Lesson 7

This week's assignment is to design a 3D mold, machine it, and cast parts from it.

Lesson 7

When I started off, I wanted to design a ball valve for this assignment, but I soon found out it's more complex than I thought. I decided to make a toy car and looked on the internet for a nice model to mold. I found one of the web of an AC Cobra Daytona and imported it into inkscape. In openscad you can linear_extrude an image, so i made an outline of the shape of the AC cobra daytona 1963 without the wheels and saved the dxf file. Later i found out that in openscad you can download different layers and give them different dimension, but for now i just outlined one image/layer and exported that dxf file to openscad. In openscad i extruded this 2d image with the command:

linear_extrude(height = 300,file = "daytona1.dxf", center = true , convexity=10);

I made the basic shape of the car with the dxf file and added wheels and two tilted boxes to keep the wheels in place. I substracted the half of the car with a cube and from there i took the difference of the car shape with another cube. After this procedure, I ended up with the negative half of the car in a box. Then i added some supports where the two mold will be fixed together. You will find the code and an image of the openscad model below.

Openscad code

module car(){
union(){
linear_extrude(height = 300,file = "daytona1.dxf",  center = true , convexity=10);

difference(){
translate([220,80,-160]) cylinder(h=320,r=60);
translate([220,80,-160]) cylinder(h=15,r=50);
translate([220,80,145]) cylinder(h=15,r=50);
}

difference(){
translate([610,72,-160]) cylinder(h=320,r=56);
translate([610,72,-160]) cylinder(h=14,r=49.4 );
translate([610,72,146]) cylinder(h=14,r=49.4 );
}


translate([220,80,-160]) cylinder(h=15,r1=5, r2=10);
translate([220,80,145]) cylinder(h=15,r1=10, r2=5);
translate([610,72,-160]) cylinder(h=14,r1=5, r2=10);
translate([610,72,146]) cylinder(h=14,r1=10, r2=5 );


rotate([0,0,-5]) translate([120,80,-120]) cube([200,100,240]);
rotate([0,0,-1]) translate([510,72,-120]) cube([175,75,240]);





}
}


/// top of car
difference(){
translate([0,0,-250]) cube([800,250,250]);


translate([400,230,-20]) cylinder(h=20,r1=8,r2=10);
translate([20,20,-20]) cylinder(h=20,r1=8,r2=10);
translate([780,20,-20]) cylinder(h=20,r1=8,r2=10);
translate([20,230,-20]) cylinder(h=20,r1=8,r2=10);
translate([780,230,-20]) cylinder(h=20,r1=8,r2=10);
car();
} 

The next step was to take another difference from this model and a slightly bigger cube to end up with the mold of the mold. Then i had to mirror the resulting image to get the other side of the mold and change the pins into holes on this second mold. On the back of the car i added an air and pouring hole on both parts of the mold.

Openscad code

module car(){
union(){
linear_extrude(height = 300,file = "daytona1.dxf",  center = true , convexity=10); // car shape


difference(){
color("red") translate([220,80,-160]) cylinder(h=320,r=58); //back wheel
translate([220,80,-160]) cylinder(h=15,r=48); // back wheel inside
translate([220,80,145]) cylinder(h=15,r=48);
}

difference(){
color("red") translate([610,72,-160]) cylinder(h=320,r=54); // front wheel
translate([610,72,-160]) cylinder(h=14,r=47.4 ); // front wheel
translate([610,72,146]) cylinder(h=14,r=47.4 );
}


translate([220,80,-160]) cylinder(h=15,r1=5, r2=10);
translate([220,80,145]) cylinder(h=15,r1=10, r2=5);
translate([610,72,-160]) cylinder(h=14,r1=5, r2=10);
translate([610,72,146]) cylinder(h=14,r1=10, r2=5 );


rotate([0,0,-5]) translate([120,80,-120]) cube([200,100,240]);
rotate([0,0,-1]) translate([510,72,-120]) cube([175,75,240]);

