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Last Updated on October 1, 2023 by Larry Edwards
Calculating the right speed can do wonders, especially when turning wood.
When the lathe is too slow, cutting the wood requires a more challenging and tedious experience.
On the other hand, if the lathe is too fast, it can be too dangerous, as high-speed projectiles can pose a hazard to everyone near the area.
But when speed is proper, you allow wood blanks to turn without vibrations resulting in a cleaner surface.
As a result, you have yourself a smooth and safer woodturning experience. Here’s everything you need to know about calculating the lathe speed.
Key Takeaways
- Calculating lathe speed involves dividing the RPM or the speed at which the lathe is running by the diameter of the piece.
- Cutting speed has a great impact on tool life.
- Surface speed is measured in surface feet per minute (SFPM)
- It takes fewer revolutions for a bigger piece to cover the same distance in the same amount of time compared to a smaller wheel.
- Cutting speed refers to the speed of the outside edge of the cutting tool.
What are Lathe Speeds?
There are two answers to this. The first answer is rpm or the revolutions per minute that the lathe is running, and the second is how big the turning diameter is.
Most lathes sold today have cutting speeds of around 600 to 3000 rpm.
In addition, there are usually charts written on the lathe to tell you what rpm you can get from different pulley arrangements.
What is Cutting Speed?
Cutting speed, commonly known as surface speed or spindle speed, refers to the speed of the outside edge of the tool when it’s cutting.
Cutting speeds are often measured in surface feet per minute or (SFPM). Cutting speed and spindle speed are proportional too.
However, cutting speed is highly dependent on a few factors, such as the hardness of the materials, the type of material, and the cutting tool.
The hardness of the material indicates the right cutting speed, which means the recommended cutting speed for harder materials should be slower.
In addition, the higher the hardness of the drill or the cutting tool, the faster the recommended cutting speed.
On the other hand, recommended cutting speed is lower when dealing with a softer cutting tool material.
Also, it has a greater surface speed when the cutter is cutting larger work sizes and turning at the same rpm as the smaller tool.
One thing to note that can affect the cutting speed is the depth of the cut and the feed rate, but not to the extent of the hardness of the work. These three factors are what I like to call cutting conditions.
In addition, these conditions are also determined by the machinability rating or the ability of materials to be cut through or machined.
Woodturning Lathe Speed Chart
If you look at the calculation above regarding how to calculate lathe speed based on the highest rpm, the speed chart below should give you a good idea of what cutting speed you should have based on the diameter.
The calculation is as follows: 9000/diameter in inches = high RPM
Here’s a good example of the recommended cutting speed chart based on the range of diameters:
- 10-inch diameter bowl = 900 RPMs
- 11-inch diameter bowl = 815 RPMs
- 12-inch diameter bowl = 750 RPMs
- 13 Inch Diameter Bowl = 690 RPMs
- 14 Inch Diameter Bowl = 640 RPMs
- 15 Inch Diameter Bowl = 600 RPMs
NOTE: This chart is not meant to be followed religiously. It should serve as a guideline or an example for woodturning and cutting speed purposes only. Always check the quality of the wood for turning!
ADDITIONAL NOTE: Always remember these aren’t universal cutting speeds for all materials. Cutting is highly dependent on many things, such as the type of materials, the cutting tool used, the machinability of materials, and depth.
How to Calculate Wood Lathe Speed
It’s easy to calculate the lathe speed if you’re turning wood. To get the low RPM, you have to divide the diameter of the workpiece by 6000 rpm to get the lowest rpm possible.
To get the high rpm, divide the diameter of the workpiece by 9000 rpm, and you’ll get the right lathe speed.
For example, if your workpiece is 3 inches in diameter, your cutting speed should be around a minimum RPM of 2000 and a maximum of 3000 high RPM.
These calculations are only good if you deal with imperial measurements such as inches, feet, and miles.
On the other hand, if you are working on metric measurements, you have to divide the diameter of the workpiece by 15000 to calculate the rpm.
What if Vibration Is Present?
There are many factors why it is important to calculate the cutting speed involving several materials.
One factor is to reduce vibration to a minimum when the workpiece isn’t properly balanced.
Another factor is rotating involves centrifugal force, which is responsible for pulling the object apart.
As a rule of thumb, if you are dealing with flawed wood, then excessive speeds can break the wood apart due to the centrifugal force created. Here are some of the best woods you can work with!
