"Mirror" welding??

On Tue, 11 Aug 2009 10:40:53 -0500, Ignoramus17579


Ah, but many people do rub their fingers over their face while shaving to tighten the skin so they shave closer :-) But my point was that half the population learns to shave using a mirror with no problems.
When I was in the A.F. there was an authorized repair to the inlet guide vanes on the B-52 that allowed welding cracks, in situ, which required mirror TIG welding on the aft portion of the vane. We sat one of the shop's welding booths up to practice the "art" and most of the troops learned how fairly quickly but I must say that there were one or two guys who never learned it.
Cheers,
John B. (johnbslocombatgmaildotcom)
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"Ignoramus17579" wrote: To the person who said "you shave in the mirror every day", I would

^^^^^^^^^^^^^^^^^^^^^^ I think the main difference is that most of the movement in shaving is up-and-down and side-to-side. The guy in the mirror moves pretty much the way you do. The motion that causes trouble is front-to-back, where the direction is reversed. This is complicated further when front-to-back is combined with one of the non-reversed directions.
Off-topic, but amusing: Why are your left and right hands reversed in the mirror, but not the head-to-foot direction?
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Because eyes are placed horizontally
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wrote:

^^^^^^^^^^^^^^^^^^^^ Ha-ha. Keep guessing.
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says...

Your hands are not reversed, they are reflected, in a mirror. Left is still left, Right is still right, so to for up and down. It would take several mirrors (lenses, too could be used) to actually reverse an image.
It's difficult for some to weld in a mirror because your mind perceives a reversal due to the reflection. The physical directions don't reverse, your perception of motions do. It's your brain fighting itself.
If the original question is asked from the reflected image's viewpoint, Which side is right and left?
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"TinLizziedl" wrote. Your hands are not reversed, they are reflected, in a mirror. Left is

^^^^^^^^^^^^^^^^^^^^^^^^ If you lay a glove down, can you tell whether it is a right or left? Of course. If you lay a right glove down and look at its reflection in a mirror, do you see a right glove or left glove? The handedness of the reflected glove is opposite to the real glove.
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says...

Bilateral Symmetry. If the handedness of the reflected glove were not opposite, you would be "interesting" looking. It's still a reflection of light rays, not a refraction of them.
How does your eyeball work? If you could see the image on your retina (rather than your brain's interpretation of it), you would see the world upside down.
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"TinLizziedl" wrote: Bilateral Symmetry. ^^^^^^^^^^^^^^^ By this I assume you mean symmetry of the space in front of the mirror with the "space" behind the mirror. This is the key to the whole thing. If you reverse one direction in three-dimensonal space, you change the handedness. ^^^^^^^^^^^^^^^^^^^^^^^^ It's still a reflection of light rays, not a refraction of them. ^^^^^^^^^^^^^^^^^^^^^^^^ True, but I don't see the relevance. ^^^^^^^^^^^^^^^^^^^^^^^ If you could see the image on your retina

^^^^^^^^^^^^^^^^^^^^^^^ Only until your brain catches on, and then the perceived image would turn right side up. This was pointed out in an earlier post, but the thread is too long for me to search for the poster.
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says...

Have you ever seen someone standing at the very edge of a mirror? If the person's placement is just right, it appears you are looking at a full person, not half a person and a reflection. Some levitation tricks use this.
Semantics. The use of "handedness" here is dependent on the location and orientation of a moving observer. Why else would we come up with "driver-side" and "passenger-side?" Only by establishing stable reference points in space (following contours of an object) can an object be accurately described. A reflection is not an object. It is a bunch of light rays bouncing off an essentially 2-dimensional plane.
Place a glove on a glass table and walk around the table, look up from underneith, down from above. From all perspectives the glove is the same. Its "handedness" does not change, no matter how often you "reverse" the vector you are using to view the object. Only by using a mirror can you fall prey to the logical fallacy of believing the reflection is the equivalent of the object.
My reference to bilateral symmetry is the the fact that most, if not all, mamalian life on our planet looks the same from midline to left as from midline to right. The reflection of a glove being of a different "handedness" is because we have our own "mirror-images" built in. Could you say the handedness of a reflected glove was different if we all had 5 fingers on our right, and 6 on our left?
The original post stating the "handedness" changed is a logical fallicy, not a physical reality.
Sorry, I tend to get a little (ok, a tiny itsy bit) pedantic about stuff like this.
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"TinLizziedl" wrote: (clip) Could

^^^^^^^^^^^^^^^^^^^^^^ It's not your pedantry I am troubled by--it is the fact that you don't seem to understand what handedness means. Let me make a final effort:
Look at a right hand glove. What makes it right hand? Not the number of fingers, not which shoulder it hangs from. It's the way the fingers and joints wrap around something. Now turn the glove inside out. A right hand glove becomes left handed. It now resembles the reflection of a right handed glove in a mirror. Turning it inside out or reflecting it in a mirror are both methods of doing a spacial transformation of one axis only.
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says...

