Design Rules for CNC/casting

Most important is to know your cutter shape and size. Any features have to be designed in accordance to the diameter of the cutter as this will restrict how far and deep the slots between features can be.

Try and keep the cutter as big as possible as it will reduce cutting time as well as the chance of the cutter breaking or chipping.

If you are making a mould;just like the vacuum former; you will need to use draft angles even on small/short features.

Absolutely no undercuts as the CNC milling machine is unable to rotate the bed or spindle

Milling Guide from fablab

CNC design restrictions

Making a 30 x 30 x 5mm tile in Fusion 360

• See SolidCAM post for a similar setup on SolidCAM
• Using the design rules above create your tile using Fusion 360 – with the front place in CAD actually the top of your model – CNC’s use a different coordinate system
• Move to the ‘MANUFACTURE’ tab

CAM Setup

1. Click on ‘Setup’  > Ensure ‘Stock box point’ is selected in the origin box
2. Under the ‘Stock’ tab > Selected ‘Fixed box size’ Set to 50x50x10mm with ‘Centre’ selected for the other boxes ; I always select ‘Fixed size box’ as then I always know that the scale isn’t messed up and can work with the size of the raw stock much more easily
3. Under the Post-Process tab choose a suitable ‘Program Name/Number’ which includes enough details to not get confused with other versions of the file, in case you need to come back and change tool paths etc later

Milling Operations

1. Right click on Setup > New Operation > ‘2D Milling’ > ‘Face’ ; For the first operation to get down to the correct level
2. Select ‘Tool’ to create the new tool > Edit the highlighted settings which are the important ones for making this tile > click ‘OK’ and turn off coolant – OR choose from the existing tool selection > Samples > Metric – High Carbon Steel > φ3mm – flat (3mm Flat Endmill) 
3. Go to the stock selections tab > ‘Select’ box next to ‘Stock Selections’  > Click on the top surface > this picks the level you want to mill down to
4. Heights tab > Set them all to from ‘Stock Top’ apart from depth which you can set to ‘Model Top’  > These are the levels your tool will retract to after each operation and shouldn’t be too close to the stock top nor too far to waste time
5. Passes tab > This sets how much material the tool can remove with one pass > General rule is half the width of the tool for step over and 1/3 of tool diameter for ‘stepdown’
6. Linking tab can be left as is. This controls the lead in and lead out
7. Complete these steps again for the next tool operations – the only difference should be the levels and the geometry > all of the tool data and offsets remain the same as above
8. I use the ‘Ramp’ tool to cut the contour and pocket in the same operation > this just cuts out time in Fusion 360 by remove the need to go through the set up process for the operations separately
9. Check the tool path with the simulate function > Set the tool to be Shaft to remove the large tool holder blocking your view > Show stock > click play button at the base and ensure that all the material is removed > make sure that the mould left is exactly what you want to be left behind

Post Processor

Relatively simple > Click Generate and choose Grbl as the post processor > check you save it in a place that you can find it and check the file name is correct and identifiable

Here is the model with the tool path

And the .nc file

Cutting the tile on the mini CNC machine

1.  Prepare the mill, by removing any waste from previous milling operations and apply double sided tape to the block of wax that you will cut your tile on – attach the tile to the centre of the bed
2. Choose the cutter – use the one which you programmed in Fusion 360 – and install it on the machine. Two grub screws need to be tightened (one either side of the bit, as well as two for attaching it to the spindle)
3. Load the GRBLcontrol program and use this to set the X and Y coordinates in the centre of the block of wax – click set home before moving on
4. Do a test cut by raising the z home position and cut the air above the wax for about a minute just to check speeds, feeds and if anything looks amiss.
5. Setting the z height – Very similar to that of the PCB milling machine – turn on the spindle and slowly move the z height down until it just kisses the wax, then set the home and lift it up slightly so it doesn’t crash when you start your operation
6. Depending on the material override the feed to ~75% for pink foam or ~30% for wax
7. Click send
8. I stopped the milling machine at the end of the occasional operation to give it a vacuum and remove the waste material – This will improve the finish especially of harder materials as well as reduce the chance of the tool breaking.
9. Keep watching it until the operation has finished then remove from the machine bed using a pry tool
10. Vacuum the bed, ways, screw threads etc as well as the surrounding area, ensuring any tape is also remove from your part as well – leave it as you want to find it
11. Deburr you wax part using a fingernail or the end of a ruler – but don’t press too hard

Problems

Ensure that the milling cutter is tightened down before each use – it wasn’t checked in my case and it just popped off when it had nearly finished (no harm done!)

This meant that the operation had to be started again and therefore took much longer than it should have

Casting with food grade condensation cure silicone

1. Prepare a work area with some covering on the work surface as it can be quite messy
2. Wear disposable gloves as well as a lab coat to protect clothing
3. Use scale to measure out the silicone and curing agent in a coffee mug 1 to 10 ratio much easier to mix in than a disposable cup (excess can be peeled out after it has set)
4. Mix well using a plastic stirring rod and leave for 10 minutes to let the bubbles rise to the surface
5. Push into your wax mould using the stirring rod and fill up any others it you have any silicone left – try and remove any pockets of air
6. It is quite good at self levelling so leave the silicone proud of the top of the mould so there is material left to fill any voids as it settles.
7. Leave to set for at least 24hrs
8. Peel it out of the wax mould and you should have a mould to cast into

Chocolate Moulding

Moulding with chocolate is relatively easy, just take your time and don’t burn it by putting too much heat into it too quickly. I tried to use the microwave to melt it but in suck small amounts found that difficult.

The best way is to melt the chocolate in a glass bowl over boiling water. Seeing as its a small amount I only needed to boil the kettle and transfer the water to the jug and place the bowl over this (with the water level just below the level of the bowl).

The chocolate came out of the mould relatively easily, and the details were quite prominent. One issue however is the flashing on the outside of the chocolate indicating that I overfilled the mould. It is also down to the ball nose end mill not leaving a square edge at the base of the chocolate, instead leaving a flange.