Major Types of Special Purpose Machines (SPMs)

Broad Design Classifications of SPMs

Mar 27, 2009 C. Varada Rajan

SPMs are made in various shapes and sizes with umpteen varieties and combination, but there are some general and popular types of construction. They are explained here.

This is Part:2 of article on Special Purpose Machines (SPMs). Please read Part:1 first before proceeding further in this article.

Modular Units – Building Blocks of SPMs

Though SPMs are custom-built to suit specific components and operations, it is not always that all the elements of the machines are built from scratch. Any metal cutting machine will require spindle(s) to hold and rotate the component(turning spindles) or the cutting tools (drilling, boring, milling, tapping etc), slide units to hold and move either the fixtures or the spindles, base units and columns to hold the slides etc.

Such basic building blocks of machines, pre-designed and manufactured to varying capacities and sizes are known as “modular units”. Either the SPM builders themselves or dedicated manufacturers of such items will have an array of these modular units in their kitty.

Cost and lead time can be saved by readily incorporating these modular units in SPMs. In drilling, reaming and tapping applications, which are the most widely done operations in any mass production, such spindles are also available readily with self-feed arrangement built in. Sugino, Suhner, Drillmatic etc are some very popular international names offering such modular units. These spindles will also come with an array of mounting accessories – to facilitate setting them horizontally, vertically or inclined.

There may also be peculiar applications of SPMs where highly specialized spindles and slides may also be required, which will call for designing them from scratch.

Some Popular Types of Construction of Special Purpose Machines:

1. “End” Type Machines:

For example, a “double ended boring SPM”. In this type, machining spindles carrying rotating tools (boring bars) will be placed on two ends. The work piece held in a fixture on a slide will shuttle from one end to another to get both ends bored.

In this type, the operations are sequential. The cycle time will be more (and perhaps matched with the output requirements). Only one slide is involved and the cost is comparatively less.

2. “Way” Type Machines:

For example, a “two way boring machine”. In this type of machine, machining spindles carrying rotating tools (boring bars) will be placed on two independent slides. The work piece held in a fixture will remain stationery. Both the boring spindles will approach the workpiece simultaneously and do the boring operation and then retract.

In this type, as the operations are done simultaneously, cycle time is less than a corresponding End type machine. Here two slide units are involved and hence the cost of machine will be more than an End Type machine. Where shorter cycle times are needed, a way type machine is a better choice over the end type machine.

More number of “ways” are possible in this type of machine construction – like 3- way, 4-way etc. Since workpiece is stationery, many spindles can approach it from different directions to do several operations both simultaneously and independently (if there are going to be some interference problems).

3. Linear Indexing Machines:

For example, a “two-station drilling and tapping machine”. Here, the component to be machined is placed in a slide that can linearly index from one station to another. In the first station, a self feed drilling spindle ( way type) will approach the component and do the drilling operation. Afterwards the component will move to the next station where a tapping spindle will approach it and do the tapping on the previously drilled hole. Now the work slide will retract to the starting position again to get the component unloaded.

More than one operation may also take place in each station in such machines, depending on the application.

In this type of SPMs, operations are sequential. Cycle time will be more, since linear indexing is involved.

4. Rotary Indexing Machines (Dial Type Machines):

For example, a “4-station drilling counter-boring and tapping machine”. In this machine, a rotary table will be the main work holding arrangement, where a set of 4 identical fixtures are placed. The operator will load and unload the component in one station. The rotary table will index by 90 degrees and take the component to the subsequent station. In station:2, drilling will take place. In station:3, counter boring and in station:4 tapping will take place.

Thus (after 4 cycles) all the 4-stations will keep working simultaneously and doing all the operations. The operator will keep on unloading and loading a fresh component at the end of each rotary indexing at his station.

In this type of machine, the rate of output will be the highest. The cycle time is decided by the longest machining time taken by any one of the 3 stations (or the loading and unloading time if it is longer than machining time) plus the rotary indexing time.

Rotary indexing machines demand very high accuracies of indexing, fixture design and interchangeability and alignment of machine elements. They are normally costly and are best suited for very high volume production of components.

5. Multi-spindle Machines:

Typical examples are “Multi spindle drilling machines”. In these machines, several holes either arranged in a pitch circle diameter or in a matrix of preset pitch distances are drilled simultaneously; the drilling spindles will be powered by a single motor and the drive to each spindle will be taken through universal jointed shafts or by meshing gears.

It is also possible to develop multi-spindle machines in way type, end type and indexing types, with other operations combined along with multi-drilling.

6. CNC SPMs:

With the wide ranging advantages perceived in using Computer Numerical Control (CNC) in machine tools, application CNC technology to SPMs too is gaining popularity. The main advantages of CNC are the high degree of flexibility, accuracy and control. CNC SPMs (single axis, 2-axis, or more) prove to be advantageous in short batch production and also in applications where product design revisions and variations are more pronounced.

These machines will normally incorporate typical CNC machines' mechanical features like LM guides, ball screws, Servo Motor control etc.

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