Dear Doc: We have two dressers on our machine — one mounted behind the wheel and one mounted on the bed. Some operators like the one behind, and some like the one on the bed. Which do you prefer?

The Doc replies: My answer is the always popular "It depends." There are four factors to consider, probably in this order: stiffness, cycle time, chatter and alignment. Let's take them one by one.

First, rear-mounted, or top-mounted, dressing units consistently have less stiffness than those mounted on a bed. This may not be a problem. If it is, several issues can arise from a lack of stiffness.

Chatter could occur during dressing. With plunge rolls, this puts straight-line chatter into the wheel, which ends up being imparted into the workpiece in the form of straight-line chatter. With traverse-style dressing, this puts a "fish scale pattern" into the wheel, which imparts a fish scale pattern into the workpiece.

Longer dwell times could be required when rotary plunge dressing to get the final form, which can dull the wheel.

There could be deflection in the dresser, which can result in a taper in the wheel and therefore the part.

Again, if stiffness isn't an issue, don't worry about it. But if it is, you might pull out your hair trying to fix it.

Second, rear-mounted dressers allow you to dress while doing other stuff: cycling back, changing parts or even grinding itself during continuous-dress grinding. Eventually, these seconds can add up to weeks of increased production time.

Third, there's a weird type of chatter called "2N chatter," which is caused by differing stiffness in the x-axis versus the y-axis — assuming that the z-axis is the axis of rotation of your wheel. This makes the wheel pulsate twice per wheel revolution. Dressing on the same side that you grind on doesn't eliminate this pulsation, but it dresses the pulsation pattern into the wheel, lessening the problem. Dressing on the opposite side that you grind on actually doubles the pulsation problem. Again, if it's not an issue, it's not an issue. Unless it is.

Fourth, if the bed of your machine has an alignment error, dressing on the same side that you grind on can sometimes — but not always — hide this error by imparting the same one into the wheel. It's not textbook grinding, but it does improve things. On rear-mounted dressers, if there's an alignment error in either the dresser or bed, the error isn't going to correct itself. This is a frequent cause of "barber pole" in cylindrical traverse grinding.

That's why old-school machinists have been known to say, "Dress and grind on the same side." It doesn't always matter. But when it does, it does.

So I'm all for rear- or top-mounted dressers as they can save time. But they do add potential issues that you need to be aware of.