Week 7, Computer-Controlled Machining¶

Assignment¶

Group Assignment¶

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

Individual Assignment¶

Make (design+mill+assemble) something big

Checklist¶

from Nueval

Linked to the group assignment page
Documented how you designed your object (something big)
Documented how you made your CAM-toolpath
Documented how you made something BIG (setting up the machine, using fixing, testing joint, adjusting feeds and speeds)
Described problems and how you fixed them
Included your design files and ‘hero shot’ of your final product

Group work¶

For group work, we split possible tasks. My work was to look at fit of joints and see the effects of kerfs. See my comment on our group page.

While detailed on the group page, I had some very specific lessons on cutting with the CNC. Most notably, my piece failed to cut all the way through. I was left with a very thin onion skin holding the cut piece. It was nearly cut through, but still was slightly attached. The underlying reason was that the bit was not sufficiently tightened, and the bit moved slightly up. I had to use a sharp knife to free the piece. With a little cleaning and sanding, the piece was sufficiently cut well enough to test the effects of the kerf clearance.

My piece for this gave me knowledge on running the CNC, and what is needed for a good cut. Safety is paramount, and several steps need to be done in order. Though I had a slight fault in my cut, I never felt at risk with this powerful machine.

My biggest personal lesson was to ensure the bit is sufficiently tightened. In our safety training we were told to not overtighten the bit, but I erred too low. I developed better muscle memory on the amount of tightening needed. In my individual work (below), I had no issue with the bits being loose.

Safety¶

As a group, we had safety training from our instructor, Andri. This is absolutely necessary for anyone that wants to use the CNC in the lab.

On the group page for this week, details are given. While there are detailed considerations, it is important to realize this is a powerful machine, and that safety is paramount. One needs to ensure that body parts are not near the moving machine, and that safety protection is needed. The machine can not be left alone while running, and there could be a fire hazard.

Specific details on all the safety aspects are detailed on our group page.

Individual work¶

Design¶

For a large project, I am making a relatively simple stool. Never having made a larger project, particularly one reliant on interlocking parts. I gained experience making finger joints and interlocking parts for my group work (see link above.)

Starting from a simple sketch, I drew a stool with three pieces. There are four legs, which will come from two flat pieces. The legs are upside down Ys, with slots to interlock. When interlocking, this will result in a cross. The cross will fit into a cutout in the circular top.

Working in Fusion, the design will be fully parametric. Among other things, I will be using stock at the FabLab, and I do not know the thickness of what I will be using. Also, by making a parametric design, the height and top diameter can easily be changed.

The parameters include:

seat_diameter: Diameter of the seat
ply: Thickness of the stock
height: Height of the stool
leg_inset: The distance from the edge of the seat to the top of the leg
overlap: Overlap for the top of the leg pieces, which will form a cross
foot_width: Width of the leg (at the bottom)
cross: Length of the leg top to make the cross
offset: Factor to add an offset for the joints (use depends on observations from group work)

To start, I drew a circle of seat_diameter. (Via the ‘Create/Create Sketch’ command)

Then the circle was extruded by ply. (With the Extrude (E) command)

Then I made a sketch for one leg. As one leg, after making the design, this can be mirrored to make a symmetric pair of legs. The sketch is perpendicular to the top, and relies upon several parameters (height, leg_inset, overlap, foot_width, and cross).

Prior to mirroring, the leg drawing was extruded. This is done symmetrically for 1/2 ply. (The top is made not visible for this work.)

Then this single leg was mirrored across the axis, by selecting the leg body and using ‘Create/Mirror’ command. This creates a new body with two legs.

The other two legs are exactly the same, so using the Copy command, a copy is created and rotated 90 degrees around the central axis.

The copied and rotated legs viewed from the top

Now, these two legs are overlapping with the top, and slots need to be made for the leg. This done by first making a new sketch from the top. Using the Project Sketch command (P), choose the top of the legs (with the stool top hidden). Selecting the overlap, create a new sketch. This sketch is extruded (cut) into the top. The removes the overlapping portion of the top.

The top after cutting out the leg overlap.

The two legs pieces also overlap. A similar process needs to be done to cut out slots. For this, make a projected sketch on one leg piece. Then, draw a sketch which goes down half the distance of the overlap. The width of the sketch is ply, and is centered on the middle axis. This sketch is extruded (cutting the leg body). The process is also repeated for the perpendicular leg, though this slot goes from the bottom and is halfway towards the top.

The entire design steps can be seen via replay in Fusion

Video of design process

Then, to make the cut lines for the CNC, the pieces can be laid flat. To do this, make a new component, and add a flat piece to represent a piece of plywood. This is by making a rectangle and extruding it. Also make a copy of the chair (with the copy command) and paste it into this need component. Move the new copy to offset it from the original.

