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Letter from John Sherman (Jumpshack)

 

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Unstable  (D 28930)

Mar 25, 2011, 8:18 AM
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Letter from John Sherman (Jumpshack) Can't Post

******************8
The Next Step



It seems to me that any cutter, which has the potential to close a door and trap, the all-important retaining loop would fail to meet the requirement of “Must not interfere with the normal operation of”.



When CYPRES first was being introduced Helmut Cloth came to me and requested I redesign the reserve closing loop to his specification, which was to allow it to float in a channel over the pilot chute. I refused because of a near tragic occurrence on a misrigged system. His reasoning was because of a known failure rate of the cutter. I felt that was his problem and was not going to compromise my design not realizing that the failure mode to which he was referring might “interfere with the normal operation of”. I changed the loop material to his requested specification because he told me the Kevlar I was using would shred and not cut cleanly. He assured me that his material would always cut cleanly. Well now it allegedly has not and I wonder if we (the industry), haven’t made a mistake in approving any guillotine cutter which does not fully open again after firing. This would be a simple matter to accomplish with of a small reactive charge in the end cap to drive the cutter piston back into the cylinder, when struck, at the end of the cut cycle, leaving an open hole and never trapping the loop.



Failing the deployment into the field of a “Fail-Safe” cutter I don’t see how any of us can continue to believe that the current AAD cutter design does not potentially “interfere with the normal operation of”.



My recommended course of action is to require all the AAD manufacturers to design and develop a “Fail-Safe” cutter, which would be open at the completion of the cycle. This could be coordinated through The PIA Technical Committee. The Committee should poll the AAD manufacturers (whether they are members or not, local or foreign) for estimates of development and delivery time. In consultation with the AAD and Harness & Container manufacturers, establish a date for the beginning of transition, and the completion of transition, after which the old cutters may not be used. The “beginning of transition” means all new sales would be so equipped and field replacement begins. This course would continue to allow the use of the current design, as the potential for failure is statistically low, and less injurious than the grounding of all AAD’s.



I understand that the cutter industry, which supplies all of the AAD makers, already has designs for “Returnable Pistons”. Somebody is going to make a lot of money selling replacements.



John


roq  (D 105)

Mar 25, 2011, 9:18 AM
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

I agree with the John, I don't understand that the dynnema (polyetilene fibres) was utilized for aad reserve loop for all time. The superior elasticity and resistence of the dynnema can avoid the normal separation of the loop filaments .This problem is incresed because the design of the AAD cutters of today
Is urgently necessary to make another cutter design with guilhotine free after fire. Also I think too that the kevlar (aramide) is more suitable for to make the reserve aad loops because the aramide have less broken resistence and less elasticity

roq


Premier billvon  (D 16479)
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Mar 25, 2011, 1:00 PM
Post #3 of 77 (5287 views)
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

>This would be a simple matter to accomplish with of a small reactive
>charge in the end cap to drive the cutter piston back into the cylinder,
>when struck, at the end of the cut cycle, leaving an open hole and never
>trapping the loop.

This would not have helped the situation in any of the failures so far with the Argus cutter. If the cutter cannot cut the loop, the piston does not reach the end of the cylinder, and the reactive charge will not fire.

One can envision a two-charge cutter, with the AAD first firing the cutting charge, then firing the clearing charge say 100 milliseconds later. Ensuring a way to inspect the cutter to see if it had fired would be an important feature of this design.

(Needless to say, designing a cutter that will cut reliably during the first firing would be ideal.)


roq  (D 105)

Mar 25, 2011, 1:07 PM
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Re: [billvon] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

(Needless to say, designing a cutter that will cut reliably during the first firing would be ideal.)

Or, and may be one cutter that remain loose (without pression) inside the hole after firing.


(This post was edited by roq on Mar 25, 2011, 1:11 PM)


Skybear

Mar 25, 2011, 1:41 PM
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

Sorry to say, but this idea is complete bullshit. The cutting blade does not only hit the anvil, but also intrudes 1 to 2mm into it. This is due to the big forces which are needed, to make sure that the blade completely severes the loop. A reactive charge which is big enough to push the piston back will severely damage your rig. The reason is simple. The charge that drives the piston forward can go no other way than this, pushing the puston inside a closed containment. A reactive charge from the other side will loose most of its force through the open loop hole. Even if it is a cylindrical cutter it is not tight enough at the containment to keep the pressure. The only way out is taking a bigger charge, which will make you carrying a little bomb on your back. I prefer the knife and anvil type cutter, which simply cuts the loop into two pieces.

