Grinding and honing
A long journey of over 40 years in search of solutions for the most complex machining operations has enabled us to acquire the technical and meterological skills to manufacture and inspect components with geometrical and dimensional tolerances of a few thousandths, and roughness down to Rt of less than 1 µ.
14 adjustments of different types
- 4 CNC for external or internal/external max 130 mm
- 2 for external hydraulic with electronic meter in process
- 1 CNC centerless max 80 mm
- 4 without hydraulic centers
- 1 CNC tangential max 650 mm
- 1 CNC vertical honing machine max 120 mm
- 1 horizontal honing machine max 25 mm
Studer – Tacchella – Mar – Morara – Palmary - Delta
Precision grinding and honing
Grinding is the finishing operation that brings the component to its final dimensional and geometric tolerances after turning, milling and heat treatment. It works by abrasion, removing hundredths or thousandths of a millimetre per pass, and achieves precision levels impossible with chip-removal machining: roundness to 2 µm, controlled cylindricity, roughness down to Ra 0.1 µm.
Honing is a super-finishing process that follows grinding when the specifications require even lower roughness, down to Ra < 0.05 µm (Rt < 1 µm), on hydraulic sealing bores and sliding seats where the surface topography is critical for the component functionality.
How CNC grinding works
A grinding wheel made of abrasive grains (corundum, CBN or diamond) rotates at high speed and removes minimal quantities of material from the surface of the part. The numerical control manages speed, feed and wheel-wear compensation automatically. Coolant is essential to prevent surface burns (grinding burn) that would alter the hardened layer.
Grinding is the final operation after heat treatment (hardening, case-hardening, nitriding), when the material has reached its definitive hardness and turning and milling can no longer operate with the required precision.
How bore honing works
Honing uses an expandable tool (lap) coated with abrasive paste that oscillates and rotates inside the bore, removing micro-quantities of material down to Ra < 0.05 µm. Unlike grinding, it does not generate significant heat and does not alter the metallurgical properties of the surface, which is why it is preferred where the surface topography is critical for oil-film retention and dynamic sealing.
Difference between grinding and honing
Grinding and honing are both finishing operations but with different principles. Grinding removes hundredths–tenths of a mm per pass with a high-speed wheel, achieving roughness Ra 0.1–0.4 µm and tolerances of ±0.002–±0.005 mm. It generates significant heat and requires coolant. Honing removes thousandths of a mm with abrasive paste at low speed, achieving Ra down to 0.05 µm and tolerances ±0.001–±0.002 mm. It generates no heat.
In short: grinding defines the dimension and geometry of the part, honing perfects the roughness of the bore. On the most critical components they are performed in sequence. Performing both within the same facility ensures consistency between ground and lapped dimensions.
Types of grinding
- Cylindrical grinding machines external cylindrical surfaces (shafts, pins, bearing seats) and internal ones (bores, bushings, liners) with the part rotating slowly between centres. Multi-spindle grinders mount external and internal wheels simultaneously, reducing repositioning.
- Centerless grinding supports the part on a work-rest blade between two wheels: ideal for long, thin cylindrical parts, pins, dowels and small parts that cannot be clamped between centres. It can operate in plunge mode (for profiles and shoulders) or through-feed (for constant-section cylinders in large batches), with very high productivity thanks to continuous feeding.
- Surface grinding (or flat grinding) machines flat surfaces with the wheel running tangentially over the part fixed on a magnetic table. It is the reference process for achieving flatness, parallelism and perpendicularity with tolerances of a few thousandths on flanges, plates and sealing surfaces. It is often the last step after milling.
Workable materials
The choice of wheel and parameters depends on the hardness and structure of the material.
| Category | Materials | Common applications |
|---|---|---|
| Hardened steels | 100Cr6, C45, 42CrMo4 (56–62 HRC) | Bearing seats, guides, hydraulics |
| Case-hardening steels | 18NiCrMo4, 16MnCr5, 20MnCr5 | Pins, gears, case-hardened layer |
| Stainless steels | AISI 304, 316, 420, 630 | Medical, food, fluid-contact components |
| Quenched and tempered steels | C45, 39NiCrMo3, 42CrMo4, 40CrMnMo7 | Shafts, flanges, structural components |
| Aluminium alloys | Anticorodal (6082), Ergal (7075) | Lightweight components, housings |
| Copper and alloys | Brass, aluminium bronze | Bushings, propellers, electrical components |
Precision and tolerances
The achievable tolerances depend on the type of grinding, the rigidity of the machine and the thermal stability of the process.
