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Capstan And Turret Lathes
The center lathe is a general purpose machine tool,it has a number of limitations of center lathes are:
(i) The setting time for the job in terms of holding the job is large.
(ii) Only one tool can be used in the normal course. Sometimes the conventional tool post can be replaced by a square tool post with four tools.
(iii) The idle times involved in the setting and movement of tools between the cuts is large.
(iv) Precise movement of the tools to destined places is difficult to achieve if proper care is not taken by the operator.
All these difficulties mean that the center lathe cannot be used for production work in view of the low production rate. The center lathe is thus modified to improve the production rate. The various modified lathes are turret and capstan lathes, semi-automatics and automatics.
Improvements are achieved basically in the following areas:
(a) work holding methods
(b) multiple tool availability
(c) automatic feeding of the tools
(d) automatic stopping of the tools at precise locations
(e) automatic control of the proper sequence of operations.
The main characteristic feature of the capstan and turret lathes is the six sides (hexagonal) block mounted on one end of the bed replacing the normal tailstock. This allows for mounting six tool blocks, each one of which can contain one or more tools depending upon the requirement. Further on the cross slide, two tool posts are mounted, one in the front and the other in the rear. Each one of them can hold up to four tools each. Thus the total carrying capacity is a maximum of 14 tools when only one tool is mounted in each of the locations.
The turret lathe consists of an all gear, heavy duty headstock with a greater range of spindle speeds. The turret is mounted on a saddle which in turn is sliding on the bed. When the saddle moves on the bed during the return stroke it would automatically be indexed to the next tool position, thus reducing the idle time of the machine.
The tools in the turret lathe are provided with a system of stops and trips on the feed rod which can precisely control the actual distance moved by the tool. Thus it is possible to set and control the individual movements of the tools as required by the component.
The type of work holding devices that can be used with turret lathe is similar to the conventional lathes, but in view of the higher productivity demanded and greater repeatability required, generally automatic fixtures such as collets, self centering chucks or pneumatic chucks are used.
The collet chucks come in a variety of designs. The actual clamping is done by the movement of the collet tube along the axis of the spindle by either pushing or pulling .Sometimes the bar material is either pushed or pulled back during the closing of the collet. This can be prevented by having an external tubular locking stop so that the axial movement is prevented.
Often a large variety of components on a turret lathe are machined from raw material which is in a bar from. For continuous feeding of the bar special bar feeding arrangements are available which pushes the bar by a precise amount against a stop provided on the face of the hexagonal turret at the beginning of the cycle.
Most of the tools used in the cross slide tool post are very similar to those used in the center lathe. Form tools are generally used in the cross slide. A large variety of special tool holders are available for use in the turret for providing greater productivity. A box tool is generally used for long turning jobs since the tool while cutting also supports the job. They have a cutting tool and also support rollers for providing the necessary support to the workpiece. This helps in machining of bars which are not well supported during the machining operation .It is also possible to have more than one cutting tool held in a box tool such that there is an overlap of the cuts while providing support for the workpiece.
Combination tool holders allow for mounting multiple cutting tools with provisions for their adjustment to suit the machining situation. They have the ability to perform more than one cutting operation at the same time, thereby reducing the actual machining time required for the operation. They can be have both the internal and external cutting tools in a single tool holder such that the workpiece support can be taken care of so that higher accuracy can be achieved.
Many turret lathes would be fitted with taper turning attachments very similar to that used in center-lathes, for machining tapers. Small tapers can be produced by form tools from the cross slide, while internal tapers are produced by taper reamers.
Thus the various differences that can be found between capstan and turret with that of a general purpose center lathe are:
(i) The headstock has more and heavier range of speeds and to allow for higher rate of production.
(ii) The tool post is indexable (four tools). Any one tool can be brought into the cutting position.
(iii) The tail stock is replaced by a tool turret with six tool positions.
(iv) Feed of each tool can be regulated by means of feed stops.
(v) Two or more tools mounted on a single tool face can cut simultaneously.
(vi) Semi-skilled operators are required.
(vii) These are used for production operations involving better repeatability.
A variation of the turret lathe is the capstan lathe, in which the turret moves on the saddle while the saddle can itself be fixed at any position on the bed depending upon the length of the job. Thus the tool travel length is limited to the length of the saddle. This type of arrangement is normally used for small size machines.