English is not my native language, so I apologize if I am using strange words t some points.


Unstable  (D 28930)

Mar 25, 2011, 5:17 PM
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Re: [roq] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

*Or simply increase the charge on the cutter by what, 10% should do the trick. To me, this seems like an easy solution.


in2jumping  (C License)

Mar 25, 2011, 7:04 PM
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

How about redundant cutters?

Stack them on top of each other and stagger the lower cutter to fire milliseconds later. Or one above pilot chute and one bottom of container and stagger the lower cutter to fire milliseconds later.


Martini  (D 23756)

Mar 25, 2011, 7:45 PM
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Re: [in2jumping] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

How about not using a blade at all? The energy spent trying to mechanically cut a material that is used for armor and cut-resistant materials might be better used as a hot knife to melt the loop. A high current on a thin wire becomes red hot very quickly, a tensioned cord will melt through cleanly in an instant, a small spring could keep the loop and hot wire in contact without creating enough force to trap the loop mechanically. Even if a Cypres cutter is instantaneous (which it isn't) and a hot knife cutter took 1/4 second (which seems unbelievably long) the difference in freefall distance at 120 mph is only 44 feet. So set the hot knife system up to fire 50 feet higher than a Cypres if 44 feet seems excessive.


ufk22  (D 16168)

Mar 25, 2011, 9:01 PM
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

Extra charges, dual charges, triple charges, hot wires. This thread seems to be based on the false assumption that every style of cutter has been failing.
Why spend this energy trying to solve a problem that doesn't exist?
Has a straight-blade cutter ever partially severed a loop and/or locked things closed?
This problem seems to be solely with the round cutters.
Eliminate the round cutter-eliminate the problem.
The save rate on the cypres


Premier billvon  (D 16479)
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Mar 25, 2011, 11:17 PM
Post #10 of 77 (4924 views)
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Re: [in2jumping] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

>Stack them on top of each other and stagger the lower cutter to fire
>milliseconds later.

Then if either one fails and traps the loop you have a reserve total.


DARK  (B 31685)

Mar 26, 2011, 4:27 AM
Post #11 of 77 (4854 views)
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Re: [billvon] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
>Stack them on top of each other and stagger the lower cutter to fire
>milliseconds later.

Then if either one fails and traps the loop you have a reserve total.

I havnt been following what has happened with the argus but if what i gather from this thread makes sense a cutter fired and failed to completely cut the closing lop which caused a total of the reserve? did this actually happen or is it just theoretical?

How can this happen if the cutter is not on top of the reserve pc?


roq  (D 105)

Mar 26, 2011, 4:29 AM
Post #12 of 77 (4853 views)
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Re: [Unstable] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

Yeah, maybe... But I think, because the guillotine is tight after shooting, it is a scenario appropriate for to grab a few loose strands of the loop and avoid the opening the container

roq


ufk22  (D 16168)

Mar 26, 2011, 5:57 AM
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Re: [DARK] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
In reply to:
>Stack them on top of each other and stagger the lower cutter to fire
>milliseconds later.

Then if either one fails and traps the loop you have a reserve total.

I havnt been following what has happened with the argus but if what i gather from this thread makes sense a cutter fired and failed to completely cut the closing lop which caused a total of the reserve? did this actually happen or is it just theoretical?

How can this happen if the cutter is not on top of the reserve pc?
This has actually happened 3 times that we are aware of. Once, in Poland, where it ended with a fatality, twice where it did not, but was discovered later,


nigel99  (D 1)

Mar 26, 2011, 6:21 AM
Post #14 of 77 (4785 views)
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Re: [ufk22] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
In reply to:
In reply to:
>Stack them on top of each other and stagger the lower cutter to fire
>milliseconds later.

Then if either one fails and traps the loop you have a reserve total.

I havnt been following what has happened with the argus but if what i gather from this thread makes sense a cutter fired and failed to completely cut the closing lop which caused a total of the reserve? did this actually happen or is it just theoretical?

