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Re: [PSUBS-MAILIST] Very Heavy endcaps
Hi Sean.
Thanks for your detailed reply. Im glad i was right about the theory of putting on a bevel, but then im also glad i dont need to actually do it!
I was going to take it to a machine shop where i think they have a large rotary table for their mill. Would have been expensive though.
I can get the 30 degree bevel on with the hand grinder and i'll add the weld material to create a smooth transition as you suggest.
Many thanks for your advice. Help like this is priceless to me as i've been dithering about wondering what to do.
Kind regards
James
----- Original Message -----
From: Sean T. Stevenson [mailto:cast55@telus.net]
To: personal_submersibles@psubs.org
Sent: Wed, 07 Apr 2010 08:53:24 -0700
Subject: Re: [PSUBS-MAILIST] Very Heavy endcaps
From a stress analysis point of view, ideally the head to cylindrical
shell interface would occur with the theoretical mid-shell
sphere/cylinder in alignment (so that with different thicknesses at the
interface, the step is identical on either side). When this is not the
case, you get a tripping moment over the interface in addition to the
ordinary shell stresses. When welding a butt joint between plates of
differing thicknesses, adding transitional weld material to create a
gradual taper between the two helps to eliminate stress
discontinuities. I always specify a minimum 4:1 taper so that, for
example, if you have a 1" plate joining a 1/2" plate, centering them
leaves a 1/4" step on each side, and the taper weld on each side would
extend a minimum of 1". Even better than welding would be to taper the
plate itself, again at a minimum 4:1, such that the weld joint is
between identically sized faces, and weld deposition is minimized.
In your specific case, it sounds like the design thickness of the head
is 3/8", so this was used as the minimum thickness during manufacture,
maintained at the apex which is why the part is thicker at the base.
This is typical of pressed or spun heads, and is the reason why
spherical hulls are often fabricated from several smaller spherical
sections welded together - the smaller the piece, the smaller the
thickness variance, so you don't end up carrying a bunch of superfluous
material.
Machining a formed head is a big undertaking for a hobbyist, and
although this would be a superior way to do it since it would eliminate
unnecessary weight, in your case I would suggest simply doing an
appropriate bevel as a weld prep, welding, and then adding additional
weld material at the joint to create your taper transition. This is
easy to visualize if you imagine the two parts as being nothing more
than shells with zero thickness, occurring at the middle of the material
thickness. You need to join them in such a way as to have a smooth
transition with no discontinuities, so if these theoretical parts don't
align, add weld material to essentially create a cone to join the two
different diameters.
-Sean
Jon Wallace wrote:
>
> James,
>
> Is the inside diameter of the heads equal to the inside diameter of
> the hull? Vice-versa? I'm trying to understand where the excess
> material is located geometrically. The end caps are not too heavy and
> I assume you ordered a minimum thickness of 3/8 inch which is why the
> sides are thicker. Kittredge used thicker heads on the K600 than the
> hull because the company he built it for insisted on it. As I recall,
> the ID of head and hull were identical and the excess material is on
> the outside of the hull, and he did not grind it down as the conning
> tower is. Looking at the head to hull connection on the outside, you
> can imagine there would be an obvious edge because of the two
> dissimilar thicknesses. However, some material was used to flair the
> transition from the head to the hull. Now, I have no idea what
> material was used but I'm sure it wasn't car body filler or anything
> like that. Without knowing, I suspect that area was built up with
> welding filler material and then ground down as smooth as they were
> able. Again, from recollection, the length of the flair, or
> transition from head to hull, appeared to be about a 2-3 inches. Also
> on the K600, the same approach was taken with the conning tower.
> Kittredge did not grind down the conning tower thru-hull as he shows
> in the K350 plans, but instead used build-up on the outside edge
> flaring it to the CT itself.
>
> Jon
>
>
> James Frankland wrote:
>> Hi All,
>>
>> Ive been making some slow but steady progress on my sub.
>>
>> Im am at the stage where i am starting work on the endcaps. A few
>> penetrations to go in and then i am ready to assemble the main parts
>> of the hull together.
>>
>> But I have what i think is a problem with the endcaps. I specified
>> the correct thickness as per the plans (3/8ths) but they are spun so
>> while they are 3\8ths (10mm approx) at the centre, they are 13mm at
>> the edge.
>>
>> I dont know the exact weight but they seem extremely heavy to me.
>>
>> Questions:
>>
>> 1. Are they to heavy?
>>
>> 2. Do i need to machine a taper onto the inside diameter to take the
>> 13mm down to 6mm (hull thickness) in a controlled manner or can i
>> just bevel it for welding? If i do not machine it, there will be
>> quite a step from 6 - 13mm. I have a feeling it should have a stress
>> reducing taper like where the main tower shell joins the hull
>> reinforcement ring.
>>
>> Endcaps link - http://www.guernseysubmarine.com/index_files/Page493.htm
>> Taper Example link -
>> http://www.guernseysubmarine.com/index_files/Page3975.htm
>>
>> What have other people done at this point??
>>
>> Many thanks for your advice.
>> James
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