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CNC Work offset always applies to ALL tools

The problem can be summed up in this observation: 

For reference, when adding or subtracting positive or negative numbers, the following table may be of help: 

Work offset always applies to ALL tools 

Calculation 

Result 

Setting the Z-axis 

The setting of the Z-axis is done either at the machine or off-machine. In either case, the amount measured is entered into the tool length offset. The frequently asked question is – why? Let’s look at the following example. It uses a setup method common to small shops and job shops, so called touch-off method. Once the particular tool is placed to the spindle and the Z-axis is at home po sition, the tool length measurement can begin. 

In the following illustration, the tool measurement is 275.627 mm in negative direction: 

Example 1b – If these are INTERNAL measurements … 

For internal measurements and part zero at the lower left corner, the edge finder center is not far enough and a negative radius must be added to both measurements (‘friendly’ numbers used for convenience only): X-axis edge = -600.0 +-3 = -600.0 – 3 = Work offset is -603.0 Y-axis edge = -300.0 +-3 =-300.0 – 3 = Work offset is -303.0 Work offset will be set to X-603.000 Y-303.000 

PLUS and POSITIVE ( ++) PLUS and NEGATNE (+-) MINUS and POSITIVE (-+) MINUS and NEGATIVE (–) 

A+ (+B) = A+B 

A+ (-B) = A-B 

A- (+B) = A-B 

A-(-B) = A +B 

From this statement, it should be obvious that not all tools used by the program will have the same length. In other words, the work offset setting of Z-273.627 is appli cable to all tools using G54 work offset. As this is not an acceptable situation, the control system offers a special offset, called the tool length offset (G43 H..). In this reg istry, each measured tool length is entered individually. This and other details relating to tool length offset are described in a separate chapter. 

For the following subjects and var- 01654 ious examples, the setting used as the X -615.375 starting point will be the one without any Z-setting (20.000). 

Y -304.540 

z 0.000 

Part Zero at Lower Right – Example 2: 

For this second example, the part zero is at the lower right of the part, using the following measurements for edge finder center, again with ‘friendly’ numbers: 

Edge finder radius = 3 mm 

Edge finder center measured in X = -600.000 

• Edge finder center measured in Y = -300.000 

Original Example Revisited 

The simple three hole part introduced at the beginning of this chapter can now be set and work offsets estab lished by measurement, using the described methods. 

Once the 3 mm adjustment is made, the settings can be input as the work offset G54 at the control. 

The original program block G90 GOO X10.0 78.0 

(81) … should be changed to reflect the fact that the setting is for the G54 offset and none other. Including the work offset results in a program that does not take chances: 

-275,627 

Example 2a – If these are EXTERNAL measurements … 

The table on the previous page shows a short X mea surement – it means a negative radius has to be added to the measurement: X-axis edge = -600.0 +-3 = -600.0-3 

= Work offset Is… -603.0 The Y-distance is too long, meaning a positive radius has to be added to the measurement: Y-axis edge = -300.0 ++3 = -300.0 + 3 

= Work offset is … -297.000 Work offset will be set to X-603.000 Y-297.000 

MULTIPLE WORK OFFSETS 

As the heading suggests, multiple work offsets will require respective settings not only in the G54 registry, but also in one or more of the other registries. There are many uses for more than one work offset, but for machining purposes, the most common use is when multiple parts are setup on the machine table, each with its own part zero, measured in and registered to any of the six work offsets available. Take an example for two vises set on the machine table (not to scale): 

PART 

G90 G54 GOO X10.0 78.0 

(81) 

Can this Z-setting be entered into on 6511 the G54 work offset? 

X-615.375 

Y -304.540 Consider this updated G54 setting: |Z -275.627 

WORK OFFSET Z-SETTING 

The following is the G54 work off set setting for the drawing example, 

01 G54 

as explained already: 

X -615.375 

Y -304.540 

Note that the Z-setting is zero. 

