8. Large format CNC (computer controlled Machining)¶
This week’s task was to design something big to cut from a large sheet of wood using the large scale CNC machine.
The machine available in Fab Lab Bahrain is a ShopBot 3 axis large CNC machine.
Machine Testing¶
Due to time constraints (and maybe forgetting about us) our group did not perform thorough testing on the machine. You can read about what the other groups did with the CNC here.
Because I was somewhat free the week when the first group did their testing, I hung out and help around the CNC and learned quite a bit.
I even have their test piece that they used to identify the optimal speeds (make it smaller next time guys 😅).
For 12mm thickness mdf, the optimal speed was 17,000 RPM and the optimal feed rate was 60 in/min.
Available Tools and Materials¶
There are different shapes and sizes of drill bits that can be used based on the application. We will be using a straight drill bit in this case. Since the part that holds the 3mm bit is missing, we will be restricted to only using the 6mm drill bit. This does mean that our design cannot have any details smaller than this.
Additionally, there are different methods to secure a sheet on the machine bed. We will be using screws and nails.
Wood¶
The wood sheet that we were provided is 2440mm x 1220mm and 12mm thick, and is either made of MDF or plywood.
Medium-Density Fiberboard (MDF) is a type of manufactured wood that is made by binding broken down fibers of other wood using wax and resin, they are then formed into their desired shapes using high temperatures and pressure.
MDF is typically inexpensive and is easy to cut, shape, paint, and glue. Additionally, because it is man made it also has a smooth and consistent surface that is free from natural wood defects.
However, it is relatively weak compared to other wood types such as plywood. Furthermore, it absorbs moisture quickly, which can lead to swelling or warping. Moreover, MDF contains some harmful organic compounds such as formaldehyde, which can be released when working with the wood. Formaldehyde is known to promote the development of cancers and can cause allergy, and eye and lung irritation.
Plywood is a type of manufactured wood that is made by gluing together thin sheets of wood.
These individuals sheets are stacked in manner where each layer’s grain direction is perpendicular to the adjacent layers (can be in steps of 45° for additional benefits).
This cross-graining of the layers gives better and more consistent strength in all directions, reduces swelling and shrinkage, and allows the sheet to be nailed at the edges without splitting.
Generally, MDF is nice to use for beginner projects because it is easy to work with, and has a smooth and uniform surface. Plywood tends to be stronger, more moisture resistant, and typically looks like its of higher quality.
MDF is more suitable for my needs because for the games I want to make, the surface needs to be smooth. Additionally, the plywood we have did not seem of high quality because its edges were very rough. Apparently good plywood is hard to come by in Bahrain.
Individual Work¶
At first I didn’t know what to make. I thought of making a basket ball hoop, but the backboard alone took up most of the sheet. I asked my parents what they wanted, but they didn’t really need anything. I also downloaded Slicer for Fusion, and thought of making a statue of some sort to then slice and assemble, but that seemed like a useless project.
In the end I got most of the ideas from my friend, Saeed Makhlooq, and decided to make several objects.
Designing¶
Monopoly Board¶
The idea for the monopoly board was that we would make the board with engraved slots for the land cards along with the community chest and chance cards, and then make these cards separately based on a theme we liked. This is so we can swap out the cards based on the theme we wanted to play.
I based the board size on the tables we use at Fab Lab Bahrain. Based on this size I made the card slots to be engraved.
I also wanted to engrave the Fab Lab Bahrain logo on the board, but it was difficult to incorporate it in a tasteful size because of the 6mm drill bit, so I dropped the idea and decided to simply paste out names using the vinyl cutter, but that also didn’t work out since its power cord was not available at that time😔.
You can download the model as an editable SLDPRT or as DXF.
The playing cards were designed by Saeed Makhlooq and myself. The land cards to be placed on the board were cut using the laser cutter on 2mm cardboard.
The parameters we used were 500mm/s and 15% for the scan, and 70mm/s and 30% for the cut.
The font we used caused some letters to not get engraved properly so we had to use a marker to fix them.
We did not finish cutting the rest of the cards as we keep having issues with laser not maintaining its power when trying to make multiple cards.
Small Boxes¶
I used MakerCase to make a small box based on some desired internal dimensions. I simply input the dimensions, the material thickness of 12mm, and selected finger joints.
I also added a dog bone fillet of 6.2mm diameter, which allows me to skip this model when adding these fillets in the CNC software. I also disabled panel labels because I thought it might interfere with the tool path.
You can download the DXF file here
In total I made 3 boxes, two of them without a ceiling because I thought it would be hard to open when closed.
I also used the laser cutter to engrave some designs on 2 boxes so I could give them to my friends. I followed the same steps detailed in week 3, including setting the z axis origin on top of the box face.
For the current case of 12mm MDF, we found that the optimal scanning parameters for the laser engraving were 500mm/s speed and 30% power.
Pucket Table¶
I wanted to make a fun game that uses pucks. At first I thought to make a carrom table, but then I realized that those tables need smooth surfaces in addition to the painted design. In the end I went with pucket.
Pucket is a game where you have to sides on a table separated by a wall with a small hole, each player has a number of pucks that he needs to send into the opposition’s field. The winner is the one who manages to clear his field of pucks.
To add the walls necessary to play the game, I had to use two pieces, one as the base board, and the other for the frame. These two pieces are held together using some small wood pieces to act as joints. I also decided to make the hole in the wall be a complete separation instead of only a hole from the floor side, this is to make it faster to machine since it doesn’t require engraving, while also making it so that the pucks can be made with the full thickness of the wood, which also saves a lot of time.
The assembled product should look like this:
You can download the parts as SLDPRT: Base, Frame, and Joint
Or as a DXF file.