When this happens, the situation can be too dangerous for you to work with as this creates projectiles and debris that can result in injuries.
In addition, rpm and centrifugal force are directly proportional, which means the higher the rpm, the higher the force.
So, be wary and always check your speed setting to avoid injuries or breaking the entire workpiece.
When vibrations are present, lathe speeds should be kept at a minimum or the lowest speed possible. This way, you keep vibration to a minimum and reduce the risk of breaking the wood.
What is the Best Lathe Speed?
For bowls and lathe speeds, there’s a general rule most woodturners follow. However, these are only applicable for small bowl turning speeds. For large bowls, the cutting speed must be lower.
So, the rule of thumb is not to exceed more than 1000 rpm. It’s a starting point for a bowl blank to go up and down if it comes off the lathe.
One reason behind this calculation is that a cutting speed below 1000rpm can dislodge a bowl blank to fall off the floor. Lathe speeds exceeding 1000 rpm can send the bowl blank upwards and into your face.
What is a Tachometer?
A tachometer is an instrument or tool used and installed on a lathe that measures the working speed of a particular object, in this case, a lathe. It’s a tool for measuring rpm if your lathe has no measuring tool.
RPM vs. Size
Here are some guidelines regarding revolutions per minute and size.
- Smaller diameter objects tolerate higher rpm.
- Small lathes and light require slower rpm
- Objects that are out of balance need a slower rpm
- Using a heavy-handed tool technique requires a slower rpm
Small Objects Tolerate Higher RPMs
Small objects like boxes and pens can often tolerate a cutting speed of up to 3000 rpm.
While this can be rare, it’s not uncommon because sometimes cuts are cleaner at a higher speed than you would normally experience at a lower speed.
At higher rpm, you are turning between centers, which adds safety and stability when cutting. However, a face shield is a must.
In some rare instances, you can involve a higher cutting speed for a short duration when shaping the outer portion of a goblet. However, turn back the cutting speed to a lower rpm if you cut the inside portion.
Bigger Objects Require Lower RPMs
There are several reasons why bigger objects or bowls have reduced recommended cutting speed.
First, a larger workpiece tends to make your lathe vibrate, which means you have to lower the speed, or else it can get very unsafe quickly.
The bigger the workpiece’s diameter, the greater surface speed the object has at the same speed or RPM.
Safety Tips to Remember
- Wear a face shield at all times when using the lathe machine
- Use protective gear at all times, like safety glasses and protective footwear.
- Reduce the surface speed for out-of-balance objects
- Always clean the surface where you will be working
- When vibration starts, reduce the cutting speed until smooth rotation can be felt
- If you are a beginner, working on a small lathe is fine as long as you practice religiously.
- Always factor in the hardness of the wood or material when determining the cutting speed.
- Do not wear gloves, rings, loose clothing, or jewelry around the neck. Wear comfortable clothing that isn’t loose.
- Adjust the drill or cutting tool when necessary
TIP: One of the best indications of the cutting speed is the color of the chip. When using a high-speed steel cutter, the chip should indicate a straw-colored one to know you are at the maximum edge. On the other hand, a dark purple color on the chip indicates you are on the maximum edge of your cutting conditions when using carbide.
Frequently Asked Questions
If you have further questions about cutting and turning speeds in general, you might find them below.
Why Is Spindle Speed Faster than Lathe Speeds?
Spindle turning involves faster cutting speeds when making wood bowls. In fact, spindle turning lathe speeds can reach up to 4000 RPMs and higher.
It is much faster because it involves a much smaller diameter of wood than you would normally experience when turning a bowl blank.
Is It Better to Cut Aluminum Fast or Slow?
Go slow and steady. Cutting quickly can be tempting as it can save you time in the process.
However, it would be best if you went slow and steady to ensure you are cutting with precision and depth.
Nobody wants to deal with inaccurate cuts due to the poor method of cutting quickly. Keep the cutting speed to a minimum, as cutting aluminum is quite dangerous.
What is the Best Lubricant for Cutting Aluminum?
One of the best-cutting fluid oils on the market today is Tap Magic Aluminum. It’s great for copper, aluminum, brass, and other soft metals.
In addition, it will not stain the surface of the work-piece, and the cutting speed should be precise for a superb finish.
Conclusion
Wood turning can be a hobby. It may sound a little bit overzealous, but turning and cutting wood has become a popular obsession for most people staying at home.
Just be mindful of the materials you are dealing with and the type of tool you use for cutting and turning.