Computer Controlled Machining

The task this week was to make something big. For this objetive i decided to use The shopbot CNC milling machine. A ShopBot is an do-all tool for cutting, carving, drilling or machining all kinds of things from all kinds of materials. With a ShopBot, you use the included software to design your parts on your personal computer, then, like a robot, the computer controls the cutter to precisely cut your parts. In the past, tools like ShopBots were strictly industrial tools and were referred to in factories as CNC (for Computer Numeric Control) tools. Now, the types of tools that create things by cutting material away or building up material in layers to create an object are called digital fabrication tools, and ShopBot's innovations have made them affordable for individuals and small shops

A. The shopbot CNC milling

A ShopBot is like a large plotter that moves pens around the surface (in X and Y axes) to create a drawing. Only a ShopBot moves a cutter around a big table (X and Y axes) and moves it up and down as well (Z axis) allowing it to make 3D movements and cut all sorts of shapes. The cutter looks like a drill bit and is spun by a motor called a router or spindle. Unlike a drill bit, a router bit is designed to cut from the sides as well as the tip. By precisely moving the cutter through material, a ShopBot CNC tool can create virtually any pattern or shape and will do it in materials such as wood, plastic, foam, aluminum and many composites.

B. Feeds and Speeds and Chip Load Calculator

One of the challenges in getting good CNC cuts is in selecting the best cutting speed (feed rate) and router/spindle RPM (speed of rotation). Feeds and speeds are a critical part of machining and should be fully understood before deviating from recommended settings. Selection can be facilitated using the guidance provided by manufacturers regarding the best chip load for a particular cutter. Chip load is the thickness of the chunk of material taken by a tooth of the cutter. This parameter has a relationship between the combination of how fast the cutter is moving forward into the material and how fast it is turning (Chip load = Feed Rate / [RPM x number of flutes]). This relationship shown in the formula gives you starting point speed values for testing to determine the most suitable parameters for any cutting situation. The ShopBot “Chip Load Calculator” simply provides a quick way to explore these values, and is most useful if you know the approximate desired chip load for a particular cutter and material.
The Chip Load Calculator tool helps us to choose the most suitable cutting tool , taking into account the cutting speed , material, and the number of threads that have the cutting tool.
If we select to Chip Load Help, will show a picture, which appear materials and possible diameter of the cutter.Notably, the Chip Load Calculator is only an aid, and does not determine or confirm any parameter court.

C. Design

First of all we design a prototipe of a furniture using press fit connections. This model will have many divisions and will be usefull.First of all we design a prototipe of a furniture using press fit connections. This model will have many divisions and will be usefull. Use a reference to Barcelona credenza by Southcone and designed my own version utlizando phenolic boards 15 mm.

D. Manufacturing

When designing your file, make sure to leave at least an inch around the margins of your material. We will need to screw the material down to the bed of the ShopBot and we do not want the bit on the machine to hit the screws.

Tool clearance between parts is determined by the thickness of the bit used.

Depth of a cut in any one pass is 1/2 the diameter of the bit used

I had to export the parts that I did in Autocad in DXF format. As my design was using a wood 15mm of thickness, I had to adapt it to the woods available.
The first thing I did was test with plywood phenolic of 15 mm thick, then for the tolerance test I had to make the cuts taking 15 in the design. I had to discard this possibility because they didn't have enough material for the 98 pieces, because the work was going out more than one wooden board.

The cutting procedure is as follows:
0. We respect and follow all safety procedures.
1. According to the design that we must choose the right cutter, according to the resolution of the design, and type of material.
2. We set the chosen material, with screws attached to the plate sacrifice. These screws should be on the edge of the plate, and keep in mind the position of these screws, since if the drill goes through these screws will break and will fragment violently.
3. After fixing the plate material, we use the tool to first calibrate X, and Y. Then also calibrate the Z axis using a metal blade. These options are in the Cut tab, XY Calibrate, Calibrate and Z.
4. We import furniture design at Software Partworks and select the vectors to be cut. We keep these vectors selected as .sbt files and then import them. Preferably it is saved in 2 files of court. In the first inner sections, and the second file in external cuts, so that the plate does not move.
5. Open the ShopBot3 software, and import the file and then send .sbt cut.
6. Once you compile the file and send it, a window that tells us that first we press the green start button, to start to turn the cutter, and once it has reached full speed arises, then we click on the OK button to start cutting.
7. Then send the file to cut almost immediately turn on the extractor cut waste.

