The inner diameter of these bearings has a plastic surface. You can get the same type of bearing with a precision ground steel inner sleeve to increase overall bearing accuracy and life. For the price these parts are very good but you can find better quality bearings.
Iverntech MGN12 400Mm Linear Rail Guide Upgrade For Ender 3, Corexy, Tronxy, Delta Kossel 3D Printers And CNC Machines With Stainless Steel Carriage Block
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