Time has become an increasingly important factor in the manufacturing industry, and minimising time throughout the manufacturing process is of key importance in raising efficiency levels. One area on which manufacturers are focusing their efforts is that of non-value added activities, including that of component inspection.
From having a discrete and independent Co-ordinate Measuring Machine (CMM) to perform dimensional inspection of machined parts, machine shops have graduated to using tool setting and spindle probes on their CNC machine tools to reduce set- up times and monitor the machining process. For some companies this includes measuring key features during the machining cycle, and adjusting the machining process as necessary. This saves time by removing the need to transfer components between differing machines, obviating the need to key offsets into the machine tool following the receipt of inspection results, and eliminating the possibilities of human error when keying offsets.
But, a great number of companies still rely on the CMM despite having probes installed on their machines, often to simply confirm the results obtained from machine tool probes. It is however possible to dispense with the CMM for such routine inspection work, by performing the final inspection of parts while they are still on the fixture. However, to do this, users must make sure that machines are capable of producing parts to specification before machining starts, and they must introduce process control and measurement methods that are traceable to recognised standards.
How can Parts be Made Right First Time?
In order to be sure that measurements made on a machine tool are accurate, a machine tool has to be calibrated to eliminate errors that may creep in owing to several external and machine-inherent factors. Using a combination of a calibration laser interferometer, telescoping ball-bar and a set of precision machine levels, the machine errors can be identified and rectified. Data can be used to create compensation values in the machines CNC to correct for systematic errors in linear and rotary motion, and to help prioritise corrective action. On larger machines where the problems of temperature and geometric errors are magnified, laser encoders can be an effective way of introducing greater accuracy to both machining and probing. Unlike conventional encoders, they are not attached to the machines structure except at one end of the travel, so distortion or growth does not affect these encoders.
Six Steps to Improve Machine Capability
1.Determine the accuracy you need - the tolerance on the drawing determines the machine accuracy required
2. Establish a baseline for current performance - run a simple 10-minute ballbar check that will detect a machines contouring errors
3.Identify and rank error sources - ballbar analysis software will isolate and rank 18 machine parameters in terms of their impact on your machines ability to servo and position accurately. Error sources include squareness, cyclic error, stick-slip, reversal spikes, scale mismatch, machine vibration, servo mismatch and backlash. The analysis helps to prioritise corrective action.
4.Eliminate or calibrate errors -the laser calibration system eliminates systematic positioning errors. If, for example, the ballbar test identifies scale mismatch, then it is possible to calibrate just the axis that needs to be corrected for linear error. Running laser calibration checks on machine tools makes them repeatable and accurate.
5.Establish a new baseline - once geometric errors have been eliminated or minimised, a re-test is run with the ballbar to set a new machine baseline.
6.Regular health checks - run regular checks to detect any deviation from the baseline.
How Do You Know Your Parts Are Correct
Before we discuss carrying our probing on a machine tool to traceable standards, a couple of key issues need to be addressed:
Can you trust the machine that cut the part to also measure it?
Often likened to a schoolboy marking his own work, the answer to the above question is simply, yes! You can use an inspection device, provided that you know the uncertainty of the measurements that they make and that the measurements are traceable. If this uncertainty is low enough relative to your tolerances, then you can probe parts and pass them off on your machine tools.
Does probing increase cycle time?
People often state that they bought a machine tool to cut metal, not carry out inspection, and that a machine tool is making money when it is cutting metal. However, this is only true if machine tool is cutting good parts. Probing reduces overall cycle times by automating set-up, allowing more time for machining, and ensures that eliminating operator error and compensating for process drift reduce scrap parts. By using probes, the downtime on machine tools associated with waiting around for offline inspection can also be significantly reduced.
The ambience of a CMM is controlled - both the humidity and temperature are within certain limits. However, when measurements are being done on a CNC machine tool, users have to contend with the rise and fall in temperature associated with metal- cutting operations. To surmount this unavoidable problem, and yet get the best out of the benefits of in-process inspection, artifact compensation or foot printing is resorted to. In the former method of compensation, a generic part or a replica of the one being machined (of the same material), is first measured on a CMM at 20C (thus one cannot totally dispense with a CMM), and then transported to the CNC machine where it resides in the same ambience as the part being machined, ensuring that it experiences the same thermal distortions as the component to be measured. Before measuring the component, the artifact is probed and differences between the known actual and measured sizes are used to make the compensations to the component measurements also taken on the machine tool.
In foot printing, the first part machined and measured on the CNC machine is measured again on a CMM calibrated to the B89 standard. By comparing the results from both devices a correction value can be calculated for all subsequent measurements made on the machine tool.
Now that you have optimized machine tool performance and established a traceable inspection process on those machines, you final]) need to establish a systematic approach to inspection and process control: Establish machining process capability - Understand the inherent variation and underlying rate of drift in your machining processes by measuring parts either on the machine, using of the traceable techniques outlined above, or on your traceable CMM. Compare this capability with your tolerances to determine how often will need to inspect the parts to keep the process under control.
Account for inspection process capability -
Know the trace, accuracy of your on-machine measurement, including measurement repeatability. Take account of when setting your process control thresholds.
Set up a process feedback regime -
You dont want to probe more features than you need, so identify those that are critical to product performance. Ideally, measure one control feature finishing tool, and use the measurement error to update the size or length o\ for that tool.
Probing software, such Renishaws Inspection Plus, make process control simple, by setting control limits and applying a damping factor to offset corrections. The monitoring and inspection need not to be restricted only to the machined parts, but also to the cutting tool. Tool breakage and wear can also be inspected and monitored, and corrective action can be subsequently taken to avoid tool breakage or work-piece damage.
Meeting the Lean Manufacturing Challenge
In todays tough business climate, it has never been more important focus on improving your manufacturing processes. Around the world, lead manufacturers are finding that they need robust machining and process control methods that result in good parts - all day, every day - with maximum efficiency in the use of capital and labour. Further, modern quality standards demand that documentary proof be maintained that processes are under control. Once the system is in place, benefits like improved quality, reduced downtime and higher productivity are there for the asking.