Multirib grinding wheels

Dear Doc: I cylindrical-grind threads into hardened steel using multiple-rib, vitrified-bond, aluminum-oxide wheels and am getting breakdown of the thread root. I increased the number of passes from four to six to even eight, but it doesn’t seem to help. Why?

The Doc Replies: Think of multiple-rib grinding wheels as a series of, say, five different wheels, each with a slightly larger diameter than the previous wheel. Let’s say you’re taking a 1mm DOC. Depending on the geometry, the fifth rib grinds a 0.025mm DOC, the fourth rib a 0.050mm DOC, the third rib a 0.100mm DOC, the second rib a 0.200mm DOC and the first rib grinds the remainder, or 0.625mm (1.000mm – 0.025mm – 0.050mm – 0.100mm – 0.200mm = 0.625mm). That fifth, final rib is going to take 0.025mm regardless of the total DOC, because that’s the material remaining after the fourth rib grinds its 0.050mm DOC. If the total DOC is 0.025mm, only the fifth, final rib will remove material.

In this example for a 1mm DOC, the fifth and final rib grinds a 0.025mm DOC, the fourth rib a 0.050mm DOC, the third rib a 0.100mm DOC, the second rib a 0.200mm DOC and the first rib grinds a 0.625mm DOC. Image courtesy J. Badger.

Let’s say you decide to divide that one 1mm pass into five 0.200mm passes. That fifth rib is still going to take 0.025mm each pass. Sure, the first rib gets a complete break, but the fifth rib takes a beating, because it’s now doing five 0.025mm passes instead of one. When taking multiple passes, if the DOC on a pass is less than 0.375mm for this example (ribs 1 + 2 + 3 + 4 = 0.025mm + 0.050mm + 0.100mm + 0.200mm = 0.375mm), then the first rib is not removing any material per pass and the second rib is only removing 0.025mm. Because the fifth rib is the one generating the final form, you’re going to lose form more quickly when taking multiple passes.