Having 30 years of experience diagnosing problems with machining accuracy, I was often asked some questions like:
- Why do I get so many machining failures with motion interpolation, despite having my machine tool checked with a laser and applying corrections?
- Why one type of machining comes out very well and another fails on the same machine tool?
For this reason, for a long time I felt inclined to consider the problem: how to translate geometric, positioning and kinematic accuracy of the machine tool into resulting machining accuracy, especially in case of special machine tools, requiring complex motion.
![](https://about-engineering.com/wp-content/uploads/2023/01/lathe-machining-accuracy.jpg)
I decided to start with lathe machine tools, because I have inspected with various methods more than two hundred such machines, including many prototypes, designed specifically for special machining. First of all I asked myself two additional questions:
- Can the measurements from conventional acceptance tests indicate the ability of given machine tool to perform accurately in all kind of machining scenarios?
- What is expected accuracy, resulting from properties of given machine tool, for given type of machining and how to asses it without producing test samples, especially in case of complex machining?
This article contains my initial considerations on this subject, resulting from my professional experience in diagnostics of machining accuracy.
The continuous development of machine control systems enables more and more possibilities for universal lathe machines and allows many machining operations to be performed combining many types of technological motion, for example:
- turning curved surfaces (interpolated feed axis movements)
- milling and drilling around circumference with spinning tools (complex rotary movements)
- machining with synchronized counter-spindle (RPM and phase synchronization)
- high performance polygon turning (synchronizing fixed translation of the spindle rotation with rotary tool and feed movement)
- machining of rope threads (synchronizing spindle movements with cyclic reciprocating motion of the x axis and z axis feed movement)
Those operations make use of combined rotary and feed technological movements to achieve required machining trajectory (contact point of the tool with the machined object). Every kind of deviation from expected/set position or trajectory results in machining failures. Such deviations in relation to characteristic mounting points of the tool and the machined part (A, B, C points in the Fig. 1) which establish expected/nominal position of the part clamped in the chuck and of the tool in working space of the lathe machine, are the cause of machining failures, resulting from machine tool’s properties. In such diagnostics errors resulting from set machining parameters, wearing of the tool or vibrostability are not taken into consideration, presuming the parameters of machining are correct.
![](https://about-engineering.com/wp-content/uploads/2023/01/lathe-base-points-transformed-Small.jpeg)
A) Point of intersection between the main spindle axis and the plane perpendicular to it, defined by the spindle face plane
B) The point defined by the cutting tool blade tip moving with the slide in the machining plane (XZ)
C) The point defined by the tailstock center tip, lying in the axis of the spindle and moving along this axis (considered together with points A and B, for the type of lathe with a tailstock)
These assumptions are the starting point for diagnosing machining failures, resulted from the characteristic of the lathe. Considering technological movements we can use different classification of machine tool’s errors, dividing them into errors causing permanent or temporary changes in desired position of the tool and the workpiece, which would allow for easier diagnostics of the reasons behind machining failures. In regard to the character of the influence on set machining trajectory movements, they can be classified as:
Static mapping errors of processed contours represent geometric, positioning or calculation model (defining set position) errors, influencing mutual situating of the tool and the workpiece at given location in the set position (with movement stopped after reaching set position).
Errors with average value for the set speed (rotational or linear) usually reflect kinematic errors with mechanical transmission. In some kind of machining operations (e.g. threading) they result in increasing error in expected angular position of the rotating workpiece or linear position of the tool. They are a particular type of servo mismatch.
Servo mismatches reflect too fast or too slow realization of the set motion value, considered as a function of time. They usually arise from characteristics, abilities and settings of servomechanism systems in relation to set values, resulting in deviations of momentary orientation of the workpiece and the tool from expected values.
Dynamic displacement errors are due to the interacting forces, set movements and machine tool’s properties (rigidity, vibrostability, kinetic accuracy). Typical diagnosed errors, differing from set positions, are caused by vibrations, backlash, friction, bearing accuracy etc.
Temperature errors result from changes in surrounding conditions and heating up of machine tool’s parts caused by heat emission during machine operation. As a result, temperature drift of the base points can appear, resulting in deviations from expected mutual position of the workpiece and the tool. Such changes, as opposed to servo mismatches and dynamic displacement errors, are slow to occur over time. The scale of temperature errors depends on the structure of the machine tool and the size of set movements.
The critical factor for machining accuracy is maintaining the trajectories of technological movements, and therefore for diagnosing the causes of machining failures it is necessary to use methods that can verify precision of those movements in time and space.
Listed below are methods that I find the most suitable for the task of assessing errors in technological movements and diagnose the causes of machining failures in special machine tools, which can be also used to review implemented corrections and/or repairs.