Note.
If any problem arises, we press the emergency stop button.
Or else you can pause the job by clicking the STOP button, then stop automatically. To resume the job, click Resume, Then Start button, to turn the cutter, and then click OK. Failure to follow these steps in the exact order, it may break the cutter, to either inherit with the cutter motionless.

Manufacturing process:
1. Open PartWorks that is the software of the Shopbot.
2. Load the file DXF
3. Appears the tab Job Setup and here we do the following:
Select the work area that is of 2440 mm by 1200 mm.
Deselect the tab "Use origin offset"
We select the units (mm)
Click on OK
4. We select everything and we click "Join vectors", so that the curves are closed. If necessary increase the tolerance to terminate to close the curves.
5. We select everything and we click "Move, scale, rotate selection", to move the vectors to the origin of the board.
6. Then we click to "draw circle" to draw the circumferences that they will serve as a reference for place the screws. As our cutter tool is 1/8" (3.175mm)
Observation: It is important that in the design we have considered the size of the cutter, because we cannot make right angles or treble interiors, in this case, we must make circumferences of larger diameter to 1/8" (3.175mm) in these corners.

7. We click the tab "toolpaths" and it will appear several options.
8. In that menu we select the operation "create drilling toolpath" and we will access to another menu.
9. In this new menu we select all the circumferences that we did with the shift key and we click "Calculate".
10. A program will take us to a 3D view and we click the option "preview visible toolpaths", we will see a simulation of what it will do the machine. Then click on Close.
11. We select the new flunge of the model and now we go to re-select the tab "toolpaths" and then the operation "create profile toolpath" and it is going to appear the following menu:
Here we chose the depth of the cuts, the type of cuts (external or internal), the tool to use, and so forth.
Start Depth: depth of start.
Cut Depth: depth of cut.
Tool: here you will select the tool, type, size.
Machine Vectors: If you want to cut on the outside, Inside if you want to cut inside.
Directions: If you want to follow a clockwise , Conventional if you want to follow a counterclockwise.
Allowance offset: allowed area between cuts.
12. Select the parameters according to the material, already described in the previous step. It is essential if you want to make assemblages, see the cuts to the union in the assemblage have the same selection in machine vectors. We have the following options:
Pass Depth: determines the depth of the tool on each pass.
Stepover: determines how much more the tool superimposed on each pass, during the rasterised.
Spindle speed: determines the speed of turn, this is based on the material, diameter, the rate of progress of the tool and the rate of immersion.
Feed Rate: determines the rate of progress.
Plunge rate: determines the rate of immersion of the tool.
13. We selected all the curves of the parts to be cut, to our pieces we choose the cut for the outside. We click on "calculate".
14. The program is going to take us to a 3D view and we click in the option "preview visible toolpaths", we will see a simulation of what it will do the machine. Then click on Close.
15. Store the files with extension sbp.
16. Open the Shopbot program.
17. Press k to use the keyboard and moving the tool (previously the material must be bolted). Use the keyboard arrows to control the position in the X and Y, position for use Z Pg Up and Pg Dn.
18. To calibrate the Z position, we click on Cuts we chose C2 - Zero Z axis w/ ZZero Plate and use the metal plate for calibration.
19. To calibrate the posixion XY, we click on Cuts and chose Zero - XY axis.
20. We load the file drill and we click to start. To start the process of milling we press the green button.
21 We move the headstock of the Shopbot to unscrew the pieces.

Downloading link for my archives:
Archives

E. Proceed to assemble

Once the lists parts, we proceed to assemble gradually. We started putting together the central box structure , then continue with press-fit connecting the central box with side studs. To join them used a hammer and grippers , do not use any connector because all parties are part of the structure. Then continue with the assembly of the upper table , and finally leaving the doors. For doors we added some haulers, and magnets for better design and finish. We then present the furniture in various positions.

Enjoy!