ID Grinding

Internal grinding is the primary process for the precision finishing of internal surfaces or bores. The bores may be simple cylindrical surfaces or may be surfaces requiring the generation of complex and exact profiles for applications such as bearing and CV joint races or fuel injection seats.

Most precision internal grinding operations require the capability to hold tolerances on size, roundness, straightness, taper, and cylindricity of the order of 0.5 to 10 μm, but for special applications such as fuel injection the tolerances have become increasingly stringent in recent years to as tight as 0.25 μm or even lower. The majority of the highest volume applications are found in the bearing and automotive industries grinding hardened steel (carbon, alloy, PM, and M50) with relatively small wheels under 100 mm on horizontal spindle grinders with finish requirements under 0.5 Ra. There are also larger diameter applications in specialty bearing and aerospace engine assembly grinding bores up to 1,000 mm. The largest such parts are usually ground on specialist vertical spindle machines.

Grinding still dominates bore finishing for all the tightest precision tolerance work because of stock-removal capability, accuracy, and cost. However, hard turning is making inroads for finishing larger bores with tolerances >2.5 μm. It is also becoming more common to see multipurpose machining centers for applications such as automotive gears that rough turn the bore, rough and finish turn the gear flanges, but then finish grind the bore in the same chucking when tighter accuracy demands. Diamond honing is also gaining favor and is reported to hold good straightness but stock-removal rates are still limited. To be cost effective stock removal must be kept to under 10 μm.

Modern CNC internal grinders provide a great deal of flexibility not only to plunge grind simple bores but also to face grind, profile grind, and even contour grind outer diameters.
Multislide machines may even simultaneously grind inner and outer diameters and flanges using two wheels.

Basic internal grinding process

In the basic internal grinding process, the wheel is fed perpendicularly into the part usually accompanied by a short-stroke high oscillation along the axis of the wheel. The part is rotated in the opposite direction to the wheel (down grind) in virtually all applications except occasionally in finish grinding with large interrupted cuts if roundness is an issue.
Internal grinding is a very weak system where the primary weakness is the wheel mount or quill. This can readily deflect during the grind leading to problems of taper and shape. The grinding conditions are driven first and foremost by the system stiffness and the level of normal grinding force. The key factors to consider are shown here.

Considerations

Large contact area
Rigidity of the spindle
Importance of good lubrication
Few abrasive grains on the periphery of the wheel

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