Measurement technology is key to automation

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Advances in machine accuracy, on-machine touch probing technology and noncontact tool setting provide powerful tools for automating and speeding mould machining, says Barry Rogers.
Note: A free brochure or catalogue is available from on the products in this news release.
Drives to faster, leaner, more flexible manufacturing are shifting industry focus away from traditional post-process quality control. The most expensive, non-value-added process in most shops is part inspection. Inspecting good parts - parts that meet all print specifications - is a waste of time, money and manpower.
Rather than back-end detection, attention is shifting to front-end prevention.
The aim is to make 100% good parts, right the first time, to ever-tighter tolerances in the lowest possible total processing time.
Under that mantra, a variety of practices and technologies are being applied to machine tools to achieve greater process control.
Automated process checks can keep process and parts in control, while minimising downtime for operator intervention.
These process control improvements can be particularly vital for mouldmaking.
The one-off nature of most mould/die work and the high accumulated value that can go into a complex mould demand right-the-first time processing.
At the same time, shorter lead times and global competition force the need for faster mould processing.
By minimising need for operator intervention, these process controls give mouldmakers an 'eye on the job' during long machining runs and lightly staffed second and third shifts.
Front-end prevention takes three forms: identifying and maintaining machine capability; in-process probing; and automated tool monitoring.
A technology leader in all three areas, Renishaw offers single-source expertise and assistance in creating an integrated programme of mouldmaking process control.
To move from defect prevention, you must be able to document your process capability and the accuracy of your machine tools.
To do this, inspect them to a nationally recognised and accepted standard, such as ISO230 or ASME B5.54.
Both call for a ballbar and laser interferometer to be used with a recommended procedure for checking machine tool accuracy.
The purpose of these standards is not to specify an accuracy the machine must meet, but to find out what accuracy level it can meet - its process capability.
The part print dictates the accuracy your machine must have to make good parts - where to set the accuracy bar.
Testing tells you how high your machine can jump.
As long as your machine can top the bar, you have process capability.
Test and calibration technology are now available - and affordable - to enable shops to ensure the accuracy and health of their machine tools.
Plants and large shops increasingly maintain their own laser interferometers and electronic levels, while rental equipment and diagnostics services are commercially available to small shops from various sources and competitively priced.
Renishaw's QC10 ballbar system is readily affordable by virtually any shop and provides a fast, 15-minute check-up for prevention and diagnosis in maintaining machine accuracy.
The ballbar test allows precise assessment of machine geometry, circularity and stick/slip error, servo gain mismatch, vibration, backlash, repeatability and scale mismatch.
Renishaw's Ballbar5 software provides diagnosis of specific errors in accordance with ISO230-4 and ASME B5.54 and B5.57 standards, then provides a plain-English list of error sources rank-ordered according to their overall effect on machine accuracy.
This allows maintenance people to target those factors which most need attention.
Periodic ballbar testing enables trend tracking of machine performance.
Preventive maintenance can be scheduled before a machine drifts out of process capability.
The industry trend is to calibrate the machine on need, not time.
There is no reason for maintenance to pull a perfectly good machine out of production for calibration.
Let the ballbar and the accuracy of your parts determine when something has gone awry.
Meantime, run production.
Today's standard machine tools can deliver accuracy and repeatability approaching levels formerly available only on CMMs.
This enables the machine tool itself to be used for probing checks of workpieces during critical stages of the machining process.
Once a machine tool's performance as a measuring instrument has been established, the touch probe becomes the operator's CNC gauge.
Probing routines can be programmed as part of the machining process and automatically run at various points to check feature dimensions and locations and apply necessary compensations.
This saves operators from using dial indicators and shim stock, or eliminates errors in manually entering fixture, part and tool offsets into the control.
Probing on the machine makes it part of the process - a powerful process improvement tool for making parts right the first time in the shortest throughput time.
Used to locate the part automatically and establish a work co-ordinate system, probing cuts setup time, increases spindle availability, lowers fixture costs, and eliminates nonproductive machining passes.
