Palmary IGM-25

Universal cylindrical grinders with internal grinding attachments can perform ID grinding, but purpose-built internal grinders like the IGM-25 are typically more productive and easier to use for dedicated bore grinding work. A purpose-built internal grinder is optimized for the specific setup requirements of ID grinding: the workhead is designed for chuck holding (ring and sleeve workpieces, not shafts between centers), the machine layout provides better access for loading and unloading bore workpieces, and the internal spindle mounting and speed range are optimized for the ID grinding wheel sizes and speeds required. For shops where ID grinding is the primary operation, a dedicated internal grinder is more efficient than a universal machine used primarily for OD work.

ID grinding requires very high wheel RPM because the grinding wheel must be small enough to fit inside the bore being ground — typically 50–80% of bore diameter as a maximum wheel size rule of thumb. A small wheel diameter means the wheel must spin at very high RPM to achieve the required peripheral speed (30–45 m/s for conventional abrasive, higher for CBN). For example, a 10 mm bore requires a grinding wheel of 6–8 mm diameter — to achieve 45 m/s peripheral speed on an 8 mm wheel requires approximately 107,000 RPM. This is why ID grinding spindles are either air turbine (very high RPM, lower torque, for small bores) or high-frequency electro-spindle (10,000–20,000 RPM for medium bores). Larger bores allow larger wheels and lower RPM spindles.

As a general rule, the maximum practical bore depth for ID grinding is 1.5–2x the bore diameter before spindle deflection becomes problematic. For a 25 mm bore, this means practical grinding depth up to about 40–50 mm. Beyond this, the grinding spindle (which must fit inside the bore — smaller than the bore diameter) becomes a long, thin cantilever that deflects under grinding forces, causing diameter taper, roundness error, and vibration. The IGM-25's 200 mm maximum bore depth is achievable for larger bore diameters (100 mm+) but not for minimum bore sizes. Palmary applications engineers can advise on specific bore depth-to-diameter combinations and the appropriate spindle configuration.

In ID grinding of rings and sleeves, the workpiece is held by its OD in a 3-jaw chuck, and the bore (ID) is ground concentric to the OD datum established by the chuck jaws. This achieves OD-to-ID concentricity within the chuck runout — typically 0.005–0.020 mm for a standard self-centering chuck. For tighter concentricity requirements (bearing rings, precision sleeves), a precision collet or soft-jaw chuck bored in situ achieves better concentricity. The best concentricity between OD and ID is achieved by grinding both in the same chucking (on a universal OD/ID grinder) — the IGM-25 is an ID-only machine, so OD-to-ID concentricity depends on the chuck accuracy.

Air turbine spindles use compressed air to drive a turbine wheel at very high speed (30,000–100,000+ RPM) — they are compact, light, and achieve the RPM needed for very small bore grinding, but have limited torque and cannot be used for aggressive stock removal. Electro-spindles are direct-drive motor spindles that integrate a motor rotor inside the spindle housing, driving at 3,000–20,000 RPM with higher torque — better for larger bores with more stock to remove. The Palmary IGM-25 typically offers both options: air turbine for small bores (10–50 mm) where very high RPM is needed, and electro-spindle for medium to larger bores (50–250 mm) where torque and stock removal rate are more important than maximum RPM.