NC Lathe] What is constant peripheral speed control? An active lathe operator explains about the rotation speed of a lathe!

NC lathes have a function called "constant peripheral speed control" that keeps the cutting speed constant.

This is an essential and very useful function when using an NC lathe, but it is also a dangerous control method if not understood and used.

In this article, I, as an active lathe operator, explained "constant peripheral speed control" and lathe speed!

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What is constant peripheral speed control?

What is constant peripheral speed control?NC lathe function that automatically adjusts the rotation speed to maintain a constant cutting speed (peripheral speed) when the diameter to be cut changes.It is.

When constant peripheral speed control is applied

Below is a video of a trial run of a used machine, and at the beginning of the video, an air cut (bare swing) of the end face machining with constant peripheral speed control is performed.

The speed of rotation increases as the tool rest approaches the center of rotation.I think you can see that.

If this is a manually operated general-purpose lathe, it will continue to rotate at the same RPM unless the RPM is manually switched.

What is the peripheral velocity?

To explain constant peripheral speed control, it is first essential to understand "peripheral speed.

Peripheral speed (cutting speed) is the speed at which the blade strikes the workpiece.refers to the following.

For example, 100 m/min means that the workpiece is hitting the blade at a speed of 100 meters per minute.

Literally, the control that keeps this peripheral speed constant is the constant peripheral speed control.

At the same rotation speed, the peripheral speed (cutting speed) is different around the periphery of the workpiece and near the center.

Here's the point,Even at the same rotation speed, the peripheral speed (cutting speed) is different between the workpiece periphery and near the center.This is the point.

As shown in the figure below, when a φ150 (150 mm diameter) workpiece is rotated at 1000 rpm (1000 revolutions per minute), the peripheral speed is 471 m/min when the cutter is applied to the φ150 portion of the outer circumference, while the peripheral speed is only 31 m/min when the cutter is applied to the φ10 portion.

Circumferential velocity is lower in the center at the same RPMThe center of rotation is the center of the rotation. And at the center of rotation, the peripheral velocity is zero. So the real center point is not moving at all. (But this one point is only the real center, so it is a virtual point.)

This is easy to understand if you imagine peeling a large apple and a small apple.

If you want to peel an apple in the same amount of time, you would have to move the knife faster on a large apple than on a small apple. Similarly, for the same number of revolutions, a workpiece with a larger diameter means that the knife and workpiece collide faster.

Constant peripheral speed control allows constant cutting speeds both near the periphery and near the center.

The constant peripheral speed control is a function that compensates for differences in cutting speed depending on the diameter to be shaved and adjusts the rotation speed so that the cutting speed is always the same.

For example, in the figure above, if the cutting speed is set at 100 m/min, the machine will automatically adjust the speed to 212 rpm for a φ150 section and 3183 rpm for a φ10 section.

If the end face is to be shaved from the X plus direction to the X minus direction, the rotation is controlled to start at 212 rpm and gradually increase to a faster speed.

incidentallyThe relationship between cutting speed, cutting diameter, and number of revolutions can be obtained from this formulaOh my god!!!

Cutting speeds are almost never calculated manually in actual machining operations. When necessary, it is possible to usecutting calculatoris recommended to use!

Cutting speed" is more important for lathe/milling than RPM

(From here on, for the sake of clarity, we will refer to the peripheral speed as the cutting speed.)

What happens if the cutting speed is too high?

earnestCutting speed is more important in lathe/milling than simple rotation speed.It is.

Specifically, it has a great deal to do with tool life. If the cutting speed is too high, the tool will wear out in a short time and the cutting edge will become unusable.

Even if carbon steel is sharpened at the same rotation speed of 2000 rpm, φ20 can be sharpened without any problem, but φ300 will damage the cutting edge and break it in an instant.

What happens if the cutting speed is too low?

forIf the cutting speed is too low, a phenomenon called "composed cutting edge" is likely to occur. (When this occurs, the cutting surface becomes crumbly and the dimensions are unstable.The following is a list of the most common problems with the

Raising the cutting temperature is an important measure for the configuration cutting edge.It is.

There are many ways to raise the cutting temperature, but the most common and easiest isIncrease cutting speedThis is to say.

The configuration blade edge is explained in detail in this article!

Lathe] Troublesome component cutting edges were explained in detail!

Have you ever experienced that the surface of a lathe tool is plucked when the speed is lowered, or that the lathe tool has been shaved a little too much when the speed is lowered? These problems are almost always caused by the cutting edge configuration. This article describes the component cutting edges! Compositional Cutting Edge

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What happens if there is no constant peripheral speed control?

When cutting at a constant rotation on a general-purpose lathe or similar machine without constant peripheral speed control, the cutting speed is too fast on the area near the periphery of the workpiece and too slow on the area near the center.

