CNC

• The CNC can cut in many different materials, and create different shapes.
• the CNC either runs a program which reads instructions from g-code, or takes user input through the keyboard. controlling the CNC from the keyboard is called jogging, and this is rarely used to cut trough material.
• jogging is mostly used to verify that the g-code will run without problems and to calibrate the CNC before execution of the g-code.
• as the program runs the g-code, each line contains a command (or comment, which will be ignored by the program)
• the program may need user interactions for events such as tool-change.

Material

We have milled in many different materials and learnt what works and what does not.

Plastics

we have a stock of different plastics at CRF. Price list is found in the workshop. if you don't find what you need, you can visit | Plastmästarn i Mölndal, they sell a big assortion of plastics, this is where we buy our plastics. make sure to bring cash and ask for waste pieces

Polycarbonate PC

we mill PC more than any other material, it's very easy to mill, strong and durable when in use and has a nice finish when done. there might be a problem with using PC in water for longer time (months/years), but we haven't managed to confirm this.

Acrylic plastic "plexiglas"

this plastic is more brittle than PC, and easily chips or cracks. it's easily identified by looking at the edges where it has been cut, the surface is chiped.

Not recomended for milling. Drilling holes however usually works. use laser cutting in this material instead there is usually a stock of this plastic at CRF, mostly sheets of 3mm thickness. you can buy yourself at Plastmästarn

Nylon

very tricky plastic to mill because it's though and easily melts, so it doesnt easily come off in tiny pieces, which is favourable. With low rpm (see below) and high K, you can get sucessfull

Acetal

Acetal might be the best material in terms of milling quality and strength, because it's very hard but not tough, so the tool makes a nice controlled cut. you can expect the result to be precise and hard. not as easy to get your hands on, and doesn't come in large sheets, like PC and Acrylic does.

Metal

can be bought at | Metalservice at Hissingen but they have fairly high prices. we get most of our aluminium at the dump

Security-rules

1. You must have taken the CNC-course by Chalmers Robotics to run the CNC-mill
2. Always follow the Checklist

Specification

Brand	Optimum
Modell	BF20L
Max-X-travel	ca 480 mm
Max-Y-travel	ca 128 mm
Max-Z-travel	?
Max-RPM	ca 3000 ( vid hög växel)
Max-Jog-speed	1500[mm/minute]

some hints:

• Max travel does not determine the maximum dimension of the piece you can manufacture, you have to take into acount that the tool has a diameter which you need to subtract from max "work-area".
• It's recomended to not attempt to manufacture a piece bigger than 110x400mm in one run.
• If you need to manufacture a piece bigger than 110 in both x and y direction, you may manufacture it in two runs ( but this is difficult and time-consuming).

CNC Milling calculating-rules

RPM [Revolutions per Minute]

• RPM = M / Tooldiameter

Material	M
Polycarbonate	20000
Aluminium	10000
Nylon	7000
Steel	5000

• A typical RPM could be 3000

Cut Feed-rate [mm/minute]

Cut feed-rate is the speed which the tool will travel in the XY plane while cutting through material

• Feedrate = K * ToolNumberOfTeeth * RPM
• K is the width of each teeth-stroke in the material in mm.
• K is usally 0.05
• K is maximum of 0.2 for rough finish in soft materials
• K is maximum of 0.1 for aluminium
• K is maximum of 0.05 for steel
• K should be 0.025 for high finish (almost glossy) DON'T use a lower K!
• K defines how fine the cut should be
• A typical Feed-rate could be 300 mm/min

Plunge Feed-rate [mm/minute]

Plunge feed-rate is the speed which the tool will travel in the Z-axis while cutting through material

Små frässtål (3 - 5 mm)	Plunge feed-rate = 20% av cut feed-rate
Medelstora frässtål (5 - 8 mm)	Plunge feed-rate = 10% av cut feed-rate
Stora frässtål (8 - 14 mm)	Plunge feed-rate = 5% av cut feed-rate

• No more than 100 mm/min