After taper, the next most common bore-grinding problem is roundness. Maintaining roundness to within the specification is critical to the ultimate performance of the component —affecting such things as lubricant retention and distribution, leakage, vibration levels, friction and ease of assembly. We want our parts to be as round as possible, within the specification, without compromising efficient grinding cycles.
Roundness is defined as the amount of deviation from the nominal shape to the actual shape after grinding. These deviations shown in the image produced by your roundness measurement system can look like a jagged mountain range. However the sharpness of these peaks and valleys are generally of no concern unless they fall outside of the concentric control limits or form an unusual shape.
Even nominally round parts are not perfectly round. Some are oval, some are lobular, and some have other “non-circular” shapes. (See figure-1 for examples of non-circular measurements.) Complex scientific analyses of these characteristics performed in the quality lab can reveal how certain irregularities within your grinding process (such as harmonics and vibration) are impacting the parts roundness profile and its performance. (That’s not our concern here.)
Our concern is this: You have a grinding process that has been producing round parts and now they are drifting out-of-round. What are the most likely fixes for this problem?
- Increase Sparkout: Roundness problems are often caused because there’s still deflection or forces built up in the quill or the wheel during grinding that need to be relaxed. The simple solution here is to make sure everything has relaxed properly by increasing your sparkout before retracting the wheel.
- Speed up The Workhead: Sometimes increasing the rotational speed of the part will fix the problem. This is especially true if roundness issues are resulting from a harmonic, which is eliminated by changing the workhead speed. Going faster also reduces the depth of cut per revolution which often reduces grinding forces, deflection, vibration, etc.
- Sharpen The Wheel: Sharper wheels cut more efficiently. Sharp wheels require less force and generate less vibration, both of which have a negative impact on roundness. So sharper wheels produce rounder parts. Optimizing your dress is a great way to ensure you are getting a sharp wheel. See our Dressing Calculation Tool for more information.
- Turn Down the Coolant Pressure: If you have a long part to grind it is a good idea to push in a substantial amount of coolant to improve cooling and material removal. Unfortunately, within the narrow confines of a long cylinder, grinding wheels can actually hydroplane on highly pressurized coolant. This causes the wheel to deflect and the part to be out-of-round. We have seen many instances in which backing off on the coolant pressure has fixed a roundness problem.
Those are the most common and easiest fixes for roundness problems. Next problem up …Shape.