Milling Machine – Work Holding Devices
It is necessary that the work should be properly and securely held on the milling machine table for effective machining operations. The cutting pressure exhorted by milling cutter is quite high comparing the single point tool of a lathe machine. Therefore the workpiece has to be secured rigidly to avoid any vibration. The following are the usual methods of holding work on the milling machine.
In the Milling machines, fixtures are still required to locate and hold the workpieces while machining.
Features of Work Holding Devices :
The work holding devices should have the following uniqueness :
(a) Work holding devices must have required accuracy and must have matching reference surfaces with the reference system.
(b) Work holding devices are allowed to perform a number of operations on different faces in a single setting.
(c) Work holding devices must enable quick loading and unloading.
(d) Work holding devices must be fool-proofing to avoid incorrect loading of the job.
(e) Work holding devices must be sufficient rigidity to fully withstand the cutting forces.
(f) Work holding devices must be safe in use and loading and unloading.
(g) Work holding devices must have a sufficient clamping force for the use of full roughing cuts.
(h) Work holding devices must be simple in construction maximum as possible.
Work Holding devices used For Milling Machines :
Various types of work holding devices are used for milling machine operations they are explained as follows :
- T-bolts and clamps
- Angle Plates
- V – Block
- Machine Vices
- Dividing Head
- Special Fixture
- Circular Table or Indexing Table
- Magnetic chuck / Vacuum chuck / Collet chuck
T- Bolts and clamps –
Bulky workpieces of irregular shapes are clamped directly on the milling machine table by using T- bolts and clamps. Different types of clamps are used for different patterns of work. The common types of clamps are shown in Fig 10 of chapter 5. All these clamps carry a long hole, through which clamping bolt passes. This hole permits the bolts for adjustment according to the size and shape of the job.
Angle plates –
- When work surfaces are to be milled at right angles to another face, angle plates are used for supporting the work.
- The angle plate is made from high-quality material (generally spheroidal cast iron) that has been stabilized to prevent further movement or distortion.
- Slotted holes or “T” bolt slots are machined into the surfaces to enable the secure attachment or clamping of workpieces to the plate, and also of the plate to the worktable.
- Angle plates also may be used to hold the workpiece square to the table during marking-out operations.
- Adjustable angle plates are also available for workpieces that need to be inclined, usually towards a milling cutter.
V block –
The V blocks are used for holding shafts on a milling machine table in which keyways and slots are to be milled.
Vices are the most common appliance for holding work on milling machine tables. According to its quick loading and unloading arrangement. Vices are of three types,
(a) Plain Vice –
The plain vice is directly bolted on the milling machine table is the most common type of vice used on plain milling operations, which involves heavy cuts, such as in slab milling. Its especially low construction enables the work to remain quite close to the table. This reduces the chance of vibration to a minimum. The base carries slots to accommodate ‘T’ bolts to fix the vice on the table. Work is clamped between the fixed and movable jaw and for holding workpieces of irregular shape special jaws are sometimes used.
(b) Swivel Vices –
The swivel vice is used to mill an angular surface in relation to a straight surface without removing the work from the vice. It has got a circular base graduated in degrees. The base is clamped on the table by means of T- bolts.
(c) Universal Vices –
It can be swiveled in a horizontal plane similar to a swivel vice and can also be tilted in any vertical position for an angular cut. The vice is not rigid in construction and is used mainly in tool room work. It enables the milling of various surfaces, at an inclination to one another, without removing the workpiece.
Dividing Head –
Dividing head or indexing head used to hold the workpiece and divide the periphery into the number of divisions required. These are of three types:
(a) Plain dividing head
(b) Universal dividing head
(c) Optical dividing head
Special Fixture :
Work directly mounted on table or Special fixtures
Work directly mounted on the table for heavy nature of jobs or odd-shaped jobs which is not possible to hold by other holding devices, with the help of slots, T- bolts, and nuts. The fixtures are special devices designed to hold work for specific operations more efficiently than standard work holding devices. The fixtures are especially useful when large numbers of identical parts are to be manufactured.
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