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Scuba Forum / Scuba Equipment / January 2005Opinion on tank neck threads please ?
One of my 2 liter aluminium cylinders has just been condemned. They tell me that there are no Visual Plus probes for taper thread cylinders. Luxfer are unable to confirm which alloy it was made from and want me to ship the cylinder to them - Branded SM Gerzat, a subsidiary of theirs and made in '84. The strange thing is that there are 3 "cracks" exactly 120 degrees apart and that the "cracks" only appear on every second 'peak' or 'ridge' of the actual threads. Leading up to the "cracks" the material gets "dull" and almost looks smeared, and then it continues all bright and shiny on the other side of the "crack". I have taken it to several local inspection facilites and some engineering shops and the answers remain inconclusive. Some suggestions that it could be the 'tool start marks' from when the thread was cut in the cylinder neck ? There has been a suggestion that I should try to get it x-rayed ? Pictures are here : http://www.scubaculture.co.za/tank_neck_crack.html rgds monty Tapered thread in an AL scuba tank??? From looking at the picture, it looks like those are marks left by an interrupted tap when the cutting was stopped, but what is strange is that you generally never see a three flute tap of that size. More questions than answers I am afraid. Could also be from the male threaded part. Why don't you try chasing the thread, and see what you get? "Scott" <scottk@localaxes.com> wrote in message > > Tapered thread in an AL scuba tank??? Yeah, quite common here - especially in medical oxygen cylinders. > Why don't you try chasing the thread, and see what you get? I have been told repeatedly by several people here that that is "illegal" or "unacceptable". No modifications of threads allowed whatsoever - not even for the purposes of 'cleaning up' the threads ? rgds monty In my experience, X-raying will cost more than a new tank. > One of my 2 liter aluminium cylinders has just been condemned. > [quoted text clipped - 27 lines] > rgds > monty > One of my 2 liter aluminium cylinders has just been condemned. > [quoted text clipped - 27 lines] > rgds > monty Why are you putting so much effort into an iffy 20 year old tank with obvious damage on the tank neck threads? Jon C <news@jonnythan.com> wrote : > Why are you putting so much effort into an iffy 20 year old tank with > obvious damage on the tank neck threads? 1. Because you cannot buy 2 liter cylinders here, they simply don't get imported. 2. Buying stuff over the Internet isn't easy since we have "Exchange Control" ( have to apply to the reserve bank for money to leave the country - I live in South Africa ) 3. Because I am sick and tired of dive shop staff telling me what I can & can't do when they don't know what they are talking about. rgds monty > 3. Because I am sick and tired of dive shop staff telling me what I > can & can't do when they don't know what they are talking about. Then you should get along fine with them.  Signature"We're going to rush the hijackers." -Jeremy Glick, aboard United Airlines flight 93, September 11, 2001 1. If you have to worry about the tank, why continue to use it. Replacements are not all that hard to come by. 2. Send it to Luxfer. If it's defective, you have a chance of getting help with the cost of the new tank. 3. Scott's an expert compared to me, but if I saw something that looked like a crack, surrounded by dull metal, I'd assume it was a load induced crack. That's what the edges of cracks in aluminum I've seen look like. Lee > One of my 2 liter aluminium cylinders has just been condemned. > [quoted text clipped - 27 lines] > rgds > monty Lee wrote: > 2. Send it to Luxfer. If it's defective, you have a chance of getting help > with the cost of the new tank. > 3. Scott's an expert compared to me, but if I saw something that looked like > a crack, surrounded by dull metal, I'd assume it was a load induced crack. > That's what the edges of cracks in aluminum I've seen look like. I got a reply from Luxfer UK: "These mark are not thread cracks but are a machining feature that occurs when a fluted tap is used to cut the thread. These are common when using this thread machining method. Reference is given to this in BS EN 1802 : 2002, Periodic inspection and testing of seamless aluminium alloy gas cylinders. The thread is a 6 degree 52' taper, 19.8 DIN 477 thread and is satisfactory to guages" rgds monty |
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