Technical Sharing | Poor Surface Finish? Machining Allowance Could Be the Root Cause
SNSTC 2026-01-05
In machining, surface finish (or surface roughness) is one of the most critical indicators of part quality. It affects not only the appearance of a component, but also its wear resistance, corrosion resistance, fit performance, and fatigue strength. However, many engineers and operators—especially those new to manufacturing—often assume that leaving more machining allowance during the finishing stage will allow the cutting tool to remove more material and thereby achieve a better surface finish.
However, reality is far more complex. There is no simple proportional relationship between machining allowance and surface finish. Multiple factors influence surface quality, and their interactions will be examined in the following analysis.
Machining Allowance vs Surface Quality: Not the Only Determining Factor
First, it must be clarified that machining allowance is an important factor influencing surface finish, but it is by no means the sole determining factor. Final surface quality results from the combined interaction of the machine, cutting tool, workpiece material, cutting parameters, and machining allowance. Blindly increasing or reducing the allowance rarely improves surface finish and often leads to additional process-related issues.
1.Negative Effects of Excessive Machining Allowance
If the finishing allowance is set too large, the following issues may occur:
①Significant increase in cutting force
When the tool is required to remove an excessive amount of material, cutting resistance rises sharply. This can easily induce vibration in the machining system (machine–tool–workpiece–fixture). In cases of insufficient rigidity, chatter is likely to occur, leaving periodic or random chatter marks on the machined surface and severely degrading surface quality.
②Concentration of cutting heat
A higher material removal rate leads to increased cutting heat generation. Localized high temperatures may soften the workpiece material, accelerate tool wear, and even promote built-up edge formation, resulting in defects such as burrs, tearing, or surface smearing on the finished surface.
③Tool deflection and elastic deformation
Under large cutting forces, elastic deformation of the machining system occurs, causing fluctuations in the actual depth of cut. This leads to uneven surface finish and adversely affects dimensional accuracy.
Therefore, an excessively large machining allowance not only fails to improve surface finish, but may instead produce a rough surface with pronounced chatter marks.
2.Problems Caused by an Excessively Small Machining Allowance
Conversely, reducing the machining allowance to an extremely small value also presents several challenges:
①Ineffective cutting action
When the allowance is smaller than the cutting edge radius of the tool, the cutting edge cannot properly penetrate the material. Instead of true cutting, the tool primarily causes surface extrusion and friction, resulting in an abnormal ploughing or rubbing-dominated cutting state.
②Surface work hardening
Severe plastic deformation leads to work hardening of the workpiece surface, increasing tool wear and making subsequent cutting more difficult. As a result, scratches and non-uniform bright bands are likely to appear on the machined surface.
③Inability to eliminate errors from previous operations
One of the key objectives of finishing is to correct dimensional errors and surface defects left by roughing or semi-finishing. If the allowance is too small, these pre-existing imperfections may not be completely removed and can remain on the final surface.
Therefore, an excessively small machining allowance is also detrimental to achieving an ideal surface finish.
3.Determining a Reasonable Machining Allowance: Finding the Balance
Between “too large” and “too small”, there exists an optimal range of machining allowance—often referred to in process engineering as the “sweet spot”. Within this range, the cutting tool can operate under stable and appropriate cutting conditions, removing material with moderate cutting forces and heat generation, thereby producing a high-quality surface finish.
The selection of a reasonable machining allowance should take the following factors into comprehensive consideration:
①Workpiece material properties
Different materials require different optimal depths of cut. For example, the suitable finishing allowance for aluminum alloys, low-carbon steels, and high-temperature alloys can vary significantly.
②Tool geometry parameters
Tool nose radius, rake angle, and cutting-edge sharpness directly affect cutting performance. In general, it is recommended that the finishing allowance be slightly larger than the tool nose radius to ensure true cutting rather than surface extrusion or rubbing.
③Machine and process system rigidity
Machines with higher rigidity can withstand larger cutting forces, allowing for a relatively wider acceptable range of machining allowance.
④Surface condition from previous operations
If the roughing or semi-finishing surface exhibits significant unevenness or a hardened layer, the finishing allowance must be sufficient to completely remove these defects.
4.Multiple Factors Affecting Surface Finish
To systematically improve surface quality, machining allowance must be controlled within the context of the entire machining process. The following are the key factors that influence surface finish:
① Cutting parameter combinations
Cutting speed: Appropriately increasing the cutting speed helps suppress built-up edge formation and improves surface texture.
Feed rate: This parameter has the most significant impact on theoretical surface roughness. The roughness value is generally proportional to the square of the feed rate, making feed reduction a common and effective approach to improving surface finish.
Depth of cut: In finishing operations, this corresponds to the machining allowance and must be maintained within a reasonable range.
②Tool-related factors
Tool nose radius: A larger nose radius helps “smooth” or “iron out” the surface, reducing the theoretical residual height.
Tool wear condition: Worn cutting edges can damage the machined surface and should be inspected and replaced regularly.
Tool geometry: Appropriate rake angle, clearance angle, and inclination angle contribute to stable and smooth cutting.
③Stability of the machine tool and process system
Vibration is one of the primary “killers” of surface quality. Spindle accuracy, guideway condition, fixture rigidity, and workpiece clamping method together determine the stability of the cutting process.
④Cooling and lubrication conditions
Effective cutting fluids reduce cutting temperature, minimize tool wear, and suppress built-up edge formation, playing a particularly important role in finishing operations.
5.Practical Guideline for Finishing Allowance Selection
For finishing operations such as turning or milling of common materials (e.g., carbon steels and aluminum alloys), the following empirical guideline can be applied:
Finishing allowance ≈ Tool nose radius × (1.0–1.2)
For example, when performing finish turning with an insert having a tool nose radius of 0.4 mm, the finishing allowance on one side can typically be set to 0.4–0.5 mm.
In practical applications, this value should be fine-tuned based on the specific workpiece material, tool manufacturer’s recommended cutting parameters, and the actual machine tool and process conditions.
6.Summary
Surface finish is not determined solely by the magnitude of the machining allowance. The key takeaway is that the finishing allowance must fall within a reasonable range—this is a necessary condition for achieving a high-quality surface, but it is not a sufficient one.
The final achievable surface quality fundamentally depends on the systematic coordination of cutting parameters (particularly the balance between feed rate and cutting speed), the overall performance of the cutting tool (including tool geometry, wear condition, and tool nose radius), the dynamic stability of the machine tool, and the rigidity of the entire machining system.
Therefore, attributing surface quality issues solely to allowance selection is an oversimplification. Scientific process control lies in the holistic balancing of cutting speed, feed rate, depth of cut, and tool geometry. Only through coordinated optimization of these variables under stable machining conditions can both superior surface finish and dimensional accuracy be achieved simultaneously.
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