Manufacturing high precision parts at lower costs and higher efficiency requires integration of complex dimensional measurement within the manufacturing system. This article by Ray Karadayi, President & CEO at Applied Automation Technologies, discusses how a manufacturing machine can be made to perform complex dimensional measurements traditionally performed on coordinate measuring machines and how this metrology information can be used to achieve adaptive manufacturing and automation.
Introduction to On Machine Measurement
Dimensional measurement on the machine tools has been used ever since parts are being made by automatic machining cycles. To locate a part on the machine tool work volume requires some simple measurement of points to determine its X, Y, Z coordinates. A part being turned on a lathe requires some basic measurement of the diameter of a cylindrical part being manufactured. These measurements were done by very simple methods which are dependent on the operator skills and sometimes performed manually by just using a hard tool, a dial gage or an electronic probe.
Measurements are either manually recorded by the operator or in the case of probing macros, could be directly entered into the machine tool memory. With the advance of the machine tools, scales, and controllers, more advanced measurement routines are now possible to apply within the manufacturing machines. Bi-directional and fast interface capabilities already available on many of the machine tool controllers allows a metrology software to be utilized to process the measurement data received and calculate advanced geometrical and tolerance characteristics of parts as they are being made.
Having this capability directly on the manufacturing machine opens up the possibility of using the metrology calculations as a feedback loop to perform important corrections within the manufacturing cycle. By strategically using measurement programs through the manufacturing process enables ‘adaptive manufacturing’ resulting in improved part quality at much less total manufacturing cycle time.
Integrated Measurement Probes on Manufacturing Machines
As the need to gather faster and more accurate data grows, new probes and non-contact sensors are being developed and integrated within machining centers.
Kinematic Probes: Traditionally, kinematic probes have been used for simpler applications. Although these probes produced repeatable measurement for a unidirectional measurement such as measuring along X axis, their trigger points vary when measuring along other directions which are required for more complex shape measurements. This characteristics known as lobing can be calibrated and its effect can be minimized by compensating the trigger points.
Strain Gage Probes: Although the non-linear profile error can be calibrated and reduced by a software, it can still produce inconsistencies while measuring surface points. This is because the non-linearity in the trigger point is due to the par to probe pressure vector and not the motion vector. To eliminate this, a strain gage probe is used for the trigger mechanism and provides an extremely uniform measuring profile. The strain gage probe provides very repeatable measurement data which can be used by a Coordinate Measurement Machine (CMM) style measurement program producing very fast and high accuracy metrology data within the manufacturing machine.
Analog Scanning Probes: Analog probes that can collect high density data by sweeping over the part surfaces has been used on CMMs for many years. Several manufacturers have developed analog probes for manufacturing centers so that part measurement cycles can be reduced without having to sacrifice measurement accuracy.
Non-Contact Sensors: Laser sensor are integrated with the manufacturing system to help collect even higher density data at shorter times. The recent surge especially in aerospace manufacturing helped fuel the innovation in integration of laser sensor for on machine quick data collection.
Using Machine Tool As Coordinate Measurement Machine
In order to apply metrology feedback during a machining cycle, certain procedures have to be put in place to assure the reliability of the dimensional measurements. There are a number of factors that could change the machine tool characteristics and affect the quality of the measurement data. These procedures help monitor these changes and adapt the system parameters to achieve measurement results within the specified uncertainty budgets.
Sensor Calibrations: For an automatic in process measurement cycle, measurement sensors such as touch, analog or lasers are adapted and used by the machine tool very similar to a cutting tool. Therefore a tool must be defined within the machine tool controller to identify it to the system. This definition can be done as a nominal definition defining the geometry of the tool and the actual calibrated values are held at the metrology software used. The parameters calculated and compensated during the measurement cycles are:
– Probe Runout: This determines the probe tip deviation from its center line.
– Effective radius: Since the trigger point received by the system has a delay, depending on the measurement velocity the probe radius would be smaller than its physical size. This value is used to compensate for probe radius during measurements.
– Lobing: This value is similar to effective radius except a different value is calculated for each approach vector.
– Head offsets: While utilizing a 5 axis head, the actual probe position is calculated and used as the correction value during measurement cycles. This would eliminate any errors due to head misalignments.
