The goal was to develop a drill which could be deployed from one of the small payload containers embedded in the wheels of the Jet Propulsion Laboratory (NASA/JPL) Axel prototype Mars rover and drill into the side of a cliff. These are the basic requirements given when I began my internship:
- Drill 2 cm deep
- Drill at 1.5 mm / second
- Fit into the JPL prototype rover ‘Axel’
- Rotary-percussive mechanism
- Collect powdered rock sample
The drill bit I needed to use had already been selected, a Relton Groo-V 1/8 in bit, which is very effective drilling into hard rocks, especially when combined with percussion. Most of us are familiar with rotary drilling, it is what is commonly used for drilling wood, metal, and other materials. Rocks are extremely hard and brittle. Hammering the drill bit into the hole introduces cracks in the rock, making the material much easier to cut. This results in significantly faster penetration rates, lower energy consumption, and a reduction in bit wear. The drill bit is much tougher than the rock, even though it may not be much harder. This is important because some rocks (such as saddleback basalt) have a compressive strength of 280 MPa, significantly stronger than even the best concrete. Percussion is even more important for drills going to other planets, because water cooling isn't an option.
In the design of the drill, it was decided to use off the shelf parts wherever we could, so a much larger Hilti hand-held rock drill was disassembled to see what parts might be useful. We chose to reuse the percussion clutch components because of the difficulty that it would take to manufacture new ones. The percussion clutch components in the Hilti drill were made out of hardened steel and also had a coating on them, making them very strong and abrasion resistant so the pieces could be slammed against each other many times. Recycling these components saved money and time.
For the drill rotation and percussion I chose a 100 Watt Maxon motor with a diameter of 22 mm, and a 10 mm Maxon motor for advancing the stroke. The drill motor was integrated to the 3D printed chassis and the stroke motor (Z-axis) was mounted to the back of the linear slide. The drill motor had an integrated 2 stage gearbox but still needed an additional reduction of 3:1 for the output. It was a packaging challenge to fit the gears in such a small space but I was able to make it fit, and the flexibility of 3D printing to fabricate any geometry helped.
The final product was delivered to the Jet Propulsion Laboratory in 3 months. During field trials, it was proven capable of drilling through multiple rock types and collecting samples from horizontal and vertical surfaces. Honeybee Robotics had never made a smaller drill, nor one that was as inexpensive as this one, the project was far under budget.
While the project was a success, it did not come without some lessons that were learned the hard way. Recycling components from other products can be great, but you don't always know what you are getting. Our machinist found that the rotary percussive mechanism was hardened not just on the percussive surface, but almost everywhere. As a result one of the tapped holes had to be made with an EDM. Luckily, this operation was only $75.
In the first picture you can see a bright nickel coating on the housing of the drill. Although this certainly added some style, I regret doing it and wish I had skipped it. The plating mask was complicated and the vendor was not able to do it. The vendor should have refused, but attempted anyway and added a month long delay to getting my parts. The plating build-up was much thicker than it should have been and the part required light machining so the motor could fit. As a result the vendor learned a lesson as and stopped offering that service with masks. I learned not to make intricate masks and get a sanity check before trusting if a vendor can perform as advertised, especially when it is through a subcontract. Finally, I learned to do more planning when taking on a project, and resist the urge to begin modeling and engineering right away.