Cold Rolled Confusion

[John TV]

A steel question.
So another nuub question. I am doing my best to get a YouTube doctorate in machining or at least a high school shop certificate. That said...

I know there are literally thousands of steel types and to date I have only used a few. Seems like most often I read or see mild steel used in
fabrication and cold rolled being used for machining.

I get the mill scale problem and the inferior finish of mild steel but then I ran across this video. ( link below ) and maybe I need to change my thinking.

I have not experienced ( I don't think) significant stress issues with the minor nuub projects I have made so far. Is this something we need to consider for most projects?

Looking for the groups thoughts.

This is a link to "the tool and die guy" video on "A few things you should know about cold rolled steel"

Sorry, don't know how to insert actual YouTube video.

John in Minnesota


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[Latinrascalrg1]

Thanks for posting, thats really good info to know so you don't end up chasing your tail!

 

[Mitch Alsup]

Cold rolling induces stress in the material (not just at the skin) when some other section of the part get machined, the stress redistributes itself.

 

[francist]

Is this something we need to consider for most projects?

I think it is, and I've run into it directly on a couple occasions.

Once when I was making some replacement fingers for my sheet metal brake. I chose cold rolled because I didn't want to have to surface the mill scale off of hot rolled material but when I cut the nose taper section (and removed a fair percentage off of one side of the segments) I noted a slight bow on the underside of the pieces. The untrimmed pieces were nice and flat the way I had wanted them to be. Granted it wasn't a huge difference -- maybe 2 to 3 thousandths on the wider of the fingers -- but still there and still measurable. And if you're expecting flat when you're done, it might not be.

The second occasion was when I was remaking the arm for my Keller power hacksaw. The bar had been severely abused and worn into an arc so I decided to trim the worn edge flat again and call it good. Well, every time I'd take some off the one edge it would kind of move around in weird ways. Took me a bit to realize I was chasing an induced stress -- each time I dressed the edge it would move some more -- like a dog chasing it's tail. When I took an amount of the opposite edge, it miraculously straightened!

So I do always think about it now. It may not always be a valid consideration, but it's something in my awareness when selecting stock or choosing methods. You can make your own decision on whether it's something you need to consider for your uses or not.

-frank

 

[Tinkertoy1941]

Have your steel materials stress relieved prior to machining to reduce the possibility of the material moving after removing large sections.
If extreme accuracy is required rough machine the steel material then stress relieve your near net shape item for gage quality finish work
Most industrial Heat Treat Facilities do Stress Relieving

 

[John TV]

Also thinking about lathe work with cold rolled. How might internal stress show up there, or is it uniform enough with the circular shape to hold its form better?

Say turning a bushing with a large thin shoulder, would that shoulder cup?

Thanks for your thoughts.

John in Minnesota


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[homebrewed]

When I'm machining cold-rolled I try to remove approximately equal amounts of material from opposite sides of the work. This based on the premise that the stresses in the as-manufactured material are balanced, more or less, so you should remove equal amounts to maintain that balance. But you still should figure on some warping to occur, unless the material has been stress relieved.

Some judicious bending to minimize the warp usually is good enough for my purposes .

Since the built-in stress is not necessarily perfectly symmetrical, lathe work on cold rolled likely won't totally prevent warp. Your approach will depend on your tolerance requirements.

 

[John TV]

All my work is hobby work so it's unlikely I will ever go to a commercial heat treatment, and I have no idea what that might cost. That said...

Is there any home treatment, such as home annealing that would reduce some of that stress?


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[homebrewed]

I have heard of folks putting the metal into a burning bed of charcoal and letting it go until the fire is stone-cold out. I haven't tried it so can't vouch for the result. The piece may warp due to uneven heat and/or support, too.

The surface of your CRS may well look like HRS after that.....

 

[Illinoyance]

Hot rolled steel or cold finished that has been annealed or stress relieved can best be described as gummy.
Cold finished stock is nicer to machine. In lathe work stock is removed symmetrically so distortion is minimal.