Automatic Lathe Cutting Pros & Cons

July 11, 2018 Joshua Jablons

Is an automatic lathe a practical choice for 2-axis metal cutoff?

In the world of precision metal cutting, the automatic lathe is most commonly used for the shaping of metal parts. So the question is, can and should you use a lathe for the process of 2-axis cutoff?

Automatic Lathe Processes

With an automatic lathe, the workpiece is held and rotated while a tool bit performs the cutting action — usually, to produce screw threads, tapers, drilled holes, chamfers, and other complex shapes. Typically the last step of the process is parting off, which is used to remove the finished end of the workpiece. The cut is made at right angles to the axis of the lathe.

A Swiss-type automatic lathe requires the starting material to have a diameter tolerance that is at least ±0.0005. This style of lathe is engineered for more complex purposes than 2-axis cutoff. However, a simple, single-spindle lathe — also known as a chucker — certainly is capable of doing high-volume, cut to length parts.

Some Uses for a Chucker

A chucker-type lathe can be programmed to send material through for 2-axis cutoff. It can be use for rod or tube cutoff where other methods, such as shearing or cold sawing, are impractical.

A single-spindle automatic lathe requires a bar puller or bar feeder to repetitively move the metal to be cut. A bar puller is the lower cost option and it does what it says: It automatically pulls a single bar of metal through the lathe for the cutoff action. However, once the cut pieces drop down, a human being needs to manually reload the bar puller.

A bar feeder, as the name implies, is cable of feeding multiple consecutive bars of metal through the automatic lathe. This eliminates the need for a person to stand by to continuously reload the feeder. However, a bar feeder is a more expensive upfront capital equipment cost.

Besides thinking about how to feed the material through the automatic lathe, you need to consider whether you are cutting a solid rod or a tube, since each has its own characteristic response to being cut to length while being spun. Because a lathe is a spinning process, you would not likely use it for an extruded profile. 

Some Disadvantages with Cutting Tubes and Rods

In a single-spindle scenario, using an automatic lathe to cut a tube to length will most likely produce burrs. With good programming, the tool can both cut off the part and break the outer edges, eliminating the OD burr. However, the ID burr is more troublesome. Besides requiring further programming, it requires an additional tool or a second spindle to break the ID edges.

When cutting solid rod to length, the OD burr can be remedied through programming. However, the pip remains an issue. You would think that because the workpiece is spinning in the automatic lathe, the feed of the cutting tool only has to go through the radius rather than the entire diameter, saving time.

But smart machinists know that is not true, because cutting tools are directional. After you pass the center point, the direction of rotation presents the workpiece to the cutting tool in the opposite direction. Therefore, it is no longer cutting — in fact, it is hitting against the supporting structure. It’s a mess.

This creates a problem unique to cutting solids on an automatic lathe: That is, you can get close to the center, but you cannot cross it. Therefore, it is very characteristic to have a tiny little pip — or sometimes a very noticeable pip — in the center of the part.

Because cut to length parts have two sides, this pip on a solid is actually a complicated problem requiring additional action. In theory, you could just face off the side of the cut that is the material itself; in practicality, the backside of the part will still have a pip.

The solution is to come in with a parting tool with two cutting edges plus another spindle that is supported and spinning. This will allow you to pass the center point and remove the pip on a cutoff rod; however, it will also add to the time and cost of using a lathe.

Other Issues for 2-Axis Cutoff

An automatic lathe can only cut one rod or tube at a time, and does not allow materials to be bundled. In addition, the efficiency of the process varies depending on whether you using a bar feeder or a bar puller.

With a bar feeder:

  • If the material is bent, it is not going to feed. Period.
  • If the material is wavy or not quite straight, it may not feed properly —  causing problems that defeat the purpose (and increase the cost) of using a bar feeder.
  • An extremely rough surface finish may have a positive or negative impact on feed, varying with the material and its surface condition.
  • The smaller the material diameter, the more expensive and potentially problem-prone a bar feeder will be.

A bar puller, on the other hand, can handle smaller diameters and some bend in the material. But again, a person needs to reload the material into the puller one bar at a time. In addition, the length of the spindle must be limited so that it does not create “whipping” — both a safety issue and material condition and cutoff issue.

Is an Automatic Lathe Your Best Choice?

As with laser cutting, you can do simple 2-axis cutoff with an automatic lathe. However, the truth is you may be better off reserving its use for more complex, multi-step operations.

Ultimately, deciding whether to use a lathe for cutoff requires looking closely at your unique application and its specific parameters. Making the best choice also requires an understanding of the different 2-axis precision metal cutting options.

How does automatic lathe cutting compare with other precision metal cutting methods? Keep reading to learn more. 

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