Straight stock

Back at the beginning of this year, I wrote about how the brass 1/8" bar stock that I purchased was not straight. While the steel 1/8" bar stock was straighter, it also was not straight:


You can see light through the 0.06" gap (about 1/16") between the table and the bar. The bar itself is 18" long.

Since I took the metalforming class at NextFab, I realized that the slip roll machine might be used to straighten the stock.


These machines are normally used to bend sheet metal into a tube:


Normally the thickness of material going into the rollers is limited by the machine specs. In this case, the limit is 20 gauge steel, or 0.036". However, Dan at NextFab agreed that although my bar stock is 0.125" thick, the machine should easily handle it because (a) it is only 0.25" wide, so the force required to bend it is vastly reduced, and (b) I'm not bending into a circle, but only to an extremely large (and hopefully infinite) radius.

After running one bar through and adjusting the bending amount, I was able to reduce the bend to nearly nothing:


Now, theoretically, if I turned the bar around and fed it through, I should end up with a similar amount of bend going the other way:


Oh no, what happened? I believe that what we have here is a case of internal stress. Machining produces internal stress in metals, which can cause distortion. In the bar stock, imagine that there is an invisible rubber band pulling on the ends of the bar, thus causing the bar to bend. That represents the internal stress. Applying a force opposite the distortion is resisted by the internal stress, but applying a force in the same direction as the distortion makes the metal bend even more because the internal stress helps it along.

Ideally, I would "process anneal" the stock to remove much of the internal stress. This, however, involves heating the metal to about 650 degrees C for an hour, which is impractical. I will just have to take the risk that straightening the metal will cause just enough internal stress to balance out the existing internal stress, and that after straightening and machining the metal, it will not significantly distort.

One has to be careful when using the slip roll machine. Its rollers can eat your fingers:


NextFabber Dan showed me another warning sticker on some other device, which made me burst out laughing when I saw it: