Re: [PSUBS-MAILIST] fabrication question

[Phil Nuytten <72020.572@compuserve.com>]

Hi, Alec:
        As Carsten has already told you . . .your ring will distort when
welded. It is not simply a matter of a larger mass versus a smaller . .the
ring by itself will often distort if heated unevenly to welding temperature
and then allowed to cool. High pre-heat helps in marginal situations, but,
unfortunately, is not a panacea . The Newtsub series uses 4" wide by 2"
thick by 30" diameter stainless rings welded to a steel sphere.(that is
finished size - rough blank has greater dimensions)and we have to machine
after welding - sometimes up to 125 thou (1/8th inch). We use a radial arm
milling machine rather than turn the sphere - the real trick is mounting it
absolutely rigid! We have used a rotary table, in the past ( on Sea Otter
-we refaced the hatch seal by turning the entire 16 foot sub on a rotary
table at a shipyard!) as well as a radial arm grinder - which then had to
be hand finished with progressively finer abrasive. I even did a hatch ring
with no machinery a long while ago, Used a spider jack 'in the hole' ,
clamped a rotary bearing to the center ( some fun) made up a rigid bar with
a tool carrier at the end, put in a carballoy tool bit and swung the sucker
'round and 'round by means of a vertical bolt stuck in the toolholder end -
every couple of passes - move the tool in the holder and when out of
travel, move the whole assembly towards center and reset. Boy, talk about
blisters!! 
        If I had to do it somewhere away from a large machining center, I
would look at a Wajax rotary pipe-end dresser. These are around anywhere
where pipelines are laid - which is just about everywhere! the cutter head
clamps onto the pipe-end by means of a chain - like a super heavy-duty
motorcycle chain and can handle even very large pipes with a suprising
degree of accuracy - certainly enough for a low pressure seal. There are
other makes, also . . .check out pipeline companies, or enquire at welding
suppliers, or on the net. 
        Finally, you might want to look at your seal design - seems like
almost everybody uses the same old 'o-ring in a groove'  even when it is
not appropriate for the problem. Certainly, simple hatch seals are the
easiest to seal and the most forgiving of surface tolerance mismatch if the
seal is well designed - a side seal, for example . . no, not a radial seal
. . . a side seal.
Anyhow, some food for thought . . (today is a holiday in Canuckland, so
sitting on my  . .in front of the devil box)
Phil Nuytten