Spindle control

Both types of CNC machines, machining centers and lathes, use spindle rotation when removing excessive mate-rial from a part. The rotation may be that of the cutting tool (milling) or the part itself (lathes). In both cases, the activi-ties of the machine spindle and the working feedrate of the cutting tool need to be strictly controlled by the program. These CNC machines require instructions that relate to the selection of a suitable speed of the machine spindle and a cutting feedrate for a given job.

There are several methods to control the spindle and cutting feedrate and they all depend mainly on the type of the CNC machine and the current machining application. In this article, we look at the .spindle control and its programming applications. 

The program command relative to spindle speed is con-trolled in the CNC system by the address S. The program-ming format of the S address is usually within the range of 1 to 9999 and no decimal point is allowed: 
S1 to S9999 
For many high speed CNC machines is not unusual to have spindle speed available up to five digits, in the range of 1 to 99999, within the S address range: 
S1 to S99999 
The maximum spindle speed range available in the control must always be greater than the maximum spindle speed range of the machine itself. It is quite typical that virtually all control systems support a much greater range of spindle speeds than the CNC machine allows. In programming spindle speeds, the limitation is always caused by the machine unit, not by the control system.

 Spindle Speed Input

The address S relates to the machine spindle function, and must always be assigned a specific numeric value in the CNC program. There are three alternatives as to what the numeric value (input) of the spindle function may be: 
1) Spindle speed code number 

2) Direct spindle speed

3)Peripheral spindle speed 

On the CNC lathes, all three alternatives may exist, de-pending on the control system. For the CNC milling sys-tems, peripheral spindle speed is not applicable, but the spindle speed code number and the direct spindle speed are. The spindle speed selection by special code number is an obsolete concept, not required on modern controls.

The spindle speed designation S is not sufficient to be programmed by itself. In addition to the selected spindle speed address, certain additional attributes are necessary as well. These are attributes that control the spindle function environment. For example, if the spindle speed is specified as S400 in the program, the programming instruction is not complete, because the spindle function stands by itself in the program. It does not include all information the control system requires for the spindle data. A spindle speed value that is set, for example, to 400 r/min or 400 m/min or 400 in/min (depending on the machining application), does not contain all necessary information, namely, the spindle rotation direction. 
Most machine spindles can be rotated in two directions -clockwise or counterclockwise, depending on the type and setup of the cutting tool used. The spindle rotation has to be specified in program, in addition to the spindle speed func-tion. There are two miscellaneous functions provided by the control system that control the direction of the spindle M03 and M04. 

Thinking in terms of right and left, up and down, clock-wise and counterclockwise, and similar directional terms, is thinking in terms that are relative to some known reference. To describe a spindle rotation as clockwise (CW), or as counterclockwise (CCW), some established and standard reference method is needed, in this case a reference point of view (reference viewpoint).

The direction of spindle rotation is always relative to the point of view that is established from the spindle side of the machine. This part of a machine that contains the spindle, and is generally called the machine headstock. Looking from the machine headstock area into the direction along spindle center line and towards its face, establishes the cor-rect viewpoint for defining CW and CCW rotation of the spindle. For CNC drills, milling machines and CNC ma-chining centers, the reference point of view is quite simple to understand. For CNC lathes, the rules are exactly the same.

Direction for Milling

It may be rather impractical to look down along the centerline of the spindle, perpendicularly towards the part. The common standard view is from the operator's position, facing the front of a vertical machine. Based on this view, theterms clockwise and counterclockwise can be used accurately, as they relate to the spindle rotation 


Direction for Turning

A comparable approach would seem logical for the CNC lathes as well. After all, the operator also faces the front of a machine, same as when facing a vertical machining center.Figure below shows a front view of a typical CNC lathe.

NOTE-To establish spindle rotation as CW and CCW look from the headstock towards the spindle face.





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