WHAT MAKES BORE GRINDING SO DIFFICULT?
Bore grinding is one of the most difficult manufacturing capabilities to master. In fact, compared to bore grinding, external surface grinding is a piece of cake. Here’s why:
On the face of it, bore grinding seems like a simple operation. You send a spinning wheel inside a spinning part and oscillate it back and forth until the desired dimensions and surface finish have been achieved. Unfortunately to reach into the part, the wheel must be mounted on a quill (sometimes a long one) which is, in essence, a cantilever.
You want the bore to be perfectly straight and perfectly round. However, as soon as you put pressure on the end of a cantilever, no matter how stiff, there’s going to be some amount of deflection. Then weird things are likely to happen.
Another challenge that external grinding does not have is getting coolant down into the bore while you are grinding it. Grinding creates a lot of friction and heat, which is bad for both the wheel and the part. Getting coolant into the bore is essential, but the wheel itself is an impediment to coolant flow. The problems created by insufficient coolant along with cantilever effect problems are compounded with deeper bores.
With external grinding speed, surface speed is not much of an issue. If you need more of it you can use a larger diameter wheel so that the surface of the wheel is really flying even at moderate RPMs. With bore grinding you don’t have this option. The diameter of the wheel is limited by the diameter of the bore. Even with a high speed spindle, surface speeds for bore grinding wheels are relatively slow so grinding is much less efficient.
So you’ve got low cutting speed. You’ve got cantilever. You’ve got difficulty getting the coolant to where you want it, especially when you are down there at the bottom of the part. These are all things that make bore grinding more difficult.
There are ways to compensate for all of them, which we will explore in detail in future posts. If you have bore grinding problems that can’t wait check out our short course.