Polishing a steel tank - GIF

[hemmjo]

I am having trouble believing that tank is so perfectly round. In oder for that to happen, the tank and all 4 of the supporting rollers have to be perfect. I suspect that wheel is flexible rather than a solid grinding wheel. OR, there could be a "follower" of some kind that we cannot see which controls distance from wheel to tank.

If you examine closely, you see a spring and pneumatic cylinders on the device that seem to be controlling the grinding pressure. This would allow the wheel to touch the tank with very little pressure, rather than being a rigid mounted wheel. So the wheel itself can follow the variations in diameter.