When purchasing a CNC lathe, there are several questions that you need to ask yourself before you begin the process.Some of these questions will be quite obvious: How much axis travel do I need?What size chuck should I look for? How many tool stations are on the turret? What is the spindle bore size, etc.? However,there are other specifications that are just as important, but not always so obvious: What is the maximum swing distance that my work will require?What is the maximum turning diameter necessary for my family of parts? What kind of spindle horsepower and torque will my type of work consume? The first set of questions above is relatively easy to answer, but the second group requires a better understanding of lathes in general.

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I am often amazed at the number of highly skilled CNC machinists and operators who can accomplish almost anything on a milling machine, but who are very uneasy and intimidated around a lathe because they don’t really understand the meaning of basic lathe terminology.

Michael Cope, Senior Applications Engineer & Product Specialist at Hurco Companies, Inc.

That is the purpose of this article. I will try to clarify the meaning and benefit of a few of the “not-so-obvious” features that exist on a typical lathe spec sheet, and attempt to clarify their definition and explain why they might be an important consideration when purchasing a CNC lathe.

Maximum Turning Diameter: This simply indicates the largest size of part that can be turned on the machine – using standard length tooling – without interference or collision with guarding or other machine components.

With the X-axis retracted all the way positive, what size of part can be turned safely, as it relates to X-axis travels of the machine tool. For example: if you are looking at a machine with a max turning diameter of 16″, and the parts that you run on a regular basis are 15″ in diameter or larger, then you would probably want to look at a machine with a larger maximum turning diameter.

Even though, in our example above, the part would technically “fit” in this case, you must realize that you are running on the very edge of the envelope, and if you had to hang a tool out of the turret farther than normal – for one reason or another – you would likely NOT have enough X-axis travel to accommodate the part.

Maximum Swing: Refers to the largest diameter part that can be spun in the chuck without mechanical interference with guarding, cross-slide, or other machine components located near the chucking area. Depending on the style and design of the machine tool in question, this value could be larger than the maximum turning diameter mentioned above, however this does NOT mean that you can turn a part larger than that specified in the maximum turning diameter specification.

Horsepower & Torque: Horsepower and torque are obvious considerations when purchasing a new machine, but their necessity may not be so obvious in all cases. If you are running work such as castings and forgings, drilling large diameter holes in steel, or generally turning features on large diameter parts, then horsepower and torque are going to be very important to you, and you should be certain that the machine in question has enough for your application. However, if you are more focused on high production or general turning of small to medium sized parts, then spindle RPM may become more important than power in your case.

Just as we have seen in the milling arena over the past several years, high-speed machining is quickly making its way into turning as well. As the technology of turning tooling is advancing, and through the tool coolant options are more prevalent, the principles of cutting shallower but faster are becoming more common. Spindle speed, rapid traverse, and maximum programmable feed rates become much more important than sheer horsepower and torque.

Maximum Turning Length: Very similar to the maximum turning diameter, this specification indicates the longest part that can be turned based on the mechanical limitations and axis travels of the machine tool. Keep in mind – the effective maximum turning length, for a particular part, can be less than specified by the use of larger or deeper chucks, or tooling that sticks out from the face of the turret farther than what is considered “normal”. In both cases you would be introducing the possibility for mechanical interferences – which would restrict the length of the part that could be machined, even though the physical travels and limits of the machine have not been changed.


Bed design 

Now let’s discuss the ins and outs of the two main bed designs – the true slant bed and the flatbed “flying wedge” configurations.

First we will dive into the true slant bed design. Unlike the flatbed flying wedge design – where the slant is achieved by the addition of a bolt-on wedge that is mounted on the cross slide – the true slant bed machine casting is manufactured with the slant built in. This not only offers more rigidity and thermal stability, but also proves to give the casting more overall mass, and means you have a much heavier machine with a smaller footprint. Typically the true slant bed design is offered in one of two slant angles, 30 degree and 45 degree, but there are also some 60 degree models available.

There are many advantages to the true slant bed design, and it is probably the most common configuration in modern CNC lathes. One of the most well-known and obvious advantages to the true slant bed is better chip evacuation. As the chips are created during the machining process, they are immediately washed down toward the chip bed by gravity and the normal flow of the coolant. This keeps chips from accumulating on flat surfaces, which not only helps control the chips in high volume production applications, but can also aid in prolonging the overall life of a machine – by reducing undue wear on the ways and other moving parts.

Another advantage to the true slant design is larger X-axis travels. Unlike the flatbed lathes where guide rail length is limited to the horizontal depth of the casting, the true slant bed design allows for longer X-axis rails. Just like in a square box, the straight sides of the box are one specific length, but the angular distance from one corner to the other is much longer. The same is true for the slant bed casting design which obviously means a larger part capacity in a smaller machine footprint. Although the flying wedge design, with the bolt-on slant, can also offer some increased X-axis travels over traditional flatbed machines, it can also magnify the lack of rigidity that is present in the bolt-on approach. You just cannot substitute for a sturdy casting design.

Thermal dynamics are also a big consideration in any machining process. The angular configuration of the base casting, and extended X-axis guideways, also offer better rigidity and part accuracies. Since the linear rails are longer, the base saddle casting that carries the turret can also be longer, providing a much sturdier base of support for the turret. And as the machine components begin to heat-up during the machining process, the headstock, tailstock and cross slide will all begin to grow along the same 30, 45, or 60 degree plane as the X-axis – unlike the flatbed flying wedge design, where the X-axis is mounted on a slant, but the rest of the machine components are mounted on the horizontal flatbed plane.

For more information, contact Hurco – Tel: (011) 849-5600.

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