Week 7 - Computer-controlled machining
Assignment
Make something Big
For this exercise I decided to make an auxiliary cabinet with wheels for my lab. The auxiliary cabinet serves to accommodate the electronic equipment, so they can be used anywhere in the laboratory and not be anchored to one place.
Designing the auxiliary cabinet
Fitting tests
For the object that I will design I have chosen two different joints I but i had to discard the number 1 because we will use OSB for the assigment. I don't think joint number 1 will resists enough. I need a functional cabinet
If we use joints in a design to cut with a CNC machine it is necessary to bear in mind that not all materials respond equally, we also have to look for a balance between an easy fit and a narrow one. For this it is necessary to make fit tests and evaluate, in this last point it will be the experience that guides us to decide the right "Clearance" (space between male a female part of of joint)
For this part of the work I have to give the credits to my lab partner Gregoire Durens, he did a great job with grasshoper and then taking it to the shopbot to finally get the results we all wanted
A 0 mm or 0,25 mm clearance gap seems nice for my design. I will apply this clearance gap to my design. Thanks to Greg I can go directly to the design work.
Design
I used the MicroStation design program to design and locate the different parts within the board. To design this object I have taken into account the material that will provide for the exercise and the requirements of the auxiliary cabinet (working height, storage and size of electronic equipment that will host.
Before applying joints I had to make sure that the different pieces fit into the board
The design is ready! now I have to export to dxf to fit the different pieces inside the board and to make chamfes
After making the chamfers I have drawn a few points to make the drills. I have separated the different work by levels.
Basics before milling
Workflow.The workflow for CNC milling is similar to Milling PCB, just bigger!.
Make Design / create setup and toolpath / Generate Gcode / Milling
Feed, speed and flutes.
Feed rate: When milling or drilling, or creating a tool path for a CNC machine the feed rate must be determined. Materials have rated surface speeds for a given type of cutter. The harder the material the slower the speed. Given the diameter of the tool and the surface speed, the RPMs of the spindle can be calculated. Then if the tooth load for the cutter is know, and the number of teeth, the feed rate can be determined.
Speed: Most spindles (the term for the router attached to your cnc router) will go from about 7,000rpm to 18,000rpm. This speed is termed ‘spindle speed’ and is directly related to the feed rate or surface speed, which most machines are capable of doing up to about 200ipm
Flutes: Flutes are the sharp slots that corkscrew upwards along the length of a milling bit. They are responsible for doing the cutting work when the bit is spinning.
Bits come with 2, 3, or 4 flutes / A bit with more flutes has more cutting edges and consequently cuts faster and smoother than a bit with fewer flutes / The more flutes a bit has, the thinner these flutes are and the more likely they are to get clogged with material when cutting
There are 4 main types of flute patterns for router bits, plus many types of specialty bits.
ChipLoad x CutterDiameter x NumberOfFlutes x SpindleSpeed = FeedRate
Where chipload is the amount of material cut per tooth (feed per tooth). Feed rate is the surface speed of the cutting tool in inches per min, spindle speed is the rotational speed of the cutting tool in revolutions per min, number of flutes and cutter diameter are determined by your tool.
Ther's alot of online calculator for chipload, Fedd rate,and milling speed
Also you can find vendors values like Shopbot
I think the best thing to do in a first time is asking to someone who knows about CNC milling, some one with experience, and then contrast the information.
Working with Vcarve
I've select a 6mm end mill for drilling and setup a 3mm cut depth.
After drawing the dogbones i have setup the diferents works (pocket, inside profile and out side profile. evrething it's ready to export to sbp format.
I have not been able to complete the assignment for today, because the Vcarve trial version does not allow export to bsp format. I tried to save and open the Vcarve file on the pc that is connected to the shopbot but the versions are different and can not be opened. I will have to redraw and set parameters again.
Milling process
Day 2. I've reconfigured the parameters in the version of VCarve of Fablab Valldaura. Ready to start working with the shopbot
There's a complete wiki about this machine (made by Fab Lab Barcelona ), take a look!Fab Lab Bcn Wiki / Shopbot
You can also look for more information at the shopbot official page. shopbottools webpage
This machine is the PRSalpha CNC, this machine has different sizes but this one is (3.05m x 1.52m x .15m) and has a Total Movement Area of (3.10m x 1.57m x .2m). You can also see the machine brochure
Start milling / workflowTurn on the machine with the key
If the machine is ON is better to turn it off and turn it on again to reset the machine, this machine has a "reset Button". There's also a "emergency stop button"
This is what we see when we open the shopbot software.This machine has proximity switches, setting the x/y 0 is easy. there's a command called "set the x/y 0 with proximity switches". it's good to use the machine 0, if a job is interrupted for whatever reason it's possible to restart it without loosing the precision.
Before setting the x,y and z axis we need to change the milling tool (if is not already placed) Click on the yellow button and the KeyPad window will appear.
Now we can move the bridge and the router close to the border in order to change the milling tool. Be carefuul the tools are very sharp
Now it's time to set the Z axis
To set the x/y 0 there's a command called "set the x/y 0 with proximity switches". In a CNC machine you can set the machine coordinates or user coordinates, it's good to use the machine coordinates, if a job is interrupted for whatever reason it's possible to restart it without loosing the precision. You can also set the x y potion manually moving the axis with the Keypad and setting the position in the Position window
Load the .sbp file generetad in vCarve and press "CUT PART"
I didn't make enough drills and the board has not been sufficiently attached to the cutting table. or maybe the board thickness may be nonuniform. I sent another job changing the number of passes by one and changing de thickness of the board by 17mm
Work completed, now it's time for the assembling work.
Assembling process
Day 3. The Dremmel it's a fantastic tool!.
The joints are perfect, every thing it's going ok.
I would have liked to use other types of joints but board material is not good enough to use these connections. I wanted a functional furniture and here it is!.