0.000 

G54 X 

The answer is yes, as shown, the setting is correct. In the program, there will be an additional command, mov ing the tool along Z-axis to a clearance position of 2 mm above the part top face (Z2.0): G90 G54 GOO x10.0 Y8.0 

(H1) 

22.0 

(CLEARANCE) 

A 759 

G55 Y 

Example 2b – If these are INTERNAL measurements … 

For internal measurement at the same lower right part zero, the calculations must be reversed – positive radius will be added to the X-measurement, and a negative ra dius will be added to the Y-measurement: X-axis edge = -600.0 + +3 = -600.0 + 3 

= Work offset is … -597.0 Y-axis edge = -300.0 +-3 = -300.0-3 

= Work offset is … -303.0 Work offset will be set to X-597.000 Y-303.000 

G55 x 

Based on the previous distance-to-go calculations for the X and Y axis motion, the same formula will be used for the Z-axis motion: 

Target: 

22.0 (all holes) Current location: 2-275.627 (from machine zero) Difference = Distance-to-Go = 2.0 +-275.625 

= 2.0 – 275.627 

= -273.627 The setting is correct – yet, there is a problem. 

Where is the Z-setting? 

One very natural question can be asked at this point. What about the work offset for Z-axis? Having it set to zero does not seem to make much sense, you may ask. 

Typically, Z-axis is the axis that controls the depth of cut. As such, it is measured differently than the XY set tings and requires a different offset, called tool length offset. This important subject requires thorough knowl edge, and will be discussed in a separate chapter. 

In some special cases, the Z-setting does not have to be zero. This will be explained briefly in this chapter as well as in the chapter covering the tool length offset. 

Schematic drawing only – not to scale Although both parts appear to have the X-setting iden tical for G54 and G55, that does not mean only one set ting is required for the X. The X-origin for one vise will not likely be the same as the X-origin for the other vise. 

Part zero for the other two corners can be calculated in a similar way. Hopefully, the explanation and some ex amples have made this important subject clear – always think twice before committing a particular calculation to any control system setting. 

84 

CNC Control Setup for Milling and Tuming WORK OFFSET SETTINGS 

CNC Control Setup for Milling and Turning 

WORK OFFSET SETTINGS 

83 

Height of Part 

Using the example of a two-vise setup (G54 and G55) for the three holes will illustrate another common prob lem that has to be identified and corrected. The problem is the difference between the Z-height of the upper vise and Z-height of the lower vise. 

Ideally, using the same type of vise for both setups, the top face of both parts should be identical. That may be the case in some cases – but what about the cases where there is a difference? Consider the following setup as it relates to the part heights of the upper and lower vise, as shown already for the three hole example: 

The difference may be small, but for precision work it WORK OFFSET ADJUSTMENTS is absolutely necessary to have two offsets available. 

If one or more work offsets have been set properly at 

Settings for the 

| 01 G54 and G55 work 

654 02 655 the beginning, there will not be any need to make adjust 

X -615.375 

offsets may be sim 

X -614.859 ments once the machining has started. For various rea 

Y -304.540 

ilar to this: 

Y 441.363 

sons, this ideal situation may not always be the case. One 

z 0.000 z 0.000 

common example is to use an imperial example on a metric job or vice versa. 

The dimensions show the part in the lower vise is 0.298 mm higher than the part located in the upper vise. How can we tell whether the part is higher or lower? The answer is in the setting amount itself. 

During touch-off, the tool tip has to travel a certain distance required to touch the Zo face. For long tools, this distance is short, while for short tools the distance will be longer. In the example, the travel of 275.329 is shorter than the travel of 275.627, therefore the part lo cated in the lower vise is slightly higher. 

This explanation may be interesting by itself, but it is critical for understanding the necessary offset adjust ment. Let’s evaluate the situation and possibilities. 

Both vises contain the same part I The same tool is used to move between vises i Upper vise uses G54 work offset 

Lower vise uses G55 work offset I Both work offsets in X and Y are set correctly 

Both work offsets have Z-setting as 20.000 

Program Entry 

In the program, both offsets have to be used in order to reach the proper hole: 

(H1 – UPPER VISE) 

(CLEARANCE) 

G90 G54 GOO X10.0 78.0 22.0 <… drill Hi …> 22.0 G55 GOO X10.0 78.0 <… drill H1 … > 

(CLEARANCE) (H1 – LORER VISE) 

Mixing Units of Measurement 

In North America, use of the metric system in manu facturing has progressed very significantly in last few years, but the imperial units are still the norm in many companies. In such situations, it is not unusual to pro gram a metric job and use imperial tools for machining, including the shaft type edge finder. For example, a 0 0.200 inch edge finder tip is used to set metric work offset. In such cases, the edge finder radius has to be converted to metric units, base on 1.0″ = 25.4 mm: Ø0.2 mm / 2 = R0.1 inch 0.1 inch x 25.4 = R2,54 

When you consider the very low cost of edge finders, it makes sense to purchase a metric edge finder and a suitable collet and use them for all metric jobs. Any con version creates a possibility of a error. Even a small error will have serious consequences. 

