Week 7: Computer controlled cutting

This week I designed a planter box that can be made with a cnc router.
I also worked with other students in my lab to produce a test piece to determine the tolerances of our lab’s machine. here.

Group work

In the group work assignment I learned about the tolerance of the cnc mill. I had assumed that the tolerance would be tighter than 1/32 of an inch, which is what we measured as a group, so that was a bit of a surprise. I had previously used a shopbot cnc router with Vcarve, so the tools and process were already familiar to me. Another thing I did learn was the use of a vibrating blade power tool to cut through the tabs. This was much more efficient than using a chisel which is how I had done it in the past.

Moss planter box

Fusion file (download)

Pencil

I started off by making a sketch of my design on paper. I colored the different pieces with different colors. The idea of this was to make a small planter box for growing moss in my dorm room, so that I could more easily test water and light level requirements for my final project.

Fusion 360

Next I started modeling it in Fusion. First I made parameters for the design. Among these were the thickness of the material, the tolerance of the router, and certain dimensions of the design.

The fusion model consisted of only sketches extruded out by the material thickness. I used the include tool to align the position and dimensions of the joints.

I used the mirror tool to take advantage of symmetries in my design whenever possible. I decided not to add curves or dogbones into the sketches, as those would be easier in illustrator and Vcarve respectively.

Illustrator

[SVG download 1]
[SVG download 2]
[SVG download 3]

Next I made new sketches on all of the faces of the model and exported them as .dxf files to move to Illustrator. In illustrator I arranged the parts on 2 sheets of the size we had available in the lab. I used the direct selection tool and the vertex tool to turn some of the hard corners of the design into curves.

I wanted to try out the V-carve toolpath, so I made a leaf profile and some text, and then converted the text to vectors using the [create outlines] feature.

Vcarve

I exported the vectors as SVG files, and imported them into V-carve. I then scaled them by a factor of 1.25, because the files had been shrunken by 20% between Illustrator and V-carve for some reason. I started by selecting the leaf profile and created a shallow pocket toolpath with a low depth of cut, and an ⅛” upcut bit. Next, I selected all of the text and the happy face, and created a 90 degree v-carve toolpath. I then selected all of the profiles of areas that would be cut out of the wood, and then all the profiles of the edges of pieces being cut out, and made two separate profile toolpaths. The interior cuts I set to Inside, and the exterior pieces I set to outside. I added 3-dimensional tabs to the sides of the larger pieces. I like the 3 dimensional tabs because they are a much smoother motion for the CNC to do than the rectangular tabs. Vcarve has default settings for the feed rates and speeds for different types of bits. When I selected a ¼” toolpath for example, the program automatically set the toolpath to do 2 passes, ¼” cut depth for each path. It is generally recommended to have a cut depth not more than the width of the bit you are using. It’s good to use lower cut depths and lower speeds for harder materials.

I used the preview tool to make sure the toolaths were doing what I wanted them to do.

Cutting

Once I was satisfied with the toolpaths for all 3 sheets, I put them on a USB and transferred them over to the CNC milling machine. Our lab has a 24 x 48 AXIOM mill, which is what I used to cut all of the parts for this project. This job involved 2 tool changes, and 5 z-leveling operations. It is important to zero the Z-axis after changing the bit, or swapping in for a new material.

Changing the bit involves two wrenches. One to hold the collet, and one to hold the fixture it is attached to. When unscrewing the collet, you want the bottom wrench to be to the right of the top wrench, so that you can pull the two wrenches together with one hand. This is an easy way to loosen it without pinching any part of your hand between the wrenches when they suddenly move. Once it has been loosened enough to slide the bit out, you can then put the new bit in. Make sure to slide the bit in far enough that the collet can grip it well, and the bit won’t be under too much stress. Tighten it thoroughly so that the bit won’t slide out.

Next you need to zero the z-axis to the material you are cutting. This process can vary slightly depending on the machine, but with my machine it is done by plugging in a metal puck into the gantry, and hitting a “tool set” button to have it lower down until the bit touches the puck and completes a circuit.

Fit

The wood boards were held in place by drilling wood screws into the corners. The entire planter was cut out with 8 separate toolpaths. Once it was done, I cut all the tabs with a hand saw, and then sanded the edges. I dry fit it together and it stays together well without glue. I plan to come back to it later to weather seal it, and glue it together with wood glue.