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7.Computer-Controlled Machining:

10-03-2021 | Hanani

1. Objectives:

2.Group Assignment: test runout, alignment, speeds, feeds, and toolpaths for your machine

3.Individual Assignment:

• make (design+mill+assemble) something big (~meter-scale)

4. Files, Planning, Tools:

Files:

• The DXF File: Final Cut

• Do the group Assignment
• CAD Design a metre-scale model
• Cut it using laser-cutter
• Cut it in wood
• Assemble

Tools:

• Shopbot: here
• Fusion 360
• Inkscape
• Trotec Laser Cutter
• Dog Bone here

5. CNC Machine:

A CNC machine is a motorized maneuverable tool and often a motorized maneuverable platform, which are both controlled by a computer, according to specific input instructions. Instructions are delivered to a CNC machine in the form of a sequential program of machine control instructions such as G-code and M-code, then executed. The program can be written by a person or, far more often, generated by graphical computer-aided design (CAD) software and/or computer aided manufacturing (CAM) software. In the case of 3D printers, the part to be printed is "sliced", before the instructions (or the program) is generated. 3D printers also use G-Code.

In our lab at Kochi, we have the Shopbot. Shopbot is a 3 axis machine the Z axis is perpendicular to the bed. The X and Y axis are the length and the breadth of the machine respectively.To have a rigid supporting layer on the bottom of the workpiece we will place an additional piece of plywood above the bed called sacrificing layer. The sacrificing layer also protects the millig bit from damage by avoiding unwanted intereference with the metal body. It can be used to cut timber, plywood, soft aluminum etc and has a bed size of 4x8 feet. The shopbot also has a dust collector that sucks in all the dusk while cutting.

6. Assignments

Maybe owing to the fact that we couldn't access the lab for 2 days, or that I had an exam over the weekend, this was the hardest week for me, yet. At the same time, I learned the most in this week.

7. Design & Inspiration:

I, prelimarily, wanted to build a squat-rack for my gym.

After talking to my instructor, I realized the infeasability. The wood I will be given is just 12 mm. Such thin wood cannot handle that loads.

Then, I decided that I will build a height measuring board. Something like this:

There was a lot of trial-error in my design, this week.

This particular one was a tad-bit structurally unstable. I had to change it.

This was the second-iteration.

I used acrylic to cut out the design in a Trotec laser-cutter. Here's the result.

There was still some translational motion, thus wobbling. I altered the deisgn, and made it simple and to-the-point.

I was not convinced of the stability of the current design, I altered it further.

Cardboard-Cut:

Still unsatisfied of the stability of the structure. I sat down started sketching it, with dimensions.

I have altered the base. I was satisfied with its stability

I have filleted it at the top, to reduce the weight

Since, this is a digital height checker, and I am gonna use a ultrasonic sensor, I have realized that I can do away the slot in middle. Further, I chamfered the top for further weight reduction

I have pulled the planck down, for further stability.

Final Version:

All the iterations, I had to go through:

8. Drill Bit vs Mill Bit:

CNC machining is a subtractive process that uses rotational cutting tools called “end mills” to remove material. An end mill, while similar in appearance to a drill bit, is far more versatile. However, in practice the terms “bit” and “end mill” are often used interchangeably

9. Flat End vs Ball End:

Flat end leaves flat surface profile on the stock and are good for removing large volume of material, but steps are formed when used for making curved surfaces. Ball end leaves curved surfaces and forms smooth curved finish while cutting cavities. They are used for finishing cuts.

• Bit material: Router bits used in Shopbot are made from a variety of materials such as solid carbide, carbide-tipped steel, and high-speed steel.
• Flute type : There are four basic flute types: Straight, spiral up-cut, spiral down-cut, and compression
• Chip load :- This is the amount of material that is removed in each chip. The value is approximately = feed rate (inches per minute) / (RPM x number of flutes)
• Cut depth :- This is the measure of how deep the end mill should go in each step while milling. Ideally cut depths should be less than 1/3rd of the length of the tool. They are mentioned in the product manual of the tool bits.
• Step over :- In a pocket cut the machine will traverse the entire area of the cut. It does that with a series of paths that cover the entire length and width of the cut. While doing this the step over determines how much the adjacent paths overlap each other. Usually we use it at 50%, ie the adjacent end mill cuts will overlap 50%, which gives a better finish.

