Adaptive machining is the adaptation of NC programs to accommodate part to part variation enabling more stable and consistent production of complex or variable geometries. Repeat component manufacture requires a repeatable machine and process to ensure the product is machined to the same standard every time. With a repeatable machine, stable fixturing and consistent material supply condition a production process can be set and left to run with minimal operator input but this level of input stability is not always possible. Part to part variation of stock material may cause inconsistent loading that cannot be accommodated by the machining process. Multi-axis toolpaths may not align when the part is approached from different orientations. The result is increased set up time per part, higher scrap rates or manual clean-up operations after machining.
The left of Figure 1 shows a chamfering program with the expected part profile. The top right shows the result if that program was used to cut a part with different curvature. The lower right shows the how the program can be adapted to the new geometry to produce a conforming feature:
Figure 1: Nominal and adapted chamfering program on varied part.
There are several levels of complexity in adaptive machining which can lead to increased cost and difficulty in setup and configuration, making it important to only apply the methods that are necessary to achieve stable production. The methods discussed here are:
- Measurement and Alignment – the program is not changed, instead the loaded part is measured and the work origin point is shifted to a best fit location.
- Cycle input – The part is measured in cycle and the measurement result is used to update a machining cycle. For example, changing the start point of a roughing cycle.
- NC Program update – The part is measured and the measurement data is used by CAM software to regenerate a part model and NC program for machining.
Measurement and Alignment
Verification of part loading and alignment of coordinate systems to loaded components can reduce or eliminate setting errors, greatly improving process repeatability. This does not require any change to the NC program that produces the component, only the addition of an alignment program before machining to reposition the work coordinate system.
Figure 2: Alignment of work offset to part.
Touch trigger probe systems for machine tools are available from a range of suppliers including Renishaw, Blum and Hexagon and are becoming more commonly used. Probe tools can be sourced with standard tool holders (E.G. BT, HSK and CAPTO) and loaded to the machine spindle or tool changer like any other tool. The probe is triggered when it makes contact, sending a signal to the machine control. When a probing system is installed on a machine it will usually include a set of pre-programmed cycles that allows simple programming of standard measurement operations with results stored in machine control variables. These allow coordinate systems to be set using the measurement results, either by setting the offset in the probe cycle or by writing the measurement results directly to machine offset tables. If more complex alignment is required, custom macros can be written to perform calculations on a set of measurements and output a coordinate system offset or rotation command.
- Probe hardware and integration
- Programming time and CAM integration
- Cycle time increase to include probing operations
- Established proven technology
- Simple setup and configuration
- Confirmation of part locatio
- Update of work offsets
- Update of tooling offsets
For certain geometries or more variable stock materials, the preconfigured probing cycles may not be sufficient to fully align the part and achieve the required tolerances. It is possible to use more advanced alignment software using the data connections available on machine tool controllers to send the probing data to an external PC. Examples include MSP NC-PerfectPart and Autodesk PowerInspect. With MSP NC-PerfectPart reports can be generated and displayed showing the operator how far out of alignment the component is and determines whether a conforming part can be achieved within the current setup. If the part can be aligned to produce a conforming component, then the offset will be written into the machine allowing the program to continue.
- Probe integration and programming costs
- External PC with connection to machine
- Software license costs
- Initial connection setup and machine configuration
- Removal of manual alignment processes
- Potential for improved yield
- Verification of stock material compliance - avoid machining out of specification material
- Statistical data on stock and loading accuracy
In an environment where production is constrained by machine capacity, it is essential to maximise productive time on machine tools. Alignment may be essential in ensuring a good part is produced but the alignment routines may take up valuable machine time. In these cases, it may be beneficial to use a pre-alignment method where a palletised fixturing system is combined with a CMM alignment. Palletised fixturing systems such as those available from Erowa, Schunk, or included as part of the machine bed enable quick swapping of pre-loaded fixtures with precise repeatable locations.
The part must be loaded to a fixture with a suitable pallet system. Both the measurement system and the machine need to have the same pallet system. This provides a common datum coordinate system allowing the fixture and loaded part to be transferred between the CMM and machine tool without losing the datum reference.
The pre-alignment process has the following steps:
- Part is loaded to a palletised fixture.
- Fixture is loaded to the measuring system.
- Measurement cycle is run establishing the exact position of the component relative to the pallet system datum.
- Position data is saved for transfer to the machine.
- Part is loaded to the machine.
- Position data is loaded and used to update the machine work offset.
- Part is machined.
The pallet coordinates on both the measurement device and machine tool must be precisely calibrated to ensure they are in the same place. Any difference in pallet coordinate location will result in a corresponding error in offset position.