How can this happen if the cutter is not on top of the reserve pc?
This has actually happened 3 times that we are aware of. Once, in Poland, where it ended with a fatality, twice where it did not, but was discovered later,

I may have missed some detail but I have a question. There is clearly a difference between a reserve container being "locked closed" where even pulling the reserve handle does not release the reserve and the reserve simply not launching as a result of an AAD failure.

If I understand correctly, the AAD has not cut the loop correctly on a number of occasions and as a resulted prevented manual activation. Is that correct? If you pulled the reserve ripcord in the 3 instances cited would nothing have happened, or was there a high probability of nothing happening?


Divalent  (C 40494)

Mar 26, 2011, 6:38 AM
Post #15 of 77 (4772 views)
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Re: [billvon] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

Wait, wouldn't *both* have to fail to get total failure? If I've correctly envisioned what he's suggesting, if the first fires but doesn't cleanly cut the loop, it would still open if the second one cleanly cut it. (Obviously, the fire order would have to be correct: second one would have be located closer to the pin than the first.)


jojo0815  (D 28155)

Mar 26, 2011, 7:18 AM
Post #16 of 77 (4739 views)
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Re: [roq] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
... I don't understand that the dynnema (polyetilene fibres) was utilized for aad reserve loop for all time. ...
.... Also I think too that the kevlar (aramide) is more suitable for to make the reserve aad loops because the aramide have less broken resistence and less elasticity

roq

the Cypres closing loop is made from Spectra (UHMWPE) which is a PE and has very little elongation. In fact it has less elongation as a stainless steel wire of the same tensile strength. PE is very easy to cut especially when compared to any aramid. The problem with the cylindrical blade is that instead of actually cutting the loop it gets sheared. The filaments of spectra are so thin that it would be possible for them to wedge between the blade and the anvil. Using an aramid in this situation would make matters only worse and actually damage the knife edge during the cutting process.


roq  (D 105)

Mar 26, 2011, 7:50 AM
Post #17 of 77 (4714 views)
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Re: [jojo0815] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

I don't agree
For several years of experience with armides (kevlar) and spectre (PE) lines in all situations the kevlar broken more easy than spectre.
You can make a simple experience. Try to cut one line of kevlar and other of the spectre with scissors blunt
You can see that the line kevlar broken and cut all and the spectre don't cut all wires

One of reasons tha the kevlar is refused for loops is exactly because the less abrasion and elasticity resitence that can make expontaneous broken when suject for fast and strong tension or pression, that can cause the open container out of the time when the loop is in poor conditions or it is suject a very surge strong forces.

roq


ufk22  (D 16168)

Mar 26, 2011, 7:57 AM
Post #18 of 77 (4712 views)
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Re: [nigel99] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
In reply to:
In reply to:
In reply to:
>Stack them on top of each other and stagger the lower cutter to fire
>milliseconds later.

Then if either one fails and traps the loop you have a reserve total.

I havnt been following what has happened with the argus but if what i gather from this thread makes sense a cutter fired and failed to completely cut the closing lop which caused a total of the reserve? did this actually happen or is it just theoretical?

How can this happen if the cutter is not on top of the reserve pc?
This has actually happened 3 times that we are aware of. Once, in Poland, where it ended with a fatality, twice where it did not, but was discovered later,

I may have missed some detail but I have a question. There is clearly a difference between a reserve container being "locked closed" where even pulling the reserve handle does not release the reserve and the reserve simply not launching as a result of an AAD failure.

If I understand correctly, the AAD has not cut the loop correctly on a number of occasions and as a resulted prevented manual activation. Is that correct? If you pulled the reserve ripcord in the 3 instances cited would nothing have happened, or was there a high probability of nothing happening?
Depending on the location of the cutter as specified by the container manufacturer. If the cutter is on top of the pilot chute this could cause a total container lock. If the cutter is below the pilot chute but above at least one of the container flaps, this could cause a PCIT or at least a hesitation in deployment until forces from the pilot chute managed to break the remaining strands and open the container.


lopullterri  (D 20814)

Mar 26, 2011, 8:29 AM
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Re: [ufk22] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

So...place all cutters in the bottom of the pack tray (besides the obvious, which is to correct the fact that they are NOT cutting through the loop on the Argus). Never liked a cutter on top in the first place...for esthetic reasons, if for no other.