///translate([15,15,-15]) cylinder(h=30,r=10);



}
}


rotate([180,0,0]){
//mirror([ 0, 1, 0 ]) {}
translate([0,6,0]){
scale(0.125,0.125,0.125){
difference(){

translate([0,-50,-230]) cube([850,400,240]);
translate([25,25,-25]){
			difference(){
translate([0,-50,-250]) cube([800,350,250]);

translate([25,25,-30]) cylinder(h=60,r1=17,r2=25);
translate([775,25,-30]) cylinder(h=60,r1=17,r2=25);
translate([25,225,-30]) cylinder(h=60,r1=17,r2=25);
translate([775,225,-30]) cylinder(h=60,r1=17,r2=25);
translate([425,225,-30]) cylinder(h=60,r1=17,r2=25);
car();
translate([0,150,0]) rotate([0,90,0]) cylinder(h=70,r1=25, r2=15); // fluidhole
translate([0,100,0]) rotate([0,90,0]) cylinder(h=90,r=15);// airhole
				}
				}
}
}
}
///////////////////////////////////////

mirror([ 0, 1, 0 ]) {
translate([0,6,0]){
scale(0.125,0.125,0.125){
difference(){

translate([0,-50,-230]) cube([850,400,240]);
translate([25,25,-25]){
difference(){
			difference(){
translate([0,-50,-250]) cube([800,350,250]);


car();
translate([0,150,0]) rotate([0,90,0]) cylinder (h=90,r1=25, r2=15); // fluidhole
translate([0,100,0]) rotate([0,90,0]) cylinder(h=70,r1=15, r2=15);// airhole
				}//difference
				}// translate

color ("blue") translate([25,25,-30]) cylinder(h=50,r1=25,r2=17);
color ("blue")translate([775,25,-30]) cylinder(h=60,r1=25,r2=17);
color ("blue")translate([25,225,-30]) cylinder(h=60,r1=25,r2=17);
color ("blue")translate([775,225,-30]) cylinder(h=60,r1=25,r2=17);
color ("blue")translate([425,225,-30]) cylinder(h=60,r1=25,r2=17);

}

} // difference
} // scale
} // translate
} // mirror
} // flip and rotate entire model

Lesson 7

The next step was to put download the stl generated from openscad to the shopbot. The material we used for cutting was machinable wax and we used a 3.125 inch mill to cut out the rough and fine parts of the model.

After 3 hours of milling the model was finished. The model took longer than i expected. The relatively high walls of the model took a long time to mill and as i needed two days to mill, there is a clear line where i restarted the machine on the casted model. I copied the x,y settings and restarting line, but still this gave a little deviation on the model.

Lesson 7

After finishing the mold above, i proceded to the next step. I needed to fill this model with OOMOO 25 to get the mold to cast the car. I filled the mold with water to determine how much OOMOO i would need. This appeared to be exactly right, but i did not take into account that a considerable amount of OOMOO will stick in the container, so it's be wise to make a little be more than you think you need.

The two components needed to be mixed on a one to one ratio on volume basis and i mixed it until the OOMOO was smooth and evenly colored. Then i poured the mixture in the machinable wax mold and let it rest. The package said 75 minutes, but i needed at least 3 hours to get it sufficiently hard (and non sticky) to take it out.

Lesson 7

I took the mold out the machinable wax and went to the next step in the process. After taking all the safety precautions (mask, gloves, goggles) for working with hydro-stone i measured the exact amount of hydro-stone i needed relative to the amount water and mixed it with a mixer and drill until i had an even and smooth substance. Then i poured in the hydro-stone mixture in the OOMOO mold, which i had reinforced with some triplex. The back walls of the OOMOO molds were a little thin as i had been very tight with my OOMOO mixture. I put the mold into the oven at 65 celcius/150 fahrenheit and waited for 45 minutes.

The car cast was hard and dried and i suspected that i could have taken it out sooner. You can see a line on the side of the cast, where i stop the milling at the end of the day and continued the next day.