Ok. Your final effort is a much better, more precise, explanation. Now I understand what you mean by the term, "handedness."
I'm sorry I drove you to distraction and I appreciate the effort you have made in clarifying your prior statements.
I have never heard the term used this way before.
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It's the standard use of the term in physics and chemistry, and I expect just about any of the other sciences as well.
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Who said I was in any of the sciences? I'm just a dumb grunt on the waterfront, arcin'N'sparkin....
In my welding apprenticeship (the only real college level courses I've taken) I don't think anyone ever mentioned the word without meaning definition #1 in the Merriam-Webster dictionary. I had never heard of meaning #2 until your posts. Non identical mirror image indeed!
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says...

Ok. Your final effort is a much better explanation. Now I understand what you mean by the term, "handedness."
I'm sorry I drove you to distraction and I appreciate the effort you have made in clarifying your prior statements.
I have never heard the term used this way before.
I still have some trouble with the idea that a reflection is in fact a single axis spatial transformation of a three dimensional object. A mirror acts as a plane defined by only x and y coordinates. The reflection is a bunch of rays of light that have bounced off that plane.
The image we see in the mirror only posesses two dimensions- we infer the third by object placement and foreshortening of apparent distances.
Do you use Autocad-type programs alot? The only people I know who treat mirrors as you do deal with a lot of drafting.
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"TinLizziedl" wrote: I still have some trouble with the idea that a reflection is in fact a

^^^^^^^^^^^^^^^^^ As you say, you have not studied physics. The practice of ray tracing is used to determine where an image will be, how large it will be, etc. This applies to reflection in mirrors, both flat and curved, to lenses and to combinations of lenses and mirrors. The image seen in a mirror is a "virtual image." It seems to exist, even no light travels to or from the apparent location of the image. If you constrtuct an image reflected in a plane mirror, using standard ray-tracing techniques, you come up with one axis turned around (the front-to-back axis.) This is what causes a right hand to look like a left hand, or a right-hand thread to look like a left-hand thread. ^^^^^^^^^^^^^^^^^^^^ Do you use Autocad-type programs alot? The only people I know who treat

^^^^^^^^^^^^^^^^^^^^^ No. I am a mechanical engineer, so I have done a lot of drafting, but the treatment of mirrors is universal in the sciences, and particularly critical in optical design, such as telescopes, camera lenses, periscopes, fiberoptics, photo-copiers, etc.
Have a nice day. I think this discussion has been interesting and worthwhile.
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"TinLizziedl" wrote: I still have some trouble with the idea that a reflection is in fact a

^^^^^^^^^^^^^^^^^^^^^^^^ I have been thinking about this, and I have another idea that might help clarify things. Do this "thought experiment." Take a pair of side view mirrors from a car, and place them opposite each other with a plate of glass in between. Now take one side view mirror and place it in a similar position against a mirror. The one on the far side of the glass plate will look like the reflection in the mirror. Each one is a "mirror image."
There can be no argument that the real pair (against the glass) are alike except that one dimension is reversed. By inference, then, the image seen in the mirror is also the same as the real one with one dimension reversed.
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Because your hands aren't reversed, either. You reverse when you turn around to face somebody, not when you look in a mirror.
Imagine someone standing in front of you, looking in the mirror. His left shoulder is in front of your left shoulder both in space and in the mirror; same with the right shoulder.
Now suppose he turns around to face you. Most likely, he turns by rotating in the vertical axis: this is when his hands "reverse", but his head and feet don't.
But now suppose this thought experiment happens on the ISS instead of on earth, and when he turns around he does it by doing a half-somersault. Now it's his head and feet that have reversed, while his left and right sides are still lined up.
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Also see 7-29 post by Tin Lizzie in the "Newbie Stick Welding Question"
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snipped-for-privacy@one.com says...

I knew I posted it, just didn't save a copy...
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says...

I posted a few tips here not too long ago- try searching for it- I posted it about a week ago...
It's used alot more often than most weldors are comfortable with.
Practice,practice,practice. A little patience helps, too.
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Tin Lizzie
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