When cutting under such conditions, problems due to inappropriate cutting speeds, as explained in the previous section, are inevitable.

Therefore, with constant peripheral speed control, the cutting speed is never too fast or too slow, and the cutting speed can always be maintained at an appropriate constant.

Constant peripheral speed control increases the speed of blade movement toward the center.

In the video at the beginning of this article, you can see that the movement of the tool post becomes faster as it goes to the center of rotation in constant peripheral speed control. (The video is posted again.)

The feed speed of the NC lathe turret is commanded in terms of feed rate per revolution. Unit is mm/rev.Rev is rotation, so for example, 0.1 mm/rev means a feed rate of 0.1 mm per revolution.

As explained, in end-face machining from X plus to X minus direction, the number of rotations increases as you move toward the center of rotation.

The same 0.1 mm/rev for the feed rate because of the command per revolution,As the rotation speed increases, the speed of blade movement increases accordingly.The following is a list of the most common problems with the

To be able to increase the speed of blade movement,Advantages of using constant peripheral speed control include reduced machining time.The first is that it is a very good idea to use the same type of equipment as the second!

Advantages of constant peripheral speed control

Shorter processing time

Without constant peripheral speed control, it is necessary to set the rotation speed according to the diameter where the cutting speed is highest for the life of the cutter.

If this is the case, the cutting speed will be lower than the appropriate value when cutting a smaller diameter workpiece. In many cases, rough machining can still be performed without problems, but the lower rotation speed also results in slower blade movement, which lengthens the machining time.

Using constant peripheral speed control, the rotation speed increases as the cutting diameter of the workpiece decreases, allowing machining to be completed more quickly.The following is a list of the most common problems with the

Prevents configuration blade edge.

This phenomenon tends to occur when the cutting speed is lower than appropriate. →Cutting speedClick here for an explanation of the component cutting edges.

Although there is some overlap with what I wrote in the previous section, without constant peripheral speed control, the cutting speed becomes lower than the optimum as the cutting progresses, and it is easy for the cutting temperature to drop, which can cause component cutting edges.

Constant peripheral speed control adjusts the rotation speed according to the cutting diameter and keeps the cutting speed constant to prevent configuration edgeThe following is a list of the most common problems with the

Easy to program

When a constant rotation is programmed, the rotation speed must be set higher if the diameter to be shaved is larger, and lower if the diameter is smaller.

Even when modifying a program for a similar workpiece to one that has been programmed in the past, if the diameter is different, it is necessary to re-set the rotation speed to match the cutting diameter.

On the other hand, with constant peripheral speed control, for example, if a cutting speed of 180 m/min is entered, the program can be set to the same numerical value even if the diameter changes.The following is a list of the most common problems with the

This is much easier because there is no need to think about rotation speed every time.

Disadvantages of constant peripheral speed control

Constant peripheral speed control also has the following disadvantages

RPMs may be too high

Constant peripheral speed controlwill continue to increase the rotational speed as far as it will go if the cutting speed is to be kept constant. Therefore, theMaximum RPM must be set.It is.

For example, in end-face machining, it is often necessary to cut to coordinate X0, or the center of rotation, but when the cutting speed is constant, the theoretical rotation speed of the center of rotation is ∞ rpm. In other words, the machine will rotate to the limit of its capacity. For example, the Okuma LB3000EX I use has a rpm of 5000 rpm.

If the clamps are too loose or if a long workpiece is being machined, turning the machine by as much as 5,000 rpm can often result in the workpiece being blown off, the machine being damaged by vibration, or in the worst case, serious personal injury.

I often turn the engine at about 3500 rpm, but I still feel that the rpm is quite high, so even in good conditions, I think that turning the engine at 5000 rpm would be a level where I would feel danger.

It is not good for the machine to force it to produce maximal force in the first place.

The constant peripheral speed control should be used with caution, because if not used with care, the rpm can rise to that level.

To reiterate,When using constant peripheral speed control, be sure to set the maximum speed. (I usually set the maximum speed to 2000 for processing.The following is a list of the most common problems with the

Constant peripheral speed control is not suitable for turning a small diameter at low rpm.

In setting the maximum RPM, there is a key point.

It is,Set the maximum speed to the number of revolutions you want to turn the fastest throughout the process.This is to say.

Use a constant rotation command for processes you want to rotate at a low RPM.The following is a list of the most common problems with the

For example, in internal diameter machining, you may want to reduce the rpm only for a part of the tooling process.

For example, when machining a small-diameter workpiece that rotates at 2000 rpm for most of the process, you want to limit the internal diameter machining only to 1000 rpm. In such a case, if the maximum speed is set at 1000 rpm to match the lower speed, the other processes that need to be turned at 2000 rpm will also be turned at only 1000 rpm.

