CNC Joints

CNC Joints: Ashley Byars, Ryan Huber, Brett Rottinghaus


Overview:

Digital Prototypes: Main Designs: 5

Total Iterations: 20 CNC Prototypes: MDF: 1 Oak: 5

Total Working Prototypes: 3

Total CNC Time: 22 hrs Total File Prep Time: 28 hrs Finishing Time: 4 hrs

Total Time: 54 hrs


Main Design Concept:

We began our CNC joint prototypes study with three main designs and two supplementary designs. Each of these designs explores two main concepts: 1) the use of a peg and hole joint connection and 2) the ability for multiple connections using one joint.

3 CNC Joint Prototypes:

The first joint features a basic peg and hole joint system in combination with an advanced, curvilinear gear-like connector. Again, the pegs and the gear are radiating around a central point allowing the joint to accommodate materials coming together every 22.5 degrees.

The second joint features the basic peg and hole joint system. The pegs are radiating around a central peg-hole joint. This allows for the joint to adapt and allow two materials to be joined at many different angles at a spacing of every 22.5 degrees.

The third joint features an elongated peg and hole joint system. Again, the pegs are radiating around a central point. However, this joint only allows materials to come together every 45 degrees. The reasoning for this is that the pegs and holes are larger, occurring every 22.5 degrees around a central point, and alternate every other one being the male and female ends.

Tooling Specifications:

For all joints created, we used four main RhinoCam milling operations:

1. Profiling: 1/8” diameter-straight flute bit, various cut-depths of .09-.125”, Cut-feed Rate of 20, and Spindle-speed 14000 RPM.

2. Pocketing: 1/8” diameter-straight flute bit, various cut-depths of .09-.125”, Cut-feed Rate of 20, and Spindle-speed 14000 RPM.

3. Engraving: 30-degree etching bit, various cut-depths of .01-.03”, Cut-feed Rate of 20, and Spindle-speed 14000 RPM.

4. V-Carving: 30-degree etching bit, various cut-depths of .01-.03”, Cut-feed Rate of 20, and Spindle-speed 14000 RPM.

Joint One - A CAD/CAM Process:

To start out testing the tolerance of the CNC, we first did a quick prototype out of MDF. We learned several key things. First, we have allow for the tolerance between hole and peg of more than 1/64” and also the tolerance of .001” between the x and y axis on the mill. Also, MDF is too soft of a material to prototype our joint out of. When creating the pegs, the force of the CNC rips them right off the stock.
Time: 20 minutes

We made several changes for our next, and first oak, prototype of this joint. We offset the female side of the peg by a diameter of 1/32”. We also minimized our step-down cut distance from .125” to .09” in an attempt to put less pressure on the pegs during the milling process. Unfortunately, when this iteration was finished, we discovered that the joint would not “easily” fit together and come apart again. The joint would work if pounded together at great force; however, there would be no adjusting of the bit. We left the two parts separate for the sake of this exercise.
Time: 90 minutes

On our third attempt, second oak prototype, of this joint, we once again made adjustments to the tolerance of the peg and hole and the gear from 1/32” to 1/16”. We also adjusted the step-down distance back to .125”.
Time: 120 minutes

Joint Two – A CAD/CAM Process:

After our first mdf iteration of Joint One, we stepped our design back to a more straightforward peg and hole connection. The main adjustment we made when we set up this file was enlarging the female ends of the connections by 1/32” in diameter. Once the prototype was done milling, the joint worked well. While the joint is tight and holds well, we can also easily separate the joint, rotate the components, and secure the joint again. Time: 75 minutes

Joint Three – A CAD/CAM Process:

The first, oak iteration of joint three was carried out using a created tolerance of 1/32”. Since the joint was larger than both Joint One and Joint Two, we kept the tolerance of 1/32” based on the successful prototype of Joint Two. We used a shallower step-down distance, .09” instead of .125”, to help protect the pegs throughout the milling process. However, when this iteration was done, we discovered that the tolerance of 1/32” did not work in the example of a more linear peg.
Time: 75 minutes

The second iteration of Joint Three featured a tolerance of 1/16” increase in the diameter of the female component. Similar to iteration three of Joint One, we increased the cut-depth to .125” from .09”. This iteration was successful, and actually fits better than Joint One and Joint Two.
Time: 90 minutes

Findings Overview:

Throughout the process, we discovered several key considerations and findings when using the CNC to create joints:

1. Using oak on the CNC, there will always be the necessity of post-milling clean-up due to the fraying edges inherent to the wood type.

2. A tolerance of 0 is not achievable with the machine.

3. The additional tolerance required to form the joint varies depending on the formal conception of the joint in addition of the CNC.

4. Using bridging creates a flaw that must be compensated for manually, not digitally.

5. A thorough understanding of the CNC machine is needed in order to create a joint that works in exact tolerances. The slightest miscalculation of settings can dictate the precision.