Now before starting off with the actual milling/cutting process, there is a lot to learn about CNC machining. Here are the basics about CNC.

What is CNC milling?

CNC milling is a specific form of computer numerical controlled (CNC) machining. Milling itself is a machining process similar to both drilling and cutting, and able to achieve many of the operations performed by cutting and drilling machines. Like drilling, milling uses a rotating cylindrical cutting tool. However, the cutter in a milling machine is able to move along multiple axes and can create a variety of shapes, slots, and holes. In addition, the work-piece is often moved across the milling tool in different directions, unlike the single axis motion of a drill.

Cutting speed

Cutting speed is the peripheral speed of the tool. Sometimes you may need to calculate cutting speed mathematically. Or, you might use a cutting table. Whichever way you do it, you need to know the cutting speed and RPM for the tool you have selected on the CNC machine you are using. Sometimes cutting speed is confused with feed speed. Just remember that cutting speed is in relation to the tool itself while feed speed is in relation to the movement of the tool.

Spindle and feed speed

Feed speed is the rate at which the workpiece moves into the cutter. It is always determined in relation to the spindle speed. Using the wrong feed speed can produce too much dust or burn the workpiece. To determine the optimum feed speed you can use a feed speed table or calculate it mathematically.

Cutting speed

Cutting speed is the peripheral speed of the tool. Sometimes you may need to calculate cutting speed mathematically. Or, you might use a cutting table. Whichever way you do it, you need to know the cutting speed and RPM for the tool you have selected on the CNC machine you are using. Sometimes cutting speed is confused with feed speed. Just remember that cutting speed is in relation to the tool itself while feed speed is in relation to the movement of the tool.

Spindle and feed speed

Feed speed is the rate at which the workpiece moves into the cutter. It is always determined in relation to the spindle speed. Using the wrong feed speed can produce too much dust or burn the workpiece. To determine the optimum feed speed you can use a feed speed table or calculate it mathematically.

Understanding Drill Bit

CNC routers need bits. They determine the kind of carving you can do, the resolution of your finished designs, and how fast you can move through the material. They come with cutting edges that pull up or push down (sometimes both), they have square or shaped ends, they are made for speed or accuracy, and they come in diameters from a pinpoint to over two inches for standard CNC routing.

Anatomy of a Milling Tool

Milling is done using a cylindrical milling tool mounted in a milling tool holder that is then mounted in the tool spindle on the machine.

Types of drill bits

End Mills

End mills are the most common milling cutters. End mills are available in a wide variety of lengths, diameters, and types. A square end mill is used for most general milling applications. It produces a sharp edge at the bottom of pockets and slots.End mills have cutting surfaces called flutes. The most common end mills have two to four flutes. Generally, fewer flutes evacuate more chips from your material, keeping the bit cool. However, more flutes produce a finer edge finish. There are four basic flute types, each optimized for different materials and edge finish.

Downcut bits ensure a smooth top surface on laminates, assist with holding down your thin parts, and possibly avoiding tabs on larger parts. A single "O" flute is key for plastics like HDPE and acrylic when clearing materials. The flute helps avoid excess heat buildup, which may cause the material to stick to and ruin the end mill and your part.

Ballnose Mill

These bits have a rounded tip and are ideal for 3D tool paths. When combined with a “roughing” bit to clear large areas of material, this end mill will result in smooth 3D surfaces, especially with two or more passes.

V-bit

A 60° or 90° V-bit is great for what’s called V-carving, in which the tip of a V-shaped bit is used to cut into narrow spaces, and the wide bottom is used to cut into larger spaces. V-bits can also create sharp corners that other end mills cannot because of their radiuses.

Procedure to be followed from Design to CNC

These are the steps to be followed to mill a design

1.Make a 2d Drawing of the design

-Consider tolerance

-Put the drawing in layers as per the cutting scheme

-Arrange pieces ti minimize material wastage

-Leave ample distance between pieces to avoid overlap

2.Export it in .dxf format

3.Import it into software that creates tool path(Fusion 360, Vcarve, Pathworks etc.)