– Sensor Attachment Coordinates: If a non-contact laser sensor is used which produces a number of points per measurement, its exact attachment coordinate system to the machine tool must be calculated and used as a part of the calibration.
Although machining centers are made to be very rigid in its structures, its geometry can change due to temperature fluctuations and large cutting forces applied. On a standard 3 axis machine, there could be 21 sources of geometry errors. Although these machines have periodic calibrations, to achieve best results for an adaptive manufacturing application, its best to monitor and correct for these errors. Since a complete calibration of a machine tool can take a long time and could not be feasible, other quick and automated methods are adapted.
In order to perform the complex mathematical calculations required for the metrology based real time decision making, a powerful CMM software needs to be integrated within the manufacturing system. Since the system is expected to function by itself without human interaction, it also needs to work very autonomously within the manufacturing process.
The following characteristics are required from a software to truly make a machine tool function similar to a CMM.
Offline Programming: A CAM style programming environment with a good machine tool virtual model, simulation capabilities, automatic path generation with collision avoidance and complete geometrical fitting and tolerancing functionality is required. Programming languages such as DMIS (Dimensional Measurement Interface Standard) also allows to interface and collaborate with CMMs for efficient programming.
Bi-Directional Interface: A direct and bi-directional interface is a must to be able analyze data in real time as soon as the measurement of a feature is completed. The calculated metrology characteristics are used as a part of the on the fly decision making and written back to the machine tool controller as a part of the adaptive cycle.
Ease of Operation: The measurement programs must be integrated into the machining center similar to any other cutting programs. This allows the programs to be integrated as a part of manufacturing cycles and can be automatically started by itself. A G-Code NC program is created by post processing the DMIS measurement program and resides in the controller. This program like any other cutting programs in controller native language is used as a part of the manufacturing cycle allowing multiple programs work along with the cutting programs.
Adaptive Manufacturing Control
In order to achieve a self-adapting manufacturing process, dimensional measurement programs can be utilized at several different stages of the manufacturing cycle, from preparing the machine to be at its best condition before any part machining to performing a final metrology analysis of the finished product for final inspection reports and statistical trend analysis.
Machine preparation and maintenance: A suitable master artifact and a measurement cycle can be used to quickly verify machine geometry. Any changes detected can be applied to machine kinematic table to assure machine is prepared to produce a good part. This is a ‘preventative’ approach making sure the machine is in its best condition before starting its cutting process.
Tooling balls: A simple tooling ball that is kept in the machine volume can be measured quickly to verify machine scale integrity or detect any changes due to thermal expansion. This could be applied to machine reference or simply incorporated into work offset calculations. Recording these measurement results would allow to view changes of the machine over time and perform trend analysis.
Ball bars: A quick measurement of a ball bar can verify the machine geometry and
detect its linear and squareness errors.
Volumetric Ball Bars: These artifacts can very quickly verify many kinematic and dynamic errors of the machine and provide a very detailed information about its current status.
Machine Axis Check: For multi axis machines such the machines with table-table configurations, mill-turns or 5 axis head-head machines, the changes in the rotation axes would cause problems in machine tool performance. These parameters can also be measured to verify their acceptability and updated back into the controller to adapt the machine tool to its changing environment.
Work piece setting is one of the most important part of the manufacturing process where mistakes could be made or time can be wasted to assure perfect setup. Cutting programs are created for parts to be at an assumed nominal position. A work offset has to be created to let the controller know the parts exact location and orientation.
A part could be extracted from a block of metal or carved to its final shape from a cast or forged rough shape. In the example above by simply measuring at least 6 points around the part, its correct location, orientation and center can be measured and updated to the controller preparing the machine to finish the part at its exact location.
A cast impeller can be finished to its exact shape on a 5 axis machine by
creating a coordinate system and automatically updating to the machine tool controller. This is done by measuring a number of points around its shape and best-fitting its CAD model to create part alignment so that all part profiles are positive avoiding under cutting and damaging the part.
A forged blade or an airfoil that is being re-worked can also be measured to prepare a work offset or help in deciding the best cutting method. By measuring critical datum points, part location is measured and verified to continue with its cutting programs. If part location is out of acceptable position, the measurement values can be sent back to a CAM system to create a cutting program for the actual part position on the machine.
