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How to Avoid Poor Surface Finishes on Machined Parts

Scrapping parts due to poor surface finish is an outcome no manufacturer ever wants. Let’s take a look at some tips, tools, and processes that can help limit the number of visible tool marks and to ensure excellent as-machined surface quality, every time.

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By Team Xometry
 5 min read
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Scrapping parts due to poor surface finish is an outcome no manufacturer ever wants. The parts may meet all dimensional specifications and function as intended, but if the surface finish is anything short of impeccable, it’s enough for customers to reject them. This is especially the case for parts that are visible on a final product and parts that serve as components of a mold. Due to the nature of the CNC machining process, minor tool marks will always be evident on as-machined parts. However, certain factors, like excessive tool and workpiece vibration and incorrect feed rates, can also contribute to poor surface finish. This article will discuss tips, tools, and processes that can be applied during and after the machining process to limit the number of visible tool marks and to ensure excellent as-machined surface quality, every time.

Why is poor surface finish unacceptable?

Aside from being unsightly, poor surface finishes can have a detrimental effect on corrosion resistance and can make certain manufacturing processes, like painting, more difficult and time-consuming. With a poor surface finish, it becomes difficult for parts to be adequately treated by an appropriate post-machining, finishing process like anodizing or powder coating. That is,  it can be more difficult for coatings to adhere - leading to pockets of uncoated portions where corrosion can creep in or on entire, uncoated portions. Consequently, the parts become more susceptible to corrosion due to the poorly treated (or non-treated), exposed metal on the surfaces of the part. Poor surface finishes can also lead to more friction effects between the part and other objects, as well as decreased strength and wear resistance.

What is an “as-machined” surface?

When a completed part is removed from a CNC machine, the surface of the part (before any post-processing surface finishing) will be dull and have visible tool marks. This is called an as-machined surface. The quality of a finished surface is quantified by measuring the average surface roughness (Ra) of the part. The standard commercial machine finish for parts has a surface roughness of 3.2 μm. This is typically the cheapest surface finish option, as no additional finishing processes are needed. As-machined surfaces may be acceptable for some customers. However, others may require parts to have a lower surface roughness. 

For this article, only tips related to improving as-machined surface finishes during CNC machining operations are discussed. However, several other surface finishing processes can be applied to a part after machining to achieve the desired surface roughness.

Four Tips to Improve As-machined Surface Quality

Improving as-machined surface quality involves revisiting the basics of machining. By modifying tool setups and optimizing tool paths and cutting parameters, manufacturers can ensure excellent surface quality on their parts. Here are 4 tips on how to avoid poor surface finish on machined parts:

1. Increase Speeds, Reduce Feeds

To achieve the desired as-machined surface, the CNC machine needs to be set up to work within the prescribed feed and speed ranges for the material being cut. By increasing the spindle speeds of the cutter, the contact time between the cutter and workpiece is reduced. This reduces built-up edges (BUE) and prolongs tool life. While removing material quickly is the goal of roughing passes, removing material slowly to achieve precise dimensions and excellent surface quality is the goal of finishing passes. By reducing feed rates, lighter and shallower cuts can be taken - allowing for a more uniform surface finish. Through a combination of both increasing spindle speeds and reducing feed rates, surface finishes can be improved due to the reduction of BUE on the part.

2. Minimize tool chatter and deflection

Chatter is the vibration that occurs on the tool due to the forces experienced during a CNC operation. Deflection, on the other hand, is the distance traveled by the end of the tool from the axis of the tool shank. Both are significant reasons for poor surface quality. While unavoidable, here are two tips to minimize tool chatter and deflection:

  1. Make sure that the right tool and tool holder for the job and material are being used to ensure rigidity. Ensure that cutting parameters and feed rates are within the range of recommended values. With the right tool holder, tool overhang - and thus deflection -  is minimized.
  2. Ensure that the tool is secured tightly in the collet and that the workpiece is tightly clamped. Doing both will minimize the possibility of tool or workpiece movement or vibration during a machining operation.

Applying these two tips will go a long way towards minimizing tool chatter and deflection and achieving a quality as-machined surface.

3. Control chip removal

Improper chip breakage and removal can have adverse effects on surface finishes, as chips can abrade the workpiece and can even cause damage to cutters. To remedy this, consider using a chip breaker. Chip breakers are built into or clamped onto the cutting face of a tool. They facilitate chip breakage by intentionally bending the chips, shorter and less string-like, and easier to break off. Reducing the chip load alleviates the forces experienced by the cutter. By controlling chip removal, manufacturers can move one step closer to obtaining high-quality as-machined surfaces, avoid BUE, and increase tool life.

4. Increase the rake angle

The rake angle is the angle between the rake surface (surface of the tool into which chips flow after removal) and the plane perpendicular to the direction of the velocity vector of the cutter. An increase in rake angle helps facilitate chip removal and helps minimize BUE. By positively increasing the rake angle, cutting forces are reduced and chips are continuously formed and evacuated - which also helps reduce the occurrence of poor surface finishes.

Poor surface finishes on as-machined parts are a headache. Despite being machined to precise dimensions, parts with poor finishes are seldom accepted and can be a drain on a company’s profit. However, by applying the tips described above, surface finishes that meet customer requirements in the as-machined condition become possible.

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Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.