Programming of CNC Machines. Ken Evans
Figure 1&2 Common Millling Tool Holders
The tapered portion of the holder is the actual surface that is in contact with the mating taper of the spindle. These tapers are standardized by the industry and are numbered according to size (No. 30, No. 40, No. 50).
One benefit of these tapers over the standard R-8 Bridgeport style of tool holder is the increased surface area in contact with the mating taper of the spindle. The increased surface area makes the tool setup more rigid and stable.
Figure 3 Tool Holder with Retention Knob
Another feature on the tool holder is the notch or cutout on centerline of the tool (there is an identical cutout on the opposite side). This enables axial orientation within the spindle and tool changer. As the holder is inserted into the spindle, the cutouts enable it to be locked into place in exactly the same orientation each and every time it is used. This orientation makes a real difference when trying to perform very precise operations such as boring a diameter. These notches also aid the spindle driving mechanism.
On CNC machines with a manual tool change, the holder is inserted into the machine and rotated until the holder pops into place (axial orientation is done by hand) and then, the draw bar is tightened to clamp the tool holder in place. Finally, another component of the CNC Automatic Tool Changing system is the Retention Knob or Pull-Stud. Machining Centers need the retention knob/pull-stud to pull the tool into the spindle and clamp the holder. This knob is threaded into the small end of the taper as shown. Note: There are several styles of knobs available. The operator should consult the appropriate manufacturer manual for specifications required in their situation.
Figure 4 Retention Knob
Many tool and work holding methods used on manual machines are also used on CNC machines. The machines themselves differ in their method of control but otherwise they are very similar. The major objective of CNC is to increase productivity and improve quality by consistently controlling the machining operation. Knowledge of the exact capabilities of the machine and its components as well as the tooling involved is imperative when working with CNC. It is necessary for the CNC programmer to have a thorough knowledge of the CNC machines they are responsible for programming. This may involve an ongoing process of research and update training with the ultimate goal of obtaining a near optimum metal-cutting process. From this research and training comes a decrease in the cycle time necessary to produce each part lowering per piece cost to the consumer. Fine tuning of the machining process for high-speed production gives more control over the quality of the product on a consistent basis. The following are some of the most important factors that affect the metal cutting process.
The Machine Tool
The machine used must have the physical ability to perform the machining. If the planned machining cut requires 10 horsepower from the spindle motor, a machine with only 5 horsepower will not be an efficient one to use. It is important to work within the capabilities of the machine tool. The stability, rigidity and repeatability of the machine are of paramount importance as well. Always take these things into consideration when planning for machining.
The Cutting Fluid or Coolant
The metal cutting process is one that creates friction between the cutting tool and the workpiece. A cutting fluid or coolant is necessary to lubricate and remove heat and chips from the tool and workpiece during cutting. Water alone is not sufficient because it only cools and does not lubricate, and it will also cause rust to develop on the machine Ways and table. Also, because of the heat produced, water vaporizes and thus compromises the cooling effect. A mixture of lard-based soluble oil and water creates a good coolant for most light metal-cutting operations. Harder materials, like stainless steel and high alloy composition steels, require the use of a cutting-oil for the optimum results. Advancements have been made with synthetic coolants, as well. Finally, the flow of coolant should be as strong as possible and be directed at the cutting edge to accomplish its purpose. Programmers and machine operators should research available resources like the Machinery’s Handbook and coolant manufacturer data, for information about the proper selection and use of cutting fluids for specific types of materials.
The Workpiece & the Work Holding Method
The material to be machined has a definite effect on decisions about what tools will be used, the type of coolant necessary, and the selection of proper speeds and feeds for the metal-cutting operation.
The shape or geometry of the workpiece affects the metal-cutting operation and determines the type of work holding method that will be used. This clamping method is important for CNC work because of the high performance expected. It must hold the workpiece securely, be rigid, and should minimize the possibility of any flex or movement of the part.
The Cutting Speed
Cutting speed is the rate at which the circumference of the tool moves past the workpiece in surface feet (sf/min) or meters per minute (m/min), to obtain satisfactory metal removal.
The cutting speed factor is most closely related to the tool life. Many years of research have been dedicated to this aspect of metal-cutting operations. The workpiece and the cutting tool material determine the recommended cutting speed. The Machinery’s Handbook is an excellent source for information pertaining to determining proper cutting speed. If incorrect cutting speeds, spindle speed or feedrates are used, the results will be poor tool life, poor surface finishes and even the possibility of damage to the tool and/or part.
The Spindle Speed
When referring to a milling or a turning operation, the spindle speed of the cutting tool or chuck must be accurately calculated relating to the conditions present. This speed is measured in revolutions per minute, r/min (formerly known as RPM) and is dependant upon the type and condition of material being machined. This factor, coupled with a depth of cut, gives the information necessary to find the required horsepower necessary to perform a given operation. In order to create a highly productive machining operation all these factors should be given careful consideration. Refer to the formulae below needed to calculate r/min.
where
CS = Cutting Speed from the charts in Machinery’s Handbook
π = 3.1417
D = Diameter of the workpiece or the cutter.
Many modern machine controllers have a feature that allows automatic calculation of feeds and speeds that is based on appropriate operator input of the cutting conditions.
The Feedrate
Feedrate is defined as the distance the tool travels along a given axis in a set amount of time, generally measured in inches per minute in/min (formerly known as IPM) for milling or inches per revolution in/rev (formerly known as IPR) for turning. This factor is dependent upon the selected tool type, the calculated spindle speed and the depth of cut. Refer to the Machinery’s Handbook for the chip load recommendations and review the formula below that is necessary to calculate this aspect of the metal-cutting operation.
where F = R × N × ƒ
F = Feed in in/min or mm/min
R = r/min calculated from the preceding formula
N = the number of cutting edges
ƒ = the chip load per tooth recommended from the Machinery’s Handbook
The Depth of Cut
The