Using magnetic edge finders requires only a precision dial test indicator attached to the face of a free spinning spindle. By rotating the spindle and moving XY axes po- sitions as required, the objective is to make the indicator run true in one or both axes, depending on which edge finder is used. At this point, the edge location or the cor- ner is known (based on spindle centerline) and can be entered as the work offset for current part setup.
Using much more common mechanical shaft type edge finder, the setup procedure is a bit more involved, and some additional calculations will also required.
The simple three hole part, introduced earlier in this chapter, will be used to illustrate the concept of using shaft type edge finder in detail. Metric edge finder with a 6 mm shaft diameter will be used for the example.
Prerequisites
Regardless of the actual fixture used, one setup condi- tion is absolutely essential:
A vise has to be mounted securely to the machine table and its jaws have to be parallel with an axis motion.
Magnetic edge or corner finders do not require additional calculations during setup
Distance-To-Go calculation (total distance) is used for all available offsets
All parts of the batch must be located at exactly the same fixture position
WORK OFFSET SETTINGS 79
CNC Control Setup for Milling and Turning
Individual Steps
Using a mechanical shaft type edge finder is divided into three general steps:
n Step 1 Eccentric rotation of the edge finder (rotation)
n Step 2 Concentricity at the part edge (centering)
n Step 3 Shift from the center (often called a ‘kick’) In Step 1, the edge finder is positioned close to the measured edge, but far enough so its bottom tip can be shifted off its center and rotate eccentrically, without ac- tually touching the part edge. A suitable depth from edge top is also required.
In Step 2, the operator uses the setup handle and gently moves the rotating edge finder closer and closer to the measured edge. At a certain point, the diameter of the edge finder tip will touch the part edge. As the operator continues the axis movement, the eccentricity of the tip will become smaller and smaller, until it disappears completely and the 6 mm tip is concentric with the spindle center line.
The actual measurement takes place between Step 2 and Step 3. By having the handle set to the smallest in- crement and moving one division, there will be a ‘kick’
- that is a common description when the edge finder tip will become eccentric again. By moving the handle back by the same amount, the ‘kick’ is removed and tip is con- centric again. At this point, the measurement is within about 12 microns (0.0005”).
As every CNC operator approaches part setup in dif- ferent ways, the following steps reflect the general sug- gestions present so far. Once the fixture (such as a machinist’s vise) is properly set and the edge finder is mounted in the spindle, the procedure begins from ma- chine zero (spindle is stationary at that time):
01 Make sure X and Y axes show 0.000 (0.0000) position 02 Work with one axis at a time
03 Manually throw the end tip off center
04 Move the edge finder close to the edge to measure 05 Start spindle rotation 800 r/min (or your preference) 06 Move 3 to 5 mm below the edge top face
07 Using setup handle, gently move towards the edge 08 When contact is established, eccentricity gets smaller 09 Move handle until the shaft runs continuously true 10 Wait for the 'kick'(see explanation above and below)
At this point (item 10), the shaft diameter is in contact with the part edge at all times, and the 6 mm shaft is exactly 3 mm (its radius) away from the measured edge.
Because of the natural resistance between the two sur- faces, the shaft will be thrown off its center - this 'kick' means the precise edge location has been established.
Write down the position measured for the selected axis, and repeat the procedure for the other axis. You should have two known locations, exactly measured. This pro- cedure takes a bit of practice, so it is best to try the first attempts with an experienced person to assist and guide.
Work Offset Calculations
Keep in mind that all measured dimensions are from machine zero to the spindle centerline, which means the actual edges (required for work offset settings) are still off by the actual tip radius (3 mm in the example). This radius amount must always be taken into consideration.
The illustration below shows into which direction the measured dimension should be adjusted, based on the part zero and the axis of measurement. Examples follow.
One of the most common errors in setup is to forget adjusting edge finder diameter to radius
X-
Y- Y-
Y- Y-
X- X- X-
X- Y-
Machine Zero
zero measurement tip radius R direction Part Work offset Edge finder R-adjustment
R
Positive Negative
PART
+R -R -R +R
+R -R +R -R
+R -R
Setup errors also happen even if the tip radius is ac- counted for. Common reason is that the tip radius was added rather than subtracted, or vice versa.
The last illustration identifies common setups for four different part zeros on vertical machining centers. Typi- cal X and Y work offsets are both measured from ma- chine zero into the negative direction, which requires both work offsets to be stored as negative amounts.
Think of the edge finder center as being either too far (long travel), or not far enough (short travel) from the edge measured. All eight possibilities are listed here:
For the final work offset, you either add or subtract the edge finder radius to the measured negative dimension.
The calculation is simple and always the same:
The most important part of the formula is to remember that we are always adding the radius, whether it is posi- tive or negative, to a readout that is always negative for the machine zero position as shown.
Part Zero at Lower Left Example 1:
Setting the part zero at the lower left corner is very common - it makes all XY part locations to be positive.
For this example, sample X and Y measurements will be used - keep in mind that actual numbers will be different for each setup:
n Edge finder radius = 3 mm
n Edge finder center measured in X = 618.385
n Edge finder center measured in Y = 307.540
Based on these dimensions, an example can be used.
The first one is for the part zero at lower left corner.
Example 1a If these are EXTERNAL measurements ...
The table shows both measurements as long, meaning the edge finder center is too far and a positive radius will be added to both measurements:
X axis edge = 618.385 + +3 = 618.385 + 3 = 615.385
= Work offset X is 615.385 Y axis edge = 307.540 + +3 = 307.540 + 3
= Work offset Y is 304.540 Work offset will be set to X-615.385 Y-304.540 This example may represent the settings for the three holes as per initial drawing:
Work offset registry has to be set accordingly:
The next few setup examples do not refer to a particu- lar drawing, but show calculations for other possible set- tings that may exist.
Part zero location
Axis Edge finder distance as measured
External Internal
Lower Left
X LONG + R SHORT - R
Y LONG + R SHORT - R
Lower Right
X SHORT - R LONG +R
Y LONG + R SHORT - R
Upper Right
X SHORT - R LONG + R
Y SHORT - R LONG + R
Upper Left
X LONG + R SHORT - R
Y SHORT - R LONG + R
NEGATIVE READOUT + RADIUS ±R
0
0
MACHINE ZERO
8
10
PARTZERO
MACHINE WORK AREA
-304.540 (G54 Y)
-615.375 (G54 X) -618.375 measured
-307.540 measured
R3
R3
0.000 0.000 0.000 X
Y Z
EXT 00
-615.375 -304.540 0.000 X
Y Z
G54 01
0.000 0.000 0.000 X
Y Z
G57 04
0.000 0.000 0.000 X
Y Z
G59 06
0.000 0.000 0.000 X
Y Z
G56 03
0.000 0.000 0.000 X
Y Z
G55 02
0.000 0.000 0.000 X
Y Z
G58 05
WORK OFFSET SETTINGS 81
CNC Control Setup for Milling and Turning
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 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:
n Edge finder radius = 3 mm
n Edge finder center measured in X = 600.000
n Edge finder center measured in Y = 300.000 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 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 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.
For reference, when adding or subtracting positive or negative numbers, the following table may be of help:
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 G00 X10.0 Y8.0 (H1)
... 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:
G90 G54 G00 X10.0 Y8.0 (H1)