Subspindles on today’s CNC turning centers can be part of any manufacturer’s key to producing finished turned parts in one machine setup. A subspindle or “pickoff spindle,” as referred to by most in the screw machine industry, has been around for a while. Early screw machine manufacturers saw its value and have been offering this option on many of their models for years.
Today, big or small, almost every CNC lathe manufacturer around the world has incorporated this simple but innovative idea into their product lines. So if everybody is doing it, what questions should you ask when it comes to subspindles? To answer this, let’s look at some “bells and whistles” that are available and you decide what you really need from a subspindle.
1. Spindle RPM Synchronization: If you plan to operate both spindles at the same time for cutoff and transferring the workpiece, you must have spindle rpm synchronization.
This means that the machine’s control system must be capable of matching and maintaining the rpm of both spindles. An out-of-sync situation would cause one spindle to drag the other and could twist off or damage the part.
2. Spindle Phase Synchronization: This is a synchronization that not only matches the rpm of the two spindles, but also matches the angular position of the two spindles. This is a “must have” for parts where a milled feature on the first side must be aligned to a milled feature on the second side. An example would be side holes that are produced on the main spindle and then face holes that are drilled on the subspindle that need to meet and match the positions of those side holes. Another dramatic demonstration of phase synchronization is the transferring of Hex or irregular-shaped polygons from spindle-to-spindle while rotating. Without this phase sync, it would be virtually impossible.
3. Subspindle Ejection or Part Evacuation: Once the part is in the subspindle, there must be a way to get it out. If it was necessary to stop the machine cycle to manually remove the part, this would be defeating the purpose of having a subspindle. A simple part ejector can be employed. This ejector is normally pneumatic or hydraulic and is sent forward to push the part out of the subspindle after the machining work is completed. Some subspindle manufacturers use a spring loaded system to push the part out once the collet is opened. The problem with these systems is that they are not a closed loop. There isn’t any feedback to control whether the part has left the subspindle or not. If the spring was jammed or the collet did not open and the part remained in the subspindle, the system is likely to fail.
4. Ejector Switch Confirmation: This is a necessary part of any subspindle system if the machine is meant for untended use. The switch can tell the machine if the parts ejector is all the way forward past the collet opening, and if it is not, to alarm out, thereby preventing any damage from occurring.
5. Air Blast and/or Coolant Through: All machining operations produce some type of chip, so it is important to try and keep these chips out of critical areas of the machine. A very critical area is the collet or workholding device. Even the smallest chip in the collet can cause a part to become scrap if they are crushed into the surface of the part and damage the part or the setup and tolerances. Air and/or coolant flushed through the subspindle greatly reduce the chance for this occurrence.
6. Clearance, Rigidity and Capacity: These three items tell you who has provided a subspindle engineered for it’s specific use and who is a “me to” supplier. Clearance is an area where if the subspindle is too large, there isn’t enough room for cutoff operations or room to machine between spindles. If the subspindle is too small, then the capacity for size and power can be lacking. A careful balance of all three of these items is dependent on the overall planned use of the machine. For chucking work, many times a builder will design matched spindles and capacities. Clearances are less important between them because no parting off is being done. For bar work, the main spindle is normally designed with a larger capacity and power for large-diameter 12-foot-long bar and the subspindle is designed around the machining of the machine’s largest bar diameter. Clearances are such to allow cutoff and machining between spindles.
7. Collet Systems Versus Chucks: Again, the biggest factor is what the machine is being used for. Collet systems are easier and faster to install and change-over versus chucks. Chucks are normally heavier and require bored jaws and additional set up. If it’s bar work, collets are the first choice and if it is chucking work for castings, forgings or billets, a chuck is the first choice. Collet systems are normally smaller in diameter and provide better clearances for bar machining. If an odd size is needed and cannot be found from a collet manufacturer, then an emergency collet can be bored out on the machine.
8. Stress Control and Part-Off Confirmation: Stress control is the ability to check for stress or interference when moving to or from a part. An example is when moving over a part that is finish-machined in the main spindle. If the finishing tool that was cutting the part O.D. broke and left the O.D. oversized to a degree that the collet in the sub spindle could not fit over the diameter, the subspindle would try to push over this part resulting in damaging the part, the collet and even the machine to some degree. To prevent this, a stress control system can be used to monitor the stress as the sub spindle is moving over the part. If the stress rises above the allowed limit, the machine alarms out with no damage to the part, setup or machine. This same system can be used in reverse to check as cutoff confirmation instead of using additional wire or probe type checking systems. When checking with stress control for cutoff confirmation, a small feed move back of the subspindle is employed with the stress control on. If the cutoff tool broke, the bar would not have been separated and the load would rise when trying to move the sub spindle away with the stress control check on and the alarm will signal. Stress control and part-off confirmation is a must for untended operation.