Introduction

In CNC parts manufacturing achievement of dimensional accuracy and precision is at the cornerstone. Both rough and finishing are the important processes that create finished parts with accurate dimensions and tolerance. However not everybody knows these two processes in detail and their significance.

This blog explains all the details about roughing and finishing. It is a complete guide for machinists if they want to know how and when to use roughing and finishing to manufacture precise and accurate finished machined parts.

What is Roughing in CNC Machining?

In simple words roughing is just the beginning of every machining process. Roughing has various names in the manufacturing industry. You can call it CNC roughing, rough milling or simply the first step in the machining spectrum.

In terms of definition roughing is the removal of excess material from a workpiece. Removal of excess material brings the workpiece close to its original design. Roughing is the bridging step. Bridges the gap between the raw material and the later precise machining stages.

Roughing prepares the workpiece for the later finishing stages. You can say that it acts as an intermediary between the raw stock and the final finished product. Manufacturers prefer to use large tools for roughing. It is because large tools are capable of making deep cuts. The main reason behind using large tools is that a substantial amount of material is removed from the workpiece. Roughing quickly removes blank allowances. Removal of unwanted material results in coarse surface finish of the workpiece.

A major advantage of roughing is high feed rate i.e. removal of large waste material in a shorter period of time. This makes the process more efficient. Roughing however impacts the desired surface finish, and dimensional parameters to a greater extent. Whatsoever the products after going through roughing are poorly finished, have rough surface finish and lack accuracy and precision. So, if you want to achieve high tolerance and dimensional accuracy in your products then roughing is not at all the recommended procedure.

Benefits of Roughing

Every process has its advantages and limitations in the machining industry. Same is the case with roughing. It has several advantages for the manufacturer that is why it is being implemented at such a large scale. In this section we shall have a look at the various advantages of roughing in the machining process.

Rapid Feed or Material Removal Rate

A major advantage of rough machining is the rapid feed rate. Although this makes the process very efficient yet it has the probability of producing errors. Finishing in the later stages can reduce these errors. Use of large machining tools are helpful in the achievement of rapid feed rate.

Preserve Processing Time and Production Cost

Roughing saves a lot of processing time and production cost. It is because in rough and finish machining several errors of the workpiece are identified. These errors include sand hole, air hole, and insufficient machining allowance. Early identification of these errors makes it easy to rectify them on time. Therefore, it saves a lot of processing time and production cost.

Enhanced Tool Life

Roughing is the preliminary step before a product or workpiece is subjected to finishing. Therefore, it paves ways for a more convenient finishing process. The tools in finishing are delicate and vulnerable to more wear and tear. Removal of excessive material during roughing makes the operations of finishing tools easier and more convenient. This enhances the tool life of the finishing process.

Creation of Difficult Cuts

Creation of wider and deeper cuts is very risky during the finishing phase. Deeper and wider cuts are therefore created during roughing. When working on high speed steel, with super hard materials and complex geometric designs roughing before finishing is a very desirable process.

Improved Efficiency

Roughing takes the workpiece closer to its final form. Therefore, finishing or other subsequent machining processes take less time for implementation. This makes the depth of cut whole machining process more efficient and less time consuming.

Preparation for Finishing

As mentioned several times, roughing prepares the groundwork for finishing. It brings the workpiece closer to its finished form. This enhances the precision and accuracy of the final finishing process.

Considerations for Roughing in Machining

Roughing helps manufacturers fabricate the expected shape of the workpiece before the finishing process. However, for accurate roughing certain considerations need to be implemented. Following are them:

Machining Parameters

Technology has made everything more convenient. Same is the case with the machining parameters. You can now preselect the machining parameters by using the CNC roughing tool software. Set all the parameters such as the cutting speed, velocity, feed rate, and depth of the cut by default in the software.

However, since each roughing process is unique depending upon the various process parameters and the material of the workpiece default settings are not recommended. It is better to pre-select all these options every time before you start a roughing process keeping in view the material type and the required design. This will optimize the whole roughing process and reduce the possibility of process errors.