The inner workings of a bit:

10. Upcut vs Downcut

In an Upcut type end mill the teeth on the flute will point upwards. This means that the end mill is cutting and drawing out the wood through the flute. This is good for cutting deep into the stock. But this leaves a bad surface finish on the top of the surface. A downcut type end mill has teeth that point downward on the flute. This means that the end mill will cut and try to push the material into the stock. This will give good surface finish on the top, but it is not very efficient at removing material. You can slide over the bit using fingers and differentiate between up cut mill and down cut mills.

11. Shopbut Cutting:

Export the .DXF file created previously, and send it to VCarve software. It will convert your file into G-Codes, which shopbot can use to mill the wood.

Make sure the bit-size is compatible with the material you are cutting:

• First, turn on the CNC Machine
• Warm up the machine
• Make sure the router bit is safely above the table
• Move the router to the table using the toggles in yellow control panel
• Unlock the Interlock Wrench and grab its counterpart
• Loosen the vacuum hose brush
• Loosen the collet using the two wrenches as shown
• Change bit or hand tighten a new collet with a bit needing a different shaft size
• Tighten collet and retighten the vacuum hose brush
• Finally, choose the speed for the router based on the bit

Setting up the Z-Zero height:

• First pull the Zeroing plate and alligator clips off the router
• Attach the Alligator Clip to the Zeroing Screw on the CNC Router
• Place the Zeroing plate directly above the router
• Call C2 on control panel of the ShopBot software, and it asks you to check whether the aligator clip and plate are in correct position
• Watch the Zeroing process to ensure the bit will hit the plate
• Another pop-up will appear on the screen asking you to return the Alligator Clip and Plate.
• The router should stay at a safe height above the table after being zeroed, however, if need be, raise the router safely above the tables

X-Y Axes:

Use keypad to manually choose X and Y co-ordinates, and then click the zero-axis button in the software.

Setting up the material:

• First, Clean the sacrificial layer to avoid any height fluctuations.
• Carefully, place the wood, and nail it to the aforementioned layer. It is imporant that the material does not move at all times.

The common time-frame for any shopbot work is from 1-6 hours.

12. Dogbones and Toolpaths:

1. Why do we need dogbones?: When cutting materials with the CNC, a known problem is that while the CNC can make a perfect outer corner, the inner corners can never be more sharp than the diameter of the cutting tool. So if you are e.g. using a 6mm mill, all the inside corners will not be perfect right angles, but will instead have an inside diameter of 3 mm (half the cutting tool's diameter) Sometimes these small rounded corners don't matter, but they are almost always an issue with joints - where often another piece of plywood, which by nature is square, must fit precisely in a hole or slot. If the hole or slot has very rounded corners, the mating piece just isn't going to fit in.
2. Fusion 360 Python Script for Dogbones: here
3. Generating toolpaths for Shoptbot using Vetric's VCarve Pro Software:
• First, Setup the dimensions of the workpiece in the software:
• Then, import your DXF:
• Scale it by an appropriate factor
• Since, I manually nested the objects, I did not had to do any nesting in VCarve.
• Next, I joined all the vectors in the .dxf using the "Join Vectors" command, and have provided "0.5mm" tolerance, upon my instructor's recommendation.
• Next, Select the cutting tool. My selections were: (Tool Bit: 6mm Dia; Step Over: 4.8mm; Pass Depth: 5mm). I had provided cutting depths as 18.3. I inside cut for holes, and outside cut for rest.

  Tool Database & Pass Depth:

  
• I manually added tabs, wherever I needed.
• Check the generated toolpath in the simulation tab. If everything is fine, generate the .crv file.

Send the file to cut, using Shopbot Controller Software.

Inner Cut:

The holes:



Tabs:



Final Cut:



From the Back:



How it fits into the Final Project?:

Group Assignment:

This week We need a test runout, alignment, speeds, feeds, and toolpaths for our machine by part of our assignment. So for testing some parameters in shopbot, we have designed a structure in Fusion 360.

This was our group assignment to find the press fit dimension for the 12mm and 18 mm plywood .So we cut slots from 18.60 to 17.80 in steps of 0.1mm and for 12mm from 11.70 to 12.50 in steeps of 0.1 mm. We found out that the right press fits comes somewhere near 18.6 mm and 12.4 mm. The material thickness does vary a bit with temperature, moisture etc.