• Meier, H., J. Zhu, B. Buff, and R. Laurischkat. “CAx Process Chain for Two Robots Based Incremental Sheet Metal Forming.” Procedia CIRP 3 (2012): 37-42. Science Direct. Web. 2 Oct. 2013. <http://dx.doi.org/10.1016/j.procir.2012.07.008>.
  • Robot based incremental sheet metal forming, also called Roboforming, is a dieless incremental forming process. It is especially suitable for rapid prototyping and manufacture of small batch sizes with low costs. This paper introduces the whole process Chain of this method, including CAD, CAM and CAE. With the use of this process chain, not only the robot programs for forming experiments, but also the simulation results of formed workpieces can be obtained. Furthermore, the CAE-results are used in the CAM-process to realize the tool path planning considering both CAD-models and many real influencing factors on the forming accuracy.

Robot Based Incremental Sheet Metal Forming == Roboforming:

• “a dieless incremental forming process” (Meier, 37)
• a new rapid prototyping method to form sheet metals by means of two universal forming tools driven by two industrial robots” (Meier, 37)

Roboforming “is especially suitable for rapid prototyping and manufacture of small batch sizes with low costs.” (Meier, 37)

“CAE-results are used in the CAM-process to realize the tool path planning considering both CAD-models and many real influencing factors on the forming accuracy.”  (Meier, 37)

“the final shape of workpiece is kinematically generated by the movement of the forming tool along the lateral direction and its gradual infeed in depth direction.” (Meier, 37)

“In many incremental forming methods, workpiece-dependent dies are often used for complex workpieces with convex and concave structures in order to maintain good geometric accuracy” (Meier, 37)

“In Roboforming the supporting tool driven by the slave robot supports the part on its backside according to individual surface structures.” (Meier, 37)

“the use of an industrial robot system enables large-sized structures to be formed with lower equipment costs compared to a CNC-machine with a large working area.” (Meier, 37)

“Therefore low cost, fast production speed and high manufacture flexibility are realized in this method by entirely abandoning dies and geometry specific forming tools.” (Meier, 37)

Process Chain

Target Geometry -> CAD -> Surface Points -> CAM -> Animation -> FEM -> MBS -> Experment

CAD

Target geometry needs to have supporting structures added around its edges. (Meier, 38)
“main structure can also be rotated here to get an average distributed draw angle.” (Meier, 38)

CAM

“Since Roboforming is an innovative incremental forming process, there is no existing CAM-solution to quickly and accurately generate two synchronized tool paths according to the above mentioned forming strategies.” (Meier, 38)
Software needs to be developed to ease the process of creating toolpaths.

1. Generate tool paths for forming tool and supporting tool (Meier, 38)
1. User determines “tool radius, sheet thickness, forming strategy, and other detailed requirements”
2. “Then the tool paths representing positions and orientations of the tool center points (TCP) can be directly viewed in the software interface”
3. “Determination of relevant robot parameters”
4. “adequate programs for both robots can be generated by the CAM software.”
5. “The tool path for each robot is directly displayed in the user interface.”

Animation

“The path planning is only responsible for movements of the tool which is mounted on the robot’s effector.” (Meier, 39)
“Because of the large serial structure of the used robots, a collision with the fixture frame or other experimental equipment is possible.” (Meier, 39)
It is necessary to simulate the robot movement with the environment accurately modeled

FEM

“incremental sheet forming (ISF) minimizes the time and cost for the forming of prototypes.” (Meier, 39)

• Disadvantages (Meier, 39)
  • “reduced sheet thickness caused by the fixed boundary”
  • “increased geometrical deviation due to the subsequent deforming and the springback effect of the sheet”

“If these negative effects could be predicted, effective methods would be used to improve the forming results.” (Meier, 39)

“FEM-model is built in software LS-DYNA” (Meier, 39)
Maybe the engeering school has a License for LS-DYNA

“geometrical features cause the maximum deviations at 6 areas of high curvature” (Meier, 39)
A nice description of the problems with accuracy.

“The tool path has a continuous infeed of 1 mm per revolution in the depth direction.” (Meier, 39)
Very technical description of the tooling process

“simulation lasts about 10 hours on a personal computer” (Meier, 39)
Uh, so maybe simulation is not really a viable option before every new part that is designed.

“the geometrical deviation of the experimentally formed workpiece was taken by a white light fringe projection sensor.” (Meier, 39)
White Light Fringe Projection Sensor

“The deviations are caused mostly by the subsequent deformation of the sheet and sheet springback.” (Meier, 39-40)

“the FEM-model still needs to be further improved to raise the accuracy of geometrical prediction.” (Meier, 40)

If deviations are correctly predicted and sent back to the CAM-program, some special measures could be adopted in the path planning even without an experiment. (Meier, 40)

“infeed depth can be reduced in these areas to get smaller forming forces, which leads to reduced subsequent deformation.” (Meier, 40)

Or based on some algorithm, the toolpath can be shifted according to predicted deviations in order to achieve reduced deviations. (Meier, 40)

Or a sensor approach to be explained in chapter 6

“Compared to conventional machine tools the low stiffness of the robot’s kinematic results in a significant deviation of the planned tool path and therefore in a shape of insufficient quality”
Robot kinematics over the 6 joints causes a significant deviation of the planned tool-path.

“A multi body system (MBS) model has been built to predict and compensate the deviations caused by the robot compliance.”
They developed software to predict deviations and compensate the robot’s path accordingly.

The simulation does not account for tool-head movement due to the inexactitude of robot-kinematics, but still produces an accurate enough prediction of the forming forces. (Meier, 40)

However, the accuracy of Roboforming is more affected by the forming forces than the dynamic behaviors of robots. (Meier, 40)

Therefore “it is more important to consider the forming forces during the processing” (Meier, 40)

They used a truncated cone as the verification shape.
The tool-path is directly related to the graphs outputed. The tool-path is about 52 circles, or 18720 degrees (360 x 52)
In order to make the comparison between the experimental and theoretical results clearer, the x axis of the graph is mapped to the rotation angle around the center point; y mapped to the distance between the path point and the center point
sqrt(X^2+Y^2+Z^2) (Meier, 41)

“When the target tool path and the forming forces from the simulation are first given to the MBS-model and the adjusted tool path considering robot compliance is used as inputs for the robot system the path deviations and the oscillations of the curve are reduced in all directions. The rest of the oscillations with a small amplitude of about 0.6mm are mostly caused by the inaccuracy of the used robot itself.” (Meier, 41)

“the average deviation of the path without the compensation of the MBS-model is 1.081mm and after the compensation is 0.205mm.” (Meier, 41)

“the compensation method with the use of the MBS-model reduces the deviations at an average of 81.04%” (Meier, 41)

Conclusions

“Through the expansion of a commercial CAM-system designed for milling, additional functions have been developed to get two synchronized tool paths according to different forming strategies.”
“The tool paths are first sent to a simulation model, which can provide the animation of robot movements and ensure the experimental safety”
“The tool paths are then sent into an established FEM-model to forecast the forming results”
The tool paths are then sent back to the CAM-program which compares the results with the target geometry to adjust the tool-path.
Then a simulation of the robot is run to record and adjust the tool-path according to joint movement deviations.

“The FEM model needs to be improved in future work, in which reducing the calculation time and raising the simulation accuracy are the research focuses.”

“Additional conditions like the relief of the clamped sheet, the cutting and even the warm forming process could also be considered.”
I can experiment with plasma cutting the sheet before and after forming.

“the whole CAx process has not been applied to a large complex geometry yet”
I will be creating a large complex geometry with this method, which can fill the gap in this paper.

Definitions

Single Point Incremental Forming (SPIF): “backplates with geometry specific outlines are added to the backside of the sheet for a stiffer support.” (Meier, 38)
Two Point Incremental Forming (TPIF): “either a partial or a complete die with geometry specific structures is used on the backside of the workpiece. Although better geometric accuracy can be achieved, a longer product delivery time and higher material cost are unavoidable.” (Meier, 38)

• Roboforming:
  • Duplex Incremental Forming with Peripheral supporting tool (DPIF-P): “a supporting tool is synchronized with the forming tool moving on the boundary of the part to substitute the backing plates.” (Meier, 38)
  • Duplex Incremental Forming with Locally supporting tool (DPIF-l): “the supporting tool moves at the other side of the sheet directly opposite to the forming tool, generating a forming gap between both tools. Especially for complex geometries, through the exchange of both tools’ forming and supporting functions, convex and concave structures can be formed.” (Meier, 38)

Effector: Robot tool-head. Also called end effector.