X25.0 (H2) Current location: x10.0 (H1) Difference = Distance-to-Go = 25.0 – 10.0 = 15.0
this type of edge finder is that there is no need to adjust the distance displayed on the control screen. The display is the work offset for the measured axis.
Their purpose of either device is to align the spindle centerline with an part edge or a corner. In small fine ad justments, the operator moves the XY position of the indicator, until it runs true.
Whether using the single edge finder of the corner finder, both are precision ground devices and the part edge can be indicated with high degree of accuracy, without any adjustments. Keep in mind that in order to get correct distance measured, you have to start from machine zero and axis position set to zero as well.
All four device groups are generally used to establish only the XOYO part origin (part zero point). Zero setting in the Z-axis uses a different method, described in the next chapter. Establishing the XY program zero using a probing device is not a common practice and requires a special macro program – this method is outside the topics covered in this book. The first three methods are used quite frequently, mainly by preference.
Y22.0 (H1) Current location: Y8.0 (H2) Difference = Distance-to-Go = 22.0 -8.0 = 14.0
CNC system will use the same calculation to find the distance between both holes. Some math formulas may accept a lot of data, but the core calculation is never changed.
Ball type wigglers used in manual machining are not recommended for precision work
Magnetic edge or corner finders do not require
additional calculations during setup
Distance-To-Go calculation (total distance)
is used for all available offsets
In addition, using a dial test indicator with these two magnetic finders, you can also check the squareness of the setup. Both types work well on metal surfaces only and should not be used for plastic or other non-magnetic materials.
Understanding the concept of distance-to-go is an important aspect of understanding all types of offsets.
Face with built-in magnet
Before discussing practical use of an edge finder (regardless of its type), it is important to understand the concept of what Fanuc controls call Distance-To-Go.
Many CNC systems use distance-to-go as a description for an axis motion in progress (some controls call this motion increment or total travel distance). When an axis motion is active, the control shows the exact length of the remaining motion needed to reach programmed target location.
Follow the following two blocks, based on the motion between the first and second hole of the drawing provided at the chapter beginning:
SHAFT TYPE EDGE FINDERS
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.
USING EDGE FINDERS
For most jobs, a revolving mechanical edge finder is the most suitable device. It is inexpensive even for high end brands and very accurate for most work. The fixed main body is mounted in a collet, and the shaft end is internally connected, using a strong spring. Electronic edge finders are even more accurate but cost a lot more. Also, they can only be used on conductive materials only. Although only marginally similar, these two devices work on the principle of physically touching the part edge that happens to be a zero location.
Modern technology also provides laser edge for virtually any CNC machine. This special method is also worth further study. The last option, using magnetic edge finders or comer finders, offers an excellent alternative in many situations.
Two most common edge finders are single end with the edge finder only and a double end type (described later) that also includes a conical center finder.
Edge finders are used to locate an edge of the part in relation to the spindle centerline. They are handy for parts that have straight edges, such as shoulders, slots and grooves.
Magnetic Center Finder
Another type of a similar device also requires the use of a dial test indicator. It is called magnetic center finder. As the name suggests, this device will find the corner of a part that will become the part zero. Both axes are measured simultaneously and no adjustments are required.
Regardless of the actual fixture used, one setup condition is absolutely essential:
G90 GOO X10.0 78.0
(H2) In the first block (for H1), the absolute tool position is at X10.0 Y8.0. The following block (for H2) contains a tool motion to the next absolute position of X25.0 Y22.0 (H2). As both positions are in G90 (absolute mode), they are measured from part zero. The control system has to calculate how much the actual motion between the two holes will be for each axis. To complete such calculation, a suitable mathematical formula has to be built into the CNC software.
Using basic math knowledge, you should see that the actual tool motion length is the difference between two absolute locations – target and current tool location:
All parts of the batch must be located at exactly the same fixture position
Magnetic Single Edge Finder
A very common type of an edge finder is called a magnetic single edge finder. As its name suggests, this device is attached to the metal part with built-in magnet – see illustration in the next column.
In order to find an edge, a dial test indicator is used. While it is rotated by hand in the free moving spindle, the axis of measurement is adjusted by the handle, until the reading at both inner walls is identical. Each edge must be measured separately. The main advantage of
A vise has to be mounted securely to the machine table and its jaws have to be parallel with an axis motion.
Faces with built-in magnet
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. Typ 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:
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:
Using a mechanical shaft type edge finder is divided into three general steps:
Step 1 Eccentric rotation of the edge finder (rotation) Step 2 Concentricity at the part edge (centering) Step 3 Shift from the center (often called a ‘kick’)
At this point (item 10), the shaft diameter is in contact with the part edge at all times, and the Ø 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
-615.375 (G54 XI
One of the most common errors in setup is to forget adjusting edge finder diameter to radius
Edge finder distance as measured External
Internal LONG + SHORT -R LONG +R SHORT -R SHORT
LONG LONG +R SHORT -R SHORT -R LONG +R SHORT R LONG LONG +R SHORT SHORT LR LONG + R
-304.540 (G54 Y)
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 Ø 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):
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.
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:
– MACHINE WORK AREA
Work offset registry has to be set accordingly:
NEGATIVE READOUT + RADIUS ER
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.
X 0.000 Y 0.000
Z 0.000 01 G54 0265503456 X-615.375 || X 0.000 || X 0.000 Y-304.540 IY 0.000 IY 0.000
0.000 I|Z| 0.000 || Z 0.000 04657 05 G58 06G59 X 0.000 X 0.000 | X 0.000 Y 0.000 IY 0.000 || Y 0.000 Z 0.000 | Z 0.000 || Z 0.000
00 PART 0
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:
• Edge finder radius = 3 mm
Edge finder center measured in X = -618.385 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.
| Z |
01 Make sure X and Y axes show 0.000 (0.0000) position O2 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)
The next few setup examples do not refer to a particular drawing, but show calculations for other possible settings that may exist.
Part Work offset | Edge finder zero measurement tip radius R