The most common reason a contact wheel gets "out of flat" is because people will leave a belt on the machine, with tension on it. (If you're not using the grinder, take the belt tension off!) This will often cause a "flat spot" on the wheel. Corners also get rounded just from day to day use. Although there are many different ways a person can TRY to true up a contact wheel....you'll be money and time ahead if you take it to a good machine shop and have them do it.