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Lapping

Mechanical finishing · Mechanical

Lapping is a precision loose-abrasive process used to improve flatness, parallelism, and very fine roughness.

CNC machiningHigh cost $$$$Unchanged
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Reference page — Lapped is not in the instant-quote configurator. Upload the part, name the finish in the notes, and an engineer quotes it with the part.

Lapping on a machined part

What is Lapped?

It is common on seal faces, gauges, valves, ceramics, and optical or metrology components.

How it works

The workpiece moves against a dimensionally controlled lap charged with loose diamond, alumina, or silicon-carbide slurry, typically specified by micrometre particle size rather than grit alone. Abrasive particles roll and slide between surfaces, removing microscopic high spots rather than following the prior tool path. Production lapping commonly reaches Ra 0.01–0.10 µm (0.4–4 µin), including the requested sub-0.1 µm class, while flatness depends on lap conditioning, geometry, and thermal control. Measure Ra under ASME B46.1 or ISO 21920.

Why specify it

  • Achieves Ra 0.01–0.10 µm (0.4–4 µin)
  • Improves flatness and parallelism
  • Produces high bearing area
  • Finishes hard metals and ceramics

Where it falls short

  • Slow and costly
  • Can embed abrasive in soft materials
  • Rounds unsupported edges
  • Requires rigorous cleaning

Lapped at a glance

Lapped is strongest on cosmetic quality (70 of 100), at a high cost. It adds little corrosion protection. Surface hardness is not rated: the finish leaves the part's own surface.

Corrosion protectionMinimal0 / 100
Surface hardnessNot rated—
Cosmetic qualityVery good70 / 100
LubricityLow30 / 100
Relative costHigh70 / 100
CorrosionHardnessCosmeticLubricityEconomy

Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.

Lapped key numbers

Typical values from the governing specifications and shop practice; bars are this finish against the highest value among mechanical finishing finishes.

Thickness0 µm0 mil
Thickness
No coating; typical resulting Ra 0.01–0.10 µm (0.4–4 µin).
Electrical
Unchanged
Appearance
Very smooth matte-to-reflective surface with nondirectional fine marks.

Specifications for Lapped

The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard.

Drawing call-out

LAP DATUM A TO FLATNESS 0.002 mm AND Ra 0.05 µm MAX; FINAL THICKNESS 8.000 ±0.003 mm; NO EMBEDDED ABRASIVE

Which materials take Lapped?

Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.

Material familyVerdictNotes
Ceramic
YesDiamond lapping is common for hard technical ceramics.
Stainless Steel
YesCommon for seal and hygienic components.
Steel
YesCommon for gauges, valves, and seals.
Aluminum
With careSoft aluminum embeds abrasive; use a dedicated plate and rigorous cleaning.
Brass
With careSoft brass embeds abrasive and rounds edges quickly.
Copper Alloy
With careSoft copper embeds abrasive; use a dedicated plate and thorough cleaning.
Magnesium Alloy
With carePossible with non-sparking media and fire-safe dust control.
Titanium
With carePossible; control galling and abrasive embedment.
Zinc Alloy
With careSoft die-cast zinc cuts quickly and can embed media.
Polymer
NoAbrasive lapping is for metals and ceramics, not polymers.

How Lapped is applied, and what it does to the part

  1. 1Pre-grind with allowance
  2. 2Condition lap
  3. 3Charge with selected loose abrasive
  4. 4Lap under controlled pressure and motion
  5. 5Clean and inspect flatness thickness and Ra
DimensionsLapping removes small but measurable stock and can hold precise thickness, flatness, and parallelism only when each is specified and inspected.
Build-up per side0 µm — No buildup; micrometre-scale stock is removed and allowance must be planned.
MaskingRarely needed. Only designated faces contact the lap; protect holes and crevices from slurry ingress.
ElectricalUnchanged
AppearanceVery smooth matte-to-reflective surface with nondirectional fine marks.
Added lead time3–10 working days on top of the bare part

Design rules for Lapped

What to decide on the drawing before the part goes to the finisher.

  1. Specify flatness and Ra independently.Neither guarantees the other.
  2. Provide lapping allowance.The process must clean up the pre-ground face.
  3. Control edge condition.Unsupported edges can roll.
  4. State cleaning acceptance.Loose abrasive may remain in pores and holes.

Lapped versus the alternatives

Finishes engineers weigh against Lapped, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
Lapped
this page
0—70$$$$0 µm—
Surface Grinding
Lapped vs Surface Grinding →
0—35$$$0 µmwhen faster stock removal and Ra above 0.2 µm are acceptable
Superfinishing and Isotropic Superfinishing
Lapped vs Superfinishing and Isotropic Superfinishing →
5—65$$$$0 µmwhen a low-friction bearing surface is primary
CorrosionHardnessCosmeticLubricityEconomy
LappedSurface Grinding

The radar overlays Lapped with Surface Grinding, its most common alternative — chosen when faster stock removal and Ra above 0.2 µm are acceptable. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Lapping is a high-cost finish ($$$$): more expensive than about 75 % of the finishes FabDigit quotes.

Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates. The exact figure appears on the instant quote once the part is uploaded.

Lead time and compliance

Added lead time3–10 working days
RoHSCompliant
NotesSpent slurry contains fine abrasive and substrate particles and requires controlled collection.

What Lapped is used for

  • Mechanical seal faces
  • Valve plates
  • Gauge surfaces
  • Ceramic seats

Lapped — frequently asked

what Ra can lapping achieve

Production lapping commonly achieves Ra 0.01–0.10 µm (0.4–4 µin). Material, abrasive size, lap condition, and cleaning determine the actual result.

lapping vs grinding

Grinding uses a bonded wheel for faster removal and commonly gives Ra 0.2–1.6 µm. Lapping uses loose abrasive for finer flatness and Ra below 0.1 µm.

does lapping improve flatness

Yes, when the lap is correctly conditioned and the part is thermally stable. Flatness must still be stated as a separate tolerance.

what abrasive is used for lapping

Diamond, aluminum oxide, and silicon carbide are common. Abrasive type and size are selected for hardness, removal rate, and embedment risk.

does lapping change dimensions

Yes. It removes micrometre-scale stock and can control final thickness precisely, so allowance and final thickness must be on the drawing.