Servo - April 2017

Given the awesome dangerous power of the 6,000 psi canister, the Action Gas guy was curious about what we planned to use it for. So we told him, and he thought it was awesome. We hoped it would be — assuming we could get the ballistic missile of a canister back to Robot Central safely. It was a very cautious ride back from Lake Elsinore.

Back at Robot Central, we strapped the canister to the leg of the built-in workbench to keep it secure in case the “Big One” hit. If that ballistic missile hit the ground the wrong way, we would need more than The Rock in a helicopter to save us.

Once the canister was secure, we still had a few more errands to run. Specifically, we needed to find a solution to a problem we didn’t have with low pressure testing. For the PVC prototypes and for the low pressure baseball tests, shooting at our plywood target was fine. It was starting to crack a bit, but the baseball would bounce fairly harmlessly off of the target and roll lazily down the hill in the backyard — our makeshift shooting range. This time, however, we were hoping to shoot a steel slug at up to 300 miles per hour. The plywood target wouldn’t stand a chance, and we were worried that after ripping through the target that the slug would keep on going and either ricochet back off the hillside and hit a defenseless car sitting in the driveway or keep traveling up the hill and into the wild blue yonder.

We wanted something that could absorb the force of the projectile; something that our slug could burrow into instead of ricochet off of. So, we hopped back in the truck and headed to the local feed store for some bales of straw.

The yard guys at the feed store were a little incredulous. The quality of the straw bales, they warned us, was subpar. It was the last of their current stock, and had gotten a bit damp and muddy in the unusually wet weather Southern California had experienced in the last few weeks. The yard guys asked what we planned to use the bales for — maybe bedding or something like that. Our answer of “target practice” was truthful, though we think more likely conjured images of an archer with a bow and arrow rather than a high pressure robotic air cannon.

We hauled the straw bales back to Robot

Central and stored them in a horse stable in the backyard. The bales were not nearly as heavy as the nitrogen canister, but were certainly over 100 pounds each. Everything about this project was turning out to be heavy.

A Clockwork Test with Orange Straps

With our nitrogen strapped down and our straw bale target set up, we were almost ready for testing. The last step was to prep our slugs. McMaster had a nice selection of large diameter steel rods, so we ordered some to have a few options for potential projectiles. One potential slug was a 2-7/8” diameter 6” long rod of chromoly (numerically known as 4140). We were concerned that diameter could be too small for our 3” diameter cannon barrel, so we also ordered a few 3” diameter rods ( 3” long), thinking that the tolerance on the diameter could end up giving us the perfect fit.

We had a few alloys to choose from among the 3” long slugs, too — 1018, 4140, 1045, and 8620. The softest alloy was the 1018, and that rod had a few dents from shipping. That wasn’t ideal because it would probably deform so much upon impact that we wouldn’t be able to reuse it. As it turned out, we didn’t need to hem and haw about the virtues of different alloys because the 2-7/8” diameter chromoly rod ended up being a perfect fit for our cannon barrel. Plus, it weighed about 10 lbs, which had a nice round appeal.