Premier billvon  (D 16479)
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Mar 26, 2011, 8:51 AM
Post #20 of 77 (4648 views)
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Re: [Divalent] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

>if the first fires but doesn't cleanly cut the loop, it would still open if the
>second one cleanly cut it.

Hmm. If the first cutter traps the loop completely then no matter what the second cutter does there will be a piece of the loop holding all the flaps closed between the first cutter and the loop anchor point that remains.

One problem here is that the cutter is never located quite where the pin is. So you can trap the flaps below the cutter point or above the cutter point. Having a second cutter can clear the loop above or below the first cutter, but not in both places.

One way to get around this is to do a continuous loop, with cutters on both sides of the continuous loop. That way if any one segment of the loop is trapped, the loop can still feed out and clear the rest of the flaps.

Another way is to always put them at the loop anchor, but then you have the problem with that loop clearing all the flaps, which has been problematic on some rigs. But the problems caused by incomplete cutting go down; the reserve simply does not open, but pulling the pin will still initiate deployment.


DARK  (B 31685)

Mar 26, 2011, 9:26 AM
Post #21 of 77 (4623 views)
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Re: [lopullterri] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
So...place all cutters in the bottom of the pack tray (besides the obvious, which is to correct the fact that they are NOT cutting through the loop on the Argus). Never liked a cutter on top in the first place...for esthetic reasons, if for no other.

Yes this is what I was thinking is the easy solution and if this is what the argus problem is then it seems a little unfair that all the flack is getting sent their way when it is essentially a container design problem that is causing the problem.

obviously the cutters should work and have as close to a zero failure rate as possible but in the event that they do fail it is a container design problem that is causing the total mal

or am i missing something?


Premier billvon  (D 16479)
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Mar 26, 2011, 10:42 AM
Post #22 of 77 (4569 views)
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Re: [DARK] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

>obviously the cutters should work and have as close to a zero failure rate
>as possible but in the event that they do fail it is a container design
>problem that is causing the total mal

Wow, I don't think that's the case at all. When a cutter traps a loop on an upper flap so that it cannot feed through the grommets of a rig and allow it to open, then there is nothing the manufacturer can do to ensure the rig opens properly. It would be like criticizing PD because someone went in when a rigger left a molar strap around a PD reserve. "PD, this is your fault! You should have designed your reserves to open even when riggers make mistakes!"

On the other hand, if Argus does come out and say "our cutters have a failure mode where sometimes they trap the loop" then manufacturers can try to relocate the cutter where the risk of a cutter-induced reserve total is minimized. Or they can decide that such a failure mode is incompatible with their rigs, and refuse to approve it for installation (which many have done.) But they would have to know this to design for the eventuality.


DARK  (B 31685)

Mar 26, 2011, 1:22 PM
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Re: [billvon] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

In reply to:
>obviously the cutters should work and have as close to a zero failure rate
>as possible but in the event that they do fail it is a container design
>problem that is causing the total mal

Wow, I don't think that's the case at all. When a cutter traps a loop on an upper flap so that it cannot feed through the grommets of a rig and allow it to open, then there is nothing the manufacturer can do to ensure the rig opens properly. It would be like criticizing PD because someone went in when a rigger left a molar strap around a PD reserve. "PD, this is your fault! You should have designed your reserves to open even when riggers make mistakes!"

On the other hand, if Argus does come out and say "our cutters have a failure mode where sometimes they trap the loop" then manufacturers can try to relocate the cutter where the risk of a cutter-induced reserve total is minimized. Or they can decide that such a failure mode is incompatible with their rigs, and refuse to approve it for installation (which many have done.) But they would have to know this to design for the eventuality.

Ye I suppose, the way it appears to me though is argus is getting all this flack though(probably rightfully so) but this could easily happen with a cypress in your rig if it is designed to have the cutter on top. were as it could never happen with my rig regardless of the aad because its under the entire reserve and pilot chute.

anyway I was just curious as to exactly what the issue is, I understand that now


RiggerLee

Mar 26, 2011, 5:35 PM
Post #24 of 77 (4425 views)
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Re: [DARK] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

 
This thread is a fun read perticurly looking back on the history of this. First when Mirage moved there cutter above the pilot chute. The demand that this be done with all rigs. Then the cries of out rage after poland demanding that all cutters be moved bellow the pilot chute. Now that we redesign loops, use multiple cutters, continues loops, etc. Good laughs.

