Clutch bells will not get worn unless there is a problem. For some reason the clutch shoes must not be able to rotate the bell easily so are just scraping the inside away. This could be for example if the brake is to tight or is coming on when it is in the idle position. You should make sure it rolls forward easily when in the idle position. Another reason would be say if the mesh between the engine and gearbox was to tight which again would make the clutch bell stiff. Alternatively it could be that your clutch shoes are worn so instead of making a firm grip they rub.

If you do not check the setup of your model when new and at regular intervals this can happen. If it does you would need to order a new clutch bell and possibly new clutch shoes which are available from the spare parts section.