Stool copy, ready to make flat on plywood

For each body of the stool (top and two leg pieces), use the Joint (J) command to join the piece with the plywood. Hit J, and then choose the two faces to join. Do this for all three pieces. When first joined, they are not aligned optimally.

Using the move command, arrange the pieces efficiently on the plywood. This arrangement can be used to make a new projected sketch. Hit P, and select the three stool pieces. Those will make a new sketch. This sketch can be exported as a DXF, which can be imported into VCarve for work on the Shopbot.

Note, because all the work is parametric, the parameters can be changed and then the appropriate model can be exported. This is most notably true for the thickness of the stock.

Fabrication¶

With a completed design, it was time to move to fabrication. At the time of design, I did not know the thickness of the wood I would be using for the project. For that, I found a piece of 12mm plywood in the shop scrap pile which would be sufficiently large for the stool pieces.

I went back to Fusion, and used 12mm as the thickness in my parametric design. I exported the DXF for the pieces as noted above.

Those DXF files can be read into VCarve, where further preparation is needed.

First, dog bones were added in VCarve to places where the pieces come together. This includes the top of the slots, the cross on the table top, and also the tabs at the tops of the legs.

In the screen shot the pieces are laid out on a template, which is a bit smaller than the piece of wood to be added.

From this, cut paths were determined in VCarve. The bits to be used are 6mm downcut and 6mm upcut. Two tool paths are needed. To start, it will be with the downcut bit, going down 3mm. This helps to lessen splintering at the top. This is made as an outside cut against the path. This is down in a single pass. For the remaining cuts, the upcut bit is used. This is set to start at 3mm, and does 4 passed of 3mm. For the path, tabs were added, which hold the piece after being cut.

To tell the difference between the upcut and downcut bit, pinch the cut slots with one hand, and then rotate it clockwise with the other. If the rotatation makes your pinched fingers go up, this is the upcut bit, and for down that is the downcut bit.

The paths were exported (Gcode for the Shopbot). These are exported as two separate paths. The first one is for the top 3mm down cut. To make it obvious for the first cut, the file name starts with “1”. The second upcut has name starting with “2”.

To cut on the shopbot, first the rotor needs to be warmed up. For that, we have a ~10min routine that spins the router faster and faster until it is ready to cut. Then, the wood is attached to the spoilboard. For this, 6 screws were put on each of the long sides of the plywood. These are outside the cut paths near the corners and edges of the plywood.

The X.Y. zero point needs to be set. With the bit inserted, the router bit is moved to the corner of the plywood. It is slight in from the corner. With that in position, use the Shopbot software to set 0 for the X and Y axes. Then, the zero height needs to be set. The Z zero height can be either from the top of the board, or from the top of the spoilboard. Since we are cutting all the way through in this cut, we set the Z zero to the top of the spoilboard (as that is defined to flat). Even more, it is best to set at the places that spoilboard is screwed down. To set the Z zero, move the bit to be over the position for zeroing. Then, place the aluminum tool under, and hit the Z zero protocol in the Shopbot software. The bit moves down 2x, which then sets the Z zero height. When zeroing, the rotor is fully disconnected, so there is no possiblity of starting it.

The physical start button (green). Panic button is the big red one, though this is only for emergencies. Normally, one should be able to stop (pause) in the software

The position interface. The Z icon starts the Z zero routine. The yellow button activates the keyboard control. The position of the router is seen in the X/Y/Z position

With that, the G code for the 1st cut is loaded. The machine is then activated. (The start button is physical.) With the rotor spinning, the vacuum is turned on, and then the software is started to make the cuts.

I had a problem in my first attempt. I did the steps above, and then the rotor attempted to move off the end of the shopbot. It turned out that the previous user had an offset for the X,Y, and this wasn’t noted. The shopbot needed to be immediately shutdown and reset. I had to remake the G code with the correct X,Y zeroing. In the previous image you can see the option for offset, which is not being used in this screen shot. (I didn’t capture an image when having the issue.)

With the correct Gcode, the cutting started and worked as it should.

After cutting the tabs were severed with a chisel, and then the pieces were taken from the cut piece.

Finished cut, outlining the removed pieces

With the finished cut pieces, I used a hand router to put a roundover on the edges. I also sanded the pieces.

I then put the stool together. It was a tight fit, but with a rubber hammer, it all came together well.

I sat on the finished stool, and it held me. However, because I used relatively thin plywood, the wood wasn’t firm. If I was making a stool for real use, I’d have thicker stock.

DXF file for the stool: Stool DXF.