A lathe holds a workpiece between two centers. The workpiece is mounted on a spindle, which rotates and spins the workpiece. This is why lathe work is often referred to as “turning;” a piece is “turned” on a lathe, and the tool removes material from it to produce the desired part.
So how fast does the spindle spin? What speed does a lathe need to run in order to cut a particular part? There’s no set answer, but instead, machinists rely on a series of calculations. In this article, we’ll talk about the ideas behind those calculations and how machinists use them.
Two kinds of speed
When you begin to cut a workpiece on a lathe, there are actually two speeds that come into play. There’s the speed at which the workpiece spins, the spindle speed, and the speed at which the tool moves down the workpiece, the feed. Together, these are known as the feed and speed.
Most lathes are variable speed; the workpiece can be spun faster or slower, depending on the needs of the job. The feed rate can also vary, so machinists need to calculate the right combination of the two variables in order to achieve the fastest overall cutting speed for the workpiece.
Why are feeds and speeds important?
Calculating the “correct” feed rate and spindle speed can seem like an advanced concept, and in many ways, it is. Even experienced machinists admit to struggling with the idea. However, everyone admits that getting the calculations right is important.
Why is it important? Incorrect feeds and speeds can lead to:
- Excessive chipping of the workpiece
In lathe work, a lot depends on the angle of the tool to the surface of the workpiece. The speed of rotation effects that geometry, leading to chipping rather than cutting, and a correspondingly poor finish.
- Cuts not done to the correct dimensions
Along with a rough finish, the incorrect speed can cause too much or too little material to be removed, leading to a finished piece with incorrect dimensions.
- Tool wear-and-tear
Too much speed causes excessive wear and tear on the tools themselves, degrading cutters and bits and consequently resulting in more expensive operation.
On the other hand, finding the perfect feed and speed can help you:
- Extend tool life
- Achieve a perfect finish
- Find the fastest machining speed through the ideal Material Removal Rate (MRR)
There are good speeds, and then there is the ideal speed. Calculate it correctly, and you’ll be able to remove the largest amount of material in the shortest time, making each project go by more quickly.
What factors determine the correct speed?
What factors determine the correct speed? Is it simply as fast as your lathe can spin? There are several items you’ll want to consider as part of your calculations.
- Lathe specifications
This includes whether or not your lathe is variable-speed, CNC-equipped, or generally able to handle higher speeds; older machines may work best at lower speeds.
- Quality of tools
Better, newer, or sharper tools can handle higher spindle speeds and faster feed rates, but too much can soften or dull the tools.
- Material properties
Each metal cuts differently; some are softer, some harder, some denser, and some more brittle. For that reason, each metal will respond differently to higher or lower speeds.
Finding the sweet spot
Balancing feed rate with spindle speed is all about finding the sweet spot for each workpiece. Slower spindle speeds are better for tool life but remove material more slowly. Faster spindle speeds and faster feed lead to quicker projects, but mean more wear and tear on your tools and the possibility of a poor finish on the workpiece.
How do you find the ideal feed and speed rate for a given material? Rather than eyeball it, or trying to guess by the sound your lathe makes, use a dedicated feeds-and-speeds calculator. This tool takes the guesswork out of it, making your next project that much easier.
How fast does a metal lathe spin? The answer is that it spins as fast as it should, in order to find the sweet spot and get the job done as quickly and efficiently as possible.
