Week 7 Assignments - Computer-Controlled Machining

Shopbot Cut of Table Stand Legs

Group Assignment

The group assignment for this week was to:

Complete your lab’s safety training
Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
Document your work to the group work page and reflect on your individual page what you learned

Outcomes

Link to Group Site

The group assignment page for this week is on the 2025 Charlotte Super Fab Lab group site for Week 07 - Computer-Controlled Machining.

What Was Learned

In the group assignment, we covered safety training and Shopbot CNC router capabilities, including runout, fixturing, speeds and feeds, materials, toolpaths, and workflows.

Training highlighted important safety considerations and provided experience with the workflows needed to use the Shopbot, both for preparing toolpaths in software used by the lab (VCarve, Shopbot), as well as for operating the CNC router (spindle movement, bit setup, material setup / fixing, axis calibration / zeroing, dust collection, job start, and safety cutoff).

Individual Assignment

The individual assignment for this week was to:

Make (design+mill+assemble) something big

Outcomes

I considered a number of possibilities for design - primarily furniture options - including tables, planters, leaning desks, and custom shelving. The two main candidates were in support of the whereabouts clock project. For best viewing, a whereabouts clock display needs an elevated position - in the direction of the face display for a grandfather clock. The "clock tower" also needs to be in a sensible location in the house and fit in that area. Since an internal mechanism (weights, pendulum) would not be needed, the options coalesced down to: a custom corner cabinet - providing shelving as well as an elevated spot for the whereabouts clock, or an elevated table stand - providing a streamlined small footprint, primarily for clock elevation.

Design Exploration - Concepts

As part of the design exploration, I considered the types of joints that might be appropriate for CNC milling. In particular, joints need to account for the There are a wide variety of resources, but the "50 Digital Wood Joints" by Jochen Gros provides a very nice resource for consideration. In addition, there is a concise visual poster

There is a particular corner location in the abode that seemed most appropriate for incorporating a sort of clock tower.

I made some measurements and explored options for both candidate designs - cabinet and stand - in Autodesk Fusion. But it was clear that the custom corner cabinet would be a much more involved - I designed the basic shape, but not the joinery. Given supply-side time constraints, I opted for an elevated table stand design to serve as the "clock tower."

Clock Tower Table Stand Design

For the Clock Tower Stand, there are a wide variety of examples and resources online for creating CNC tables, stools, and pedestals (search terms in various combinations - CNC, "Flat Pack", furniture, stool, table, design - as well as autodesk, fusion - for more tool specific). Some examples include:

For more detailed design steps with Autodesk Fusion, I used Taylor Stein's Flat-pack furniture design as a reference.

I chose to follow a table stand design with 2 legs and a circular top that would not require fasteners or glue. I began by considering and defining fundamental design parameters to be used throughout the design. These included parameters for:

Material
- board thickness (1/2 inch)
Design
- table stand top diameter (14 inch)
- table stand overall height (40 inch)
Production
- bit diameter (1/4 inch)
- clearance (1/100 inch)

I began by creating the basic top design. Primary design steps included:

Sketching the overall shape - center circle
- parameter: top diameter
Extruding the shape
- parameter: board thickness

I then created the basic leg design. This included a tenon (projecting piece of wood) for connecting to the top. Primary design steps included:

Projecting top profile for reference
Sketching the outline for the right side of the leg
- Adding construction lines for center reference and overall height
  - parameter: overall height
- Sketching lines for basic leg shape
  - parameters: overall height, panel height, foot size
- Sketching curve for the outer leg shape
- Sketching lines for the top tenon
  - tenon size / inset
Mirroring the right side outline to create the left side and complete the sketch
Extruding the shape
- parameter: board thickness

The basic leg design would be the first of the two leg pieces. To get the second leg piece, I copied the first leg design and pasted it as a new component - so that further changes to the second leg design would be independent of the first. I then rotated the second leg design to be perpendicular to the first.

In order to connect the legs together, I went on to design the connecting notches - one on the upper half of the first leg panel, and another on the lower half of the second leg panel. Primary design steps for each of the leg notches included:

Adding construction lines for center reference
Sketching the outline of the right side of the notch
- parameters: panel height, board thickness, clearance
Mirroring the right side outline to create the left side and complete the sketch
Extruding the notch shape
as a cut, with a distance to the opposing side face, in order to cut through

In order to connect the top with the legs, I went on to design the mortise opening (slot cut into the material) for connecting with the leg tenons. This consisted of first designing a partial mortise area and repeating that pattern 4 times around the center. An alternative would be to use boolean operations with the legs as tools for cutting, but this would also need to account for clearance on the leg with the top-notch. Primary design steps included:

Adding constructon lines for center reference
Sketching a side profile for one of the 4 mortise sides
- parameters: tenon size, board thickness, clearance
Mirroring the side profile to outine one of the 4 mortise sides
Creating a circular pattern with 4 repetitions to create all 4 mortise sides and complete the sketch
Extruding the mortise shape
as a cut, with a distance to the opposing side face, in order to cut through

