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From: Frank Krygowski <frkrygow@sbcglobal.net>
Newsgroups: rec.bicycles.tech
Subject: Re: Machine Shop
Date: Thu, 6 Mar 2025 12:10:27 -0500
Organization: A noiseless patient Spider
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On 3/6/2025 9:31 AM, AMuzi wrote:
> On 3/5/2025 2:46 PM, Frank Krygowski wrote:
>> On 3/5/2025 1:51 PM, cyclintom wrote:
>>> ... instead of actually looking into the science.
>>
>> What a weird idea. I took two courses in metallurgy as part of my 
>> first engineering degree. I taught basics of metallurgy as part of 
>> manufacturing courses at two different schools. I'm betting I'm the 
>> only person here who has used a metalloscope (although Andrew might 
>> have), I'm sure I've done and observed more heat treating than most 
>> here, and I'm sure I've done and taught more physical testing of 
>> materials, including testing for modulus of elasticity, elastic limit, 
>> yield point, ultimate tensile strength, hardness by at least 3 or 4 
>> different methods, etc.
>>
>> Tell us about your education and experience in metallurgy, Tom. 
>> Especially, tell us where you were taught that plastically deformed 
>> metal can spontaneously return to its undeformed shape, and how that 
>> can happen. Nobody here believes you.
>>
> 
> No, I have not observed metallic structure directly but textbooks are 
> rife with diagrams and micrograph photos along with the text. (cheap at 
> any used bookstore).
> 
> This is short and clear:
> https://www.slideserve.com/angeni/single-crystal-slip
> 
> I think the misunderstanding here is to consider metals as amorphous 
> like liquids and polymers. They are crystalline, with an ordered 
> repeating structure like ice not like water. Permanent deformation 
> exactly describes crystal slip which is a permanent change and can be 
> removed only by remelting (recycling) the material as Nucor does. Or for 
> a small area, heating and cooling only a portion (see link at end).
> 
> Because crystal slip inherently makes the material larger by some 
> degree, any reshaping (such as working a dent or straightening a bend) 
> will not and cannot return the piece to exactly the same shape. Close 
> enough for many actual applications, but not perfectly the same by its 
> nature.
> 
> Short overview of practical application:
> https://www.hotrod.com/how-to/0709rc-straightening-sheetmetal/

Interesting. I hadn't heard of the Shrinking Disk.

Not that I do any bodywork these days. My last related adventure was 
using a heat gun to soften a plastic rear fender to allow pushing out a 
dent. Much easier than metal!

-- 
- Frank Krygowski