Types of Machine Tools and Software’s

The machine tools used in roughing are tough, rigid and require high power. They need to produce deep cuts. Therefore, roughing is not possible with manual equipment. Manual tools are insufficient to produce the required tool movements for roughing.

Machinists and manufacturers need to be very careful in selecting the cutting tools and control software’s in roughing. Both machine tools and software should be adaptable to the challenges of CNC roughing.

Heat and Cutting Fluid

Roughing produces high feed rate and faces high cutting resistance leading to greater heat generation during the process. The heat from this resistance gets transferred to both the workpiece and the cutting tool. This causes tool wear and thermal distortion in the workpiece.

This can result in machining complications.

The solution to this issue is effective heat control measures during the roughing process. Considerable lubrication during the process can cause reduction in the heat. Use of cooling agents can also reduce the heat intensity during the process. Machinists often use water-based cutting fluids as an effective heat management system.

What is Finishing in CNC Machining?

Contrary to roughing which is the first step of the machining process finishing is the final and last step of the machining process. Finishing is the modification of the workpiece for the satisfaction of specific requirements. These requirements include appearance, smoothness and dimensional correctness. It might seem that machining operation is simple but it requires a lot of complex processes and systems to help the product achieve its final shape.

Following five process are commonly used in the machining industry for finishing of materials:

Surface Coating

Surface coating technique is evident from its name. It involves coating different layers of materials onto the surface of the workpiece. The main purpose of surface coating is to make the appearance of the CNC machined parts more attractive.

Several methods are used for the surface coating process. These include:

• Electroplating

• Anodizing

• Painting

Other than enhancing the aesthetics, surface coating makes the machined parts more resistant to corrosion and more durable as well.

Precision Machining

Precision machining involves the use of extra machining operations. This machining accuracy helps the workpiece achieve the right dimensional tolerance. And accuracy. Precision machining involves the use of secondary CNC machining operations such as:

• Tapping

• Boring

• Reaming

Assemble and Finishing Operations

To make the machined parts suitable for their intended use they need to be combined with inserts, fasteners or other machining components. This can be achieved through additional assembly and finishing operations. After the final assembly the machined parts are ready to use.

Surface Smoothing

Use of smoothing processes such as grinding, polishing and sanding to make the surface of machined parts smoother is known as surface smoothing. Machined parts often have marks on them. There can be spots and surface irregularities as well. Therefore, surface smoothing eliminates such surface irregularities and gives poor surface finish the machined part an attractive finish.

Deburring

The use of deburring blades, tumbling machines and blades to remove the sharp edges on the machined parts is known as deburring. Sharp edges or burrs reduce the functionality of the machined parts. Deburring makes the surface of the machined parts more refined and thus increase the functionality.

Benefits of Finishing

A lot of advantages of the finishing process are evident from the definition and explanation of the process. Let’s look at the advantages of the process one by one in detail:

Better Appearance

The primary goal of finishing process is to make the machined product more visually appealing. For this purpose, it is essential to well execute the finishing process. The aesthetic appearance and quality of the [products are given due importance in several industries. These include:

• The automotive industry

• Consumer electronics

• Aerospace industry

Better appearance is the result of removing all the surface imperfections such as burrs, spots and uneven surface. Consider the example of a perfume bottle manufacturer. If the bottle has scratches and marks on it then it wouldn’t be able to attract customers. On the other hand, a shiny scratch free bottle would look more attractive to customers.

Increased Dimensional Accuracy

Dimensional accuracy is of prime importance in the machining industry. Especially in the finishing process one of the prime objectives is the achievement of high dimensional accuracy, and precision of machined parts.

It is dimensional accuracy that makes the machined part capable of operating in the designated applications.

Durability

Finishing makes the machined parts more durable. Consider the example of surface coating where machined parts are coated with layers of different materials. This makes the machined parts more attractive and increases their corrosion resistance. It also makes them more resistant towards wear and tear. This leads to an increase in the durability of machined parts.

