Designing an Automated Assembly Station - Part 1- Designing a CNC Turntable in Modular Assembly

This is part 1 of a multi-step Instructable where I go over the designing, constructing, and the operation of an automated assembly station. The idea of the assembly station is to build an integrated set of operations around a central turntable axis. This way, it's possible to automate the assembly of multiple different future projects.

The turntable will be CNC controlled, and so will the various tooling sub-assemblies. The design of this turntable is intended to be low-cost, and highly modular. It will be possible to add future sub-assemblies in the future. The reason to go with this design, over machines with more degrees of freedom will become evident. However, it's worth noting that keeping the machine to a high degree of accuracy, while not introducing unneeded features is of utmost importance.

This is a rectangular formation of extruded aluminium profiles.

I particularly prefer these, as we can build very dimensionally accurate assemblies for low-cost.

The turntable is intended to be 3D printed in Carbon-Nylon. Furthermore, I intend to use a regular ball-bearing. However, as these are laterally unstable, we're going to have to resort to clever planetary gear arrangement to stabilise this motion. This assembly borrows this concept from Christopher Helmke's YouTube video on 3+2 axis CNC, you should check it out.

In short, as the planetary gear sets act against the lateral load of the turntable's perimeter, we get an extremely stiff assembly, without having to resort to expensive, specialty bearings. This will only work however, if we don't overstrain these gears, but for our application this is more than adequate. However, if you were to use this assembly for something else, particularly in metal machining, I would advise you to purchase steel gear sets and more appropriate bearings.

The design is based on a NEMA 23 stepper motor. This motor is low-cost, favoured in the CNC-community, and easy to source. There's a reduction gearbox on the NEMA. This is the first axis of the assembly station, and will form the basis for the entire design. This axis will perform many duties. However, it should be noted that this design works best if there are circular assemblies attached to it. Meaning, if what you're assembling is circular, this design has many advantages. However, you might require more degrees of freedom otherwise.

To add more rigidity to the system, I will add plating. This also helps keep the various parts of the system apart from each other.

While it's certainly possible to add vice jaws. For this step I'm intending on something more bespoke for each application. This system will utilise a negative jig, 3D printed to index the part directly. As we don't intend to do any milling of the part once it's in the assembly system, this will work well. However, if we add milling capabilities, we'll increase the support by adding proper fixtures.

One of the most important design constraints for this system is speed. Not needing to spend time with clamping is a great time saver, so we'll go with that for as long as we can. Most of the strain on the system will be directly on the z-axis, and as such there is no need for much lateral support. However, we will use the turntable to spin constant torque springs, so some lateral support is definitely important.

The next step is to add the next sub assemblies. While this design is modular, and can be taken anywhere, there are some features that are more important for the time being.

These are:

• Heat-set insert press
• Automatic screw driver
• Hold down bridges

For the next part I will go over the design of these features.