Bar Feeding Mechanism on Capstan Lathe.
Tools used in Capstan and Turret Lathe:
- This is used for griping or you can hold any small bars in Capstan and Turret Lathe (Mainly when we do Mass production).
- The size of collet chucks is different corresponding to the bar sizes.
- The jaws of the collet chuck are gripped the workpiece by its springing nature.
- It is a thin steel brass bushing having slots on the outer side throughout its length.
Roller Box Steady Turning Tool:
- This type of tool is used on bar work and when a considerable amount of stock is to be removed from the job.
- Roller box consists of the backrest or traveling two roller steadies that can be adjusted as per requirement.
- A single point cutting tool is present in front of two rollers and gives rigidity to the workpiece.
- Due to this rigid support, depth of cut, turning, etc. can be performed very smoothly.
- This is a costly tool only used in mass production.
Self-opening Die Head:
- This tool is used especially for cutting external threads.
- The pitch of the cutting edges is determined according to the required thread pitch to be cut.
- Chasers may be triangular, tangential, and circular types.
- The function of the self-opening die is it opened automatically when the tool travel is stopped after the screw cutting operation.
Difference between capstan and turret lathe
Capstan and Turret lathes are the advancements of the Engine lathes and center lathes in which the tailstock is replaced by a hexagonal turret tool head with 6 different tools in the turret where the turret tool is rotated according to process requirements in each operation. The capstan and turret lathe although appear to be identical at first sight but a lot of differences in construction, operation, and use. In this article, you can check it out some difference between these two lathe machines.
- Lightweight machine.
- These are usually horizontal lathe.
- Turret’s head is mounted on a slide called ram which is mounted on the saddle.
- Suitable for bar work.
- The saddle is locked at a particular point and the ram is moved to provide feed to the tool.
- Only a limited amount of feed and depth of cut is provided for machining.
- The cross slide is mounted on a carriage which rests on bed ways between the headstock and the ram.
- The turret tool head is indexed automatically.
- Feed stop screws are used to control the distance of tool movement which is at the rear side of the turret.
- No such facility for moving turret at right angles.
- Feed rod gives for longitudinal feed.
- Used for mass production of small size equal part.
- It has hand-operated collet chucks.
- Heavy cuts on workpiece can’t be given because of non-rigid construction.
- Heavyweight machine.
- Turret tool head is directly fitted on the saddle and both of them appear like one unit.
- Suitable for heavier chucking work.
- The saddle is moved to provide feed to the tool.
- They are heavy and durable.
- More feed and depth of cut are provided for machining.
- Some turret type lathes are equipped with side hung type carriage.
- To index the turret tool, head, a clamping lever is released and the turret is rotated manually.
- Limit dogs are used to control the distance of the tool movement.
- Some turret lathes have the facility of moving the turret at right angles to the lathe axis.
- Feed rod does not give for longitudinal feed.
- Used for mass production of large size equal part.
- It is accommodated with power chucks.
- Heavy cuts on a workpiece can be given because of the rigid construction of a machine.
- These are available in the horizontal and vertical lathe.