Debugged mill cuts

Continuing with my DARwIn-OP clone robot, I am now cutting the shoulder parts.

After most of the parts done, I am finally getting a better setup for cuts in my CNC Sherline mill.

First I started fixing the aluminum sheets over small MDF board, cutting with 3mm endmills, a center drill, and several drills (1.6mm, 2.0mm, 2.05mm, 2.5mm, and 2.6mm), later I moved to a smaller 2.5mm endmill for better score’s finish. Following the DARwIn-OP Fabrication Manual, I placed checks on the cutoff path so the parts would not come loose. Later I replaced the MDF base board with an aluminum plate. Then I tested only using a single 1.5mm endmill for everything, but I didn’t like the surface finish and the holes weren’t perfect enough.

Now I think I have a better setup:

  • Following a friend’s setup, I replaced my mill’s aluminum matrix plate with a MDF board with the same size and attachment.
  • I fix the aluminum plates to the MDF board with several 2.5mm x 10mm screws around the parts to be cut. For this, I drill the MDF with a 2mm drill, 9mm deep. After several cuts, the MDF can easily be changed with another one.
  • I went back to milling with a 2.5mm endmill (or 2mm endmill depending on the detail of the part, the Fabrication manual recommends a 3mm endmill, but some of the designs require 1mm radius cuts).
  • I went back to using drills of  size 1.6mm, 2.0mm, 2.05mm, 2.5mm, and 2.6mm. But since these are carbide drills with a wider (1/8″) shaft, I am not using a center drill any more to mark the holes. (After a few tests, I saw no hole displacements).

    Drill
    Drill
  • I am not leaving checks on the cutoff paths anymore. Now I screw the part to the MDF before doing a complete cutoff. I use some of the original holes of the part for this.
  • For any big slot that need to be emptied on a part, I mill it completely so no sizable aluminum bit that gets loose could break the endmill. If the slot is too big, then I would also place some screws on it so I don’t have to mill it all off.
  • Also, I replaced the Sherline mill headstock with an ER-16 model (from Sherline also). This allows for much easier tool changes, and ER-16 collets are available such as Techniks collets with very good accuracy (low TIR) which is important to keep small endmills well centered.
  • I upgraded the mill to 10k RPM (with a 10k RPM pulley set from Sherline).
  • I am cutting with the 2.5mm endmill at 5k RPM, 200mm/s speed, and 0.4mm depth of cut. For the 2mm endmill, I change the depth of cut to 0.25mm.
  • I don’t use any lubricant, only an air blower to keep the cutting area clean (see photo).

The following picture shows the setup while milling the last cutoff:

Cutting DARwIn-OP shoulders
Cutting DARwIn-OP shoulders

The older gcode files I wrote don’t follow this setup, but can easily be updated.

DARwIn-OP body

My DARwIn-OP clone is taking shape. I mounted the chest section.

DARwIn-OP de los pies a los hombros.
DARwIn-OP up to the shoulders.

The chest parts (where the computer is mounted) are pretty big and my mill doesn’t have enough range in the Y axis to mill it in one cut. Luckily a friend that I met through this blog helped me with the cuts. He lives in my city and has a modified Sherline mill with bigger range. Also he is a Sherline dealer in Chile and works on CNC projects and has several interesting posts on his own blog at metaltronics.wordpress.com.

Fresado en Sherline modificado
Milling on the modified Sherline mill.

For the chest cuts, we tested using only a 1.5mm endmill, even for holes (1.6mm the smallest). For future cuts I will use bigger endmills such as 2mm o 2.5mm as in my previous cuts, the 1.5 endmill requires a depth of cut too small (we used 0.2mm) and are more fragile.

Since I am not going to use the original DARwIn-OP electronics (Fit-PC + CM-730 controller), I modified the chest parts to allow mounting an Odroid-XU3, in particular for access to the rear ports and mounting points.

Partes del pecho con Odroid-XU3 y servos de hombros y cuello.
Chest parts with mounted Odroid-XU3 (and USB2AX) and shoulder + neck servos.

The Odroid-XU3 fits without problem, but to mount the USB2AX (interface to servos) I had to adjust the TTL connector to a vertical position.

In the rear, I exposed the Ethernet port, 4 USB ports and a micro-HDMI (with a 15cm extender). Probably I’ll leave only 2 USB ports exposed, removing one connector to allow for more internal USB ports.

Puertos del Odroid-XU3 expuestos
Odroid-XU3 exposed ports.

The upper chest part was modified to mount a 10-DOF, centering the accelerometer chip in the same place where the original CM-730 controller has its own. Additional holes tapped to 2.5mm were added for future components.

10DOF montado
10-DOF mounted on upper chest part.

Because of the change of electronics, a 6 port TTL dynamixel hub is needed to connect the servos. In the rear there is just enough space to mounted.

Hub dynamixel de 6 puertos
6 port TTL Dynamixel hub.

DARwIn-OP Leg Mounting Brackets

I milled the PR13_B_SPACER_PELV mounting brackets which go on top of each leg.

DARwIn-OP Mounting Brackets
Leg Mounting Brackets

The brackets are necessary to attach the legs to the chest section where the main computer resides. Currently I am redesigning the chest parts to fit a Odroid-XU3 computer, the main changes are in the position of external USB and Ethernet ports.

The cut files for the mounting bracket are in SourceForge here. In the README file is the list of gcodes files.

For future parts I’ll try to change the cutting strategy for milling. Instead of using several drills and endmills, I’ll try to use a single 1.5mm endmill for all cuts, avoiding tool changes.

DARwIn-OP Covers

I just received the plastic covers for my DARwIn-OP robot clone. I got them 3D-printed on a SLS printing service in orange.

The 3D models are published in this ZIP file in the original DARwIn-OP SourceForge repository.

I tested the assembly of the left leg and it locks very nice compared to the bare-bone right leg.

DARwIn-OP Leg Covers
Leg Covers