Like many in the industry, when I started in the machining world it was on a manual lathe. I have a special place in my heart for lathes. That includes CNC lathes, the next step up for aspiring machinists. In my experience, I have found there is a lopsided ratio of CNC mill machinists to CNC lathe machinists, with about 10 times as many being mill machinists. My theory is this is partly because the CNC lathe is more difficult to operate.

I have heard the argument that the CNC lathe only has two axes and, therefore, can't be that difficult to use. After all, the mill has at least three axes so it must be more complicated.

But that's not so. Imagine on a 3-axis mill that every time you do a tool change the entire tool magazine rapid traverses into the work zone. As it does, it approaches the mill table, which is spinning at 4,000 rpm along with the workpiece. The spindle mass of steel is about to shear off all the tools hanging from the magazine. This would be the mill equivalent to the CNC lathe.

Early in the history of CNC mills, a tool holding and changing system was adopted that simplified and helped eliminate the danger of spectacular crashes. In this system, all the tools are held in individual holders, which have a common mounting position and are safely stored well away from the work area. This small factor, I believe, is the major reason why there are so many more CNC mill machinists than CNC lathe machinists; it is so much easier to crash a CNC lathe than a CNC mill because of this basic design difference.

In the standard 2-axis CNC lathe, most of the tools are held stationary in a large turret while a workpiece of unknown size and weight revolves to provide the cutting action. Each tool is indexed and brought into the work area with all its companions mounted in the turret. Also, because most of the tools are stationary, the workholding device, which is a large mass of steel, must spin. This provides another obstacle that you must plan and account for to avoid a crash when operating a CNC lathe.

If machine tool builders had their thinking caps on back in the day, they would have adopted a system similar to the setup used on most CNC mills. Only in modern times, though, do we see a trend, in some CNC lathes, where this kind of system is starting to materialize.

In the job shop environment, a CNC lathe can be either a real moneymaker or time sinker. It's almost always a tough call whether to set up and run a small lot of parts on a CNC or a manual lathe. CNC lathes are intended for high-volume production and often don't lend themselves to short runs or lots of changeovers.

So what is the appropriate job size for setting up and running a CNC lathe vs. running one manually? Some of what drives this decision is part geometry and quantity, but more often than not, it's the setup and programming time. If the CNC lathe was as easy as the manual lathe to set up and program, the threshold job size to justify running a CNC lathe would be reduced. Therefore, it's important to focus on ways to reduce the amount of setup and programming necessary to get a job out of the starting gate and into the CNC lathe. Keep in mind that the thinking is completely different on a production job, where the setup and programming is amortized over a large number of parts.

The following are some questions to answer when running a small quantity of parts on a CNC lathe.

• Is it likely the job will repeat?
• If it comes back, will it be the same?
• Can we run extra parts and store them for future orders?
• Do we already run a similar part that we can leverage our tooling or setup with?
• Is the part geometry difficult enough to warrant the setup and programming effort?
• Can the operator do secondary operations on the parts while they run unattended in the CNC lathe to telescope the schedule and manpower used?
• Will the customer pay for it?

Answering yes to any of these questions is a good reason to spend the extra effort required to perform the job on a CNC lathe and potentially generate more profits. CTE