Sunday, May 27, 2012

Pneumatic Toe Pincher Coffin, Part 4

I decided to revisit this project and make some improvements. The solid plywood coffin lid tends to "flap" when opened, and the pneumatics are not smooth. It takes a while for the cylinder to overcome the weight of the closed lid, and if I open up the flow control too much the lid opens with too much force.

This year I am replacing the lid with a "torsion box" style lid, which will include a nice routed edge profile. Torsion box construction will result in a very rigid lid, eliminating "flap" - and I am also making some changes to the pneumatics.

The problem is that since the coffin is laying on the ground horizontally, the weight of the lid decreases as it is opened. It takes more air pressure to start the lid opening than it takes to complete the arc to the fully opened position. The cylinder needs an initial burst of air to overcome the weight of the lid, then a slow flow of air to open the lid slowly.

Bimba makes accumulators (air tanks) in a variety of sizes. It only takes a small volume cylinder to add enough burst pressure to overcome the lid. For my project I selected the Bimba D-1500-A-3. I also added another flow control. See the diagram below.

The air compressor connects to the coffin pneumatics via standard 1/4" industrial quick-disconnect fittings. The first component is the air filter, to prevent debris from entering any other components. Next is an optional large accumulator. If large enough, this tank can be "charged" with enough air to run the coffin for an entire evening, but at a minimum it serves to buffer any air line pressure drops due to other props consuming air from the same compressor. The check valve keeps this tank pressurized even after the air line is disconnected from the coffin.

Next is flow control #1. This flow control determines the speed at which the coffin lid will open AFTER the initial weight of the lid has been overcome by the small accumulator and flow control #2. Flow control #2 should be open a bit more than flow control #1, but adjusted so that overall lid movement is smooth after the coffin has been placed into the environment where it will operate. The angle of the coffin where it rests on the ground will affect the weight of the lid through its opening arc.

Flow control #2 connects to the 3/2 valve which connects to the cylinder. The other side of the valve connects to flow control #3 which controls the rate the coffin lid closes when the valve is turned off. This connects to a small exhaust muffler to reduce air hissing noise.

Another major change is the shift towards push-to-connect hose and fittings. Such much easier to work with than coiled rigid hose and 1/8" threaded connectors. Automation Direct sells a broad selection of push-to-connect fittings, tubing, and flow controls.

The new lid is made out of two sheets of 1/8" doorskin with 3/4" pine beams that are approximately 5/16" wide. Grid spacing is 4.5" and the "bottom" lid is 1/8" wider on each edge than the coffin. The "top" is 1/8" narrower. Edges are 1.5" x 3/4" pine, and are routed with a Whiteside 3172 "Wavy Edge" bit.

The hinge side of the lid and the bottom are made of 5.5" wide pine rather than the 1.5" so that there is support for screwing the hinges and cylinder clevis to the lid.
Coffin lid torsion box internals       Coffin lid routed and base coat  Whiteside 3172 Wavy Edge

Return to: Pneumatic Toe Pincher Coffin, Part 1

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