-Choose the type of output(cutting, pocketing etc)

-Choose the orientation of the bit

-Check all parameter

-See the preview in 3D

4.Export it as .GCODE

5.Milling machine

-Mount the correct drill bit

-Turn on the machine

-Turn on the exhaust

-Rotate spindle if not used for more than 12hrs to ensure lubrication flow

6.Import GCODE into Shopbot

7.Set x,y,z zero(Origin)

8.Start milling process

Designing

I wanted to make an adjustable table/desk. One should be able to sit as well as stand and work on the desk. That was the basic intent while I was thinking of the product. So after searching for reference I came across this product.

This design was close to what I had in mind but I wanted to make something without rods or other metal parts. So I tried designing something where the notches are also cut using CNC. I first made some rough sketches of the design and took it to Autocad to draft it out.

I used AutoCAD as I am comfortable with it. I made a rough sketch first and then detailed it part by part. I first wanted to understand the design clearly, hence I made plan and elevations for the design, imagining the support systems as well.

Once I got the initial design I had to visualize it in 3d to understand how the notches and joinery had to be designed. I took the dwg file into Fusion 360.

Using the previously made sketch I started extruding the plans with 12mm, cause that the size of ply that I had purchased.

After extruding all the elements I started assembling them together and making slots and projections where ever necessary.

Once I completed the process of modeling I disassembled all the pieces again and arranged them plat on the top plane. Though Fusion 360 had tool path I wanted to explore Pathworks for that sake. Made sketch of all the components and exported them as .dxf.

Again using AutoCAD I assigned layers for different elements depending on how I wanted to cut them and arranged them on a 1200mmx2400mm rectangle keeping a cushion of 50mm on all sides because that's the size of ply I had.

Prototyping

As suggested by my instructor and Neil I decided to prototype the model before milling it. I wanted to laser cut the model using a 3mm thick MDF sheet. So I scaled down the model by having the slots as my reference.

And then cut the assembled it. It was quite exciting to see the model in a miniature version. I did not consider the tolerance of the material as it was just prototype and I also used waste/used MDF. Assembling it was easy and I was happy to see that I had considered all joints while designing. Some of the slots required filing to fit in. This was the result after assembling

All the assembly was fine but the problem was in the stability and strength of the structure. The top assembly started swinging as the loading was not correct and the main problem seems to have been in the toothing system that I had designed to extend the table. The orientation of the toothing was wrong.

Understanding the problem I changed the system of toothing to slots and also introduce additional support members.

 But this system also didn't work because the support structures restrict the movement in the Z-axis. So in my 3 iterations, I changed the support system. Instead of two members, I replaced it with one longer member.

Even in this scenario, the stability was a question. The top layer starts to sway or rotate because of the pin support. So I decided to make the pin support larger and more stable.

This iteration worked. The model was stable and there was no pivoting. Hence I decided to make it final and use the same design to mill.

Milling

Milling is like a subtractive process like laser cutting, but the difference is milling gives the function of 2.5d and also certain milling machines allow 3d processes. Hence in laser cutting the only tool used is a laser beam, whereas in case of milling depending on the desired output the tool has to change and also the toolpath. There are certain application that allows and patchwork is one of them. This is a licensed software and now it has been changed to Vcarve. I made changes in the  CAD design before importing by increasing the notch size by 0.5 mm on all sides. This is for tolerance. This I obtained after group work testing.

The above link explains the design software used for milling.

Pathwork

On opening Pathworks the options presented are to create a new file or to open an existing file. I clicked on create a new file and the next step is to Job Setup. Here basically you are telling the software how big your cut is gonna be, depth of the material and the origin to begin cutting from.

The next step is to import the desired file. Pathworks allows vectors files such as .dxf, .dwg, .pdf, .ai etc. This gives a wide area to design from. The import menu can be found in the File-Import tab.

I imported my drawing which I had made a .dxf of.Most often than not the files get placed not in the white area, but somewhere in the model space. So all the elements need to be selected and using the move tool it can be brought into the drawing board and then using the black dots in the corner can be rotated.