| Operation type | Tolerance | Roughness | Geometric tolerance |
|---|---|---|---|
| CNC cylindrical (external) | ±0.003 mm | Ra 0.2 µm | Roundness 2 µm, cylindricity 3 µm |
| CNC cylindrical (internal) | ±0.003 mm | Ra 0.2 µm | Roundness 2 µm, concentricity 5 µm |
| Centerless (plunge) | ±0.003 mm | Ra 0.2 µm | Roundness 2–3 µm |
| Centerless (through-feed) | ±0.003 mm | Ra 0.2 µm | Straightness 5 µm/100 mm |
| Surface | ±0.003 mm | Ra 0.2 µm | Flatness 5 µm, parallelism 5 µm |
| CNC honing | ±0.001 mm | Ra 0.05 µm | Roundness 1–2 µm, cylindricity 2 µm |
Industrial applications
Grinding and honing are indispensable where fit tolerances, roughness of sliding surfaces and bore geometry are critical functional requirements.
- Hydraulics — sealing seats, bores for spools and slides, cylinder rods. Honing ensures the topography for oil-film retention.
- Machine tool equipment — sliding guides, bearing seats, spindles and collets with tight concentricity and flatness.
- Industrial machinery — shafts, pins, bushings, mating seats. Centerless grinding for series of cylindrical elements.
- Medical and food — components in AISI 316 and hardened 420 with finishes for hygiene and corrosion resistance.
- Marine — finishing of aluminium bronze and stainless steel components after turning and milling.
The Gamba grinding and honing department
The department has 14 grinders from six manufacturers (Studer, Tacchella, Mar, Morara, Palmary, Delta): 4 CNC cylindrical grinders for external and internal surfaces (Ø max 130 mm), 2 cylindrical with in-process gauging for tight-tolerance series, 1 CNC centerless (Ø max 80 mm) + 4 hydraulic centerless, and 1 CNC surface grinder with a 650 mm table.
The grinders have a granite base for thermal stability. They machine parts up to Ø350 × 500 mm with internal spindles at 75,000 rpm and a continuous B axis for complex profiles.
The honing department comprises 2 CNC honing machines: a vertical one for bores from 12 to 120 mm (Rt 1 µm, Ra 0.05 µm) and a horizontal one for bores up to 20 mm (Rt 2 µm, Ra 0.1 µm).
Quality control
The grinders with in-process electronic gauging measure the diameter during machining and stop the pass at the programmed dimension. At the end of the cycle, verification takes place in the metrology room with a motorised roundness tester, CMM, roughness tester and over 500 manual instruments certified to 0.1 µm.
Complete cycle: from grinding to the finished component
A component that requires grinding follows this flow, managed in-house:
- CNC turning: roughing and semi-finishing of diameters
- CNC milling: slots, pockets, cross holes
- Heat treatment: hardening, case-hardening or nitriding
- Grinding: finishing of diameters, planes and geometries to tight tolerances
- Honing: bore super-finishing when Rt < 1 µm
- Final inspection: CMM, roundness tester, dedicated instrumentation
Integrating all operations within the same facility eliminates transfers between suppliers and ensures dimensional consistency of the entire component.
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Frequently asked questions about grinding and honing
Grinding is a finishing process with an abrasive wheel that brings the part to tolerances of ±0.002 mm and Ra 0.1 µm. Honing is a super-finishing process that improves the roughness of bores down to Ra < 0.05 µm with an expandable lap and abrasive paste.
Grinding removes hundredths of a mm with a high-speed wheel (Ra 0.1 µm). Honing removes thousandths of a mm at low speed, without heat (Ra < 0.05 µm). Grinding defines dimension and geometry; honing perfects the roughness of the bore.
Hardened steels up to 62 HRC, case-hardening steels, stainless steel (AISI 304, 316, 420, 630), quenched and tempered steels, aluminium, bronze and HSS. The wheel (corundum, CBN, diamond) is chosen according to the hardness of the material.
CNC cylindrical grinding reaches ±0.002 mm with roundness of 2 µm and Ra 0.1 µm. CNC honing reaches ±0.001 mm with Rt < 1 µm and Ra < 0.05 µm. The level depends on the type of machine, material and geometry.
Grinding takes place after heat treatment to bring the part to its final tolerances. Without grinding, a hardened part does not reach fit tolerances. Honing completes the cycle when the specifications exceed the capabilities of grinding.
Honing holes
CNC vertical honing allows a wide range of machining operations with holes from 12 to 120 mm, to achieve very tight tolerances while respecting the geometry of the component. In parallel, with finishing operations, the CNC's high sensitivity allows finishes with Rt values < 1 µ or Ra < 0.05 µ to be realised.
Horizontal honing is suitable for honing and lapping holes from 4 to 20 mm with Rt values < 2 µ or Ra < 0.1 µ.
Other types of processing
We manage all stages of the production process internally to ensure quality control and adherence to schedules.
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