Understanding Special Purpose Machines (SPMs)
Custom-made Machine Tools For Specific Applications
SPMs are widely and popularly being used in automobile industries where mass production of cars and two-wheelers is most prevalent.
Definition of a Special Purpose Machine
A Special Purpose Machine can be defined as a “mother machine”, custom designed to do a specific operation or a combination of some operations on a specific job or on a small variety of similar jobs, in a mass scale, at short cycle times. SPMs can exist in metal forming, metal cutting, welding, in assembly, in testing and in many other mass production applications. However, generally speaking, the word Special Purpose Machine is largely identified with metal cutting machines.
Major Classifications of Special Purpose Machines
In case of SPMs in metal cutting, they can be broadly classified into (a) Job rotating type (i.e., Lathe type) and (b) Tool rotating type. Application wise, tool rotating type SPMs are very widely popular. Under tool rotating type SPMs, operations like drilling, boring, reaming, tapping and milling are done and also combinations of several such operations in one machine. Under rare cases, both job rotating and tool rotating operations too may get combined in an SPM.
Taking advantage of the popularity and convenience of Computer Numerical Control (CNC) in machine tools, CNC SPMs are also becoming increasingly popular nowadays. Depending on the extent of automation involved in the machine (including loading and unloading, clamping and de-clamping, re-positioning etc) the SPM can be semi-automatic or fully automatic.
The Major Expectations in Buying a Special Purpose Machine
- It should complete the specific operations on the job and produce the components at the shortest possible time
- It should work automatically, to the extent possible
- it should involve only the barest minimum of operator's involvement and judgment in producing parts to the required quality norms; i.e., the machine should be operated by an unskilled operator.
- It should be possible to set up and run the machine in the shortest time. Wastage of components in doing the settings and getting the desired quality requirements should be minimum.
- If it is tooled up for a few types of different components, then the set up change-over from one type of component to the other should be the barest minimum.
- The machine construction, work holding arrangement, selection of toolings, ways and means of delivery of coolant, chip disposal etc should ensure that the machine runs to the best of its efficiency for long, without frequent interruptions for maintenance.
- Considering the fact that SPMs are custom-made and hence costlier than conventional machines, their cost versus benefit should justify getting the Return On Investment at much shorter periods than in investment on conventional machines.
- Ideally, the SPM should have some reasonable amount of flexibility built into it, to ensure any minor changes in the design of the jobs or in the variety of jobs it can handle in the future. This is one ideal condition which may not always be possible in all SPMs, particularly if the design and application are more complex.
Some Salient Features of Special Purpose Machines
SPMs are designed and custom- built based on customers' specific job requirements and also taking into consideration above listed expectations for the SPMs. Based on the above, one can normally find some of the salient features of Special Purpose Machines as below:
- Work Holding: The machine will have jigs/ fixtures/ work holding devices to locate, orient and clamp the component firmly.
- Semi-automation: The operator may load and unload the component(s), but the clamping and de-clamping may be automatic. Most of other operations involved (like movement of the spindles, slides, indexing of the part from one station to another etc) will be automatic. In some SPMs, auto ejection of component may be in-built.
- Full automation: In some SPMs, picking and placing of the component into the fixture and auto-ejection / unloading will also be built in. In some specific cases, in- process gaging to measure the size of the job and adjust the cutting depths may also be found in-built.
- Actuating sources: SPMs, in majority, are controlled by hydraulic systems. On light duty applications, pneumatic units are also widely used. Thus SPMs will incorporate Hydraulic cylinders and pneumatic cylinders for slide movements and also for work clamping. SPMs with slide movements controlled by screw rods rotated by motorized gear trains and servo drives are also in existence.
- Auto - sequencing: SPMs will have proper electrical Controls (or PLC – Programmable Logic Controllers or CNC- Computer Numerical Controller), pneumatic, hydraulic and/ or servo controls to ensure the proper, sequential way of movements of slides and workpiece to complete the job.For example, in a drilling and tapping SPM, the clamping of component will trigger rotation of drilling spindle, movement of the drilling slide in rapid and then in cutting feed to drill the component; once the drilling slide retracts, the work slide will linearly index to the next station where the tapping spindle will rotate, move forward to do the tapping and retract.Once finished, the work slide will return to home position and the job will get ejected.
- Peripherals: SPMs may have, depending on application and complexity, proper coolant system, chip flushing arrangement, proper chip disposal systems with a chip conveyor (if needed), protective guards, front sliding door/ guard with or without automatic closing and opening, operator panel with switches for auto/ manual settings, electrical cabinet, centralized lubrication unit, hydraulic power pack, air preparatory system for pneumatics etc
Types of SPMs – From Design Point of View
Though SPMs are made in various shapes and sizes with umpteen varieties and combination, there are some popular types of construction of SPMs. We shall see more about them in the following article.
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