Jon's Rent-a-Crate frame
From MIT Project ORCA
This is a concept for the replacement of the current '80/20' frame.
This design retains the current paradigm of a sturdy frame which holds everything together. The thrusters, batteries, sensors, and hull are all separately attached to the frame.
This permits easy adjustment of the frame as we discover problems or new requirements.
Contents |
Summary of changes from last revision
- The main long pieces are now at the very bottom of the frame, protecting the things on the inside.
- Some screw holes have changed location, and the short pieces for mounting the thrusters have changed length.
- The batteries are now cradled in the bottom rather than hanging off the side.
- The flat bottom can be used for a 'treasure gripper'
Summary of features
- The frame is taller, wider, but shorter.
- The thrusters are _internal_ to the frame, providing protection and reducing net width.
- The frame with thrustes and sensors attached will fit into a standard rent-a-crate.
- The hull will fit with the batteries in a standard rent-a-crate.
- We can ship the entire sub, mostly assembled, in two crates.
- The hull is designed to quick-connect to the frame using locking pins.
- The frame is designed using lighter weight 20mm extrusion.
- The frame is designed using end fasteners, rather than bulky joining plates.
- All hardrware is mounted to the frame using plastic mounts which are waterjet cut.
- Lifting is by the outer corners of the frame.
- Fore and aft cross rails provide good grip locations; one person will be able to carry the sub.
- All sensors and thrusters are internal to, and protected by, the frame.
- Buoyant materials are concentrated at the top of the structure, heavy objects at the bottom.
- The frame is designed to support _three_ thrusters, with a single Z thruster centered below the hull.
- The frame can be easily modified to carry _four_ thrusters, if a single Z thruster approach proves unstable.
- The frame was designed using inch measurements
(Note: the below image is shown in 'transport' configuration. Everything is upside down to go into the crate. The dark orange is the space defined by the inside of a rent-a-crate. Blue is attached hardware, thrusters, adf, batteries. Green is the aluminium frame Red is the dry hull. Purple are mounting plates. )
Overview image and design file
File:Crate frame design file.dxf.zip
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Extrusion cutting list
The extrusion material is available from McMaster-Carr, part number 5537T13 for 8 feet. 18 feet is required for the frame, although cutting efficiency probably means that 3 lengths of material will be required.
Extrusion and connector information
The extrusions should be cut using a table saw and a 'sled' to make sure that the cuts are perfectly square.
The extrusion ends will need to be tapped to accept 5mm screws.
The extrusions are connected together using 'end connectors'. These are McMaster-Carr part number 5537T63, Item K on page 1588. The end connectors are screwed into the hole in the center of the extrusion, and require a clearance hole for the proper allen wrench. The end connectors are very secure, and clean and light. The problem is that they are _not_ stainless steel.
Making Stainless Steel Connectors
The end connectors are composed of a 'washer' and a standard button head socket screw. The 'washer' is rectangular to fit the T-slot, and has 'ears' which prevent rotation of the attached extrusion. The 'washer' can be water jet cut, and then the ears folded using a 'brake' sheet metal bending tool.
It is probably easier to make a chain of washers, and bend their ears all at once, and then to cut them apart.
The screws are McMaster part 92095A211, and the thin stainless stock is McMaster part 88885K13.
File:End connector waterjet plan.dxf.zip
The other way that connections are made to the extrusion is with something known as a 'T-nut'. This is a rectangular nut that fits into the slot, with a threaded hole in the center. Again we have the problem that the available T-nuts are not stainless steel.
These can be easily waterjet cut, but then the center hole needs to be tapped. Again, a set of T-nuts can be made, and then broken apart, rather than single nuts.
The stock used is thicker stainless steel.
File:T-nut waterjet plan.dxf.zip
Connector plate cutting data
I have abstracted the bits that should be waterjet cut as a separate single layer file.
To build the frame, we need two large mounting rings, 2 Z thruster mounts, and 4 X thruster mounts. I recommend spares.
The battery mounts need to be drilled with a clearance hole for 5mm bolts.
The large mounting ring needs to be drilled for a clamp bolt and also for mounting bolts. The mounting holes can either be 5mm bolts _or_ sized for quick release pins. The large mounting rings have a bit of material that should be cut away after drilling the bold holes.
The thruster mounts use 'aluminium binding screws' McMaster part 93121A345, 0.2" diameter, 1.0" long as supports for standard hose clamps to hold the thrusters.
The battery mounts use the same binding posts to hold nylon webbing with a buckle for mounting the battery tubes.
We should be able to machine these mounting plates using the same plastic which we have on hand for machining the end plates.
File:Crate frame plates.dxf.zip
On the off chance that we end up using the old PVC hull, I have generated a mounting bracket for the OD of the pipe. This pipe is 8.625 OD, rather than the 8" OD of the tube.
File:Crate frame old hull bracket.dxf.zip
McMaster Parts List for Above
- qty 3 Part#5537T13 20mm extrusion
- qty 24 Part#5537T63 end connector for 20mm extrusion
or make, using
- qty 20 Part#92095A211 5mm button head socket cap screw 12mm long
- qty 20 drop in T nuts
- qty 1 Part#88885K13 stainless steel type 316 0.036"
- qty 1 Part#8586K522 ABS 1" thick, or other easily machined moderately sturdy plastic
- qty 1 Part#8586K492 ABS 1/2" thick, or other easily machined moderately sturdy plastic
- qty 20 Part#93121A345 aluminium binding posts. Can substitute stainless steel, which is _slightly_ thicker
- qty 4 Part#98480A009 stainless steel 'safety pin' for attaching frame to hull mounts
-Jon
