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Lathe Speeds: How to Get it Right [Wood and Metal]

Lathe Speeds: How to Get it Right [Wood and Metal]

Lathe Speeds: How to Get it Right [Wood and Metal]

Lathe machines offer speed control to turn the workpiece at variable RPM.

Depending upon the type of material and the machining operation to be performed, you can set the optimal lathe speeds.

But what is the safe speed to machine workpieces on a lathe? And how does lathe speed affect the machining process?

Lathe speed should be under 1000 RPM for turning wooden workpieces over 6" in diameter, with slightly higher speed limits for smaller workpieces. Keeping the lathe speed under 1000 RPM is considered safe and reduces the risk of accidents.

This article explains lathe speeds in detail by discussing various factors that affect lathe speeds and provides a guide for setting the optimal speed for your application.

In the end, the article also discusses the safety concerns related to lathe speeds that you need to keep in mind when using a lathe.

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Lathe Speed for Woodworking: How to Set Optimally

Woodworking on lathe
Woodworking on lathe

When working on a wood lathe, it is important to set the optimal RPM because a high RPM can dislodge the workpiece, whereas a low RPM can lead to a poor surface finish.

Dislodging the workpiece at such high speed can release the workpiece like a projectile and injure the operator, making it important to regulate the RPM within the safe RPM limit.

The general rule of thumb for finding the optimal RPM range for a wood lathe is to multiply the RPM by the diameter of the workpiece and the product should lie within the range of 6000-9000.

This means that dividing 6000 by the diameter of the workpiece gives the minimum efficient RPM and dividing 9000 by the diameter of the workpiece gives the maximum safe RPM.

Minimum Efficient RPM = 6000 ÷ Workpiece Diameter (in inches)

Maximum Safe RPM = 9000 ÷ Workpiece Diameter (in inches)

Based on the above equations, the optimal RPM range for different workpiece sizes can be found in the table below.

Workpiece DiameterMinimum Efficient RPMMaximum Safe RPM
1"60009000
2"30004500
3"20003000
4"15002250
5"14001800
6"10001500
7"8571286
8"7501125
9"6671000
10"600900
11"545818
12"500750
13"462692
14"429643
15"400600
Optimal RPM range for turning wooden workpieces of different sizes

However, for workpieces smaller than 3" in diameter, such as pens, you can use a higher speed of around 3500 RPM, provided that you ensure firm clamping of the workpiece.

When working with degraded wood, it is advised to use your judgment and set the RPM lower than the standard recommended value.

Lathe Speed for Metalworking: How to Set Optimally

Metalworking on lathe
Metalworking on lathe

The optimal lathe speed for metalworking depends upon factors such as the hardness of the material being cut and the type of tool being used.

When turning hard metals, it is advised to use a comparatively slower cutting speed than softer metals.

Similarly, when using a cutting tool made of hard material, it can handle faster speeds without snapping.

When comparing manual lathes with CNC lathes, CNC lathes provide the ability to maintain uniform parameters due to the automation of the process and minimize the risk of tool damage.

Generally, the optimal cutting speed or surface speed for turning metal can be identified from tables and charts provided in Machinery’s Handbook or provided by the tool manufacturer.

MaterialHigh-Speed Steel ToolCarbide Tool
Free-machining plain carbon steels40 -160 SFM300 - 800 SFM
Plain carbon steels30 - 120 SFM230 - 800 SFM
Free-machining alloy steels40 - 125 SFM150 - 450 SFM
Alloy steels40 - 110 SFM175 - 400 SFM
Optimal cutting speed ranges for different types of metals and cutting tools.

It must be noted that the values mentioned in the table above provide a reference for the optimal cutting speeds and the actual value varies from one grade of metal to another.

Based on the optimal cutting speed configuration provided in the table, you can set the optimal RPM by using the following equation:

RPM = (SFM x 12) ÷ (Diameter in inches x π)

What is the Speed of a Lathe? Explained

Lathe machines work by rotating the workpiece at high speed while the cutting tool passes over its surface to perform the machining operation.

