Motions in machine tools can be classified into the following categories:
Motions for clamping the workpiece
Depending on the size of the workpiece and the operation being carried out, the workpiece may be mounted on the machine base or table, clamped in a work-holding device like a chuck or face plate, or in a vice or jig fixed to the machine table.
The workpiece is held in a chuck, supported on a face plate, mounted between centers, or fixed to a mandrel in a lathe.
The workpiece is mounted directly on the table or base of the shaping, planning, drilling, boring, and milling machine and secured with bolts and clamps.
In a surface grinding machine, the workpieces may be placed on an electromagnetic table and held in place by magnetic forces. Heavy workpieces, such as engine or machine tool beds, may simply be “put on the base of the machine without clamping.”
Motions for fixing the tool
The tool is held in place by a tool or tail stock quill on a lathe, an arbor or spindle in a milling machine, and a tool holder in a shaper, planer, or slotter.
The tool in a drilling machine can be directly fixed in the machine spindle or held in a chuck mounted in the machine spindle.
Automatic screw-cutting machines are used in production machines such as capstan and turret lathes.
A number of tools can be mounted on a turret head and indexed to the cutting position one after the other in the sequence of operations to be performed on multi-spindle drilling machines and machining centers.
Some machines, such as tapping machines, may also have the option of retracting the tool once the required length of cut has been reached by collapsing the tool or reversing the direction of rotation of the machine spindle.
Relative motions between workpiece and tool
As previously stated, relative motion between the tool and the workpiece is required for machining.
These motions are typically expressed in terms of cutting speed, feed rate, and depth of cut.
These motions determine the rate of excess material removal per unit MRR, the accuracy and finish produced, as well as the tool forces and power required.
The proper choice of these motions is critical for efficient machining.
The motions required for the tool or workpiece between cutting operations are referred to as positioning motions.
After one screw is completed on an automatic screw-cutting machine, the bar stock is advanced to the required length for the next screw to be cut.
Similarly, after one job is completed on an automatic lathe with a rotating or conveyor-type bed or a broaching machine, the bed is moved to bring the next job under the tool.
After one tool completes its cut on a capstan or turret lathe or machining center, the turret is indexed to bring the next tool into the cutting position.
Explain the primary and auxiliary motions with reference to the machine tool
The speed, feed, and depth of cut motions that are directly involved in the machining process are referred to as working or primary motions.
Auxiliary or secondary motions are those that help with workpiece handling and clamping.
Fixing and adjusting the tool in the machine to engage the workpiece, positioning the tool in the desired position with respect to the workpiece, disengaging the tool, and removing the workpiece from the machine.
Auxiliary motion includes the tool’s pre and post-contact travel relative to the workpiece.
The primary cutting motion is powered on most machine tools, while feed and depth of cut motions are given manually.
On manually operated machine tools, auxiliary motions are also performed manually by the operator.
All primary and auxiliary motions are carried out automatically by the machine tool in the required sequence in automatic machine tools.
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