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Robert Burch
01-22-2003, 02:11 PM
I machined one side of a cold rolled steel bar and of course got plenty of warpage. The machined surface is now convex. In my understanding that means the opposite surface is under tension from the rolling process.
My question is, what can I do to relieve the stress and thus straighten the bar? Will heat do it and if so how much? Will sandblasting do it?

Any help will be appreciated.
Bob

kap pullen
01-22-2003, 02:47 PM
Robert,
The general procedure for machining crs is rough one side, flip it, and rough the other side.

Then you straighten, shim and flip as required to finish overall.

Cold rolled is not a good material to use for heavily machined parts because of the stresses from cold forming.
You may be able to stress relieve before machining.

The best bet is start with Hot rolled steel which is a more stable material.
Hrs is rolled red hot so is effectively stress relieved when you recieve it.

I would straighten your part with an arbor press.
Don't think heat or sand is going to fix it now.

kap

lynnl
01-22-2003, 02:59 PM
Do the stresses in CRS go all the way thru, or just in the outer surfaces? Assuming equal layers could be simultaneously sliced off both sides would it still be prone to warpage?

kap pullen
01-22-2003, 03:47 PM
lynn

Yes, most the stresses are in the surface.
Try making a clevis out of crs and the legs may open out 1/8 inch.

You could probably do that on a Gardner Double Head Grinder (machine both sides at once).
Not something I have at home.

Best bet is plan to flip, and recut a crs job a couple times till it gets stable.

While cutting a piece of metal is constantly moving, depending on the material, because of heat, cutting away stressed areas, adding new ones, or clamp pressure.

It is sometimes hard to find where, or why a part is deforming.

This is not something the home machinist has to dwell upon. It is nescessary to think about in it precision work with exotic materials and thin walled parts.

Rudy was just talking about old machinery losing its stresses over time, compared to new Chinese machinery, at Cabin Fever.

kap

[This message has been edited by kap pullen (edited 01-22-2003).]

SGW
01-22-2003, 05:47 PM
Another thing you can do is heat up the CRS stock to stress-relieve it, BEFORE you start cutting on it.

Re: heating it after-the-fact to straighten out the piece...no such luck. Hard to say for sure, but generally, you'll be relieving the stresses induced by the bending at the same time you're relieving the stresses causes the bending, so my guess is that the net result will be a stress-relived, but still bent, piece.

kgarver
01-22-2003, 06:13 PM
The best way is to machine a little bit off of one side and turn it and machine a little off of the other side. Lay it on a flat surface (a mill table is fine) and eye ball it for any curvature. Keep milling it alternately until its relatively flat. CRS will never get completely flat. If its to be bolted down then don't worry. It will conform easily to the surface you're bolting it to. If its not to be bolted to something you may have to rethink the application. Thats one reason its so cheap! BTW brass is the same or even worse. Forewarned is prepared!

Robert Burch
01-22-2003, 06:47 PM
Thanks for the good advice. I should have known better. I notice Starrett sells ground flat stock either low carbon, water or oil hardening. Does anyone know if this stress relieved?

DR
01-22-2003, 06:52 PM
The ONLY fool proof way to avoid the warpage in CRS or hot rolled steel before extensive machining is to send it to the heat treater and have it stress relieved.

Flipping side to side may or may not be a solution. Sometimes the material moves over time. The movement may not be apparent until the steel has aged a little.

An interesting side-topic to this is observing how steel or other metals will flow over time. Setup some kind of repeatable depth stop in an arbor press. Bend a piece of metal in the arbor press quickly. Don't dwell at the stop point. Then bend another piece, but dwell for a minute or so at the depth stop point. Compare the bent pieces. The one with the dwell will be bent more with less springback. The metal had enough time to flow giving less springback in the piece. This is something well known to press brake operators, the dwell at depth must be consistant to get consistant parts.

DR

x39
01-22-2003, 07:52 PM
<font face="Verdana, Arial" size="2">Originally posted by Robert Burch:
I notice Starrett sells ground flat stock either low carbon, water or oil hardening. Does anyone know if this stress relieved?

</font>
The proper term is normalized, and yes, flat ground stock is normalized. Incidentally, you can normalize a piece of steel on the cheap at home. Get a steel bucket, perforate the sides, put the piece of steel in it and surround it on all sides, top and bottom, with charcoal briquets. Ignite it and when ahsed over (as in grilling a steak) cover the top of the bucket with a piece of steel plate and allow the fire to slowly burn itself out. Not very scientific, but it works.

wierdscience
01-22-2003, 11:25 PM
As for ground flats being normalized the answer is yes because if it where not you could grind it into eternity and not get it straight.As for cold rolled some is and some ain't the cheap stuff ain't the good stuff will usually cost you about$1.30-$2.00per# in 20'lengths,hot rolled shapes of any cross section in either a-36 or a-53 is not nor ever will be relived from the mill as the quench process sees to it that the stock will cotain stress.Also any welding will build in stress,we were told in high school shop that a weld is another term for stress concentation.I was involed in the fabrication of some heliocopter assembly jigs that were being welded up out of some tube steel,each time welding would require the whole jig to be stress relieved before machining,I think that jig was relieved about 18 or 20 times before we were through. However the cold rolled and the hot rolled sections can easily be relieved if it is winter and you burn wood regularly simply throw the material in the fire grate on top of some coals burn it for at least three hours and let it cool as the fire burns out overnight and use.As to the stress problem in castings the two common ways of reliving castings is to cast and let them season for a period of not less than 10 years or heat them in a controlled atmosphere for an extended period of time then a gradual cooling of days or even weeks.I have found from building a few steam pump kits that the problem most common in building these is the stresses in the castings you get these days.I had several that gave me fits no matter how precisely they were machined.Then one day a friend mentioned stress relieving and low and behold I did it and machined out a new casting to some pretty sloppy tolerences and the sucker worked perfectly the first time!Who'da thunk it! http://bbs.homeshopmachinist.net//biggrin.gif

[This message has been edited by wierdscience (edited 01-22-2003).]

halfnut
01-23-2003, 04:03 PM
CRS just doesn't want to stay still when machining on it. Mill a keyway in a bar of CRS and it will be bowed, mill off one side of a plate and it will be bent.

Usual method of reducing thickness of CRS bar is to mill off of both sides equally. But there are times that there is more stress in one side than the other, still bows.

Extruded aluminum bars do the same thing.

I throw CRS in the heat treat oven and heat to about 1200 degrees and let cool in the oven overnight. Heating to good red with a torch and letting cool slow does the same thing.

Quenched and tempered 4140 is wonderful stuff to work with, but alas I usually get to work with CRS.

Thrud
01-24-2003, 02:28 AM
Robert
I scored 100Lbs of Starrett Yellow Stripe on ebay for almost nothing. I got 20 bars for the price of one here in Edmonton. (W/shipping)

It is the easiest stuff to mill I have ever cut. Almost as easy as aluminum. I still have not tried to color casehardened it yet.