Gradual versus incremental cross-feed: Design & Engineering

Dear Doc: We surface-grind lots of stock (1mm per side) off of large plates (1,000mm × 500mm). Two cross-feed options are on our machine: 1) gradual, where the bed cross-feeds at a constant velocity throughout the forward and backward strokes, and 2) incremental, where the bed cross-feeds rapidly at the end of each stroke. There’s a lot of debate about which is better. What’s your take?

The Doc replies: I’m not a fan of gradual cross-feed, and here’s why.

First, let’s start with the basics. You have a lot of stock to remove. Therefore, you should use nearly the entire wheel width for roughing. This is a common mistake: using only, say, 20% of the wheel. If the wheel is 50mm wide, use nearly all that 50mm. Let’s say 90%, or 45mm. It’s ridiculous not to use almost all your wheel even if you have limited motor power. So when you do incremental cross-feed, have that wheel move over 45mm after each forward stroke and back stroke.

Different cross-feed styles for surface grinding. Image courtesy of J. Badger

You might wonder whether using more of the wheel will increase wheel wear and risk of burn. The answer is a resounding no. Wheel wear and burn risk depend mostly on the maximum Q-prime in a wheel. The equation is Q-prime = DOC in mm × traverse velocity in mm/sec. Notice that the cross-feed velocity (for the gradual cross-feed option), the cross-feed distance (for the incremental cross-feed option) or the wheel width isn’t part of this equation. Using 10%, 25%, 50% or 90% of your wheel width won’t change the risk of burn or the rate of wheel wear. The only thing it will do is increase the total spindle power and the risk of chatter. Assuming we’re OK on both of those, let’s use 90% of the wheel and reduce cycle time.