Table Stand Top with Mortise for Leg Connection

With the overall design complete for the top and 2 leg pieces, I needed to refine the design to consider the CNC milling process. The CNC mill uses a rounded bit in order to cut out the designs, so internal corners of the design will have a rounded cut. If the joint connections are designed with sharp internal corners, the actual rounded cut that results can interfer with the fit of connecting pieces. To account for cutting with the round tool, internal corners of the design can be rounded in a "dogbone" pattern. Primary steps for each of the internal corners of the top and leg designes included:

For internal corners
- Adding a construction line at a 45 degree angle
- Sketching a circle to represent the bit size
  - parameters: bit size, clearance
- Positioning the circle on the contruction line at a distance of half the bit size
  - parameters: bit size, clearance
Extruding the corner dogbone shape with a distance to the opposing face, in order to cut through
Creating dogbone corners for one of the 4 mortise sides and using a circular pattern to create the rest
Creating dogbone corners for one side of each leg, and mirroring to create the other side

Table Stand Top, Highlighting Dogbone Corners

Table Stand CNC Milling - Toolpath Creation

With the design completed in Autodesk Fusion, it was time to start the CNC milling process. The first step was to export 2D designs for each of the main parts. There are a number of ways to export 2D designs from Fusion, and I found the 4 Ways to Export to DXF in Fusion 360 to be a helpful reference.

I exported a 2D design for each of the 3 main parts - top, leg 1, and leg 2. To export from Fusion, I used the following steps:

Activate the component for the part, and hide other components
Create a new sketch on a primary face
Project the final component design to the new sketch
Complete the sketch
Export the sketch to DXF

Projected Sketch of Top for 2D Export as DXF

It was very important at this stage that the table stand design was fully parametric. The selection of material at the time I was milling was a bit limited for the better quality plywood, which was an important consideration for genuine use as finished furniture. In the process of setting up the job for milling, I found that The largest piece of better quality plywood available - to accommodate the leg height - would not fit the target height dimension. So, it was necessary to scale the height of the design down somewhat for the available material. I made several iterations of revision and export from my computer with the design to the lab computer with the software for toolpath creation.

The primary software our lab uses for creating Shopbot toolpaths from designs is VCarve. The first step is to set up a new job, with primary parameters for one/two sided, dimensions and thickness of material, and Z-Axis reference (material surface or machine bed).

VCarve - Job Setup - Material Size, Z-Axis Reference

The next step is to create the design layout. This can be done directly using VCarve drawing tools, or by importing designs. With the material selection available in the lab, there was not a single board piece with dimensions that would cover both legs and top. I repeated the process separately for the top and for the legs, laying out the legs nested together.

VCarve - Job Setup - Design Layout - Top

VCarve - Job Setup - Design Layout - Nested Legs

The next step is to set up parameters for the type of cut (profile cut around the shapes) and tool being used (quarter inch bit). As part of the setup, I added tabs to help hold the material in place.

VCarve - Job Setup - Type of Cut, Tool Detail, Add Tabs

With the job settings in place, I generated the toolpath for the Shopbot.

Table Stand CNC Milling - Milling Process

The Charlotte Super Fab Lab has a Shopbot in a separate woodworking room.

With the toolpaths for the legs and top created, I began the milling process with the Shopbot. Protective equipment for eyes and ears is stored at the entrance. I put on my safety glasses.

The Shopbot side panel has the main power switch, keyed safety interlock, and tools for changing the bit.

There is also a button control for emergency stop, reset, and spindle control.

The last main part of the setup is the dust collection system.

The Shopbot is controlled by Shopbot software. Basic controls are for manual XYZ movement, in order to facilitate material setup and job calibration. XYZ movement can be controlled through buttons in the software interface or by using keyboard arrow keys. Holding the control key during movement enables faster movement for longer distances.

In order to get the Shopbot ready, it is necessary too select and install an appropriate bit. I removed the collet, inserted the bit (1/4" upcut), and secured the collet with the Shopbot wrenches.

I secured the plywood to the work surface, using screws in the corners of the wood, positioned outside the cutting path. With the material in place, I calibrated the axes.

Z-axis calibration uses a plate and clip. For calibration to the work surface, the plate is placed on the work surface directly under the bit, and the clip is attached to the collet. Automatic Z-Axis calibtation is started using the Shopbot control software.

Shopbot Z-Axis Calibration to Work Surface

X and Y axis calibration is set manually. I moved the spindle into suitable position over the material - making sure to allow enough space for the size of the design cut, as well as to avoid material-holding screws during cutting. The position was then set using the Shopbot control software.

With the material secured and the Shopbot set up and configured for the job, I loaded the toolpath created using VCarve. I then started the cut - activating the spindle and dust collection, and initiating the cut. The first cut was for the legs.

Shopbot Cut of Table Stand Legs - Starting

Shopbot Cut of Table Stand Legs - Ending

When the leg cut completed, I disconnected the tabs with a chisel, and removed the cut parts and remaining material. I then reset with a separate piece of material for the top, following the same process.

Shopbot Cut of Table Stand Top

When the top cut completed, I disconnected the tabs with a chisel, and removed the cut parts and remaining material. I then proceeded to postprocess the finished pieces, using a combination of rotary sanding and hand sanding.

After post-processing, I finally tested assembly. The pieces fit together very well, and the overall fit is very stable - without glue or fasteners.

Application / Design File

The application / design file for this week is:

Clock Tower Table Stand - Fusion (.f3d)