Parts that are being reworked, for example an airfoil that have added material are measured to determine maximum additional thickness. This could be used to calculate a logical parameter and written back to the controller. The machine by using this parameter decides to run the optimum cutting program that matches the actual part thickness. In some cases, the actual surface profile is reverse engineered and sent back to the CAM system as a curve or surface data to create a custom cutting program that will work on this reworked part.
In Process Control
Before the final finish cut, parts can be measured again to adapt the machine to final conditions and produce a perfect finished part. A probing program is automatically loaded and part is measured to provide the decision making for the final finish. This could be do precisely align the part for a drilling operation in relation to datum features, adapting to changes in the tool size or over all effects of thermal and static forces during a turning operation.
Adapting part orientation for drilling operation: The adjacent image is an example of drilling precision holes perpendicular to its datum face on a 5 axis machine. By measuring the face vector and updating the work offset or the table rotation offsets, a perfect drilling orientation can be achieved.
Using Adaptive Pitch Comp on Turning Machines:
A precision part created on a turning machine will have errors due many factors such as part and tool bending under cutting force, tool wear and thermal effects. By measuring the final profile errors and automatically writing them into the machine tool pitch comp table, all these errors are summed into a single correction factor and the final cut is performed. This adaptive process is used to create precise profiles on turning machines.
Tool Wear and offset changes: Cutting tool size changes over time during a milling, drilling or a turning process. An electronic tool setter integrated within the machine tool can help between operations by setting the exact tool size into the controller. This is done by directly measuring the tool and not detecting the effect of the tool wear on the part. Another method used to make sure final produce has no errors due the tool wear is completing the finish cut on a brand new or preset tool. This method also does not act on the actual part shape and could be problematic or expensive. Using an automatic measuring program before the finish cut and strategically applying the part errors to tool wear, tool radius and tool offset helps achieve precision parts at the lowest cost.
In Process Re-Posting: In some cases, the cutting G-Code program may need to be regenerated do adapt the part orientation and shape. 5 axis machines that creates a precision profile or drill features in perfect orientation needs also its tool orientation adapted to the features it will create. While work offsets even with the rotational parameters can move the tool to the correct cutting location, its 5 axis parameters such as the head angles or table angles may not be possible to change causing less than perfect finish. For parts such as sheet metal or composites, the part shape may also be distorted and a new path matching exactly the part shape needs to be created.
Re-Posting: Cutting program re-posting is a process which takes a master cutting program designed for the perfect part location and shape and regenerates a similar path by applying the measured exact part orientation and part profile. The re-posted program would be same length in length but create slightly different coordinate and joint parameters. The image below shows a part and cutting path in red in nominal position. B shows the relationship of part in actual position to its cutting path before reposting. C shows the cutting path and updated tool orientation after 5 axis reposting process.
Adapting 5 Axis Parameters:
When the tool orientation must match the actual part rotation, its G-Code program must be reposted for the tool orientation as well. This could be on a 5 axis machine with a program using the A, B, C angles of the head or I, J, K vectors of the head. This is especially important when the tool cutting surface is away from the center point which could be the case for a saw blade or a grinder.The adjacent image shows an example of a part reposting to match the tool vector to exact hole locations that will be drilled.
Adapting to Part Shape:
Sheet metal and composite parts that needs to be trimmed maybe be out of its
actual shape due to fixturing of non-rigid structure or the heat treatment process. Part’s residual stresses also changes its profile while a portion is removed. Especially trimming of these parts with a water jet cutting takes a long time to prepare or causes part defects. By measuring the part profile and reposting the cutting program to its shape provides a very quick and inexpensive method which can also be automated. The image shows the profile deviation vectors of a part scanned by a blue light laser sensor.
In some applications, reverse engineering of the actual part location or part shape is necessary to create a custom cutting program. By measuring datum features on a blade that needs to be finished on a 5 axis machine might require a custom cutting program. In this case, the generated coordinate system or the measurement results can be exported to a CAM system which can then create a cutting program for the parts exact location.
For parts that had been reworked by welding material or parts being manufactured on an additive/subtractive machines, the actual welded sections can be digitized and the actual curves exported to a CAM system which can create a custom fitting cutting tool path for the parts exact shape. The above image shows an example of an airfoil repaired by welding and finished by a process like this.
Post Process Control
After a part is manufactured, a final inspection can be performed. This program is also started automatically and could be a complete metrology program making the machine tool work similar to a CMM. For multi-operation processes, this could also be measurement of critical or datum features which are shared by the next operation.