This is where the constant speed command comes in. Turn off the constant peripheral speed control and set the speed command to 1000 rpm only for that process.

If the maximum speed is set at 2000, the machine can be machined at the appropriate cutting speed up to 2000 rpm as much as possible, and then the rpm can be reduced during the necessary process.

Programming a combination of constant rotation and constant peripheral speed (constant peripheral speed control) is basic and something we NC lathe operators do on a daily basis.The following is a list of the most common problems with the

Usage of constant peripheral speed control

Then, what kind of machining should be done with the NC lathe's constant peripheral speed control?

The correct answer is all of the basic processing.If the clamping conditions, workpiece shape, material, and machine performance are met,Use of constant peripheral speed control is fundamental.It is.

Specifically, the following are some of the uses

End face cutting, O.D. cutting

In end face and outside diameter cutting, the cutting diameter basically decreases as the cutting progresses.

The use of constant peripheral speed control allows faster machining as the rotation speed is increased.The following is a list of the most common problems with the

ridged pattern usu. on circular or cylindrical objects

For slightly special tools such as knurling, I think it is often necessary to check the manufacturer's recommended cutting conditions for machining.

This is because instructions in manuals and other documents are often given in terms of cutting speed,Constant peripheral speed control, which can command the cutting speed, is more suitable.The following is a list of the most common problems with the

Of course, if the cutting speed is known, there is no problem in commanding a constant rotation.

Groove processing

Since groove machining does not require a large amount of cutting and is unlikely to affect cycle time, either constant rotation or constant peripheral speed is acceptable.

I use both, but more often use the constant frequency.

Where you don't have to use constant peripheral speed control.

Processing performed at constant rotation is often used when you do not want the rotation speed to increase, as in the following example.

Inside diameter cutting

Internal cutting is prone to chip evacuation and chattering problems.It is. To avoid such problems, the constant peripheral speed control is often turned off and the rpm is reduced to about 1000 rpm. This is because the constant peripheral speed control with a small diameter will rotate at the maximum speed of 2000 rpm or more, which often causes spookiness or chips to be caught in the cutter.

On the other hand, in the case of internal roughing with a large cutting volume, large diameter machining, or when the ejector length is short and there is no risk of chattering, internal machining is often performed at a constant peripheral speed.

drilling

Constant peripheral speed control cannot be used in drilling. Since the tool advances only in the Z direction, there is no point in using it in the first place.

thread cutting

Constant peripheral speed is not used in threading.

However, the NC control becomes complicated, and depending on the machine performance, double threads may occur, so there is no merit in using this type of screw.

Threading is a process that is prone to chattering,It is also easier to manage the bookmarks with a fixed rotation.It is.

When clamping conditions are bad

When clamping conditions are badControlled at constant rotation!If the peripheral speed is kept constant, the workpiece may move or blow off when the rpm is too high.Not only can they be defective, they can also cause accidents.

Bad clamping conditions are, for example, the following cases ↓.

-When clamping a long workpiece. -Turning at high rpm may cause bending or vibration due to runout, which may cause the workpiece to move.

When clamping with an unstable jig - for example, when a workpiece is held down by a clamping jig on a faceplate.

When the gripping allowance is small or the workpiece protrusion length is long

When machining heavy workpieces

In such cases, the constant peripheral speed control should be turned off and the machine should be turned at a constant rotation speed.

What is the appropriate cutting speed and rotation speed for each workpiece?

Cutting speed can be determined by constant peripheral speed control, but the appropriate cutting speed varies depending on the workpiece material and machining details.

Please refer to this article for an explanation of cutting conditions for each material.

We teach cutting conditions and machining methods for each material on NC lathes and general-purpose lathes!

How to cut S45C S45C is the simplest and most basic material. It is characterized by good cutting performance due to its easy chip cutting. Cutting conditions for all materials will be considered based on the cutting conditions for S45C. Since S45C hardens and hardens when the temperature rises, coolant should be applied while cutting S45C.

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summary

What is constant peripheral speed control?NC lathe function that automatically adjusts the rotation speed to maintain a constant cutting speed (peripheral speed) when the diameter to be cut changes.It is.

At the same rpm, the peripheral speed (cutting speed) differs between the periphery of the workpiece and near the center, so the rpm is automatically raised or lowered to keep it constant.

While constant peripheral speed control has the advantages of: shortening machining time, preventing configuration cutting edges, and being easy to program, it also has the disadvantage of being dangerous if not used with care because the rotation speed is too high.

It is necessary to use either constant peripheral speed control or constant rotation control depending on clamping conditions and processing details!

This is a very useful function that is indispensable for machining with NC lathes, and we recommend that you make full use of it!