On complex parts, 45 minutes of fixture alignment can be replaced by 45 seconds of touch probing - performed automatically by the CNC.
When starting with a casting or forging, probing can determine workpiece shape to avoid wasted time in air-cutting and help determine best tool approach angle.
In-process control uses touch probing to monitor size and position of machine features during the cutting process, as well as verify precise dimensional relationships between various features at each step to avoid problems.
A touch probe can be programmed to check actual machined results at various stages against the program and automatically apply cutter compensation - particularly after rough machining or semi-finish machining.
Reference probing - comparing part features to a dimensional master or reference surface of know location or dimension - enables the CNC to determine positioning discrepancies and generate an offset to make up the difference.
By probing the artefact before a critical machining pass, the CNC can check its own positioning against the master's known dimensions and program an offset.
If the dimensional master is mounted on the machine and exposed to the same environmental conditions, reference probing can used to monitor and compensate for thermal growth.
What results is a closed-loop process requiring no operator intervention.
Every machine has its own set of numerous small errors in its motions and structure.
As a result, there is always a slight discrepancy between a CNC's programmed position and the true position of the tool tip, even after laser compensation has brought the two into closer agreement.
Programmable artefact probing provides a way to further compensate for remaining machine errors.
It gives process control feedback to enable positioning accuracy that can approach the machine's repeatability specification.
Such closed-loop process control can allow a machining centre to achieve accuracies comparable to boring mills and other high-precision machines.
Many probing operations are accomplished through the use of memory- resident macro programs.
Work co-ordinate updates, tool geometry changes, part measurement etc, are automatically determined by the CNC after the successful completion of a probing cycle.
This eliminates costly errors resulting from miskeyed information or incorrect calculations.
Used to inspect parts after machining, probing can reduce the length and complexity of off-line inspection, and it some cases eliminate it altogether.
Inspecting on the machine is particularly beneficial with large, expensive workpieces, such as mould or dies, which can be especially difficult and time-consuming to move.
Here, too, reference probing against a traceable artefact can be used to compare final dimensions to the known dimensions for a metrology master.
When making this comparison, the CNC can determine if the specific machining tolerances were actually achieved.
Based on these results, an intelligent decision can be made on corrective actions, while the workpiece is still on the machine tool.
Laser tool setters provide a fast, automated means to verify tool dimensions, especially critical in checking for wear during the long machining runs in mouldmaking.
A cost-effective solution to high-speed, high-precision tool setting and broken tool detection, laser tool setters rapidly measure tool length and diameter on-the-fly, while the tool is indexing through the laser beam and rotating at normal speeds.
Laser checking at working spindle speeds identifies errors caused by clamping inconsistencies and radial run-out of the spindle, tool and toolholders - not feasible with static tool setting systems.
Renishaw's NC family tool setters can perform broken tool detection at maximum traverse to further minimise out-of-cut time.
As the tool moves through the laser beam, system electronics detect when the beam is broke and issues and output signal to the controller.
The NC systems can accurately measure tools as small as 0.2mm diameter anywhere in the beam.
The system triggers when the laser beam is broken beyond a 50% threshold by the tool being checked.
The noncontact tool setting system uses a visible-red diode laser proven reliable in machining conditions.
Advanced electronics and simplified design makes noncontact tool setting an affordable alternative to contact systems.
No moving parts make NC systems virtually maintenance free.
The design avoids the brackets and actuators with contact-based systems.
Housed in a rugged stainless steel unit, the NC laser tool setters feature Renishaw's MicroHoleTM protection system.
This uses a continuous stream of compressed air to keep out contaminants and provide uninterrupted protection from chips, graphite and coolant ingress, even during measuring routines.
Three different Renishaw NC systems enable installation on nearly any size and configuration of machine tool without impinging on the work envelope.
These proven, affordable control technologies can allow greater automation of mould machining with greater process control.
They can make it possible for mouldmakers to produce moulds faster, with greater geometric and dimensional accuracy, and less operator intervention, rework or manual finishing. Request a free brochure from