Trying to get my edges straight

I've had my Prusa working for several weeks now, and I've been climbing the learning curve of Skeinforge. I'm printing ABS on a heated bed with a Makergear hot end. I finally came up with some settings that work very well. I get a nice clean flat surface on the top of flat objects, and I can span voids nicely. However, one anomaly has cropped up recently that I'm having trouble getting rid of. My sides have a bit of a lip on the top and bottom, leaving a sort of concave sidewall.

I thought it might be due to backlash, but it goes around the entire part equally. I tightened up my belts anyway, and it didn't change.

Next, I thought my heated bed might be running too hot, its measuring around 120 C with an IR thermometer. I turned down the voltage a little, but it didn't seem to change anything.

I'm wondering if there's some Skeinforge settings that might need to be tweaked. I've noticed that its the top and bottom layers which are a little wide, layers which are 100% infilled. The layers in between seem to be OK. Could it be the packing ratio maybe?

I'm attaching pictures that demonstrate the issue. I'm open to any ideas that I can try to fix this.

I think your bed is too hot and the plastic is curling inwards. After the first layer the bed has to be below the melting point of the plastic. For ABS that is about 105C. I have my first layer a lot hotter to make it stick,

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[www.hydraraptor.blogspot.com]

So you think my heated bed is too hot? I'll try to adjust it to 105C. When you say "I have my first layer a lot hotter to make it stick" are you referring to the nozzle? Are you running the nozzle at a higher temperature for the first layer? I never thought of that, it sounds like a good idea. How do I configure that in Skeinforge?

No you run the heated bed hotter for the first few layers then reduce down.

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My Reprap blog

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OK, thanks for the clarification. I don't have automated control over my heated bed since I'm using Gen6 electronics. I guess I'm going to have to rig something up.

I adjusted my power supply down as low as it would go (10.5 V) and I got my bed temperature down to 105-110 C. I'm using the red Prusajr PCB with glass on top. I've noticed that the heat is uneven. Some spots are hotter than others. Close to the end of a test print, I was measuring 105 on one side and 118 on the other side. I read about stuffing cardboard underneath to insulate the heated PCB, maybe I'll try that to see if I get more even heating.

Anyway, the lower temperature helped, because the distortion is much smaller now. I'm going to rig up a PWM controller so I can actually get temperatures even lower and see what happens. Its too bad the Gen6 board doesn't support a bed temperature control, since the Sprinter firmware has built-in support for it. I might have to try to hack together a MOSFET and thermistor and see if I can tie it to the Sprinter code.