Case study 1: Plastic jaws for a pneumatic gripper

Automated CNC machines have long been the standard equipment in the metal working industry but connecting all the different operations in a fully automated flow still requires human intervention. Companies like DoosanRobotics have developed six-axis robots that, with the help of various end-tools (grippers, suction cups, etc), can complete many complex tasks, reducing the need for human labour in repetitive tasks. However, can these robots cope with the most common operation – loading and unloading parts from the CNC mill?

What are the barriers ahead of a fully automated workshop?

For most parts the manufacturing process is divided into two steps – processing the upper and side surfaces (1) and processing the bottom surface (2). For a robot to be able to bridge the gap between the two operations, the robot needs to be equipped with a suitable tool which can handle the blank and the processed component precisely and securely (one such tool is the Schunk gripper). Usually two sets of gripper jaws are used – standard parallel jaws to hold the blank and custom made jaws that fit onto the partly processed part (for example, a cylindrical detail on one side of the part created with the first operation in the mill). 

The custom jaws are often made of aluminum blocks with the exact shape that needs to fit in cut inside. The preparation of those jaws requires investment of time and money on its own. Once manufactured, the aluminum jaws allow almost no changes to the design of the part they hold, and if changes are made, the jaw set is no longer usable. Also, in some cases, the process cannot be automated because the standard and custom jaws need to be manually swapped between operations. The modern solution to all of these problems is 3D-printing.

Figure 1 – forces acting on the jaws during use

Quick, easy, cheap

In the first article of the series we described the requirements that a metal part should fulfil in order to be replaced with a 3D printed one. In the case of the gripper jaws, all of the conditions are met:

• Gripping and moving small to medium sized parts exerts forces less than 1500 N and the plastic jaws will have no problem withstanding those. That includes cyclic loading as well – the set of jaws we currently have at the workshop have been used for more than 20 000 cycles.
• The ambient temperature in the workshop is about 25 degrees Celsius.
• We need just a few sets of jaws so no large batches are produced.

But why would we want plastic parts after all? Here are some good reasons: 

• 3D printing allows for very complex designs without manufacturing overhead. This gives us the capacity to create jaws with multiple interfaces that will be able to grip the processed part in all stages of its production. As can be seen on Figure 2, interfaces can be located on both sides of the jaws (flat interface on one side, curved interface on the other side), maximising the number of shapes that can be manipulated with the set.
• The manufacturing time for a set of jaws with dimensions of 100 х 40 х 15 mm is only 2 hours, with zero additional processing required. According to the calculator on our website, the price is 35 BGN as of March 2023.
• In case the design of the processed part is altered in any way, the plastic jaws can be easily modified in the 3D model and reprinted in just a few hours. Similarly, a new set can be created if the jaws are damaged or worn out
• The plastics are way softer than metal and the printed jaws can never scratch or damage the processed part. The part appearance is the first sign of a good (or bad) quality work

Figure 2 – interfaces on both sides of the jaws – flat (inside) and curved (outside)

The secrets of good design

Making a good tool requires careful design in various areas – manufacturing, use, reliability, maintenance. Here is what we took into account during the creation of the plastic jaws:

• To reduce manufacturing time, warehouse management complexity and downtime lost in replacing jaws between different operations and parts, we wanted to include as many interfaces as possible in the jaw set. On Figures 2 and 3 you can see the three interfaces we managed to combine – cylindrical grip for the shape after the first operation; flat surface on the inside for the blank; a second flat gripper with different size available if the outside interface is used. The combined jaw set lets us automate the whole manufacturing process for the part on Figure 3 – loading the blank, swapping orientation after the first operation and unloading the finished product after the second operation.

Figure 3 – the same jaw set can be used for multiple operations

• A very important design consideration is the calculation of cyclic and peak forces that act on the jaws. Both the geometric design and 3D printer parameters are based on that calculation. On Figure 1 you can see the main forces acting on the jaws – vertical forces that appear during lifting (red) and horizontal forces due to gripping the part (blue). Taking into account the force calculations and following the best design practices, we developed the appropriate geometry. Regarding the print itself, we used 80 % infill density and the reliable PETG plastic. Also, the filament layers are parallel to the horizontal forces, where the most strength is required.
• Doosan collaborative robots can achieve repeatability of 0.1 mm. We needed a secure and precise way to mount the custom printed jaws to the robot in order to maximise its capabilities. As the 3D printing technology still cannot support such small tolerances, we used a well known trick to get the job done – metal inserts (Figure 4). A dedicated slot was left at the back of each jaw for tight tolerance (h7) inserts – they guarantee that the relative position between the robot and the jaws is always the same.

Figure 4 – precise jaws mounting thanks to the locating inserts

Conclusion

The 3D printed gripper jaws are definitely a success – we have been using them for months in our workshop with zero problems.

Next week’s topic is related to a seemingly insignificant activity – cleaning the mill table. Even though many operators overlook that part of the job, the presence of metal chips restricts automation and may cause misalignments and scraped parts. We have a simple yet effective solution to offer. 

See you next week!