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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|PlugLineCutter-v2c.dxf]].
The drawing is also available in [[pdf|PlugLineCutter-v2c.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 wedgh-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 wetstone.

---

Attachments:

- [[PlugLineCutter-v2c.dxf]]
- [[PlugLineCutter-v2c.pdf]]

---

Links:

[[news/2009-03-29/]]


---

There were modifications made to the origional 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:

- Transition from circular cutter to wedge cutter. We have avoide the stray thread issue
  in doing so. Also simplified process to make sharp edge. Also avoid minor problem of material
  bunching up in the center hole of the cutter.

- Moved to a 5/16 NC threaded back, simplifies igniter assembly by using a COTS
  5/16 hex head bolt. Also allows youto remove the back of the cutter if you need to
  get stuff un-stuck from inside.

- Changed stop to a single piece of HDPE round (to be used once or twice each). These
  are very easy to turn out on a lathe. Added vent holes to the side of the cord 
  to allow air to vent (was previously accomplished by hole in center of plastic tube stop.

- Ensure that the grains of smokeless powder are imiediatly surrounding the igniter. The powder
  choosen does not ignite particularly easly and thus must be in close proximity to the igniter.
  Use proper ammounts of puddy to ensure this.

---
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 appropiate e-matches for actual flights, but were will 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):

- If the sheer pin choosen has too low of a sheer force, the chamber will not build up enough
  pressure for the smokeless powder to burn entirely. During initial testing with a smaller sized
  shear pin, afte disassembling we'd find that about half the powder was left un-burned 
  (even though the cutter still fully cut the line, which may imply that we could use less powder)

- The dimensional relation ship between length of cutter, location of stop block, and location
  of vents is such that the cutter must penetrate the stop approximatly 0.020" (aka, actually be forced thru the line) 
  before the vent holes are cleared. Given that smokeless powder burns more vigeriously as the pressure
  increases, if the cutter has not completly cut the line, it will continue building pressure until it 
  finally pushes thru and clears the vent holes. Of note, it's designed such that the cutter will pene

- During testing we found that wrapping the line in somthing like electrical tape or heat shrink was helpful
  in creating a very clean cut.


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 V2c 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

- 5/8" OD 6061-T6 aluminum round, onlinemetals.com
- Coil Wire (awg?), digikey
- 5/16 NC hex bolts, Ace Hardware
- Nylon Washer, Ace hardware
- 4-40 Stop Screw, Ace Hardware
- Epoxy, Hobby shop
- HDPE Stop, Multicraft Plastics
- O2 Hardening 3/8" dia rod, where from?
- 4mm Accessory Cord, REI
- Heat Shrink, digikey
- Shear Pin, small parts inc, maybe multicraft plastics
- IMR-7828 Smokeless Powder, GI-Joes sporting goods

---

Body Jig Fabrication:

- Take a piece of steel square bar, bore out a 5/8" hole
- Drill holes, sized correctly, at appropiate locations in the jig body.
- Drill a hole suitable for 10-24 threading, thread, use set screw when drilling holes in side of body.

Cutter Jig Fabrication

- Take a piece of steel square bar, bore out a 3/8" hole.
- Drill a hole for a 10-24 set screw
- Drill a hole at the right distance from the end for drilling the shear pin hole.

Body Fabrication:

- Rough cut a piece of 5/8 aluminum round bar
- Moutn in lathe head with collet
- Face off end
- Drill undersized 3/8 hole to proper depth
- Drill F thru end of body
- Ream out 3/8 hole
- Flip in collet
- Face off rough end
- Remove, measure to find length error
- Re mount in collet
- Remove length error
- Use center drill/counter sink to chafer ID of F drill hole
- Tap with 5/16 dap, utilize tailstock jacobs chuck and tap handle to keep alignment
- Mount body in jigh and drill on drill press.

Cutter Fabrication:

- Take length of 3/8" percision ground oil hardening tool steel
- Put in cutter jig, with some length sticking out the end
- Take to bench grinder, grind 60 degree angle. Use jig to provide proper 180 degree flip and yeild a right angle cutter tip
- Re-align rod in jig so it's proper length from cutter end
- Drill shear pin hole in drill press
- Rough-cut cutter from end of rod (use metal bandsaw or other tool)
- Mount in lathe
- Finish rough cut end to proper length
- Heat cutter till red hot with propane torch
- Quickly drop in motor oil to quench/harden
- Take some 600 grit sand paper and do a couple passes by hand around OD of cutter to remove black coating left by hardening

Hex Igniter Fabrication:

- Put hex-nut on bolt
- Mount in 3-jaw lathe chuck
- Drill clearenc hole in top of bolt to proper depth
- Flip in chuck
- Face front of bolt slightly so its at a clean right angle
- Use coupler and jig for drilling wire lead holes in end of bolt on drill press
- Cuter coil wires to length (1-2 inches long)
- De-insulate ends
- Make up epoxy
- Fill clearence hole with epoxy
- Insert coil wires thru epoxy and make sure epoxy fills lead wire holes, make sure coil wires protrude thru end of hex bolt 0.15" to 0.25" as you see fit.

Stop Fabrication:

- Mount HDPE rod in lathe
- Turn down OD to proper radius
- Part off stops to proper length.

---

Assembly Instructions:


1)  Clean body, including shear pin holes, inspect for nicks and gouges

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)  Solder igniter to interior lead wires of hex bolt

7)  Place puddy below igniter up to slighly below combustable material

8) Add 55 grains of IMR-7828 smokeless powder

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

14) Use

15) Completely disassemble

16) Inspect, particularly cutter for nicks, resharpen if needed

17) Clean, particularly inside the aluminum body and 4 shear pin holes




Pictures and Videos:

- [[lineCutterTesting1t.jpg]]
- [[lineCutterTesting2t.jpg]]
- [[lineCutterTesting3t.jpg]]
- [[lineCutterTesting4t.jpg]]
- [[lineCutterTesting5t.jpg]]
- [[lineCutterTesting6t.jpg]]
- [[lineCutterTesting7t.jpg]]
- [[lineCutterTesting8t.jpg]]
- [[lineCutterTesting9t.jpg]]
- [[lineCutterTesting10t.jpg]]
- [[lineCutterTesting11t.jpg]]
- [[lineCutterTesting12t.jpg]]
- [[lineCutterTesting13t.jpg]]
- [[lineCutterTesting14t.jpg]]
- [[lineCutterTesting15t.jpg]]
- [[lineCutterTesting16t.jpg]]
- [[lineCutterTesting17t.jpg]]
- [[lineCutterTesting18t.jpg]]
- [[lineCutterTesting19t.jpg]]

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

- [[lineCutterTestingVideo11.avi]]

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 fundimentals
 of the device appeared to be sound.

- [[lineCutterTestingVideo4.avi]]
- [[lineCutterTestingVideo5.avi]]
- [[lineCutterTestingVideo6.avi]]
- [[lineCutterTestingVideo7.avi]]
- [[lineCutterTestingVideo8.avi]]
- [[lineCutterTestingVideo9.avi]]
- [[lineCutterTestingVideo10.avi]]

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

- [[lineCutterTestingVideo1.avi]]
- [[lineCutterTestingVideo2.avi]]
- [[lineCutterTestingVideo3.avi]]