This week we are working with ShopBot. The model we have here is ShopBot PRSalpha CNC . This a big machine and the biggest we have ever worked in the fab. This is a comparatively low cost CNC which can perform precision, large format, cutting, drilling, machining, and shaping in many materials like wood, acrylic etc and is controlled by a standard personal computer. The bed size of our machine is 96x48 inch.
More information about the Shopbot can be found here.
For setting up the machine refer to official shopbot
docs
For more details on feeds and speeds refer this document. This doc
will come
handy later when you are setting up a new tool in fussion for your project.
For more details on chiploads refer Detailed chip load
charts
My idea was to make a a rocking chair with around 90 degree of rotation.
When you sit on a chair like that it moves to one side giving the feel of falling. This can help relieve
pressure
and tension. Thus it is an ultimate relaxing chair. As i want to give an ultimate relaxing experiance to
anyone using
my chair i wanted to make the sitting surface with a flexible material so that it gives a more
cussioning
effect on the back. As there wasn't anything in our lab which was suited for the purpose, i plan to make
plywood
flexible and use it as the sitting portion of my chair. I also put a backup design with the flexible
plywood replaced
with plywood rails, so that i have something even if my fluxure does't work.
Initial drawing
My initial idea of the fluxure was inspired from an origami which we use to make in our
shool days.
Origami inspiration for flexible plywood structures
You can see that this kind of a design gives only a belly kind of flexure. This looks cool, maybe i can
use it for something
else, but it is not enough for my rocking chair. So i tried extending this design and see if i can get
the required results.
Extended design
I made 4 similar structures in a paper and tested out the flexibility. The structure was flexible but
was not what i wanted.
I started to search the web more intensively for what i want and finally found a couple of things which
i can use for my design.
Flexure from instructables
Flexure from msraynsford.blogspot.in
Another design from the internet
I recreated the first two designs in craft wood and the following are the results.
My re-creation of flexible plywood - design 1.
This design has less flexiblity but is more stable and less brittle.
My re-creation of flexible plywood - design 2.
This design is more flexible but is brittle.
So i have decided to use the first design on my project as it is more stable, considering
the fact that plywood is less stronger than craftwood.
I need to find a possible diamension for my chair. My friend Syed Ahamed helped me with this.
he lied on the floor so that i could measure out the length and width for the chair. The length was
estimated to be 130 cm, width 100 cm and height 110 cm.
Identifying measurements for the chair.
As always i am using Fussion 360 for my designing. The steps are Documented below. My major issue while designing was that i had to make sure that my single design has room for the actual dsign and for the backup design. Also it was not possible(to my knolwdge) to show the flexure in the tool also. So my plan was to design with the rails first and if the flexure was forming properly i will cut away the slots for the rails and fix in the flexure else i will go with the rails itself.
As i finished my designing i understood that this is a big project and as i have no prior experience in doing it i should try something small before attempting one this big.
This is a work from pinterest which i found interesting.
learning table-chair set.
As always i am using Fussion 360 for my designing. The steps are Documented below.
This model is also quite big and i want to done something still smaller as a start so i am moving on with something else
Shopbot is the biggest machine so far i have ever dealt with so as this
is my first try i will do something small.
The following is the model for a chair i am making.
A simple press fit chair.
Again i am using fussion 360 and you can find the steps below.
In our lab we were using VCarve for generating the shopbot files. We have never tried anything else. As i have done the CAD in fussion and as fussion has a full fledged CAM environment i decided to do the CAM in fussion itself. This is a trial and error process and i am documenting the process below.
While adding joints between the stock and the model parts make sure that you make the joints as 'planar' type. This will allow you to move the parts within the stock and place it anywhere you want, reducing space wastage.
This panel contains the coodinate system settings. Remember that the shopbot sets it origin at the bottom-right and x axis is is vertically up and y axis is horizantally left so you need to set the codinates of our model also to be aligned with this. Here you can also select the parts you want to print. Only the selected parts will get cut.
Stock basically means the wood size in which you want to cut out your parts. The default will the size just enough to cut out your models. There is no need to add and padding if you are setting the zero of the machine properly. Set the height to the plywood thickness and make sure that the mode is 'Fixed size box'. You can see the dismensions below, now make sure that the plywood you are planning to cut has these diamensions.
Here the tool diameter is the the most important one. You can fill this up by either refering to the bit manufactures specifications or by measurng the bit or from the above listed docs. I got the data from the docs. Flute- this is the part of the drill bit which has that engraving. More details about flute and other parts of the machine can be found in the shopbot doc
You can neglect the shaft and holder settings as they wont affect the cutting. But if you want
perfect simulation in fussion
setting them can help. The next importent settinga are the feed and speed. I have copied this
settings from the Vcarve settings which
was already being configured for the bit. You can find them here in the image.
Spindle speed is the speed of the spindle. Ramp spindle speed is the speed of the spindle while
making helical cutting operaion
into the material. Surface speed is the speed of cutting.
Cutting Feedrate is the feed used in cutting moves. Feed per Tooth is the cutting feedrate
expressed as the feed per tooth.
Other feed rates can be set as same as the cuting feed rate.
Plunge feedrate is the speed of drilling down vertically, ie how much depth is drilled in a
minute.
You can leave the post processing tab with the default settings.
The only thing you need to do on this panel is to select the tool which we have already created. This will automatically update all the required settings in this panel. Cloose coolent disabled.
Here you can choose the geometries to be cut,
make sure that you choose the bottom curves else the machine will cut in the air. Also make
sure that the arrow is outside the shape meaning outer cut. If the arrow shows from inside to outer it means inner cut.
Simply click on the arrow to toggle this.
If you want tabs you can do that here. The settings i used are shown below. Here i have
used manual placing of tabs,
you can choose to put tabs at particular distance if you want.
The default is okey except the bottom height. This i like setting to 0.5mm from the bottom of the stock to ensure that the pieces are properly cut out. Remember that all the heights are set relative to some other thing.
See the red arrows here? for the + shape it is inner cut, so arrow pointing from inside to outside and the outer cutting , we have the arrow on the outside meaning outer cut. Here i change the sideways compensation to right conventional milling and enable multiple depths and put the depth(max stepdown) to 3mm so that cutting happens in steps of 3mm each. For other settings the default is fine
Here i have just choosen the circular part and simulated that alone. You can see the tabs and the cuts and the movement of the head. This is a nice way to inspect the cutting operations before it is being done. You can fix a lot of errors in your job by looking at this.
Before moving onto the machine make sure that you have read the second and third documents listed in the begining throughly and make sure to follow all the safety measures.
This happend because i didnt place the zero of the machine in a place that the machine has enough room to cut everything. I didnt verify that the distance from the zero is enough for my model. Now i had to cut the seating again.
Plywood burned out due to the collet rubbing over the ply. The tool i selected was having a grater cutting depth than 18 mm so the bit went so deep that the collet started rubbing over the ply.
Now once i have tested fussion 360 cam and i have made this small chair i think it is time to go ahead with the first model i started with. After making the chair i realize that the plywood i was using was not hard enough to hold much weight. So if i use the same material to make the rocking chair which need to support the weight of a person, It will breakdown. I decided to redesign the model with more crossbars to make it more stable. I also does not want to use any nails to hold the parts together. I decided to use the press fit alone.