Turning Revolution (Eurotech Biglia) - written by Larry Greenawalt

Sub spindles on today's CNC turning centers can be part of any manufacturer's key to producing finished turned parts in one machine set-up. A sub spindle or "pick off spindle" as referred to by most in the screw machine industry, has been around for a while. Early screw machine manufacturer's saw its value and have been offering this option on many of their models for years.

Eurotech, (Biglia in Europe), was the first company in Europe to put a sub spindle in place where the tailstock was located on a standard 2 axis CNC turning center. The year was 1979 at the EMO show in Paris. They were able to demonstrate the machining of a part in the main spindle chuck, transfer it to the sub spindle and complete the turning on the second side. This launched the company into a fury of activity leading to more and more advancements in sub spindle and multi-axis turning designs.

Today, big or small, almost every lathe manufacturer around the world has copied this simple but innovative idea. So if everybody is doing it, what separates one builder from another when it comes to sub spindles? To answer this, let's look at what you really need from a sub spindle.

1.Spindle RPM Synchronization - If you plan to operate both spindles at the same time for cutoff and transferring the work piece, 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 sub spindle 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.Sub Spindle Ejection or Part Evacuation - Once the part is in the sub spindle, there must be a way to get it out. If we had to stop the machine cycle to manually remove the part, this would be defeating the whole purpose of having a sub spindle. 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 sub spindle after the machining work is completed. Some sub spindle 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 feed back to the control whether the part has really left the sub spindle or not. If the spring was jammed or the collet did not open and the part remained in the sub spindle, CRASH! BANG! BOOM! (you get the picture)

4.Ejector Switch Confirmation is a necessary part of any sub spindle system if the machine is intended for unattended 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 required to try and keep these chips out of critical areas of the machine. A very critical area is the collet or work holding 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 set-up and tolerances. Air and/or coolant flushed through the sub spindle greatly reduce the chance for this occurrence.

6.Clearance, Rigidity and Capacity - These three items sort of tell you who really has provided a sub spindle engineered for it's specific use or who just is a "me to" supplier. Clearance is an area where if the sub spindle is too large, there isn't enough room for cutoff operations or room to machine between spindles. If the sub spindle 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 sub spindle 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 vs Chucks - Again, the biggest factor is what the machine is being used for. Collet systems are easier and faster to install and changeover vs chucks which 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 finished machined in the main spindle. If say the finishing tool that was cutting the part OD broke and left the OD oversized to a degree that the collet in the sub spindle could not fit over the diameter, the sub spindle 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, and if the stress rises above the allowed limit, the machine alarms out with no damage to the part, set up or machine. This same system can be used in reverse to check as cut off 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 sub spindle is employed with 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 give an alarm. This is an absolute must for unattended operation.

The article this month is how to write a sub spindle cutoff and transfer sequence using Macro B programming with Fanuc. The example uses a Eurotech 710S or 420S type model. The same concept can be used for many other models. The purpose of this example is to show how to reduce or "can" your sub spindle sequences so that you don't have to write a new and unique set of instructions for each new program.

For any comments, questions, or additional information, email me at larryg@eurotechelite.com

I hope you'll be able to take away something of value from this month's article and as always, keep on turning!