What NOT to do on the CNC: DOUBLE CURVES

GROUP | ShJB

MEMBERS

| Shannon Iafrate

| Jon Bigtacion

| Ben Wilson

PROJECT | Gizmo V2 Modulation / Installation Ideation

 

Figure 1: material waste + fasteners

 

INTRODUCTION | Every machine and process has certain limitations that are directly related to the products it creates. The CNC machine is no different. Every part that is machined must be securely fastened to the bed of the machine which (due to the lack of a vacuum table) necessitates the use of screws which results in wasted material(see Figure 1.) In addition to the fasteners required, the CNC machine we have available in MakeLab has 3 axes (X,Y, Z) of movement which prevents the machining of surfaces with undercuts (see Figure 2, yellow portion is an un-millable undercut.) These limitations can lead to a restricted range of forms that have a flat bottom surface (the surface in contact with the table.) Team ShJB has rejected these limitations and seek to push their use of the 3-axis CNC machine into little known territory: DOUBLE CURVES.

 

Figure 2: cutting capabilities

 

DOUBLE CURVES | A double curve is what it sounds like, two curves. The best way to describe it is with a sketch (see Figure 3.) The first curve (top-side) of a double-curve is machined as a single curve with a flat bottom surface. There are two main problems in machining the back surface: alignment and stability/fastening. In order for the part to be machined correctly after being flipped over, it must be perfectly aligned with the original placement of the part. In addition, after cutting away waste material on the first side, there is no longer a flat surface to attach to the table or a location to place the fastening devices (see Figure 4.) This is why double curves are what NOT to do on the CNC.

 

Figure 3: double curve machining

 

 

Figure 4: double curve problems

 

CONCEPT | The current gizmo was driven by the design of the installation. The concept was to modify a space through a form that is a fluid, cohesive whole and whose motion is based upon an inverse relationship between the movement in section and the movement in plan. This movement results in a double curved shape. (see Figure 5 below.)

 

Figure 5: perspective of plan-to-section movement

 

STATUS | The issue of how to effectively construct a double curve is the main problem Team ShJB faced in the machining process. Their first attempt to create the double curve involved breaking the curve into components that could be cut separately as single curves and then pieced together to create a final double curve. The final result is crude and time consuming, as it involved numerous steps and synchronization to complete a final result that is visually fragmented. The group has observed three main problems with their current design proposal:

| The movement in plan created a form that varied in width (thinner at the points of motion) and resulted in a double curved shape.

| A 90 degree angle in plan, when rotated on its side, creates and undercut (issue addressed above)

| When scaled to full size, the shape was too tall for the router bit to reach the base of the pieces (clearance issue)

NEXT STEPS |At their critique, Professor Jim Stevens introduced the idea of a part box as a machining technique to accomplish their desired double curve form. The part box is an available setting in RhinoCAM (3d CNC toolpath software) that addresses the issues of alignment and stability/fastening. A part box is, in essence, a spaceframe around the part. It surrounds the part (creating a flat surface to attach to the table from all sides) and has strategic fastening locations to the part inside (creating stability and reducing waste and use of fasteners.) This technique is a necessary next step for the team as they continue to explore the field of double curves. Professor Jim Stevens is working on a similar project through a grant he has received to work on double curves in digital fabrication. It would be effective for these two parties to collaborate as they both explore double curves from different angles (no pun intended.)

ANALYSIS | The biggest question that comes to mind while following this groups process is intent. It comes down to this: if a machine is tailored to create certain output products, should we as designers utilize the machine to the best of its ability within these boundaries, or, should we establish these known boundaries as a launching point and attempt to explore what the machine is capable of beyond them? I think this is a critical question (while lacking of a definitive answer) that this team must face and answer as they start down a path that might not end, or might not exist.

Guest Blogger: Kyle Post

postlawrence@yahoo.com

postmosis.wordpress.com

I am a senior architecture student at Lawrence Technological University and a fellow designer in the Digital Fabrication class. Continue to follow this blog for updates on ShJB’s progress throughout the semester.


2 thoughts

  1. […] What NOT to do on the CNC: DOUBLE CURVES (via makeLab blog) by kevlarjacket on October 24, 2010 GROUP | ShJB MEMBERS | Shannon Iafrate | Jon Bigtacion | Ben Wilson PROJECT | Gizmo V2 Modulation / Installation Ideation INTRODUCTION | Every machine and process has certain limitations that are directly related to the products it creates. The CNC machine is no different. Every part that is machined must be securely … Read More […]

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