wiki/ LV2cLineCutters

The line cutters are electrically actuated. They cut the drogue parachute line at low altitude to release the main parachute.

PlugLineCutter-v2c.png

A DXF format drawing of the v2 cutters is available here. The drawing is also available in pdf format.)

The v2c cutters use an oil hardening steel piston actuator (labeled 'Cutter' in the drawings) which is explosively propelled inside an aluminum body tube. The piston has a circular wedge-shaped edge formed on a bench grinder, drilling the shear pin hole, cutting to length, heat treating the blank, and then sharpening the edge with a whetstone.


Attachments:


Links:

2009-03-29


There were modifications made to the original line cutter design in order to address the stray-thread issues, as well as simplify fabrication, reproduction and assembly of the cutter. The main differences include the following:


Testing:

All testing was done with off the shelf materials, including smokeless powder from the local sporting goods store and standard Estes igniters from the hobby shop. These igniters should be replaced with more appropriate e-matches for actual flights, but were well suited for our reliability testing.

During the testing phases there were a number of things identified as important properties for reliable operation of the cutter. As these were identified, minor tweaks were made to the design of the cutter to ensure the properties were held. Important properties are as follows (useful if/when one needs to re-design the cutters for different cord or for some other reason):

The line we cut is 4 mm climbing accessory line which we get from REI inc., a local sporting goods retailer. The 4 mm line is a good choice in several ways. It's available locally at a moderate price, it has a believable load rating, and it's a robust line resistant to abrasion, kinking, etc.

I should look this up, but if I recall correctly, the price of 4 mm accessory line is about 25 cents per foot, and it has an 800 lbs load rating. Accessory line is a low stretch line with a core of relatively straight fibers and a braided cover of very fine threads which protect the core from abrasion and other damage.

By its nature, the piston actuator used in the LV2c cutter requires fairly close tolerance to get a good gas seal, but some gap between the piston and the body must exist to allow movement of the piston.


Tentative bill of materials and sourcing


Body Jig Fabrication:

Cutter Jig Fabrication

Body Fabrication:

Cutter Fabrication:

Hex Igniter Fabrication:

Stop Fabrication:


Assembly Instructions:

1) Clean body, including shear pin holes, inspect for nicks and gouges. A couple turns by hand with a 3/8 reamer can be helpful. You may need to sand the OD of the cutter with some 600+ grit sand paper to get the deposits off of it.

2) Clean cutter including shear pin holes

3) Verify cutter will travel past shear pin holes

4) Document body and cutter number

5) Select Igniter

6) DANGEROUS: Wear safety glasses! As of yet, after doing 25-30 or so of these, we have not had a soldering iron set off an ematch, but be careful. 800 degrees on the iron seems ok... Solder igniter to interior lead wires of hex bolt.

7) Place putty below igniter up to slightly below combustible material

8) Add 55 grains of IMR-7828 smokeless powder. A small funnel with a tub off the bottom is helpful for getting all the grains into the line cutter.

9) Pack wadding over powder (about 1/2" x 1" of single ply tissue) (Wadding should gently press on the powder with the cutter in place.)

10) Use safety tool to place cutter.

11) Place shear pin, use ABS cement to soften/thin pins.

12) Insert stop and screw, verify correct stop point

13) Insert 4mm line


Pictures and Videos:

Videos of final versions of line cutter with 100% success rate:

Videos of intermediate tests which were mostly successful but had something that was sub-optimal, typically described as "annoying to fabricate", or "annoying to assemble/disassemble", "overly complex", "stickier then necessary" or various other hand-wavy "issues". Each test had minor modifications to the design to address whatever the annoyance at hand was. In each case, the line was cut cleanly and in the worst case you had to pull a little for it to come loose, but the fundamentals of the device appeared to be sound.

Videos of Early Tests with various modes of failure which were easily solved in subsequent iterations: