Lapping rubs a part against a flat cast-iron plate with fine abrasive paste or slurry between them. It removes only a few microns, but leaves faces extremely flat and smooth, flat enough to seal against each other without a gasket. It is used for mechanical seal faces, valve seats, gauge blocks, pump parts and hydraulic components.
Quick facts
- Flatness
- Down to about 1 light band (about 0.3 µm) on small parts
- Surface finish
- About Ra 0.05–0.2 µm
- Stock removed
- A few µm to about 0.05 mm (0.0020 inch)
- Materials
- Hardened steel, cast iron, carbide, ceramics, bronze, carbon
- Check method
- Optical flat and monochromatic light
- Usually priced
- Per piece or per batch on the lapping plate
How it is done
- Parts are ground or finely machined first, leaving a small allowance
- They sit in holders (conditioning rings) on a turning lapping plate
- Abrasive slurry (diamond, silicon carbide or alumina) is fed onto the plate
- The parts rub over the plate and are lapped flat
- They are cleaned and checked with an optical flat
Lapping, grinding or honing?
| Lapping | Surface grinding | Honing | |
|---|---|---|---|
| Surface | Flat faces (also bores on special laps) | Flat faces | Bores |
| Flatness or roundness | Best | Good | Best roundness in bores |
| Metal removed | Very little | More | Little |
| Speed | Slow | Fast | Medium |
What to send
- Drawing with flatness and finish required
- Material and hardness
- Which faces to lap
- Starting condition (ground, turned) and allowance left
- Quantity
- Whether a flatness report or optical flat photo is needed
Need lapping done?
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Questions
- What is a light band in lapping?
- A unit of flatness seen as dark and light bands under an optical flat. One band is about 0.3 µm with helium light. Fewer and straighter bands mean a flatter face.
- Why are mechanical seal faces lapped?
- The two faces must be flat to about one or two light bands so liquid cannot pass between them. Grinding alone is not flat enough.
- Can soft materials be lapped?
- Yes, materials such as carbon, bronze and some plastics are lapped with the right abrasive and plate, though hard materials give the best flatness.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 22 October 2026.