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CNC POSITION CONTROL

 

• Left wall too much to the left … more than 24.000 

Left wall too much to the right … less than 23.950 

Example 2 – Measured distance is Z-23.990 

This dimension is right on. There is no need of adjust ment, just monitoring the size for subsequent grooves. The offset calculation follows the same formula: Offset adiustment Target . Measurement Offset adjustment = -23.990 – -23.990 

0.000 mm 

NOTE – This section specifically addresses offset 

adjustments to control the groove POSITION only Groove position is the exact distance from the Z-axis part zero to the groove wall as dimensioned in the engi neering drawing. This location is generally the left or the right wall of the groove. Although it may seem irrelevant whether the left side or the right side of the groove is di- mensioned, keep in mind one general rule: 

Right Side Reference Point 

Setting the reference point of the insert to the right side edge may have some advantages, when the right wall of the groove is dimen sioned on the drawing. In this case, the dimension will correspond with the Z-axis value in the program, when the tool cuts that wall. 

In spite of this minor advantage, the disadvantages are far greater. The insert edge at right is harder to set at the machine and a face touch-off is not possible. 

For the last example, no tolerance was given. In this example, the tolerance has to be carefully evaluated. 

The wear offset 04 does not require adjustment, 

Note that in both calculations, the numbers were neg ative, subtracted from each other. 

Drawing dimensions and tolerances are part of the engineering design and must always be maintained 

Offset adjustment for Z-position is … EXPECTED Z – MEASURED Z 

In order to adhere to the rule, to control the groove po sition in its relationship to ZO, there is one more element that has to be considered – insert reference point (see 

page 196). 

Programmed Target Dimension 

Programmers normally use the nominal dimensions and ignore the tolerances – the initial example is no ex ception. You may find that some programmers may use a middle of the tolerance range in the program. In many ways it is a matter of opinion. The opinion promoted here is to use nominal dimension of Z-24.0 and leave the offset adjustment to the CNC operator. That also means the target dimension has to be established at the control first. What is the best Z-target at the control? Z-24.000? Z-23.950? Somewhere in the middle? Middle makes sense – but not necessarily the exact middle. 

As the same tool cuts many grooves, it gets slightly smaller by wear. Its cutting capabilities are still intact, but the tolerance of 50 microns (0.002 inch) has to be taken into consideration. Do we aim the offset adjust ment more to the left or to the right? What is better to aim for – Z-23.960 or Z-23.990, for example? Since the groove cannot be more than Z-24.000, it is better to aim further from zero, so Z-23.990 is a better choice. As the grooving tool becomes even slightly smaller, the actual dimensional shift will be further from the nominal size, but still well within tolerances. The following two exam ples will aim for the target dimension for the groove po sition of Z-23.990. 

There are other ways to find the offset adjustment amount – the method presented used the same formula for both examples. 

Unlike a typical insert for turning or boring, where only one reference point of the insert is used, grooving tool has two options: 

• Reference point at the insert bottom and the LEFT edge 

Reference point at the insert bottom and the RIGHT edge 

Reference Point Set in the Middle 

By mentioning the left edge ver sus right edge pluses and minuses, what about setting the insert refer ence point to the middle of the insert width? Some programmers like to do that, as it makes groove pro gramming symmetrical, therefore easier in some ways. 

The problem with this method is that while the programming may be easier, it is much more difficult to set such reference point at the machine. 

Use caution when adding or subtracting negative numbers 

One final note that relates to the groove Z-position. Sometimes, it is necessary to change the tool itself. For example, the programmed grooving tool of 5 mm is not available, but a smaller insert of 4 mm or a larger insert of 6 mm is available. In this case, the program itself would have to be changed. Look at the changes required for a 6 mm grooving tool (underlined). The 1 mm differ ence between tools has been distributed by +0.5 mm: 

During program development, the programmer has to make a selection – the left edge or the right edge of the insert? While reasons for selecting one over the other are often a matter of personal preference, there are also sound reasons in therms of practicality and maintenance of dimensional accuracy. 

Offset Adjustment 

Before you adjust the position of the groove along the Z-axis, keep in mind one rule: 

Position change of the groove does not change its width or depth 

Example 1 – Measured distance is Z-23.930 This dimension does not result in a scrap, because the grooving tool is smaller that the groove width. Had the tool width been the same as the groove width, the part would be a scrap. The offset has to be adjusted for the next part, only to conform to the target of Z-23.990. As the measured dimension is too far from the groove left face (closer to ZO), the adjustment must be done in the direction of the left face, which is Z-negative direction; 

The above statement indicates that the offset change takes place only along the Z-axis. The example drawing shows a 24 mm dimension to the left wall of the groove: 

(T0404 – 6 MM GROOVING TOOL) N41 T0400 N42 G96 S150 M03 N43 GOO X60.0 2-23.6 T0404 M08 N44 X48.0 

(AT CENTER OF GROOVE) N45 G01 X38.1 F0.25 (CENTER CUT + CLEARANCE) N46 G00 X48.0 

(RETRACT TO START POINT) 

