Testing and Calibration Techniques for Modern Machine Tools

Modern machine tools have and can be supplied with testing and calibration technology so that the workshop can ensure that the machines are accurate and working properly.

Increasingly, factories and large workshops have their own laser interferometers and electronic equipment, while smaller workshops have access to equipment and testing services on a commercial basis at competitive rates through a variety of channels and on a rental basis.

In fact, telescopic ballbar inspectors can now be supplied to any workshop for rapid inspection of machine tools within 15 min to maintain the machine’s machining accuracy.

Using ballbar inspection allows accurate evaluation of machine geometry, roundness and stick/slip errors, service gain mismatch, vibration, backlash, repeatability and scale mismatch.

Some ballbar software can provide a diagnosis of specific errors according to ISO 230-4 and ASME B5.54 and B5.57 standards, and then provide a simple English list of the sources of the various errors according to their overall impact on the machine’s accuracy.

This allows machine tool maintenance personnel to target problem areas directly.

Staged ballbar testing must keep up with trends in machine performance.

Preventive maintenance facilitates advance planning before the machine deviates from its process capability.

In industry, it tends to calibrate machines on an as-needed basis, rather than on a time-based basis.

There is no reason to take an intact machine out of production for maintenance and calibrate it.

When anything unusual is found, it is better to let the inspection of the ballast and the parts being produced determine it.

Production can continue during testing.

Probe testing on spot welding machines

The level of accuracy and repeatability achieved by today’s standard machine tools is close to what was once only possible with CMMs.

This capability allows the machine itself to automatically inspect the part with a probe at critical stages of the machining process.

Once the machine is equipped with a measuring instrument, the measuring probe becomes the operator’s CNC gauge.

Inspection programs can be programmed as part of the machining process and run automatically at various points, checking size and position and providing the necessary compensation.

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This eliminates the need for operators to use micrometers and plug gauges for measurement, and eliminates human error caused by jamming, part and tool offsets in the control system.

On-machine inspection has become a part of the process, a powerful process tool that has been improved to produce qualified parts the first time in the shortest possible production time.

It can be used to automatically determine the position of the part and then establish a working coordinate system.

On-machine inspection cuts set-up time, increases spindle utilization, reduces tooling costs and eliminates non-productive machining lead times.

In the case of complex parts machining, it used to take 45 minutes to set up the fixture, but now it takes only 45 seconds with the inspection device and all is done automatically by the CNC.

At the start of machining a casting or forging, the inspection device determines the shape of the workpiece, eliminating wasted time due to blank cuts and helping to determine the best angle of entry for the tool.

In-process control uses inspection devices to monitor machine characteristics, dimensions and positions during the cutting process, while verifying the precise dimensional relationships between various features of each machining step to avoid problems.

The probe can be programmed, the actual results of each stage of machining are programmatically checked and then tool compensation is automatically achieved, especially after roughing or semi-finishing.

Reference inspection, which compares part characteristics to a dimensional template or a reference surface of known position and size, enables the CNC to determine a positioning gap and then generate an offset to compensate for this gap.

By inspecting the replica before critical machining, the CNC is able to check its own positioning against the known dimensions of the pattern and then program the offset.

If the dimensional pattern is mounted on the machine and exposed to the same environmental conditions, then the thermal expansion coefficient can be monitored and compensated using reference checks.

The result is a closed-loop process that is not influenced by the operator.

Each machine tool has many small errors inherent in its motion and in its construction, so that there will always be a small gap between the CNC programmed position and the true position of the tool tip, even after the laser compensation between the two has been adjusted to be fairly consistent.

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Programmable artefact inspection is a good way to further compensate for the remaining errors in the machine.

It provides feedback to the process control and can bring positioning accuracy close to the specifications for machine repeatability .

This closed-loop process control allows machining centers to achieve the same level of machining accuracy as boring mills and other precision machines.

Many probe inspection operations are performed by TSR macro programs.
The updating of work coordinates, changes in tool geometry and measurement of parts are automatically determined by the CNC after successful completion of the probe inspection cycle.

This eliminates serious errors caused by incorrect information links or incorrect calculations.

For part inspection after machining, the length and complexity of offline inspection can be reduced, and even eliminated together in some cases.

On-machine inspection is particularly beneficial for large and expensive workpieces, as they are very difficult and time-consuming to move.

Two other methods can be used to complete the reference check:

  1. machine correlation testing, where the onboard measurement data is compared to previous CMM data;
  2. contour plate inspection, comparing the onboard data with a traceable replica of known dimensions.

when comparing, the CNC is able to determine whether the machine has actually met the specified machining tolerances.

Based on these results, an informed decision can be made as to the correct way to handle the parts that remain on the machine.

Non-contact laser tool setting

Laser tool setting provides a fast, automated method for verifying tool size, especially in tool and die manufacturing, which plays a key role in verifying tool wear after long periods of machining.

Laser tool setting is a cost effective and efficient way to set tools and detect tool breakage at high speed and with high accuracy.

In operating conditions, it quickly measures the length and diameter of a tool as it is indexed by the laser beam or rotated at normal speed.

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Laser detection operating at spindle speed identifies errors caused by uncoordinated spindle, tool and holder clamping and radial vibration, but this feature cannot be conducted with static tool setting systems.

Some NC tool setters can detect the break at the highest lateral travel.

As the tool moves through the laser beam, the system electronics detect the break in the laser beam and send an output signal to the controller.

The CNC control system can accurately measure tools with a minimum diameter of 0.2 mm anywhere in the laser beam.

The system is triggered when the laser beam exceeds the 50% threshold and is interrupted by the detected tool.

The non-contact tool setting system uses a red visible diode laser that is reliable under machining conditions.

Advanced electronics and a simplified design allow non-contact tool setting to replace contact systems.

Since there are no moving parts, the NC control system is virtually maintenance free.

The frame and actuator required for contact systems are not present in this design.

Some NC laser tool setters are equipped with a protection system, housed in a rugged stainless steel unit with an internal uninterrupted charge of compressed air to protect against contaminants, chips, graphite and coolant, even during the measurement process.

These systems can also be installed on almost any size and shape of machine with no impact on the machine’s operation.

The proven application and availability of these technologies, such as powerful tools for improving processes, are of great benefit in increasing the level of automation and achieving better process control in mold machining.

They enable mold makers to produce molds faster with greater geometric and dimensional accuracy, with little or no operator involvement, reworking or manual finishing operations.

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