A Quick Guide to CNC Turning Centers
You can trace the history of the lathe’s origins as far back as 1300 B.C. as it was used by ancient civilizations like the Egyptians, Assyrians, and the Greeks. Over the centuries, we have seen the lathe evolve from a two-person, bow-driven machine during ancient times to the explosion in its popularity during the Industrial Revolution, and with the sophisticated, precision CNC (Computer Numerical Control) turning machines we have today.
The Nuts & Bolts of Lathes
The difference between lathes and turning centers
For the most part, lathes today are commonly referred to as CNC lathes and CNC turning centers. The CNC lathe is a simpler 2-axis (X and Z-axis) machine that is computer-controlled and, generally, has one spindle. CNC turning centers are also computer controlled lathes but are more powerful and can have 3, 4 or 5-axes with more versatile capabilities and applications such as turning, tapping, drilling, milling, and reaming. Various attachments for different types of operations are also available for the turning center. CNC turning centers have higher production capabilities than CNC lathes.
Here are some of the most important parts of a CNC Lathe:
The headstock serves as the housing for the motor, pulleys, gears, bearings and main spindle where the jaw chuck is attached that holds the workpiece. It provides power to various parts and rotates the spindle. Commonly, the main spindle will include a Morse taper. The headstock is located on the left side of the lathe.
Feed Screws and Lead Screws
The Feed Screw is a driveshaft connected to a series of gears that drive the carriage along the Z-axis. Much the same, the Lead Screw on a lathe links the carriage to the rotation of the chuck and drives the carriage along the X-axis. It moves the turret in precision increments for every rotation of the screw.
The Carriage supports the cutting tool, guiding and feeding it against the workpiece. There are five parts to the carriage:
3. Compound Rest
4. Tool Post
The bed, commonly made of cast iron, is the base where various fixed and operational parts are mounted. It is connected to the headstock and spans the distance from the headstock to the tailstock. It is positioned under the workspace. Tool rests and various attachments ride along the rails of the bed, across the length of it. The bed provides a level and evenly-distant surface from which a workpiece is measured.
The Tailstock is mounted on the bed of the lathe, opposite of the headstock and slides along it in alignment with the headstock. The primary function of the tailstock is to provide support to the workpiece being machined between centers. The tailstock also has a spindle. It can include a taper to hold tooling. However, unlike the spindle in the headstock, the spindle in the tailstock does not rotate, but travels longitudinally.
When is a Lathe Considered a CNC Turning Center?
Turning Center Functions
A CNC lathe can perform many tasks but a turning center is capable of much more. For example:
1. Facing Removing the metal from the end of a workpiece (typically cylindrical) which produces a flat surface.
2. Threading This is the process of making a screw thread used to fasten or connect items.
3. Knurling A process of machining a pattern of angled or crossed lines rolled into the workpiece. A finished knurled piece allows hands to get a better grip on the object.
4. Drilling Most often, drilling holes in a workpiece will need to be done on the lathe before other tasks can be completed, such as tapping, boring or reaming. It’s always best to spot or center-punch the area to be drilled so that the drilling will be correctly aligned.
5. Boring This will enlarge the hole in the workpiece that was already drilled, achieving greater accuracy of the diameter of the hole.
6. Reaming Drilled holes in the workpiece are often not accurate (straight and cylindrical). Reaming finishes and sizes the drilled holes with a high degree of accuracy.
7. Taper Turning A taper is a uniform change in the shape of a cylinder’s diameter when measured along its axis. It is created by angling the workpiece and cutting tool relative to each other as the tool travels along the workpiece surface, cutting deeper or shallower, thus creating a tapered surface.
Turning Center Configurations (Horizontal or Vertical)
There are two types of CNC turning centers: horizontal and vertical. Horizontal turning centers are most common. Vertical turning centers are typically called a vertical turret lathe or VTL. With a horizontal turning center, the spindle is horizontally oriented and tools are mounted out of the side of the tool holder and cut across the workpiece. With this type of turning center, gravity pulls the chips away from the workpiece.
The CNC vertical turning center is virtually the same as the CNC horizontal turning center but is upended, allowing the headstock to sit on the floor and the faceplate to become a horizontal rotating table. This is beneficial for machining large, heavy and short workpieces. In this case, gravity contributes to seating the workpiece into the workholding. However, chips may become a problem as gravity will make them fall into the spindle. An inverted vertical turning center is also the same as the afore mentioned two but reverses the position of the spindle and jaw chuck.
Turning Uses and Applications
Aerospace, agriculture, and automotive are some of the industries where CNC turning centers are utilized. These machines do particularly well with machining round metal parts and gears, etc.
Turret Tooling and Gang Tooling
Turret tooling lathes give you access to more tools but it takes up to a half a second to index each tool and you need to come off the workpiece to do that. Its advantages are you have a lot more room and flexibility (greater tool clearance) so there is less interference going from tool to tool. This allows you to use larger diameter parts.
With gang tooling lathes, the tools are mounted in a row on the lathe’s cross side and is more suitable for high-volume work. Gang tooling is more common on smaller lathes, machining smaller parts. The biggest advantage of gang tooling is speed. It is very fast because it uses a slide motion to change tools and there is no tool indexing. This equates to higher productivity—shorter production time. Since there are fewer moving parts, gang tooling is less expensive than turret tooling. Another advantage of gang tooling is faster set up and better accuracy. However, with gang tooling, you cannot use a tailstock because the tailstock would get in the way.
Multitasking is the word that best describes live tooling. Live tooling allows the lathe to mill, drill, bore, turn and tap workpieces. The benefit is it reduces work-process time which increases your productivity and profitability. Kent CNC offers CNC Turning Centers with Live Tooling that will fit the more complex applications.
Automating your CNC Turning Center
You’ll notice a great improvement in productivity with an automated CNC turning center. There is a robot that loads tools into the lathe’s jaw chuck. When machining is complete, the robot takes the finished piece and places it onto a small conveyor belt at the other end of the machine. With automation, the turning center can run unattended. It’s reported that huge amounts of machinists of the baby boomer era are retiring and that there is a shortage of younger workers entering the machine tool industry. Automation can help in filling the gap to help supplement this workforce shortage.
Log Jams & Mistakes
In CNC turning we must strive for efficiency, always. There will be obstacles and issues to overcome but with the advent of CNC, mistakes are greatly minimized as calculations and settings are programmed. However, it’s up to the operator to use the skills they’ve learned and utilize the tools they have at their fingertips in the best ways possible to produce error-free pieces.
Bumps in the Road
As in any industry, there will always be things that will slow us down. The machine tool industry is no different. Perhaps you can cut at a very fast surface speed, but the fact is your chuck may not spin that fast. Be careful not to crank the speed up too much to the point where you can’t turn away from your turning center for two seconds or if the machine burns out or explodes.
Another issue is change-over. It’s essential to carefully consider your tooling systems and the time it takes to change them, as this can also rob you of your production time. You’ll produce a better, more accurate product if you focus more on quality rather than speed alone.
Making the Most of your CNC Turning Center
The key is to understand your CNC turning center’s full capabilities and then having the right skillset to utilize them. And one more thing: preparation — knowing that you’re going to have enough tools in the holder to finish the piece. Planning and mapping-out the scope of your project are keys to success.