Saturday, February 5, 2011

Building a CNC Router: Part 2

Discussion of Linear Bearings

There are several CNC router designs on the web constructed of MDF. Many of them use two 3/4” precision rods per axis with two bearing points per rod. Since the precision of construction with MDF is no better than .03” and probably worse, getting the bearings adjusted for friction free motion has to be a problem. Solving this problem should simplify the construction of the CNC router with MDF.

Starting with the Y axis, let's call the upper rod the primary rod and assume it is fixed in place. The lower rod, which is the guide rod, is located 2.5” below the primary. These two rods, ideally, should be within a vertical plane defined by the Y and Z axises and be parallel. Now, if we place two linear bearings 3 inches apart on the primary rod and attach them to a 4” by 4” by 3/4” piece of MDF, the bearings should move freely. Two points define a line. To define the plane of motion, we need one more point, so lets place a single bearing on the guide rod and attach it to the MDF. As long as the guide rod is exactly parallel to the primary rod, the MDF plate should move freely along the two rods. Of cause, because of the .03” accuracy the rods are not likely to be parallel or in the same plane. The variation in the Z direction, that is the separation between the rod is what will cause binding. The variation in the X direction will just cause the plate to swing back and forth as it moves left and right. To remove the binding, either the guide rod or the bearing on that rod must be allowed to float. It seems the easiest solution is to allow the bearing to move vertically 0.1 inches. This design should allow free motion of the Y axis plate, but does not correct any variation in the X direction. This problem could be corrected with an X axis adjustment blocks at the ends of the guide rod. The design shown here could be used for the X and Z axises also.

Bearing Features:
  • Guide-ways: Two precision steel rods per axis, one defining the axis.
  • Linear plastic or metal pillow block bearings
  • X axis, Gantry, will use 4 linear bearings
  • Y and Z axis will use 3 bearings each..

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