The TSN BasicBot being manufactured in this tutorial is a STEM teaching platform designed to be versatile enough for expansion into other, more sophisticated roles with minimal changes. This is the third installment of the series, so be sure that students have checked out parts one and two as well. Now that the TSN BasicBot Chassis has been CNC’d (see part one) it’s time to get the machine moving. Two wheel servo drive is the force behind the rear wheels, and a unique 3-D printed caster is mounted at the front. The system enables the relatively low-powered motors to move the robot easily on any smooth surface, with highly responsive turning in a minimal radius. In this STEM lesson, students will manufacture the wheels and attach the servos and caster to the front of the chassis. All CAD files are available for download so your students can quickly CNC their way to their own TSN BasicBot.
Round ‘Em Up-Manufacturing the Wheels with CNC
The wheels can be made of metal or plastic. Plastic such as ABS or Delrin can be successfully milled and is shown in this example. The CAM process is the same which ever material is chosen. The feeds and speeds when milling may be different however, so consult the recommended settings for your particular circumstance. SprutCAM is our CAM software of choice thanks to its ease of use and education-friendly pricing from the folks at SprutCAM America.
As always we begin by importing the .igs file into the desktop.
Next the faces are sewn together and the Z-axis is positioned at the top of the part.
Our first machining operation is to cut the three interior pockets of the wheel. Select the edges of the pockets by holding down the ctrl key. Then choose add pocket under the job assignment tab.
The default pockets are too deep but we adjust them to the proper value in the parameters tab. In addition to the -Z value for final cut depth, we can select the tool, adjust feeds and speeds and cut strategy here as well.
The next operation will be to drill the mounting holes in the center of the wheel. Simply select a face on each hole and choose the center option on the job assignment tab.
Double check and adjust the drill depth as necessary in the parameters setting. Here you will also set the feeds and speeds of the drilling operation.
The final machining operation of the wheel is a 2D contour cutting it free from the stock material. Select the edges of the wheel after creating the operation.
Choose add curves in the job assignment tab. The toolpath appears. Under parameters, adjust the depth of cut, feeds and speeds, and strategy.
Remember to click run and look for the happy green checks for each operation.
Next simulate the entire machining process.
Finally, clamp the stock material to your mill and enjoy the power of CNC manufacturing as the wheel is cut out.
Be sure and make two wheels for each robot.
The wheels will be mounted to servo motors modified for continuous rotation. A detailed STEM lesson on servo modification can be found here. Servos are generally packaged with several servo ‘horns’ which are mounted to the splined gear protruding from the case. We use the round servo horns in this case, drilled out and bolted to the wheels as shown. Nylon 4-40 nuts and bolts were used in this instance. Metal works fine as well of course, but in class we often use nylon bolts. They are surprisingly durable and are easily cut to length.
A Question of Balance
The front end of the TSN BasicBot is supported by a caster. The caster is a 3D printed piece with a single bolt in the center and a decorative yet functional marble as the rolling surface. A metal 5/8 inch ball bearing can be substituted as well. The marble simply snaps into position and is firmly held in place by small overhangs on the walls of the caster. It can then be placed in a variety of locations on the BasicBot chassis.
Attach the servo mounts (STEM lesson found here) to the upper and lower level of the chassis using 4-40 bolts. Bolt the servos to the mounts, making sure to route the wires forward toward the front of the chassis.
One standoff at the front of the robot is sufficient for chassis rigidity thanks to the inherent strength of the rear servo mounting system. Pull the servo wires up through the curved ‘smiley face’ for access to power and signal from the microcontroller. The photo shows two additional standoffs at the front of the robot in preparation for mounting the third and final level.
In the next TSN BasicBot STEM lesson, students will add a third chassis section, power supply, microcontroller and breadboard, as well write a simple program to get the robot moving under its own power. Stay tuned.
Author: Al Chirinian