Designing Mechanical System

My contribution for the project is in designing the mechanical system of the machine.

To be honest, this is quite a challenging week for me.

I was given the responsibility of designing the physical system because I have a decent grasp of 3D modeling, aka good, but not that great. This means, I should be able to handle the task easily while still being able to learn and practice my 3d modeling skills extensively out of this assignment.

The problem is, even though I am confident with my ability to do the modeling work, I have very minimal experience and understanding of mechanical objects, parts, and systems. Concepts related to kinematics objects and dynamics are areas I have never really paid attention to before. What I mean by that is, when discussing the idea of an automated cutting machine, I grasp the concept and understand the workflow, but I am unsure where to begin, which parts to design to which components, and how to approach the overall design process. Even though Eka had provided me with lots of references, everything were still blurry in my mind.

Another reason is also.. I had to travel halfway towards the end of this week’s assignment. It’s another challenge to coordinate remotely when doing machine design week 🙃

Lo-Fi Prototyping

Seeing how stuck I was, Eka then suggested us to first build the lo-fi prototype of our machine in order for us to understand better the mechanism, the positioning, and get a sense of the overall machine’s parts and dimensions.

From the lo-fi prototype it became much clearer to me of what I need to design and how, because I can see how each components are interconnected into a system and got a sense of their movement and operation. In general, there are 3 main mechanism that I have to design:

1. The feeder and advancer: mechanism to feed and advance the bamboo stick
2. The cutter holder: to firmly hold the cutting plier in place
3. The cutting mechanism: A mechanism to connect the servo arm to the cutting pliers, enabling the cutting action.

3D Designing with Onshape

I will not go into detail of the step by step process of the 3d desgn, but you can check my Week 2: 3d Modeling with Onshape for a more detail guides and navigation on how to do 3d modeling with Onshape. But, here’s some of the new things that I just found out through this assignment.

Sourcing 3d components platform:

• McMASTER-CARR: if you want to download standard hardware components like motors, screws, bolts, nuts.
• GrabCAD: provides 3d cad model collection in editable format, like step file.

There are some new Onshape features that I found out and learned through this assignment, such as:

• Construction Plane Feature: To create additional reference plane
• Hole Feature: To made drilled holes, for screws and such, automatically
• Circular Pattern Feature: To replicate a shape/object in circular fashion. I used this to make the gear teeth.

Mechanical System

1. Bamboo Stick Feeder & Advancement Mechanism

How it works

For this system, the main objective is to move the stick or wire according to the rotation of the stepper motor while ensuring it stays in place linearly without shifting. We can find such systems usually in a 3D printer filament feeder and advancer. In a nutshell, this is how the system works:

Basically, we will use a ball bearing and a 3D-printed gear attached to the stepper motor’s shaft. The stick or wire will be fed between the ball bearing and the gear teeth. Then we have to design it in such a way that there is a slight pressure from the ball bearing, allowing the gear teeth to grip the stick or wire and move it as the motor shaft rotates. We will use a spring to push the bearing down towards the gear.

Our main reference for this part is from this DIY Wire Cutter Machine using Arduino’s wire feeder and advancement mechanism. If we have to dissect the systems into parts, there will be 4 parts that we have to design:

Parts

1. Stick Feeder: to feed the stick or wire input and keeping it linear
2. Gear: to be attached to the stepper motor
3. Bearing Holder: to hold the ball bearing in place and push it down with the help of a spring
4. Motor Clamp: to secure the stepper motor in place

Design Developments

Version 1 Test

Design:

Test Result:

• Overhang print design

  

  I didn’t see it coming, because I made the design not flat on 2 sides, it results in an overhang. So I had to adjust it a bit before printing, making it flat on one side.

• The gear and the ball bearing not aligned in the centre

  

  Turns out I forgot to account for the screw heads’ clearance, resulting in gaps that makes the ball bearing holder had to be a bit outward, making it not aligned with the gear.

  

• Not enough clearance

  

  We tried to see if we can mitigate the previous issue by flipping the bearing holder upside down, but then this result in not enough clearance for the stick/wire.

• Forgot to account for connection plug

  

  Because I was not in-person at the time of the assignment, it’s difficult for me to check in reality how the components look like. So I learn that it’s really important for us when designing to also see the physical things in our hand.

Version 2 Test

Design:

I made the ceiling higher, beside that I also made the ball bearing holder suitable for 3d printing (no overhang - flat on one side), and adjusted clearance for the screw head.

Result:

The iteration works well and solve the previous errors.

However, in the end we have to do some small tweaks to make it work.