Therefore, the speed at which the workpiece rotates determines the quality of the cut and the cycle time, making lathe speed an important parameter for turning operations.

Almost every modern lathe machine provides a variable speed control where you either have to vary the speed manually or by the means of electric signals.

Manual speed control involves shifting the belt of the lathe drive to change the speed and torque configuration, whereas the electric system consists of a knob that can be rotated to adjust the RPM.

Generally, belt-based speed control is available in powerful lathes, such as metal lathes, as it reduces the RPM while increasing the torque, thereby making it ideal for turning heavy workpieces.

On the other hand, the electric speed control reduces the RPM by slowing down the spindle speed, reducing the output power of the lathe. This makes it suitable for small benchtop lathes.

Similarly, multi-spindle lathes consist of multiple spindles that can rotate at different speeds, suitable for different machining operations.

Machining operations on the lathe involve two types of speeds: rotation per minute (RPM) and surface speed (SFM).

Spindle Speed (RPM)

Lathe Speeds
Lathe Speeds

Spindle speed is the speed at which the lathe spindle rotates the workpiece, and higher spindle speeds are accompanied with the increase in cost of the lathe.

It determines the number of rotations that the workpiece undergoes in one minute(RPM).

Generally, the faster the RPM, the quicker the cycle time and the smoother the surface finish of the cut.

However, increasing the RPM beyond a certain limit can increase the vibrations and result in chatter on the machined surface while also increasing the risk of accidents.

High RPM exerts a high centrifugal force which can force the workpiece out of the holding device, such as a jaw chuck, and hit the operator.

Therefore, it is advised to set an optimal speed that ensures high-quality output at a quick cycle time with minimal vibrations and minimal risk of accidents.

Surface Speed

Surface speed is the speed at which the surface of the workpiece passes under the lathe cutting tool and is measured in surface feet per minute (SFM).

The surface speed during a machining process depends upon the RPM and the radius at which the workpiece is being machined.

When machining the outer surface of two workpieces of different sizes at the same RPM, the surface speed of the larger workpiece is faster than the smaller workpiece.

Although both the workpiece take a similar time to complete one rotation, the surface that passes under the cutting tool is comparatively more in the case of the larger workpiece.

Similarly, when machining a workpiece, such as a bowl, the surface speed at the outer surface is much faster than the surface speed at the center of the bowl.

This can be tricky, but let us consider an example to understand it.

Consider two circular race tracks, A and B, where racetrack A has a radius of 10 km and racetrack B has a radius of 5 km.

Graphical depiction of Race track example
Graphical depiction of Race track example

This means that the circumference or the total length of racetrack A will be around 63 km, while that of racetrack B will be around 31 km.

Now let us consider that both the cars are to complete one rotation on their respective tracks in 30 minutes.

As a result, in order to complete one rotation on both tracks in the same duration, the vehicle on track A should move faster (126 km/h) than the vehicle on track B (62 km/h), because it has to cover a larger distance.

Similarly, the surface of the larger workpiece will pass through the cutting tool at a faster surface speed to cover the larger circumference of the workpiece in one rotation.

Therefore, it is important to consider the surface speed and reduce the feed of the cutting tool as you move radially inwards towards the center of the workpiece.

Factors That Affect Lathe Speeds

The lathe speed for a machining operation depends upon various factors that directly or indirectly determine the optimal speed for the process.

Surface Finish

Surface finish during wood turning on lathe
Surface finish during wood turning on a lathe

The surface finish required during a machining operation determines the optimal lathe speed for the process.

Performing a turning operation at high speeds is generally recommended to attain a smooth finish on the machined surface.

However, it must be ensured that the speed does not exceed a certain limit that induces unwanted vibrations in the workpiece.

These vibrations can affect the quality of the cut, thereby limiting the lathe speed to a certain limit.

