ID Grinding

ID grinding finishes internal diameters to very tight size, roundness, and surface finish on hardened or difficult-to-machine bores.

Overview

ID grinding (internal grinding) uses a rotating abrasive wheel to finish internal diameters with high accuracy. It excels on hardened or heat-treated parts where boring or reaming can’t hold tolerance or surface finish. Shops use it to correct distortion after heat treat, improve roundness, and achieve fine finishes inside bores and internal features.

Choose ID grinding for tight tolerance bores, typically in the ±0.0005" range or better, with demanding roundness and cylindricity requirements. It handles hard steels, tool steels, bearing steels, and some carbides, but requires rigid fixturing and good datum control. Setup is more involved than basic turning or drilling, so it fits best for moderate to high-value parts or runs where the precision justifies the cost. Expect limits on bore length-to-diameter ratio, wheel access, and small feature transitions inside the bore. Clear prints and realistic tolerances keep cost and lead time under control.

Common Materials

  • Tool Steel A2
  • Tool Steel D2
  • 52100 bearing steel
  • 440C stainless
  • Alloy Steel 4140
  • Tungsten carbide

Tolerances

±0.0005"

Applications

  • Bearing race bores
  • Hydraulic and pneumatic cylinder bores
  • Gear and spline bores
  • Precision bushings and sleeves
  • Spindle and chuck bores
  • Injection mold and die cavities

When to Choose ID Grinding

Use ID grinding when an internal diameter needs very tight size, roundness, and cylindricity on hardened or heat-treated parts. It fits parts where the bore is a critical locating feature or sealing surface and surface finish matters. Best for moderate to high-value components where the cost of precision grinding is justified by performance or assembly requirements.

vs Surface Grinding

Choose ID grinding instead of surface grinding when the critical feature is an internal bore rather than a flat face. Surface grinding controls flatness and thickness; ID grinding controls internal size, roundness, and alignment to external datums. Use it when the bore locates bearings, shafts, or seals and drives assembly accuracy.

vs OD Grinding

Pick ID grinding over OD grinding when the functional feature is the internal diameter, not the shaft or journal outside. OD grinding stabilizes external diameters; ID grinding produces matching or mating bores that need tight clearance, press fits, or bearing fits. It is essential when bore-to-OD concentricity and internal geometry drive part performance.

vs Centerless Grinding

Use ID grinding instead of centerless grinding when you must control internal diameters or bore geometry rather than external diameter and straightness. Centerless grinding is ideal for round stock and shafts, but it cannot touch internal features. ID grinding handles short or complex parts where the bore is the functional feature or datum.

vs CNC Turning/Boring

Choose ID grinding over CNC turning/boring when the part is hardened, requires very fine surface finish, or needs sub-thousandth tolerances on the bore. ID grinding corrects distortion from heat treat and improves roundness and cylindricity beyond what single-point tools typically achieve, especially in tough or abrasive materials.

Design Considerations

  • Leave uniform grind stock on the bore after pre-machining or heat treat, typically 0.003"–0.010" per side depending on size and material
  • Dimension and tolerance the bore to clear datums so the shop can fixture from functional surfaces and control concentricity correctly
  • Avoid deep, small-diameter bores with extreme length-to-diameter ratios unless necessary; long slender bores raise cost and risk chatter
  • Call out surface finish only where functionally required and relax tolerances on non-critical bore sections to reduce cycle time
  • Minimize sharp internal corners and steps; use chamfers or generous radii so the wheel can enter and blend without burning
  • Clearly specify which bore segments control fit (bearing seat, seal land, pilot section) so the grinder can focus time where it matters