In any case this thread started out with a good sereious question of cutter design. It would seem odd that John Sherman be the voice of reason and be makeing the only practical and resonable suggestion on the whole forum. The idea of redesigning the whole universe round the AAD to try to compensate for the failure modes of bad cutters seems ludecris. It seems much more reasonable to address the root problem.

I would like to nominat a cutter for concideration. The cutter I vote for is the one on the M2 from Mars. First off let me say that I don't work for them. I'm not a dealer. I don't have a hourse in this race.

Here's what I know. I'm working with some people on a parachute recovery system. I needed a way to remote command drogue and riser releases. I was looking at diffrent electricly comanded load releases, pin pullers, etc. I was also trying to get them to include an AAD as an independent back up system. In the end they went with pin pullers but I was trying to sell them on the idea of just useing a loop cutter with a normal three ring. Well to make that fly I'd need a cutter that can reliable cut a lose loop with little or no tension on it. I was at PIA and there was a lot of talk going on about cutter desgin and I was takeing a keen interest in it. I got to talking to the guys at M2. They were telling me about their cutter. It's actually a flat faced piston. It's designed to pass all the way by the hole cutting the loop at two places in a shearing motion like two pares of sisures. and they were claiming that they didn't have a min loop tension. They'd only brought a hand full of cutters not really exspecting this level of interest in cutter design or in their cutter in perticular. So they were able to do only a hand full of demos while there. So when ever they got a few interested people they would pull out a battery and fire off a few cutters. I was there with the dutch guy that was doing a study on cutter design watching this one day. I was sceptical to say the least of their claims. But I watched them stick a cypress loop in the middle of their cutter and with it just laying lose in the middle of the table blow the fucking thing in two. Note how I frazed that. It litterally sent the two halfs of the loop shooting out in diffrent directions off the table. clean cuts. Not a single fiber cought in the cutter trapped by the blade. This was with zero, I'll say that again, ABSOLUTELY FUCKING ZERO LOOP TENTION. Then I watched them do it with 1000 lb HMA, they were kind enough to do that just for me as I had issues with aerodynamic heating on the project I was working on. Same thing. Bang! Two peaces of line, he actually cut a section of break line off his canopy to do this, go shooting off the table in opposit directions. I was sold on it.

Look it up. Talk to them. Talk to Alti 2 the US dealer. I don't know much about the AAD it self but it's been around for 8 or so years in their millitary so it's not an infant. But it's the cutter that really got me. There wasn't really time to work it into the current design but I'm still hopeing to make use of them in future test. Looking forward to shooting them into space and see how they do at 400,000 feet. With luck the M2 from Mars will be the first space AAD. And if It can do that I figure it's good enough to go in a rig.

Lee

Lee


RiggerLee

Mar 27, 2011, 5:33 PM
Post #25 of 77 (4268 views)
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Re: [RiggerLee] Letter from John Sherman (Jumpshack) [In reply to] Can't Post

 
I'll share another story with you about cypress cutters for all you nervous nellies out there.

A few years ago I built a set of recovery systems for another company where we used cypress cutters as the primary release. Every time they droped one they fired a cutter and cut a loop. I don't know the exact number but they must have done hundreds of drops with those things. They were going after a mil contract so it was worth the money to them and fireing a $75-$107 cutter to save a $20,000 antina made sence. Out of, I don't know, 500 drop test I don't think they ever had a cutter failure. If one had failed with a $20,000 package I'm sure I would have heard about it.

This was basicly cutting a reserve loop with a spring loaded pilot chute just like in your container. We had a tight pack job with very good loop tension. I'm not sure I'd trust them to cut a lose three ring loop like the current project. Also please note that this was back when the first big wave of cypresses were hitting the 12 year mark. We were buying up every 12 year plus cutter we could get our hands on. So it's not like these were freash off the shelf. You don't have to worry about your cutter getting old i can tell you that they still worked for us.

So although it's not as cool as the M2 I can bear witness to what was basically a fairly exstensive test program of cypress cutters all the way up to the 12 year mark. I have not had the chance to work with the vidgil unit so I can't speak to them but other then a few growing pains as they were getting going, which they seemed to handle in good form, I've never had cause to think badly of them.

Lee


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