When turning the workpiece at high lathe speed, a high feed rate will result in rough cuts with poor surface finish, and therefore, it is advised to maintain a slow feed rate for finish cuts.

Workpiece Material

The type of material being turned on the lathe also affects the optimal lathe speed.

Wood lathes and metal lathes, both produce high-quality cuts at high lathe speed and slow feed rate combination.

However, when working with heavy metal workpieces with large diameters, it is advised to maintain a high-torque and low-speed configuration to deliver the strong cutting forces required for machining metal.

Apart from that, machining metal at high speeds generates high frictional heat, making it necessary to use cutting fluid to prevent overheating of the workpiece and the cutting tool.

On the other hand, when working on a wood lathe, the quality of the wood being machined, affects the optimal lathe speed.

A degraded wood or wood with knots and spalting is less likely to endure the high centrifugal force exerted on it at high RPM.

Rotating this type of wooden workpiece can come apart during machining and cause severe injuries to the operator.

Therefore, it is advised to inspect the workpiece and use your judgment to set the optimal lathe speed accordingly.

Geometry of the Workpiece

The geometry of the workpiece affects the lathe speeds by determining the amount of air being turned.

When turning wooden workpieces with irregular geometries, such as a square block, the cutting tool does not maintain continuous contact with the surface of the workpiece.

Turning a workpiece with non-uniform geometry
Turning a workpiece with non-uniform geometry

It machines the edges of the square, followed by intermittent voids. The movement of the cutting tool over these voids is referred to as "turning air".

This non-uniform contact of the cutting tool with the workpiece can result in a high impact load, damaging the workpiece and the cutting tool.

Generally, it is advised to turn such workpieces at high speed while maintaining a shallow depth of cut.

Rotating the workpiece at high speed ensures that the void is passed under the cutting tool in minimal time, thereby reducing the impact load.

However, turning irregular workpieces at such high speeds requires extreme control to ensure a uniform and shallow depth of cut throughout the process.

Increasing the depth of cut will result in a stronger impact load, which can break the wooden workpiece, resulting in high-speed debris shooting toward the operator and causing damage.

Workpiece Alignment

Aligning the workpiece with the lathe axis is one of the most important steps in the operation of a lathe machine.

An offset of even 1° can result in extreme vibrations in the workpiece, which are further intensified as the turning speed increases.

Therefore, it is important to align the workpiece perfectly to minimize the vibrations.

However, attaining a perfect alignment is not possible when machining irregular workpieces.

For such situations, it is advised to align the workpiece with the central axis of the lathe spindle, turn the workpiece at a gradually increasing speed, and identify the speed and alignment configuration that produces the least vibrations.

Material of the Cutting Tool

The material of the cutting tool determines the amount of cutting force that can be delivered to the workpiece without snapping the tool.

A carbide tool is more durable than high-speed steel (HSS) tool and can be used for turning at comparatively faster speeds without affecting the tool life.

Type of Lathe

Different types of lathes
Different types of lathes

Lathe machines are broadly classified into two main categories: metal lathes and wood lathes.

While wood lathes are comparatively smaller than metal lathes, they are designed to have a higher speed-to-torque ratio because machining wood requires comparatively less torque than metals.

As a result, the speed control provided on wood lathes is electronic control, making it easy to set different speeds according to your application.

On the other hand, metal lathes with gear/pulley-based speed control provide fixed speed settings that you can choose from.

Safety Hazards With Lathe Speeds

High RPM in lathe machining results in smoother cuts and faster machining, then why is it not recommended to always use the maximum speed of the lathe?

High Centrifugal Force

The main concern when machining the workpiece at high speed is the centrifugal force that acts on the rotating workpiece.

This force pulls the workpiece away from the rotating axis of the lathe. It is applicable along the outer surface of the workpiece which can even result in exploding of a weak or defective workpiece.

The centrifugal force acting on the workpiece is directly proportional to the square of the spindle speed (RPM).

