3 point measuring is generally accepted in Engineering as the
most accurate way of measuring roundness on shafts and bores so rollers would
be a great indicator as discussed, I think by Jim, earlier in the thread.
2 sets of rollers would also give straightness a good indication
over the length. Jim’s method of using the rollers with top and bottom
indicator would be good but difficult to get true diameter. However true
diameter is almost irrelevant as differences in diameter are the crucial
indicator.
1% is the allowable which on 48” diameter is almost ½ an
inch(0.48”). If you are within ½ the diameter of an opening then you can
add 2% of the inside diameter of that opening. So next to a 24 inch hatch you
are allowed almost an inch (0.96”) difference between maximum and minimum
diameter.
To be honest that makes me nervous that much but welding of
hatches and windows pulls it out of shape somewhat. I have just had mine
measured as the hull is now finished and the maximum was 14mm. that was
allowable as it was next to a window and a hatch. I am going to try and press
it out a wee bit if possible. My template chord was almost 9 inches in length
and the deviation was 1/8 inch. My hull was really complicated and looks like
swiss cheese with holes all over the place. Hugh
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Thursday, 4 November 2010 5:35 a.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] ASME PVHO
Brian,
Yes, that's what it is. There are two tests you can perform for out of
roundness. The first is to simply measure the diameter (ID or OD) of the
cylinder at one (or both) of the ends, at various compass locations. So
let's say you identify the top and bottom of they cylinder. You might
take a diameter measurement spanning the top and bottom (0-180 degrees), then a
diameter measurement left and right (90-270 degrees), and compare them.
You might then take measurements spanning 45-225 degrees, 135-315 degrees, and
then compare them all to get an average out of roundness. This test will
give you a general idea for the out of roundness of the cylinder but will not
tell you anything about the circumferential surface.
The formula Hugh is working with calculates the chord length of a
template. The template itself is just a segment of a circle with the
exact design diameter of the hull. The template is then used to compare
the surface of the hull against an exact circle segment which will show
localized out of roundness, dents, etc. As an example, if you were trying
to measure the flatness of an eight foot counter top your template might be the
edge of a ruler. You would not necessarily require a yardstick to measure
the flatness, and in fact a yardstick might be too long of a span to make your
measurements of out-of-flatness. In this example, the formula would tell
you whether you needed a ruler, yardstick, or some other "chord
length".
Imagine the testing difference this way. If you took a ball-peen hammer
and intentionally dented the longitudinal center of your hull, the first test
checking the diameter of the cylinder at the ends likely would not indicate any
out-of-roundness since the dent is localized in the middle of the cylinder and
doesn't impact the hull at the ends. However, the second test using the
template would definitely show the dent (out-of-roundness) assuming you placed
the template over the local area where the dent existed.
We can (maybe should) assume that the rollers used to roll a hull may not be
perfectly aligned and therefore not produce a hull that is uniformly rolled
along its longitudinal axis even if the ends of they cylinder are within
tolerance. The second test with the template will identify such out of
roundness.
I'm sure Hugh or Sean can correct me if any of this is wrong.
Jon
On 11/3/2010 11:41 AM, Brian Cox wrote:
Can you explain this out of roundness evaluation, is it mearly a
tolerance that needs to kept with in a certain percentage of exactness?
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