Interpolation turning

How does interpolation turning work?

Interpolation turning has been up until now a little-known machining process, that can be used to realise turning processes on machining centres. It is used, for example, for recesses on cubic workpieces that could otherwise be produced using circular milling. However, many other geometries typical for turning, for example tapers, can also be produced using interpolation turning on machining centres.

An important prerequisite for the use of this process is that the machine has a main spindle that can be operated as a position-controlled axis. If this precondition is met, in many cases the advantages of interpolation turning over circular milling or re-clamping the workpiece in a lathe can be exploited. This article describes the machining process including the NC programming and a few important relationships for the application and the tool selection.

How does the process work?

For interpolation turning the main spindle on the machining centre is switched to position-controlled operation (also called axis operation). It can then be operated like a rotating axis. During recessing or facing on lathes, the cutting edge forms a spiral as it moves towards the workpiece. Here the feed per revolution is the pitch of the spiral. This spiral movement is normally approximated during interpolation turning on machining centres using semicircles, i.e. the feed axes move in a semicircle during circular interpolation (in the x-y plane) and at the same time the main spindle is synchronised to the movement of the feed axes (Figure 1). The centres of the semicircles are slightly displaced in relation to the central axis of the recess. The result is movement of the cutting edge that is very similar to the spiral during conventional turning on lathes. (The maximum error on the radius of the path actually covered compared to the spiral is approx. 5% of the radial feed per revolution f. At a feed of 0.15 mm the maximum error compared to the spiral is approx. 7.5 μm.)

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