- A machine tool is defined as one which while holding the cutting tools, would be able to remove metal from a workpiece in order to generate the requisite job of given size, configuration and finish.
- It is different from a machine, which is essentially a means of converting the source of power from one form to the other.
- The machine tools are the mother machines since without them, no components can be produced in their finished form.
- The machine tool is required to provide support to the workpiece and cutting tools as well as provide motion to one or both of them in order to generate the required shape on the workpiece.
- The form generated depends on the type of the machine tool used.
- A machine tool is designed to perform certain primary functions, but the extent to which it can be exploited to perform secondary functions is a measure of its flexibility.
- Generally, the flexibility of the machine tool is increased by the use of special attachments thereby productivity will be increased.
Lathe can be generally specified by
- Swing, the largest work diameter that can be swing over the lathe bed.
- Distance between tailstock and headstock
- Sometimes both the above
Basic elements of Machine tools:
The various components that are used in machine tools are discussed under:
1. Work holding device:
Work holding devices are used to hold the work-piece in the correct orientation to achieve the required accuracy in manufacturing.
2. Tool holding device
Tool holding devices are used to hold the cutting tool in the correct position with respect to the workpiece and to provide enough holding force to contract the cutting forces acting on the tool.
Example: Tool post
3. Work motion mechanism
Work motion mechanism is generally used to provide the necessary speed to the workpiece for generating the required surface.
4. Tool motion mechanism
Tool motion mechanism is generally used to support all the mechanisms and to maintain the relative position with respect to each other and to allow for relative movement between the various parts.
5. Support structures
Support structures are used to support all the mechanisms and to maintain the relative position with respect to each other and to allow for relative movement between the various parts.
Some common parts in machine tools and their features:
Important considerations in the design of bed are rigidity, alignment, accuracy and damping qualities.
- It is acting as a rigid support.
- It is used to absorb vibrations induced during machining.
- It will act as a house.
Generally, grey cast iron material is used.
Manufacturing process used:
Generally casting will be used to manufacture the beds.
Shape of the bed:
The shape of the bed is selected such that the reduction in the rigidity will be minimum and should provide the maximum amount of free space.
Example: X,I,H cross-section shapes etc.
2. Lead screw:
Lead screw is used for feeding the cutting tool in a direction parallel to the axis of rotation and this will convert rotational movement into translation motion.
The material used for making lead screw must have wear resistance.
Brass: for length less than 2 meters.
Stainless steel: Length 2 to 10 meters. For above 10 meters length rack and pinion is used.
Thread rolling or thread machining process will be employed to manufacture lead screws.
Square threads are preferable because of high efficiency in power transmission.And buttress and acme threads can also be used as the second choice.
3. Guide ways:
Guideways are used to guiding and allow the other parts moves in their directions relative to other parts.
Generally, high carbon steel will be used.
As guide ways subjected to many forces during machining low ductile like brass and high hard like stainless steel will not be preferable because of having the localized hardening.
The final manufacturing process used in manufacturing the guide ways is flame hardening to give hardened localized surface require for guide ways to avoid wearing.
Rectangle shaped guideways have the highest bearing area but it is difficult to align.
‘V’ shaped guide ways even aligned automatically but bearing area is less so the combination of ‘V’ and rectangular will be used i.e. Dovetail shaped guideways will be employed.
4. Prime Mover:
Whatever energy required for machining operation is obtained from Prime mover.
Types of Prime Mover:
- IC Engine: Due to some disadvantages of IC engines they are obsoleted.
- Electrical motor: Induction motor is the most commonly used motor. The cost of the induction motor is less compared to other motors. No reciprocating parts and no noise. So to get variable speeds of the job even though the motor is rotating is only speed we will use mechanical transmission system.