Therefore even a slight increase in the RPM will result in a four times greater increase in the centrifugal force acting on the surface of the workpiece.

Apart from that, the centrifugal force also increases with the increase in workpiece diameter.

As a result, it is advised to turn larger workpieces at a comparatively slower RPM than smaller workpieces.

Strong Cutting Force

Turning operations on the lathe produce good quality when machined at high RPM, but the quality of the cut is also dependent upon factors such as feed rate and the depth of the cut.

When machining at such high speeds, a deep cut can result in a high impact load that can damage the workpiece and the cutting tool.

This impact is more concerning on wood lathes as they consist of a hand-held cutting tool and the impact load can transfer from the hand-held tool to the operator and cause severe injuries.

Therefore, machining at high speeds is only recommended for professionals who can regulate the optimal speed, and feed, while maintaining a uniform depth of cut throughout the process.

Weak Clamping Technique

There are various ways to clamp a workpiece on the lathe.

Different work holding devices
Different work holding devices on lathe

While collets and chucks are common camping devices for metal lathes, wood lathes use clamping options such as face plates, spindle spur, etc.

3-jaw chucks or 4-jaw chucks are the most commonly used clamping technique in most metal lathes like turret lathe, gunsmith lathe, etc., where strong cutting forces are to be delivered on the workpiece.

In contrast, collet chucks are used on lightweight metal lathes like the capstan lathe.

Similarly, when working with wooden workpieces, faceplates are recommended for heavy workpieces as they provide greater holding force to overcome the centrifugal pull exerted on the workpiece.

A weak holding device like a spindle spur is suitable for small workpieces as it provides a comparatively weaker holding force that cannot overcome the centrifugal pull on larger workpieces.

This large centrifugal pull can dismount the workpiece from the work-holding device and launch the heavy workpiece toward the operator, causing severe accidents.

Therefore, it is advised to always engage the tailstock of the lathe to provide extra holding force and minimize vibrations.

High Vibrations

High lathe speeds result in high vibrations, which thereby result in poor surface finish.

These vibrations cause unwanted rubbing of the tool against the workpiece and can sometimes generate a heavy impact load, raising the risk of accidents.

The main cause of these vibrations can be a lightweight lathe chassis, an unbalanced workpiece, or a spindle runout.

Therefore, it is important to ensure a perfectly aligned workpiece and be wary of the physical limitations of your lathe when setting the lathe speed.

Final Thoughts

Lathe speeds play a crucial role in determining the safety of the process and the surface finish of the machined workpiece.

Increasing the spindle speed beyond a certain limit can result in extreme centrifugal force, compromising the safety of the process.

On the other hand, increasing the surface speed (SFM) results in a smoother cut with a high surface finish.

Furthermore, reducing the surface speed below the optimal range will affect the cutting process by reducing the cutting force delivered to remove the material from the workpiece, degrading the quality of the cut.

Both speeds are interdependent on each other, and therefore it is important to set a perfect balance that ensures high-quality cuts with safe operation.

Frequently Asked Questions (FAQ)

What lathe speed is recommended for woodturning?

Generally, a speed of around 1000RPM is recommended for woodturning. It is considered a safe speed that can produce good quality cuts with minimal tool wear on most types of wood. However, it is advised to understand the behavior of the type of wood being machined and manipulate the speed setting accordingly.

Can I perform turning at a speed lower than the recommended value?

Yes, you can perform turning operations at a speed lower than the recommended value. Especially when working on wood lathes where you have to manipulate the cutting tool manually, it is advised to set a speed that makes you feel comfortable while providing the desired surface finish.

What is the preferable speed rating for a benchtop wood lathe?

Generally, most benchtop wood lathes have a maximum spindle speed of around 4000 to 6000 RPM with variable speed control. This provides the ability to machine different types of wood with high surface finish.

About John

Hey I'm John. I talk about CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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John

Hey I'm John. I talk about CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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