What Is Roughing In cNC Machining? How is It Different from Finishing?

You’ve probably heard the terms Roughing and Finishing thrown around if you work in the manufacturing sector, especially machining. These two terms are often confused, or they are at least used incorrectly by most people.

It is critical to understand the difference between them as it will make your process more efficient, whether that be cutting metal on a lathe, drill press, or mill.

This article is to help you understand the difference between them and how they are used in your process to make CNC products sturdy.

What is Roughing in Machining?

First, we will start by looking at roughing in machining. When you hear someone say “let’s rough this part out before taking it to the finishing machine” they are talking about roughing.

Roughing is a non-uniform cutting process that removes a lot of material from a workpiece as quickly as possible. Usually roughing will take place on a machine that has a higher spindle speed and feed rate than the finishing processes to come later.

The more material you can get off quickly, without paying attention to uniformity or accuracy of your part, the faster you finish your product and move on to the next step in the process. Roughing is all about time, not quality.

What are the Benefits of Rough Machining?

There are many benefits to rough machining. They include:

• It Saves Time. Roughing will shave hours off your manufacturing time. While roughing is not very accurate nor uniform, it does produce a very close shape to how the final product should look. It sets up your finishing operations for success by removing more material in one pass than can be done with finish machining.
• It Saves Money. As stated before, it saves time, but it also helps save money. When you complete the roughing process in one setup, you do not have to pay for multiple setups on your finishing machine.
• It Saves Material. Rough machining reduces the amount of material that needs to be removed in the finishing stage by up to 50%. This is an incredible amount of material saved and will cut down your overall cost as well as the shipping weight.
• Easy to Detect Defects of Blanks. If a machine operator was then to send a workpiece that has been rough machined to the finishing process, any defects of the part will be very easy for operators to see and identify. Defects or cracks become much more pronounced once material is removed from an area of the component.

Spindle/Feed Speed vs. Feed Rate

Now that we have a better idea of what rough machining is, let’s look at how it relates to spindle speed and feed rate.

It is important to note here that spindle speed and feed rate are inversely related when it comes to roughing. That means as one goes up, the other goes down. This is because when you increase the spindle speed the cutting edges on your tools apply less pressure to the metal as they cut.

This causes a lighter cut that allows you to take more material off rapidly. The drawback is that it can lead to inaccuracies, chatter marks, burrs, and tool wear if the speeds are too high.

On the other hand, increasing the feed rate increases the amount of pressure applied by the cutting edges on your tools. This pressure allows you to get more accurate cuts and produce a better finish part. The drawback is that your machine can’t go too fast because it might cause deflection of the material or burn marks on your product.

What Is Finishing in Machining?

Finishing, also known as secondary operations, will be done after roughing on the same machine. Finishing operations are all about getting your product to look like what you designed it to be, with as few defects or burrs as possible.

The goal of finishing is to make the part look good and also match what you need from a dimensional standpoint. Examples of secondary operations include performing hole drilling, counter boring, knurling, engraving, and others.

Just like with roughing, there is no set amount of feed rate or spindle speed you should use when it comes to finishing machining. The only way to find the best settings for your part is through trial and error through testing and experience.

The most important thing for you to know about spindle speed and feed rate when it comes to finishing is that you need to run your part at the lowest speed you can while still getting a good finish. This will reduce deflection, vibrations, burrs, tool marks, and other defects.

What is the Difference Between Roughing and Finishing in Machining

If you’re to meet CNC machining requirements, you need to understand what the difference is between roughing and finishing milling. It’s easy to see that both processes are interrelated, but how do they relate?

Roughing is the first cutting operation you need to do in your production cycle. You will want to get this done as quickly as possible while still producing an accurate product. You can do this by taking deep cuts with your tools, which means you’ll use the largest spindle speeds possible.

When it comes to finishing, the focus is on getting the surface finish of your part as smooth and defect-free as possible. Since you’re using small spindle speeds for this step, you will end up taking a lot more time than you did with roughing. This is okay as long as you’re getting the surface finish that your customer wants, and it’s also where you want to focus on lowering vibrations and deflections to produce an even better product.

The specific differences between the processes can be classified as below:

• Purpose. Roughing removes material from a workpiece as fast as possible. On the other hand, finishing milling aims to achieve a good surface finish, improve tolerance, and enhance accuracy at the same time.
• Cutting Tools. Tools used in roughing are larger than those used in finishing. This is because they have to remove a lot more material quickly. In finishing, smaller tools are used to maintain accuracy and produce a good finish.
• Material Removal Rates. With roughing, you’re removing a lot of metal from your workpiece as fast as possible, which means that the cutting edges on your tools have very little contact time with the material. The shorter contact time reduces the chances of your tool enduring high temperatures.
• Precision. Roughing is done with large spindle speeds and heavy cutting loads. This allows the machine to quickly remove a lot of material, but it also means that there will be more vibrations and deflections as the part moves around on your machine. When you finish as fast as possible, all those vibrations can lead to even greater accuracy issues. Because of this, you should always aim for lower speeds and lighter cutting loads.

As you can see, the difference between roughing and finishing metal machining is what your focus needs to be during each process. The easiest way to determine which of these two processes you’re trying to do is by looking at how fast you want to remove material from your part.

If you want to remove as much material as possible from your workpiece, you’re roughing. If you want to take a lot of time and ensure that the surface finish of your part is top-notch, then this means you’re doing finishing machining.