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The challenge:
Design, mill and document a project on a large CNC mill.
Provide design files, specify tooling and materials used.

Plan of attack:
This weeks assignment doesn’t tie in with the direction my final project is currently taking. So I used it as an oppertunity to use up some of the scrap material around the lab and turn it into something useful. A table for our two MDX-40As.

Approach:
Use avaliable materials to create a table for our two wax cutting CNC mills. Adding wax catchment draws to the table to reduce the mess and make it easier to collect wax dust for remelting. I ended up doing this over a number of weeks between working on other projects.

Over the course of many jobs the shopbots sacurifical bed gets worn down. So every now and then we rotate and resurface it. This spreads the wear around evenly and increase its longervity. Flattening the entire bed with a one of our standard cutters would take a very long time so instead we use the re-surfacing bit which can clear 31mm of material each pass.

Our shopbot is setup with proxmity switches which home the X and Y axis and a Z-zero plate for setting the material height.
Before running the Z-zero script its important to check that the electrical connection between the sensor pad and the bit is working and clean, as dust build up could cause the connection to fail.


Input 1 on the control pannel should light up if there is a good connection.

Run the Z-zero plate script by selecting it from the cuts menu or using the keyboard shortcut C2.

Ensure that the Z-zero plate is flat against the bed, directly under the bit and press OK. Wait for the bit to raise and lower onto the plate twice then put the plate away when prompted to by the script.

Return the milling head to the X and Y zero points using the machine proxy switch command c3 or by manually homing the axis.

The shopbot can generate the toolpath for resurfacing onces its feed the bed size. The script outputs some g-code that looks like this. Interestingly it doesn’t move the bit to a safe height above the work surface before spinning up.

Turn on the extraction, put on ear and eye protection, enable the spindle interlock. Now we’re ready to cut.

The toolpath follows the edge of the bed, cutting ever smaller rectangles.

The whole process takes about 30 minutes and requires constant supervision.

Once complete the scrifical bed will be nice and flat, ready for the next lot of jobs.

Always disengage the spindle when not actively running a cut file.

After two years of dedication and tireless hard work the dust collector hood on our shopbot has died. Cracking along the fold between the spindle and mount arm. This provided a great oppertunity for me to have a play with the vacum former, something I’d been hoping to do for a while.

Removing all the screws highlights that a few of the parts aren’t going to be reuseable. The rivits will need to be drilled out and the blind nuts aren’t reusable so a new soltuion for holding the screws is required.

I modeled up a new mount in CAD, using the existing mounting system but adding slightly more room for the chuck to try and prevent melting of the plastic. I also added extra struts down each side to provide additional regidity to the front section of the foot.

The new foot was milled using blue foam, which machines very easily but also makes a massive mess. It also cant be collected using our main dust extraction system as it would contaminate our wormfarm so instead we use a dedicated vacume for non compostable materials.

This foam mold was milled using a two fluet 6mm square end down cutter. The foam is really light and easy to machine so a roughing pass can be really deep, if required at all.
I used a single 28mm roughing pass and then ran the fininshing pass at 120mm/s with a 1mm step over. Super speedy and the whole thing took just over 20 min.
Then it was time for the really fun part, using the vacum former to create a plastic shell over the mold that will become the new shopbot hood.

And cut out the excess using the bandsaw.

The old hood, the new hood and the mold all side by side.

The new hood attached on the head. I used nuts with spring lock washers and lock tight nuts to secure eveything. Nylock nuts would also be a nice addition.

Update:
The foam mold I made this week was also used in week 13 to make my composite form.

Toolpath Generation

The Shopbot toolpath software, partworks, takes a vector file and defines how it should be cut out. It supports the importing of layers which makes it much easier to setup files without making mistakes. My file was set out with each type of cut was on its own layer. Meaning all that was required was to select eveything on each layer in turn and apply the desired cut type and depth. For more complex files this can become quite repeaditive but overall the process is pretty painless.

Machine Settings:
Feedrate: 80mm/s
Spindle RPM: 12000
Pass Depth: 4.12mm Bit size: 6mm

Run the Cut

With the toolpath preped and ready to go its time to load the material. The orign setup is exactly the same as it was for resurfacing.

The first set is to run the screw pass to hold the sheet in place. The screw pass only drills down 2mm to indicat where I should screw. Doing it this way ensures that I know exactly where the screws on my bed are and can avoid accdently hitting them on subsiquent passes.

The bottem sheet of my table is cut out using a 6mm flat nosed down cutter.

The table is designed to sit on a recycled steel frame, which was throughly measured when table was being drawn in cad. All the same there was the possiablility that it might be out so I tested the fit before cutting the rest of the parts.

The rest of the cuts were all fairly tight on the material so instead of screwing on the waste side the screws were stragiticly placed on the material where they would be used during assembly.

The ply was quite soft and flacked and chipped in places despite my use of a down cutter for these partial cuts.

It also didnt quite cut through in some places. Most likely due to a slight bow in the timber.

To make the peices fit on a the sheet some of the cut lines were doubled up and the peices tabbed to each other.

When chisling the tabs off its important to use a sharp chisle and take your time. Rushing can lead to blow outs. Luckly with a bit of pva and a clamp blow outs can be fixed! But best to avoid them in the first place.

Test fitting the deviders.

The top is the most visable piece of the design so extra care was taken with its finish. Both down and up cutters were used to prevent blow out.

Again the tolerance was really tight with only 10mm spare across the width of the material.

A V-cutter was used to chamfer some of the internal details.

With all the parts cut it was time to test the assembly.


And finally put all the stuff back on.

With the collection draws installed all the wax is easily contained for melting down.

Tools used this week:
Hardware:
Shopbot CNC Mill
6mm down cut
6mm up cut

Software:
Partworks
Rhino 5

Materials:
18mm Ply various grades

Files Produced
Archived files
Tutorials links or references used

Good links from last year:
Robert Garita from Fab Lab Barcelona
Isaac Robles from Fab Lab ESAN