This week is for Computer Controlled Machining, we tested runout and alignment of the machine wich is a ASIMA one.  It is a CNC Router AR-3005 and its specs are:

Useful area: 1.52 x 3.10 x 0.15 m

Outside area: 3.76 x 2.40 x 1.80 m

Accuracy: 0.10 mm

Cutting Motor: Spindle

Vel. Max. Mov .: 20,000 mm / min

Vel. Max. Cut: 13,000 mm / min

Motor features:

Power: 3 1/4 HP

Power: 120 VAC @ 22Amp

Speed: Variable

Spindle: water cooled

Characteristics:

Power: 3 HP

Power: Biphasic 220 V AC @ 22Amp

Speed: Variable

Precision bearings

 Then we have to understand how the parameters for cutting wood are calculated.

Searching in the web I found that for CNC routers and wood-cutting there is sort of a standar spindle speed that is between 18,000 and 12,000 RPM, I later found out that here in the FabLab they use 12,000 so that's in the parameter.

Here they also normally use two flute cutters. I used a 1/4" cutter for my project.

To calculate the feed I found this formula:

Feed = N x cpt x RPM

N - number of cutting edges (flutes)

cpt - chip load (chip per tooth) is the amount of material, which should be removed by each tooth of the cutter as it rotates and advances into the work. (mm per tooth)

RPM - the speed at which the cutter revolves in the spindle. (Revolutions per minute)

So using the values above I calculated almost 8,000 mm/min which seemed a lot.

In the end I used the parameters they had been using in the FabLab which are 12,000 rpm for spindle speed, 2,545 mm/min for feed.

For my design I was thinking in something useful because I didn't want all that material to be useless after I was done with the assignment, so even I am pretty aware that almost every student makes a chair in this assignment I did just that, a chair.

I based my measurements in a standard chair and made some curves over it. I then made the seat and the back support with variable lenght pieces.

I didn't do fitting test, I just guessed the wood would compact a millimeter without breaking,  so I designed the slots 2 mm smaller than the material thickness, it turned out really well.

I then saved all the drawings to DXF and imported them to Vectric Aspire 4.0.

In Aspire first click on "create new file".

Then configure your material dimensions.

Copy and paste all your DXF files in the work area and open "nest vectors tool", this tool helps you to arrange your cuts so you make the most out of your material.

I had to select all the pieces, choose a diameter for my tool, I like to click on rotate parts to find best fit,  and then arrange them along Y because my sheet is vertical.

Because of the tool being a rotative cutter it will allways leave a radious instead of corners, and for this we have to compensate making something that's called "dogbone", what this does is to cut a little bit deeper in the material so the mating piece doesn't rub its corners with it. You can find the tool in Aspire object editing, as it is shown image below.

To use this tool simply select a radious bigger than the radious of your tool and click on the second option, search for all the negative corners on your drawing and click on them to change for a dogbone, repeat untill you have no more negative corners on your drawing. On the right image you can see the left arrow is how your cut will be without a dogbone, the corner of the positive piece would rub against that making it really hard or even imposible to fit the two together, the right corner shows how your pieces will mate with the dogbone.

These are the parameters for the tools.

Click on "save trajectory" select "Mach 2/3 ATC Arcs (mm) (*.txt)" on the postprocesor, click on "save tajectory" again.

I exported my project as a .txt and runned it in the router, the whole cut lasted about an hour. I had to sand all the cuts so there wouldn't be any splinters, organized all the parts by size and hammer my way to fit all the parts. Some would say my pressfit was too tight but I like the feel like it's never going to get loose.

I made some quality tests and this thing really looks and feels tough, I knew it wouldn't break when someone would sit on it but now I think this chair could easily take a beating without even getting a bit loose, I even jumped on it and it didn't even squeaked.

Download the chair files HERE