Rocket Shelf¶
Since I still had space in my wood sheet, I decided to make a storage container. After looking for inspiration, I decided I wanted to make a rocket shape bookshelf. I prioritized the shape of the rocket, hence, it has 4 sides instead only the front side, however, this may make it harder to use since it needs to be placed in a space where it can be accessed from all sides.
I made the bookshelf’s height roughly the same width as the sheet of around 1110mm. From there I just drew circles and trimmed them to get the general shape. To get the 3D shape I used two almost identical pieces and made a slot for them to join.
It should look like this:
Cutting Setup¶
After finishing all the designs I arranged them in a sketch on a single plane in order to export the dxf file. My dxf file looks like this:
You can download it here.
When arranging the parts into a single dxf file, I used a gap of 10mm between each part to ensure that the CNC does not interfere with a part while cutting another.
I used a 30mm margin in relation to the 2440mm x 1220mm size of the wooden sheet to define the safe working area, this is because there needs to be some allowance for where we secure the sheet using screws.
Before we start cutting we need to remove the previous wooden sheet if it present, and then vacuum the sacrifice sheet to ensure proper securing of the new sheet.
After cleaning the area we can then place the new sheet on the sacrifice sheet, when doing this me need to make sure that their sides are aligned (because they have the same width) and that the starting edge of the new sheet is aligned with the sacrifice sheet (because the sacrifice sheet is longer than the working sheet).
After placing it we need to secure it in place, to do this we use screws and nails. We need to screw in the corners and the middle of the long side of the sheet. Make sure that the screws are inside the 20-30mm margin that you have set. When doing this be careful of the areas that have been eaten away on the sacrifice sheet, and if the place you want to screw in doesn’t have material to hold it in place on the sacrifice sheet, simply move it slightly to the side.
To use the machine we need to use a software called VCarve Pro.
We need to first define the working area by inputting the sheet dimensions:
After defining the initial work area we need to add a margin for the screws. We do this by adding an offset:
We can then import our dxf files:
After importing your design you can move it to the corner of the offset using the arrow keys.
Many of our designs have a problem where they have open vectors, to fix this we need to use the join open vectors tool:
You need to set the tolerance to a very small amount, just write 0. and write many zeroes and then one. Then you need to select each object and join open vectors:
In the end your design should have 0 open vectors:
After joining open vectors you can create an important feature that will help you in the assembly process, and that is to add dogbone fillets to the corners:
Fillet radius should be at least half of the drill bit diameter which is 6mm. We usually set it to 3.1mm. Larger radii will be more noticeable but will make assembly easier.
These tips are for a 3mm drill bit:
Next we can define the toolpath. We do this by going to the toolpaths tab on the left, and selecting profile toolpath for cutting operations:
In this tab we can specify parameters for cutting or engraving:
For cutting you need to select everything you want to be cut. Make sure to exclude the part you want engraved.
Since the sheet is 12mm thick, we need to set the cut depth to 13mm to ensure the cut goes completely through. This is fine because we have placed a sacrifice sheet under the working sheet, ensuring no damage occurs to the machine bed.
It is better for the drill bit’s health that we set it to reach this 13mm in two passes each of 6.5mm, we can edit this by specifying the pass depths.
We need to also specify the tool that we are using. For the current case we are using a 6mm straight drill:
It is important to specify how the machine will follow the cutting path. To maintain dimensions you need to set it Outside / Right
After finishing the setup and calculating toolpath, you will get a warning about the cut and material depth. You can ignore this because we have a sacrifice sheet.
These steps are for cutting. For engraving, select pocket toolpath:
Set the engrave depth as you need. Typically half of the sheet thickness is good:
This is the final toolpath for my project:
Make sure to order the toolpaths where the engraving happens before the cutting, this is so that the sheet doesn’t move after being cut, thereby messing up the engraving.
After finishing, save the toolpath. In Fab Lab Bahrain we save all toolpaths to a file called all. This overwrites the previous file and saves space.
Cutting¶
Before using the CNC you must ensure that you are following proper safety procedure:
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Wear safety goggles or a form of eye protection.
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Stay away from the machine when it is operating.
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Make sure that no obstructions are on the machine bed.
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Ensure that the CNC drill bit is secured.
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If any faults or unexpected errors happen, press the emergency stop button (big red button).
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Ensure that the working sheet is fastened properly.
After installing the new sheet we must set the origin of the points on the CNC. This is done using a different software called Shopbot Control Software.
The x and y axis origins were already set by our lovely instructor. To do so you have to use the arrows on the control panel to move the CNC head until it reaches the close right side corner of the sheet. Then you zero the axes at that point.
For the z axis we use a tool that causes a short circuit when it touches the drill bit, thereby indicating the level of elevation.
After completing these setup steps you can start the CNC machining:
You do this by using the Shopbot Control Software and clicking Cut Part. You can then select the toolpath file you saved:
Then click start:
When you click start the spindle will not start rotating, you need to hold down the green button for the spindle to slowly rotate until it reaches its desired rpm, only then will the machine start its job:
Here is a clip of the CNC cutting my parts:
For some reason a lot of my parts popped out of their places when the CNC finished cutting 3 sides of their surrounding surface. This caused some of the pieces to have some side not get completely cut.
Look at this paranormal activity I got on camera:
Final Result¶
I was not able to cut more shelves for my bookshelf because I ran out of space on my sheet.
One of the boxes I used for storing the pucket pucks and rubber bands
The other two I engraved and gave as gifts to my friends:
The pucket game worked great and was fun to play:
When using thinner rubber bands
We did not manage to finish the monopoly playing cards, but the CNC part was fine. There was a lot more I could’ve done, such as making a space for throwing the dice:
