What are the methods to improve the machining accuracy of gray iron castings?  

I. Control at the Casting Source (The quality of the blank determines the upper limit)

The upper limit of machining accuracy is often constrained by the uniformity of the blank’s allowance and the stability of its material.

Improving the dimensional accuracy of the blank:

Method: Use lost foam casting (LFC) or resin sand molding instead of traditional green sand molding.

Effect: Reduces casting tolerances and makes machining allowances more uniform. Uneven allowances cause fluctuations in cutting forces, leading to tool deflection and reduced accuracy.

Strict aging treatment (core):

Method: After rough machining, artificial aging (stress relief annealing) must be performed, sometimes even multiple times.

Effect: Gray iron has significant internal stress. Aging treatment can eliminate over 90% of residual stress, preventing “springback” deformation of the workpiece after precision machining.

Stabilizing the metallurgical structure:

Method: Strengthen inoculation treatment to prevent the formation of white iron (hard spots) or localized excessive hardness.

Effect: Hard spots cause severe tool wear or chipping, directly leading to dimensional inaccuracies.

II. Process Route Optimization (Thermal and Cold Control)

Complete separation of rough and finish machining:

Strategy: Rough machining removes most of the allowance → cooling to room temperature → aging treatment → semi-finish machining → finish machining.

Key point: Rough machining generates significant cutting heat, causing the workpiece to expand. If finish machining is performed immediately, the workpiece will contract beyond tolerance after cooling. Adequate cooling time must be allowed.

Adopting the “unified datum” principle:

Strategy: Use the same positioning datum surface throughout the entire machining process as much as possible.

Effect: Avoids cumulative errors caused by repeated changes in the clamping datum.

III. Clamping and Positioning Techniques (Preventing Clamping Deformation)

Gray iron has a low elastic modulus (about 1/3 that of steel) and poor rigidity, making clamping force a “hidden killer” of accuracy.

Optimizing clamping force:

Strategy: “Better loose than tight.” The clamping force should be as small as possible while ensuring no slippage during cutting.

Technique: For thin-walled boxes, hydraulic multi-point floating supports can be used to distribute clamping forces and prevent workpiece deformation.

Application of auxiliary supports:

Strategy: Add auxiliary supports (such as jacks or adjustable support pins) when machining overhanging areas.

Effect: Increases the workpiece’s systemic rigidity and reduces cutting vibration.

“Unclamp and measure” method:

Strategy: After trial machining, unclamp the workpiece to measure dimensions. If springback occurs, adjust the tool compensation before final machining.

IV.Tools and Cutting Parameters (Reducing Error Repetition)

Selecting high-rigidity tools:

Method: Use tools with large core diameters and short shanks.

Effect: Gray iron machining generates significant radial cutting forces. Insufficient tool rigidity can cause bending deformation, resulting in a “concave” machined surface.

Keeping the cutting edge sharp:

Method: Use coated carbide or CBN tools and replace worn inserts promptly.

Effect: Dull tools produce a “squeezing” effect, causing work hardening on the workpiece surface and significantly increasing cutting forces, which can lead to machine spindle deflection.

Optimizing the tool path:

Method: During finish machining, use climb milling as much as possible.

Effect: In climb milling, the tool exerts a downward clamping force on the workpiece, reducing vibration. Additionally, the chips transition from thick to thin, resulting in higher surface quality.

Controlling thermal deformation:

Method: For high-precision grinding or boring, use constant-temperature cutting fluid to flush the workpiece.

Effect: Forced cooling prevents dimensional deviations caused by localized overheating.

V. Environmental Control (For Ultra-Precision Parts)

Constant-temperature workshop: For parts requiring accuracy within 0.01mm, machining and inspection must be conducted in a constant-temperature environment of 20°C ± 1°C. Gray iron is highly sensitive to temperature changes.