Sunday, November 07, 2010

Assembling a Sentry Gun

Doing some more clean up with documentation. The TF2 Sentry Gun paper model is very under documented. Magically jumps from a base stand to a level 3 within four days.

I've recently uploaded revised schematics to replace the very ineffective ones posted about two years ago. However, there's no documentation of how the parts were assembled. Time to fill in those gaps.

Building a Sentry base
The base needs to be stiff. 110lb cardstock without special treatment methods will be ineffective in supporting the loads this model will encounter. Recommend using 110lb cardstock only for tubing and using thick laminates of Magic: the Gathering to assemble every part on the base.

Magic cards offer good strength for little material. Treat them like sheet metal when building the front legs. Use thicknesses of 8 cards to create 2.5mm thick sections. The curved side frame posts were built in that manner. A thickness of 4 cards was used for minor sections like the upper parts of the front legs.

 110lb cardstock is weak when used as a plane sheet, but when rolled up with or without support of a 3.175mm wooden rod, it's sturdy. The main post and rod running through it were done with both methods: using a wooden inner rod and without.

 These rear legs are very troublesome. You're going to have minimal adhesive holding these parts together. Recommend making biscuits and joints to connect the sections. Super glue is recommended for these parts.

 Tubes, Tubes, Tubes
 A lot of the sentry will be large sections of material. The trick is to lighten up the material as best as possible. The ammo drum was built almost like a papercraft model, with lots of empty space inside. Making the drum solid would contribute excess weight to an already unstable model. Here, there's the inner drum (left), outer support drum (middle), and the outer shell (right), which is composed of a tapered cylinder. The outer shell fits alongside the outer support drum to stiffen one side, while providing the tapered shape on the other side. Magic cards were used for the flat backside, offering good planar stiffness with minimal warping.

 I built the ammo drum support arm in sections. Don't do that. Build this section as stiff as possible, and out of as few pieces as possible. This part is very problematic as it supports a lot of load and will be subject to constant bending for the rest of the life of the sentry. Don't even try using 110lb cardstock without some inner support material. Stiffer the better.

The minigun assemblies are designed to spin, but also designed for weight reduction. The arch shaped supports are fleshed out using tubes on the curved ends, and troughs made of Magic cards. Tubes offer stiffness with shape, and the cards offer stiffness with flat surfaces. The only time something needs to be completely solid is for shapes 3-4mm thick or less.

For the barrels, they span a length of 40mm, but you can support them on the edges with 12-16mm of material. This lightens up the parts significantly.

For this build, I considered using pre-existing plastic tubes to minimize friction when rotating. Not a big issue in the long run since it won't see much rotation. The tubes offered strength and volume for little effort. The barrels themselves need rework, but I've drafted new plans for a stronger set in the newer plan revisions.

 There's a bit of sag from the barrel weight, since rolled up 110lb cardstock is still heavy. There's little room for a counterweight, but adding more mass to the supports only loads the center mounting plate more. The goal is to minimize overall weight as well as keeping it balanced. The first objective helps with the second.

 The Turret
The rocket turret is another part that requires lightening. The image above demonstrates how it was built: four tubes running along the length, with tabs on the sides to allow for a cover to be placed around it. Build this like sheet metal and not like a milled object. 

These support arm parts for the turret were built light and stiff. They may appear thick, but they're mostly hollow inside.  
The amount of "large mass supported by a thin strut" situations on this model are numerous. Luckily, this is the least severe of the situations. The white support base is made of several layers of 110lb cardstock, but can be done with Magic cards. The trick is to use the thickness of the material to your advantage for small details.

Wiring the Sentry
This wire assembly can be tough. You'll need to find wire with a sleeve diameter of 3mm, or make your own by rolling printer paper around some thin wires. Printer paper is soft and bends easier than 110lb cardstock when rolled. Goal is to keep the rolled thickness to a minimum. Thicker the walls, harder to bend. 

Attaching the wires to the plate can be done by leaving bare wire on the ends and threading it through a sheet. Just glue another sheet to sandwich the wire ends in, and they should remain in place.

I've documented the ammo belts here, and with the rest of the new diagrams, that should cover all the miscellaneous aspects of building the sentry gun.

Now.. Just need to properly present the completed model this time around.

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