Because in the end we decided to utilize an existing metal frame that we previously used in another machine to mount the parys, the cutting pliers position end up shifted. As a result, we had to add an additional part to redirect the bamboo stick so it ends up in the correct cutting position. But this part was made by my colleague, Eka, as I was out of town at that time.

Final Result

2. CAM Lever Mechanism for Controlling Cutting Plier Arm

How it works

For this system, the main objective is to control the cutting plier arm to perform the cutting action. To do this, we’ll use a high-torque servo motor attached to a piece that rotates with the servo. As the servo rotates, this piece will rotate as well pushing the plier arm down to execute the cut.

Initially, we wanted to follow the mechanism in this wire cutting machine, which uses another ball bearing that rolls along the surface of the plier arm. However, we were concerned about the durability of the piece to withstand such high-torque force since we would be 3D printing it. In the reference machine, they use laser-cut acrylic for this part. After consulting with Rico, our instructor, he suggested to use the cam mechanism instead.

Cam Mechanism

A cam mechanism is a mechanical system used to convert rotational motion into linear motion or vice versa. It typically consists of two main components: the cam and the follower.

• Cam: is the rotating or sliding piece in the mechanism, usually shaped in a specific way to achieve the desired motion. The shape of the cam surface determines the motion of the follower.

• Follower: is the component that follows the contour of the cam, thereby translating the cam’s rotational or sliding motion into a specific linear or oscillating motion.

It turns out that cams can have various shapes, which are determined by the desired movement of the follower. The shape of the cam is referred to as ‘profile’. Here are some examples of different cam profiles:

If you want to explore more about the cam mechanism, you can visit this page.

Design Developments

With the use of cam mechanism, it means we can eliminate the use of another ball bearing. However, the challenge lies in designing the suitable cam profile.

Version 1

I started off by positioning the servo motor above the plier arm’s resting position, specifically in the middle part. Then I tried the snail shape with the centre of rotation more towards on the one end, because the further it rotates, the further it should push the plier arm, the longer the circular path it has to travel in order to be always in contact with the plier arm.

but… I think I designed it the wrong facing direction, haha ups.. it should be like this:

Iterated V1 design

The concern with this design is regarding to the positioning of the servo arm. The logic follows the law of the lever: “effort times effort arm equals load times load arm.” This means that the closer the load is to the axis, the more force is required, and vice versa. As a result, a higher force is needed, which may lead to increased power consumption of the servo.We explore this case –how the load of servo motor effect power consumption– on our week 9 group page.

Version 2

On the version 2 designs, I moved the position of the servo arm exactly above the endpoint of the plier arm (resting position), resulting in longer cam profiles. I designed 3 different profile variations

• Version 2A: the one we ended up using

  

• Version 2B:

  Because when you cut with a cutting plier, sometimes you have to give a bit of extra force in the end to really cut wire, so Rico suggested me to add a pointy end to further enforce the cutting until there’s a ‘click’ sound. But I think in thend, when we test it, the pointy end is too small, so doesn’t really help in giving extra force.

  

• Version 2 C:

  

Actuation Test

In this phase, I was not in person, because I had to travel out of town. So, I actually couldn’t witness on my own how it’s going on the ground. And apparently this what happened..

Test 1

The cam lobe fits and move as the servo rotates. Eka had to do some design customization from what I made for the vertical board that holds the servo in order to fits the metal scaffold. However, the cam lobe still couldn’t succesfully make the plier to cut our bamboo stick.

Test 2

At that time, we dont have much time for redesigning. Eka then had this idea to use a rubber band to connect the cam lobe and the plier to give extra force from the tension of the rubber. And finally the system works!

Final Result

3. Plier Holder

How it works

The objective of this system is to securely hold the cutting plier in place when doing the cutting operation. We solved this by designing a holder that can be secured from two directions with screws: the side and above. This way, the design is suitable for different size of plier arms. We also have to ensure the cutting plier’s blade angle position resulting in straight horizontal axis.

Design Developments

Dev V1

• how the bracket will attach to the holder body???
• unideal printing direction –> easy to break and will not withstand force to hold the plier

Dev V2

Correct the printing direction. Instead of using bracket to secure from the top, can be like this.. but will have to print 2 parts..

• too many screws needed

Dev V3

Made a gutter to slide the bracket and make the bracket legs longer with hook to be slide in to the gutter

• ups.. need more tolerance
• the screw pad that will be attach to the base can be made a bit bigger

Final Result

Design Files

• Cut-Craft: Stick/Wire Cutting Machine - Onshape