Once I had oriented my drawing I wanted to give them the T-bone junctions. Of course, it can be done in auto cad, but its time taking and Pathworks has an easier solution for that. Click on the fillet tool and a new menu opens up. On the top, the desired radius can be input and then the type of fillet can be selected. In my case I wanted the T-Bone. Apart from filleting the slots, I wanted to fillet certain junctions as well. I had already decided how I wanted my toolpath to be worked out. So based on that I knew running the toolpath would result in curves along the edges making press-fit a problem. Hence I used fillets to rectify that problem.

Once my drawing was all set it was time to design the toolpath. The toolpath can be accessed on the top right corner of the screen. Here one would find different options on how to mill depending on the design output desired.I used the create profile toolpath and using the layer option hide the other layers for easy selection and visualization.

Once selected a list of options to need to be filled. First is the depth that needs to be cut. Then select the right drill bit.

In my case, I used a 0.25-inch end mill with 4 flutes (up cut) with an rpm of 14000. Set my machine vector to cut outside.Next is to add tabs. This creates a small bridge between the stock and the component to help it not move after being milled. This makes sure that the cut piece doesn't collide with the bit.

The tabs can be added at desired location just by clicking on them and their sizes are also customizable. Once I filled all my required input I hit calculate. This calculates the tool path and takes to a 3d view screen where the movement of the bit can be seen as render.

I repeated the same process for my entire drawing but changing the parameters as required. For example, the blues lines are for inner cut and the green for slots(inner cut as well).

Once all the tool path was calculated, I saved them as shopbot(inch) file type. Another important factor to be kept in mind is to check the box that says output all visible toolpaths to one file. In case using different bits for one product milling then it can be saved as different toolpaths. Otherwise its better to save everything as one toolpath.

Before starting with the milling process, the first thing that needs to be done is to install the desired milling bit. As mentioned above, the bit has to be installed in the collar and this is done with the help of these special spanners.

I Secured my ply onto the Sacrificial layer using screws. The material was bent to a great extent, so I used clamps to hold them in place and then screwed them

The key to turn on the machine is fastened to the spanner for safety reasons. When one needs to work/ change the bit, the machine should be off. I then placed the desired material on the board and screw it to the sacrificial layer.

 Now once the bit and material are set, we can turn on the machine by rotating the red knob and the key. From the control, box hit the reset button to power on.

Connect the system with the shopbot software installed using the 2 USB cables provided. On opening the software the axial location of the bit and a command like a screen appears.

We need to move the drill bit to a point that we consider origin. To do this open the keypad option by clicking the yellow icon in the position menu.

Using the arrow key or the on-screen arrows move the bit to the desired origin point. Setting Z could be slightly tricky. The bit should touch the bed exactly at a point that it scraps the top layer.

The bit can be moved slowly by clicking on the fixed button. Once the origin is set, click on zero axes and set x,y, and z as zero.

Now its time to import the G-code into the software and this can be done by clicking the cut parts and choosing the GCODE file. Hit start. The code gets loaded up and a message appears to hit the start button in the control box.

But just before hitting the start button, the exhaust needs to be switched on.

This makes the spindle to rotate and by clicking okay to the message displayed on the screen the cutting process begins.

Things went on smoothly until it didn't. the milling was a little jerky in the middle of the board. This was because the board was bent. I did not notice this in the beginning. Yet the drilling was fine until a point where the drill bit pulled up the board, and the board got stuck to the bit. Due to the heat and stress, the bit broke.

The reason for the failure of bit.

-Cheap material (Uneven thickness)

-Not secured to the sacrificial layer properly

-Sagging / Bending of material.

I did not nail in the center of the board in the beginning because I did not know the exact path that the CNC would take. Nailing on a random spot would result in catastrophe when the bit comes in contact with the nail. Another solution would be to drill few pieces at a time rather than to mill an entire sheet.

3/4th of my cutting was done, but since the bit broke I made another drawing for the left out pieces and milled them again.