On Machine Verification: Regardless of its kinematic configuration, a machine tool can be programmed to perform complex measurement tasks which are typically done off-machine on a separate device. The calibration and verification methods discussed above allows very repeatable measurement results which produces measurement data at acceptable uncertainties. Utilizing a CMM grade metrology software directly on the machine produces real time measurement data eliminating constant dependence on a CMM.
– Automotive Die Manufacturing: Large Die and other automotive parts, whether they are being reworked or newly generated would be required to be measured for their dimensional integrity. This dimensional quality control task usually is done by a coordinate measuring machine located away from the manufacturing machine. This creates a bottle neck in the operations, takes time and ultimately increases costs. If there are defects found on the part, then it has to be brought back for additional work which increases the costs even more. Other methods such as laser trackers or portable arms are not very reliable and also interrupts the machine process, requires additional team and expertise and increases the costs as well. Using an on machine measurement software and utilizing the machining center like a CMM can very quickly generate the CMM report while the part is still on the machine.
– Aerospace Large Structure Manufacturing: In some cases, measuring very large structures by other methods may not even be possible or feasible. The measurement software can use virtual models and create measuring programs which can work on complex multi degree of freedom machines. The measurement results can be used both to adjust manufacturing parameters and create final inspection results before removing the part. Image shows a Triceps manufacturing robot used by an aerospace application.
– Final Inspection of Production Parts
Any part that is being manufactured on a machining center can be quickly measured by an automatic program and complete metrology analysis can be generated. These reports can be reported locally, saved on a server or directly passed to monitoring system for more detailed and historical analysis. Image shows an inspection of a gear component manufactured on a mill-turn.
Off-Machine Measurement Integration
In some cases, parts will need to be measured outside of the machine that makes it. This
measurement can be made by a CMM, a flexible gage or a dedicated hard gage. The
measurement results from these system can still be used to adaptively control and
optimize a manufacturing process.
Collecting Measurement Data
Measurement data gathered by various systems can be collected at a central data base and used for strategic decision making such as trend analysis or real time feedback to a machine tool controller. Measurement data received from various devices are gathered in a single database and displayed for easy view.
Current data is shown as Go/No-Go on the left side and the historical run charts are shown on the right side. This software is also linked with the manufacturing systems that are producing them so that a calculated parameter such as a tool wear correction value calculated based on a user defined algorithm is written back to the controller to adaptively adjust the cutting parameters to achieve the best results.
Adaptive manufacturing with metrology feedback is a very effective and viable method of integrating dimensional measurement directly within the manufacturing machines. The machine tool controllers interface capability, use of high resolution scales, probes and sensors ease integration within the manufacturing systems and above all the state of the art metrology software that can be used to interface with all of this makes it a solution that is easily achievable for most applications. This has many benefits and helps produce complex parts manufacturing and reduce cost of manufacturing any precision parts on many types of manufacturing systems.
Increase Product Quality – By measuring a part on the machine without having to remove it and adjusting machining parameters based on the measurement results allows to manufacture a high precision parts. Knowing the part dimensional quality and metrological characteristics before removing it from the machine has great benefits and improves the overall performance of a manufacturing facility.
Reduce Manufacturing Costs – By integrating a closed loop measurement system on the machine, the additional time invested in the measurement process actually reduces the time spent elsewhere for the same goal. The cost saving benefits can come from work piece setup time being reduced drastically especially for large and complex parts, elimination of expensive work holding, reduced off-machine measurement needs and dependency, optimized machine performance and maintenance scheduled optimally, extended cutting tool life by dynamically adapting tool wear and offsets, cast, forged or otherwise blank sizes can be minimized saving material costs and reducing machining times and scraps is avoided.
Automated Closed Loop Manufacturing – Because of the bi-directional integration of the adaptive measurement system, parts manufacturing can be automated. This can be by self-monitoring the manufacturing conditions as well as linking multiple machines working on the same part through different operations.
Manufacturing System Control – Adaptive measuring systems work with the machining center as a peer to peer secured interface but they are also on the network for measurement data collection and reporting. Measurement results from machines used in a manufacturing facility can be collected in a database and used to monitor the overall factory performance. Evaluating and comparing this kind of data allows better decision making and help plan for future manufacturing strategies.
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