N47 W-1.9 

(START OF LEFT CHAMFER) N48 G01 X45.0 w1.5 F0.1 (LEFT CHAMFER) N49 X38.1 F0.2 

(LEFT WALL) N50 GOO X48.0 Z-23.6 (BACK TO START POINT) N51 W1.9 

(START OF RIGHT CHAMFER) N52 G01 X45.0 W-1.5 F0.1 (RIGHT CHAMFER) N53 x38.0 F0.2 

(RIGHT WALL) 

N54 Z-24.0 

(SWEEP BOTTOM) N55 GOO X48.0 2-23.6 (BACK TO START POINT) N56 X100.0 275.0 T0400 (TOOL CHANGE POSITION) N57 M01 

Left Side Reference Point 

Regardless of how the groove is dimensioned, setting the reference point of the grooving insert at the bottom and the left edge is far more common than any alternative. The reason is simple and, for the CNC operator, quite welcome. Setting the reference point to the left side edge is easier for part touch-off during setup and is consistent with setting all other tools. 

The geometry measurement is to the intersection of the left side and bottom edge of the grooving insert. Tol erances related to the position and/or width of the groove can easily be handled by adjusting various offset set tings. 

Offset adjustment Offset adjustment 

Target – Measurement -23.990 – -23.930 -0.060 mm 

24 0,00 

-0,05 

CNC Control Setup for Milling and Tuming 

GROOVING ON LATHES 

203 

WIDTH CONTROL 

– NOTE – This section specifically addresses offset 

adjustments to control the groove WIDTH only Many groove designs require the groove width to be within certain tolerances, usually to allow for a physical match with another part during assembly. For standard horizontal groove used for examples in this chapter, that means offset adjustments in the Z-axis, and Z-axis only. The drawing change will also require other changes: 

Program Change 

Experienced programmers will anticipate the need for groove width and apply the necessary code to the pro gram. Unfortunately, often it is left at the CNC operator to make the change. 

The program using two offsets should contain a mes sage that two offsets have been programmed and even identify them. In the program, you should check for: 

Blocks containing offsets | Numbering of offsets « Offset cancellation 

046 

038 

The original program has been adjusted to control groove width separately from groove position, using two wear offsets – 04 and 14. Keep in mind that each offset has its own special purpose: 

• Wear offset 04 controls the groove precise location 

· Wear offset 14 controls the groove precise width 

24 

What about the depth? That is a separate issue alto gether and is not a factor here. The conclusion is: 

Settings for offsets controlling the groove position 

and groove width separately MUST have the identical X-amount 

All original dimensions have been preserved, but the grooving width of 6.8 mm has to be revised and a toler ance added. The tolerance is to the plus side, which means the groove can be wider up to 0.04 mm (-0.0015 inch), but cannot be narrower. 

In other words, if you want to control the groove depth in addition to position and width, X-offset setting for both offsets 04 and 14 must be the same. 

Here is the original example, modified for two offsets: 

Program Requirements 

The program as presented originally (see page 199), has to be changed as well. Ask yourself – why? After all, the wear offset 04 has been programmed and is active throughout the whole program. If the program remains as is, yes, you can manipulate the wear offset 04 in the Z-axis as much as you want, but that will not change the groove width, only its position. That was the subject of the previous section. 

The major change in the part program for the groove is to use of TWO OFFSETS for the same tool. While ex perienced CNC operators generally have no problems working with one geometry or wear offset per tool, they may not have the opportunity to use two (or more) off sets for the same tool. 

After all, the position is good, so why change it? It is the width that needs to be controlled and that can only be achieved by one offset controlling the position, such as wear offset 04, and another offset controlling the width, such as offset 

(T0404 – 5 MM GROOVING TOOL) N41 T0400 N42 G96 $150 MO3 N43 GOO X60.0 2-23.1 T0404 M08 N44 X48.0 

(AT CENTER OF GROOVE) N45 G01 X38.1 F0.25 (CENTER CUT + CLEARANCE) N46 GOO X48.0 

(RETRACT TO START POINT) 

N47 W-2.4 

(START OF LEFT CHAMBER) N48 G01 X45.0 w1.5 F0.1 (LEFT CHAMFER) N49 X38.1 F0.2 

(LEFT HALL) N50 GOO X48.0 Z-23.1 (BACK TO START POINT) N51 W2.4 T0414 

(START OF RIGHT CHAMFER) N52 G01 X45.0 W-1.5 F0.1 (RIGHT CHAMTER) N53 X38.0 F0.2 

(RIGHT WALL) 

N54 Z-24.0 T0404 

(SWEEP BOTTOM) N55 GOO X48.0 Z-23.1 (BACK TO START POINT) N56 X100.0 275.0 T0400 (TOOL CHANGE POSITION) N57 M01 

Study the underlined changes – cutting on the left uses wear offset 04, cutting on the right uses wear offset 14. Offsets will take effect during the actual tool motion. 

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