This alignment method presents several challenges in the storage, transfer and application of offset data. The method used will depend on:
- Capability of the measurement system to output results to a suitable file type.
- Ability of the machine controller to read and apply the offset data.
- Availability and reliability of the shop floor network.
Implementation may require support from the machine tool manufacturer or use of automation software to handle communications and data transfer.
- Palletised fixturing system
- CMM / Measurement system
- Network / data transfer setup and configuration
- Precise alignment from capable measurement device
- Reduced cycle times
Machining program editing
Some components, for example forged or extruded components, may be accurate in most dimensions but have large variability in length. A program to machine the excess material must account for the maximum possible amount of remaining material and the maximum allowable cut depth of the tool which could result in many passes. Parts closer to the minimum material limit will result in more air cuts and lost time.
The use of on-machine probing can be taken a step further by determining the amount of material to remove and then repeat a rouging operation a number of times to remove all material without overloading the tool. The end of the part can be probed to determine the length, the measured value can then be used as the start Z value for a rough turning cycle:
Figure 3: Use of measurement result in rough turning cycle.
For more complex applications, this can be achieved by placing a machining operation within a loop and incrementally shifting the WCS for the required number of passes.
Part specific NC program update
Where there is significant variation in part geometry it may be necessary to modify the NC program to match the specific part loaded to the machine. For example to follow complex formed features where the surface geometry may vary from part to part. In these cases it is possible to use measurement data to update a model of the component. The model is used to generate a component specific tool path that conforms to the unique geometry. The measurement may be in-cycle using an on-machine probe, or externally on a CMM, optical scanner, or other measurement device. The below example shows how a surface driven parametrically through a series of points can be updated with measurement data. A program specific to the part geometry is automatically generated from the updated model. In this case the adaptive surface ensures a consistent chamfer depth regardless of the changing curvature of the component.
Figure 4: Adaptation of program to measured surface points.
This method is complex to implement requiring integration of CAD, CAM, measurement systems and machine tools. It adds complexity to the programming and running of an operation.
The adaptive machining process will be application specific but typically involves the following steps:
- Part is measured to get required feature data
- Measurement results are transferred to a CAD software and a model is updated
- Updated model is transferred to CAM and a new NC program is generated
- Part specific component is transferred to the machine tool and part is machined
The first challenge in this process is creating a stable parametric CAD model that can be updated with measurement values conforming to the design intent across the range of possible dimensional updates. The modelling methods used will depend on the features being machined and the range of expected variation, but can be complex and may take several revisions of the model structure to achieve. The number and location of measurements and corresponding control points in CAD need to be carefully planned to ensure the adapted model accurately matches the measured component, while balancing measurement cycle time and accuracy.
The CAM system must be capable of remapping toolpaths to the updated model. Surface references can be lost when models are transferred between systems so integrated CAD/CAM software is recommended. The toolpath generation must be stable and produce acceptable toolpaths across the range of variation expected for the component being machined. Ideally external verification software should be included in the process to ensure the program is safe to run, for example a Vericut simulation.
The CAD and CAM software must be capable of automation either through macros, journals, or an API for custom applications. An external application will likely be required to handle the process, identifying measurement data, calling the CAD and CAM update processes, and transferring the programs to be run. This software can be expensive to buy and will take time to configure. Companies including TTL and Autodesk offer development of bespoke application specific adaptive machining systems.
- Machine and measurement integration
- CAD/CAM automation programming
- CAD/CAM software licenses
- Increased part programming time
- May be the only way to achieve a conforming component
- Enables removal of manual polishing/blending operations
- Can improve yield and process capability on complex parts
Basic alignment is becoming relatively routine but it still adds to cycle times. More advanced adaptive methods can add significant complexity to a production process with associated cycle time, software license, development and maintenance costs. Before investing in adaptive machining, the following should be considered:
Can the source of variation be removed?
- Is the stock material in a suitable condition?
- Is the machine in a suitable condition?
- Is the part fixturing sufficiently secure for the machining operation?
- Are part location features clearly defined and well seated on the fixture / machine?
- Are location features created in preceding operations produced by capable and repeatable processes?
If the source of variation cannot be removed:
- Are the component datum features well defined with correct geometric tolerancing?
- Are the tolerances appropriate for the component?
- Can the machining strategy be altered to accommodate the variation?
Collaborative robots with force and positional feedback may provide an alternative to complex adaptive machining methods. The integrated feedback enables programs or motions to be repeated until the robot achieves the programmed path with the correct level of pressure allowing them to be used for polishing operations or other operations where the number of repeat passes of a tool may vary depending on material supply. Some cobot manufacturers have included this specific functionality in their programming interfaces enabling quick programming of adaptive paths.