Making a CNC-machined robotic part from a drawing is a multi-step process. Most people starting out with CNC struggle with the first steps. The combination of software, hardware and machining know-how make what seems easy a real challenge. There is probably no cost-effectiveness in having a CNC machine for occasional prototyping, but being able to get the part you want (when you want it) and learning how the process works is very rewarding. This page provides a brief overview of how the CAD/CAM/CNC system can make a real robotic part from a drawing.
The ankle in this walking robot can be used to demonstrate CNC from drawing to metal. The ankle is machined made from 1/4" aluminum rod. The leg moves forward and backward in two slots cut into the ankle, which rotates within the foot as the robot walks. The Toddler video library has videos of a larger example of a similar part in use. The leg is held in the ankle with dowel pins and standoffs form the instep and outstep of the foot
Every CNC Part Starts With A Drawing
(a) Draw the part in a CAD program
The ankle was initially drawn in Alibre 3D to get an idea for how it would look when finished, but most parts (including this one) can be made from one or two 2D drawings. Because parts are often held more than once to make a second set of cuts, a 3D design could originate from two 2D drawings. Drawing the part you want to make seems to be the easiest step - after this you could spend a whole day getting the finished part to come off the CNC. The process takes time
(b) Export the file to DXF format
Generating G-code is done using DXF files. You can generate DXF formats from many drawing programs. I exported the 3D file into a 2D DXF format and opened it in BobCad CAM software. The CAM software will be used to generate G-code toolpath information. The smaller diameter ends are not shown in the picture because they were not part of the milling operation and will be turned on a lathe prior to milling the two slots.
Generate G - code With A CAM Software
(c) CAM software makes the G-code
Computer Aided Manufacturing (CAM) software converts the geometry into tool-paths, or commands that can be understood and executed by the CNC software running the motors on the milling machine. Creating the toolpaths in BobCad CAM is simple - click on the lines in the order you want to cut them. Next, define the type of tool you are using and the cutting depths. This will offset your tool by half of its width. The G-code is generated automatically and ready for the CNC control software. Save the file and move on.
 (d) Open the G-code in your CNC software
FlashCut Computer Numerical Control (CNC) software executes G-code from a variety of CAM programs. The CNC computer is connected serially to a signal generator/motor driver box which manages the stepper (or servo) motors on the milling machine. FlashCut will show you the toolpaths (in red, below) derived from the G-code file. Each successive pass will cut 0.05" deeper than the prior one. Note how the lines look really close together - this is because we specified an offset for a 7/64" end mill and the slot is less than 1/8" wide - causing the mill to stick close to the Y-axis.
A Quick Detour On The Manual Lathe To Turn The Ends Down
(e) Turn the ankle ends down to 1/8"
In this design the ankle rotates 10-20 degrees within the front and back of two foot pieces as the robot walks. The ankle ends were turned down to 1/8" diameter so the narrow part of the rod could rotate within the front and back of the foot. The finished length of the ankle would be 1 3/4".
(f) Comparison with a finished product
This photo shows the current progress of the ankle next to the finished robot. Each foot is composed of three aluminum parts - the ankle in the middle of foot and two standoffs on the instep and out step of the foot
Vise The Part And Find The X And Y Edges Of The Part
(g) Close milling requires lots of attention and mistakes can be expensive
You'll notice two vises on this Wabeco CNC milling machine - a $49 "machinists vise" on the left and a $350 "Kurt" on the right. In situations where end mills are cutting close to the vise jaws damage can get expensive. Therefore, the first part will be made in the less expensive machinist's vise.
(h) Firmly hold the part in vise
To prevent angled vise closure, two rods are placed in the vise. Angle blocks, step-down blocks and other helpful tools help a machinist hold the part in a vise. The only issue with this part is making sure the ankle is flat in the vise and that it won't move when the end mill cuts into the aluminum rod.
(i) Find the edges of the part
Using an edge finder, locate the X,Y edges of the part. Since you know the diameter of the rod and edge finder tip, this procedure will let you position the end mill in the right place (0,0) corresponding with your drawing's 0,0 location. The edge finder bulb illuminates when the part is located. This step is done using the CNC software. The operator takes successively smaller steps closer to the part until the light shows the edge has been detected.
(j) "Zero" the machine in the CNC software
Once the left and front sides of the part are located, the operator "zeros" the X and Y axis. The Z axis will be zeroed when the edge finder is removed and replaced with an end mill. Taking time to do these steps right means you'll have a part that matches your drawing within 0.001". Once zeroed, FlashCut places a blue dot at the 0,0 coordinate you defined in your drawing.
Run The CNC Program And Make The Part
(k) Replace edge finder with end mill
Now that the CNC software knows the location of the part relative to your drawing, you can raise the spindle and replace the edge finder with the end mill and return it to position. Lower the end mill towards the part and zero the Z axis when the mill barely touches.
(l) Press the button!
This is the reward. Adjustments to feed rate can be made while the program is running. About six minutes later you'll have the finished part. If you are interested in watching the end mill move between the slots
(m) Finished part is ready for the robot
This shows the finished ankle next to a finished foot. You'll notice that the robot legs are held in the ankle using 1/16" dowel pins. We left that step out of the process because it has its own set of challenges to manage! |