As an example used for compari- | 011 G54 son, first consider the initial XY 

X -615.375 

measurements to the part edge using 

Y -304.540 

metric edge finder with 6 mm tip: 

z 0.000 

Note that the programmer calls for the same hole H1 location, but within two different work offsets. 

In practice, the program will use a suitable fixed cycle. The following example is a complete program for spot drilling the three holes in two vises: 

-275.627 

G54 

PART – UPPER VISE 

N1 G21 N2 G17 G40 G80 T01 N3 M06 

= UPPER VISE ==) N4 G90 G54 GO0 x10.0 78.0 51100 M03 TO2 N5 G43 72.0 401 MO8 N6 G99 G82 R2.0 Z-3.3 P200 €175.0 N7 X25.0 Y22.0 NX X40.0 Y15.0 (=== LOWER VISE ===) N9 G55 X10.0 Y8.0 N10 X25.0 Y22.0 N11 X40.0 Y15.0 N12 G80 G54 225.0 MO9 N13 G28 225.0 M05 N14 M01 

(H1) 

(82) 

(83) 

Based on this evaluation, what available options are there? The most obvious answer may be to have a differ ent tool length offset for each part (vise). This solution would work well, except for a major problem in many cases. The more vises used for the setup and the more tools programmed for each part mean many more tool length offsets will be required in the program. Not only the setup time will be greatly increased in such a situa tion, but – for some complex jobs – you may even run out of the available tool length offsets. 

The solution is much simpler than the option just pre sented – change the Z-setting of the appropriate work offset. In our case, the G55 Z-setting will not be zero but will contain the amount of the calculated difference, which is 0.298 mm. 

For the operator, the difference itself is not enough, as the current Z-setting of G55 has to be adjusted UP or DOWN – in other words do we add 0.298 mm or take that amount away from the current setting? 

In the example, the G55 lower vise 01 G54 02 G55 part is higher than X-615.375 X -614.859 the G54 part, there 

Y -304.540Y -441.363 

fore the work offset 

Z 0.000 Z 0.298 

has to be increased (positive). 

The distance-to-go will be adjusted – that is shortened – by the amount of 0.298 mm for the part within the G55 work offset (lower vise). 

(81) 

(H2) 

(83) 

G55 

-275.329 

PART – LOWER VISE 

How would this setting change if an imperial edge finder with $0.2 inch tip were used? It may come as a surprise, but the offset will not change at all. The reason? 

If the setup is identical to the one described and only the edge finder is different, it is the initial XY position of the spindle center line that will be different, not the re sulting work offset – after all, the part has not moved: 

Edge finder radius = 3 mm Edge finder center measured in X = -618.385 Edge finder center measured in Y = -307.540 Now for the imperial edge finder: Edge finder radius = 0.1 Inches = 2.54 mm Edge finder center measured in X = -617.915 Edge finder center measured in Y = -307.080 

There is a difference of 0.46 mm between the radius of the metric edge finder and the imperial edge finder. 

Note that the program does not change to GOO in block N9. rapid motion command GOO would cancel the fixed cycle, which is not the intended outcome. Rapid motion between holes is the integral part of all fixed cycles. Also note block N12 – the initial G54 work offset has been reinstated. The following two tools for drilling and tapping (not shown) will have virtually the same format, with the exception of the fixed cycle used and related machining conditions. For more details, see page 127. 

Programming method using a subprogram could also be used, but it would have no influence on the subject of two work offsets discussed here. 

There is only a very small difference between the heights of the parts located in two vises: 

Upper vise: -275.627 I Lower vise: -275.329 

Difference: 275.627 – 275.329 = 0.298 mm 

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