June 30, 2021

CNC Machine Turning Tips | What To Look Out For

Do you have your beloved CNC machine, or are you looking to buy one? Most of you have probably heard of CNC machines, but what exactly are they, and how do they work? First, they are computerized machines that can make things much faster than a human can with the help of a laser that creates a cutting edge. The cutting edge is used to cut wood, bricks, and other materials. For example, with a CNC machine, you can make a house or other structure much faster than by hand. Get the most out of your new machine with these tips for turning with CNC machines.

Man standing next to a cnc cutter

Avoiding backlash

Backlash is an activity that takes place on an axis due to the clearance or looseness of mechanical parts, and it is caused by lead screw threads and movement of a screw to the axial in the bearings that hold it. 

There is a need to eliminate backlash for a productive surface finish. Machine controls such as Mach 3 get rid of backlash, and a DRO measures the backlash in your machine. Feed in the direction you will be cutting for a long distance to take up the backlash. 

Making parts bigger than the workpiece

Bigger parts than the workpiece come with advantages and require skills. There is a need for accuracy when lining up each setup. CNC machines have a weight rating that limits how much you can put on the table. 

Open up the machine by opening windows on the side parts. Setting up a big and heavy part is tricky but check for collisions. You are also allowed to remove the tool changer for more room. 

A solid model of the machine performs better. Try as many angles as you can. The weight has to be supported by stands and rollers to help the machine move. Spin round parts and avoid sliding. 

Selecting a proper blade

To get smooth surface finishes, choose a smaller blade than the nose radius, which allows you to program it for a smooth transition from line to line. Read your chips when running your final cuts. 

The size and state of your chips tell you the tools that need adjustment. A carbide blade has good thermal rigidity and abrasion resistance; however, the impact resistance is poor, which is why it breaks when you drop it. 

Alloy blades are ideal for CNC machines. The blade’s diameter matters since it must match the product and the production batch. The face of the CNC blade lies in power required by the tool. A grinding blade is ideal for fine milling and has better dimensional accuracy, and a pressed CNC blade is for processing occasions. It reduces friction and power requirements.

Avoiding chatter

Using the largest radius achieves the best surface finish and avoids chatter which is the vibration produced by the machine. A larger nose radius increases demands on the tool and causes vibration and chip breaking. 

The selection of a minimum depth of cut two-thirds of the nose radius and a maximum of one-third of the cutting edge length results in smooth chips production. In comparison, a smaller nose radius results in smaller chips that are easy to clear away from the workpiece. However, that limits the feed rate. 

Overload causes chatter as well as poor rigidity of the machine. Reducing the cross-feed and machining depth reduces the vibration. The workpiece must be clamped, and sometimes you must replace the tool. 

Speeds and Feeds

Increase your speed and reduce your feed rate. When using carbide blade tools, increase the surface feet per minute speed, which allows your material to stay in contact with the tooltip for a short time. 

The edge build-up is reduced on the tool, and edge build-up causes poor surface finishes. The feed rate is limited so that you improve the surface finish. There is less risk of flank wear and an increased life to the insert. 

Suppose you double the nose radius, and the surface finish increases. The nose radius and the cut’s depth affect the chips’ shape and direction. A tool capable of a high feed rate is ideal for roughing applications and removes material quickly and easily. A lower feed rate is for finishing and shallow cuts. 

Roughing VS Finishing

For roughing and finishing, use different tools. As some users use the same inserts for roughing and finishing, separate inserts perform better. That means you have one insert for roughing and one insert for finishing. 

The results are hardly biased. Use a coarse pitch cutter with a large nose radius for roughing and a large rake angle with a rapid feed rate. A finish-pitch finishing tool with a lead angle and a wiper flat is ideal for finishing and gives you an improved surface finish. 

The OD roughing tool is sturdy and quick. It is ideal for facing and keeping the depth of cut less than your insert’s nose radius. If your applications require a lot of material facing, consider having a dedicated Facing Tool. The OD finishing tool gives the best finish. Having different inserts for roughing and finishing gives you high performance and flexibility.  

Material

The material used determines if you should use coolant in milling applications. Sometimes, you should avoid using a coolant because it causes thermal cracking and reduces longevity, negatively affecting the surface finish. 

Using aluminum, low-carbon steel, or nickel-based alloys while using a coolant reduces the risk of the tool sticking to the workpiece. Different materials cause a varying degree of tool wear.  

Hard materials increase tool wear, and check and measure batch parts regularly. Make sure the CNC part is per requirements. When flipping your work material, use dowels that go into the bottom of your work material.

Toolpaths

A Toolpath is a series of coordinate locations that a cutting tool follows in the machining process. It is known as Roughing and Finishing, and a roughing toolpath removes material accurately and efficiently. 

The Finishing tool path finishes after roughing up the material and removing the last part of the material to complete the process. Toolpath capabilities make the performing machine better. 

There is a need for consistency, and that comes with toolpaths. They accommodate the needs of the material and the machine. High-speed toolpaths give you maximum performance, and the Pocket milling toolpath pushes volume. 

Feed Rate

The feed rate also determines the results of your finish. When the tool is light, you experience stringers, rapid flank wear, built-up edge, and uneconomical. There is less chip control, poor surface finish, plastic deformation, high power consumption, chip welding, and hammering when it is too heavy. 

Depth of Cut

When you optimize the depth of the cut, you reduce the number of cuts. Although the depth of cut has little impact on the tool, it has to be regulated. When it is too small, there is less chip control, vibration, and more heat, which is uneconomical, and when the cut is too deep, there is high power consumption and increased cutting forces. 

Speed

Reducing the cutting speed reduces heat production, making your tool comfortable to work with. When your CNC speed is too low, that affects the built-up edge and dulling of the surface edge, and it is uneconomical and results in a poor surface finish. 

The cutting speed cannot be too high since that leads to rapid flank wear, poor surface finish, crater wear, and plastic deformation. The cutting speed determines the tool’s life, and an adjustment of the CNC speed gives you the best economy. 

When working on plastics, turn off the constant cutting speed. Manual control of the speed gives you better results since you are allowed to control the chips. In addition, the chip load is increased by slowing the spindle or increasing the feed rate to thicken the chip. See our guide on How to Operate a CNC Lathe Machine.

David D. Hughes
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