Workpiece warping when milling: Why?

[jmcmullan]

I recently made "first chips" with my new Enco mill-drill. The project simply required taking 1.25mm off one side of a 6mm square rod. The approx. 5.75" long cold-rolled steel workpiece was very securely clamped in a heavy-duty milling vice and I made the cut in one pass since surface finish wasn't important. I used generous amounts of LPS Tapmatic cutting fluid and hand cranked the table. The cut went fine and neither the tool or the workpiece even got warm.

When I released the pressure on the vice, I noticed that the middle of the workpiece kind of "sprung-up" off the parallel. Examination showed that although the piece was dimensionally perfect from end to end, it was now warped into an arc: when placed on a flat surface the middle was about 3/32" off the surface!

Does anyone know what happened here? It's as if the center of the rod was held in compression by the outer surface that I milled off. I know cold rolled steel tends to be somewhat case hardened on the outer surface, but can't figure how this would cause what happened. I'd try taking 1/2 off of opposite sides next time, but I'm worried that I wouuldn't be able to get the workpiece flat on the parallels for the second cut if it warps on the first cut.

Suggestions?

[Evan]

"It's as if the center of the rod was held in compression by the outer surface that I milled off."

That is what happened. CRS has a lot of stress bound in the metal by the forming process. To avoid this when using CRS in the future buy Normalized CRS. It has been stress relieved.

[SGW]

Yep. Amazing to see how flexible a bar of steel really is, isn't it?

You can relieve the stress yourself, if the piece isn't too big, by heating the up the bar with a torch to red heat and letting it cool. (You don't have to heat the entire bar at once, just work your way along it.) This may not be as good as buying normalized steel, but it will help.

Or you can do as you suggest -- mill equal amounts from both sides, if you can figure out how to hold the piece while you do it!

[gizmo2]

Hot rolled isn't so bad about that, if you can tolerate the surface and (nominally) undersized dimension. Machine all over (or not at all) and the cold rolled works fine. Fun stuff, huh? Get all the dimensions perfect and the steel goes goober on ya!

[rangerod]

I have had that same thing happen to all sorts of different materials. I milled a 2" square piece of aluminum on one side to 3/4" thick & it sprung up like your did. I have sheared off 1/2" off of a piece of 2" X 1/4" hot rolled strapping on a 10' piece & it warped an 1" on side view. Removing material sometimes does not leave the rest of the material any where else to go other than where the removed material was. The funny thing is it does not always happen. I have been doing this for 20 years & I still don't have an answer on how to prevent it, maybe someone aout there does.

[This message has been edited by rangerod (edited 08-10-2004).]

[This message has been edited by rangerod (edited 08-10-2004).]

[Forrest Addy]

Them who been bit by that particular snake have learned to take the work piece so its neutral axis aligns with the center of the material - and even then it can be tricky.

Warm the material up to 1300 degrees slowly and let it cool even slower. A barbecue fire works great. Toss in the raw stock after the cook-out, cover it with coals and ash, and let the fire peter out in its own good time.

Distortion control is a big problem in large or long workpieces. I was once challenged to plane a couple of 8 ft square plates 2" thick flat and parallel within 0.005" over all to make a back plane for a passive phased array sonar receiver. We eventually solved the problem by hanging the plates vertically in the stress relieving furnace.

They make plate expecially for applications where good stability properties are required but of course the cost was high. I probably spent the three times price of the right stuff making do with Navy hull grade structural steel which machines like chewing gum.

It wound up a round the clock endevor making those two plates but we got 'em done and it only took five hunks of material to get two good ones.

[This message has been edited by Forrest Addy (edited 08-11-2004).]

[Evan]

It's all about internal stresses. Heat treated aluminum can develop internal stresses that are just short of yield failure. Avoiding problems with the material springing when these stresses are differentially changed by machining is all about stress relief. Unfortunately, stress relieving and solution heat treating aluminum properly is beyond the capability of all but the most advanced shops as it requires precise time and temperature controls as well a proper quenching to maintain alloy specs. With steels it is easier although it is still very important when dealing with high strength heat treatable alloys. For CRS it is simple since heat treatment does not materially change the properties other than to relieve stresses.

[PSD KEN]

Yep, I got bit by that.
Gonna true up a piece of angle, 1/4".
Don't want to mention how much metal I removed before I decided something was amiss.
Something I read assured me that all would be well by machining both sides, but nary a hint on how to flip it over w/o it springing.

[This message has been edited by PSD KEN (edited 08-11-2004).]

[Paul Alciatore]

I was just wondering yesterday why you would buy "normalized". Now I know.

As for flipping it over without springing, I would suspect that partial cuts on alternate sides would be the trick. One fourth from the first side then a forth from the second. Then 1/8 from each, etc. Sneak up on it cutting alternate sides. Lots of extra work though.

Or perhaps mark it first and saw cut the bulk off. Bandsaw can follow the line even after it's warped and hopefully it will spring back close to the original lines when the second side is cut. Leave enough to machine down to a straight surface.

I've seen lumber do this big time. Cut a very straight 2x4 into two 2x2s and they bow by over 2".

But this must be rare. I mean, I have cut hundreds of parts from CRS and it hasn't happened. Do the steel mills try to stress relieve even their cheaper products? Perhaps a quicker, cheaper process?

Paul A.

[Evan]

It depends on where you buy your steel. The supplier I buy from stocks and sells only normalized CRS.