Multiple repetitive cycles were introduced by Fujitsu Fanuc in the early to mid 1980’s, when a true computer based control system became more widespread. This was the first time when a shortcut was introduced to lathe ma chining. Basic fixed cycles for drilling on mills are a bit
15+ years, older. However, the concept date back another 15+ years, when programming languages were common.
Of the early language based programming methods, most names have been forgotten – Compact II®, Split®, APT®, GeoPath®, Numeridex®, and quite a number of others. In such a programming environment, the basic concept of stock removal was based on the removal from a defined area. Once the area had been defined, other pa rameters were used to fine tune the stock removal pro cess, parameters such as depth, stock allowance, etc.
This concept has been adapted to modern program ming, using the ‘cycle’ method.
If the point 2 in the illustration defines the first XZ CO ordinate of the contour, and if the point 3 of the same il lustration defines the last XZ coordinate of the contour – and there are other specified points between the two points, a unique contour for machining can be defined.
Apply the previous generic illustration to the actual drawing. First, the outside contour (in scale), as ma chined with an external turning tool:
LATHE CYCLES G70-G71 – G72
Stock to remove
This lathe oriented chapter is about external and inter nal stock removal on CNC lathes. It is most likely that every CNC lathe operator will machine parts that use ex ternal machining (generally called turning) and internal machining (generally called boring). On modern CNC lathes, this machining will be programmed using special cutting cycles. Operators do not have to know how to program such cycles, but they should fully understand how these cycles work. The main benefit of such knowl- edge results in the ability to make small changes at the machine that can produce significant improvements in many aspects of machining.
The best way to illustrate the concept is an illustration, a drawing. A very simple drawing shown below has been selected for further evaluation. It contains simple external and internal contours. The part size in the draw ing is the initial size, the operations required are part of a second operation only. Surfaces within jaws have been pre-machined in the first operation (irrelevant here).
These powerful and flexible lathe cycles are called multiple repetitive cycles – a kind of parametric data in put for machining. Programming these cycles and input of machining data is the job of CNC programmer, Keep in mind that the main purpose of any cycle is not only to make part programming easier and shorter, but also that any parametric cycle can be very easily edited at the ma chine, by the CNC operator. To understand how these cycles work, you need to know the format, structure, and application of the multiple repetitive cycles. This chap ter covers three commonly used cycles for turning and boring – for roughing and finishing operations. The lathe cycles covered in this chapter are:
• G70 Finishing cycle
• G71 Roughing cycle … horizontal direction
G72 Roughing cycle … vertical direction with a brief mention of G73 – Pattern repeating cycle
Stock to keep
Establishing an Area Any geometrical area can be defined by the minimum of three points that do not form a single line – like a tri angle. The illustration below shows the area of a simple taper to be machined out of a round stock:
Next is the inside contour, for a different toolpath (also in scale, but one with a smaller factor), this time for in ternal machining, using a suitable boring bar:
Stock to keep
Stock to remove
Stock to remove
Stock to keep
The emphasized word minimum in the last paragraph suggests there could be more than three points. In terms of CNC machining, a contour can have many contour change points to define the desired shape and sizes for machining. That is where the simple illustration above comes in – how is the contour represented? The part con tour is a single line between points 2 and 3 – a taper Point 1, on the other hand, represents a point to start from – a starting point.
Multiple repetitive cycles are based on the same con cept of three points minimum. Adapting this concept to the initial drawing (page 173) and changing some termi- nology, a multiple repetitive cycle can be defined.
In both illustrations there is a noticeable change from the original description of areas. The change is not in the concept itself, but in its application. The conceptual def inition does not take any clearances into effect. Yet, clearances are part of machining, and not including them in the area definition would cause severe machining problems. For that reason, any area that defines actual machining must include any necessary clearances.
During evaluation of a program received for machin ing, the location of point I may not correspond to its po sition in the illustration. This is not a requirement of the cycle itself, but a reflection of programming efficiency.
1030 STEEL 0135 x Ø28 x 82 mm Large end pre-machined
CNC Control Setup for Milling and Tuming | 175
TURNING AND BORING
The shaded portion of the next illustration shows the material to be removed during second operation.
A CNC operator will be presented with a part program to be used as the source for machining. His or her job is to interpret the program correctly and be able to make some changes, if and when they become necessary.
As the large end of the part is not relevant here, it is important to understand what shape the material will have for the second operation, which is important here. As the following illustration shows, the shaded area rep resents the material that was removed at the large end.
The program will most likely start with a facing cut. Part zero is at the centerline and the front finished face. In the program, the initial clearance will be 1.5 mm above the actual stock diameter, and only one facing cut will be required:
N1 G21 T0100 (ROUGHING OD TOOL – R0.8) N2 G96 $140 MO3 N3 GOO X138.0 ZO T0101 MOS N4 G01 X25.0 F0.33 N5 GOO 22.5
The original stock size of Ø135 x 84 x Ø28 mm has been reduced to $135 x 82 x Ø28 mm, as 2 mm have been faced off. The remaining 2 mm will be faced off in the second operation,
Both the outer and the inner chamfers have also been completed during this operation.
When facing to an existing hole, some programs may not face far enough, and the face will not be flat at the end of cut. At the control, and if necessary, you may have to extend the facing motion along the X-axis. Use the following practical guideline:
The minimum clearance programmed below an existing
diameter must be double the tool nose radius
During a two-sided lathe operation that requires facing at both ends, it may be necessary to split the facing stock dif ferently, not always by equal amounts. A smaller amount at one end may require two or more cuts at the other end.
When the part is reversed, the remaining material has to be removed – that will be the subject of further study
In the example, the tool nose radius is R0.8. Double that amount to 1.6, and when subtracted from the exist ing Ø28 mm, the X-axis end point should not be greater that x26.4. With an additional clearance, the X25.0 in the program is sufficient.