Better Surface Quality

The initial surface of machined parts is rough. Rough surfaces might produce more heat and friction. To reduce friction the surface quality finish machining part needs to be top notch i.e. polished and refined. Finishing therefore improves the surface quality of machined parts and ensures smooth functioning.

Product Longevity

Finished parts are durable, corrosion resistant and offer smooth running. This means less replacement and less wear and tear. This increases the product longevity. Of Course, why would you replace a machined part when it’s working perfectly.

Considerations for Finishing in Machining

You need to consider several considerations to successfully execute the finishing process. In this section we shall have a close look into all such considerations individually.

Cost

Cost is a major consideration during the finishing princess. If you want to have a good finish then you need to use high quality finishing products. High quality products are of course expensive. This will increase the cost of your manufacturing process. Moreover, it also requires complex procedures and tools. You need to carefully select the appropriate finishing process that complies with your budget.

Surface Inspection

During the finishing process regular surface inspections are a must. You need to make sure that even during the process there does not remain any surface irregularities such as scratches, spots and coarse surface. You can do surface inspections manually or even through metrology tools. Metrology tools include profilometers, and coordinate measuring machines.

Coolant

It might appear strange to know but use of a coolant or lubricant makes the finish look more enhanced. But how?

A coolant performs several vital functions such as:

• Easing chip evacuation

• Reducing the frictional forces

• Managing heat effectively

However, the selection of appropriate coolant depends upon two factors:

• The type of finishing process being implemented

• The material being used.

Tool Wear Requirements

To maintain consistent finish quality and dimensional accuracy you need to monitor the tool wear requirements. A worn-out tool can lead to deviations from the required dimensions. It may also negatively impact the surface finish. Machinists should therefore regularly monitor finishing tools. In case the tools are worn out they should be replaced immediately.

Roughing Machining Vs Finishing Machining, what is the Difference?

Sr No		Roughing	Finishing
1	Feed Rate	Feed rate is high because of the use of heavy tools and machinery. This also increases the process efficiency. High feed rate also results in deep cuts.	Feed rate is low. Depth of the cut is also low.
2	Surface Roughness	Roughing leaves scallop marks on the machined part. These scallop marks are the result of high feed rate and heavy cutting equipment. These scallop marks are also known as feed marks.	In the finishing process because of low feed rate and shallow cuts there are less or few scallop marks. This makes the products look better and easy to fabricate.
3	Use of Coolant	Roughing requires more use of coolant than finishing. The deep cuts and aggressive use of machinery generates a lot of heat. To cool down the heat therefore more coolant is needed.	It requires less coolant as compared to roughing. It is because of less heat generation resulting from less deeper cuts.
4	Dimensional Accuracy and Tolerance	There is no focus on the dimensional accuracy of the objects. The only purpose is to cut the material so that it compensates the final finished product or comes close to it.	Dimensional accuracy and tolerance are of extreme importance in finishing. Absence of dimensional accuracy might negatively impact the functionality of the machined part.
5	Rate of Material Removal	Rate of material removal is very high. It is because roughing removes big material parts with heavy machinery. Deep and aggressive cuts are another reason behind it.	Finishing has a very low rate of material removal. The reason is of course deep and precise cuts whose purpose is to give a finishing touch.

Conclusion

Making informed decisions has always been pivotal in the manufacturing and machining industry. Now that you have a clear understanding of both the roughing operations and finishing and the differences between them you can make decisions accordingly. Both processes have their own strengths and complexities. What’s most important is the timing. You can never perform finishing before roughing. Roughing is in fact pivotal to the implementation of finishing.Conclusion

Making informed decisions has always been pivotal in the manufacturing and machining industry. Now that you have a clear understanding of both the roughing operations and finishing and the differences between them you can make decisions accordingly. Both processes have their own strengths and complexities. What’s most important is the timing. You can never perform finishing before roughing. Roughing is in fact pivotal to the implementation of finishing.