Up vs. down grinding

Dear Doc: I grind Inconel with a CBN wheel. Does reversing the direction of the grinding wheel impact wheel life?

The Doc Replies: This is the age-old question of up grinding, where the grits travel “up” the workpiece in the opposite direction of the table movement, compared to down grinding, where they travel down in the same direction as the table movement. You don’t have a choice in many applications because of machine or part-geometry restrictions. But if you do, up grinding provides 5 to 10 percent less wheel wear than down grinding—or maybe none at all.

So, beware of outrageous claims. I’ve witnessed hearsay on the shop floor where some guy in some shop tripled wheel life by switching from down to up grinding. That’s nonsense. Controlled studies show a modest—if any—increase in wheel life with up grinding.

Dear Doc: I mostly centerless and cylindrical grind. The wheel salesman recently brought in a new “NQ” wheel. It sounds like another “SG” wheel, which I haven’t had success with, but the salesman swears by the NQ wheel. What’s your opinion?

The Doc Replies: Take a stab at it. The microfracturing SG grit from Norton is extremely tough, and many cylindrical and centerless grinding machines are either not rigid enough to handle that level of toughness or not capable of grinding aggressively enough to fracture the SG grit, so it just dulls. Therefore, Norton developed the more friable (i.e., less tough) Quantum NQ grit. 

When Norton introduced the NQ grit, a customer told me he was trying “a new SG grit” on his centerless grinder. I had my doubts, but it worked wonders because the Quantum NQ grit is friable enough to fracture on that machine. However, the grit is still tougher than, say, white alumina, so you may need to push it harder via a higher material-removal rate or a slower wheel speed.

Dear Doc: I cylindrical-traverse grind hardened steel shafts. I take deep and slow cuts with a fast workpiece speed, such as 0.001 " at 10 ipm and 120 rpm, while my colleague takes shallow and fast cuts at a low rpm, such as 0.0002 " at 50 ipm and 60 rpm. Who’s right?

The Doc Replies: It depends on the wheel width and rpm. Use the following formula to calculate overlap ratio (U):

U = wheel width ÷ (traverse speed in ipm ÷ workpiece rpm)

If a 2 "-wide wheel rotates at 50 rpm, your parameters generate a U value of 24 [2 ÷ (10 ÷ 120)], whereas your colleague’s parameters give him a U value of 2.4 [2 ÷ (50 ÷ 60)].

You want a U value of around 2 for roughing and around 8 for finishing.

Dear Doc: Is there a software program that calculates cycle times based on part geometry, material type and wheel type?

The Doc Replies: No, because there are too many other variables, including part tolerance, surface-finish requirement, thermal-damage boundaries, machine type and coolant type. The closest thing is The Grinder’s Toolbox, which I developed to help choose reasonable mrr and aggressiveness values, but it won’t calculate cycle times.