This block contains a boring bar motion from the completed diameter of 30 mm, to the clearance diameter of 28 mm. After that, the boring bar returns automatically to the start point. Note the situation carefully – there are three consecutive motions:
• Ø30 mm cutting
… Z motion NEGATIVE
12 mm run-off
… X motion NEGATIVE Return back to start point … Z motion POSITIVE
Graphically, it looks like this:
In the example, the clearances are at the start and end of each contour. Both start points in block N9 (X81.0) and in block N24 (X79.0) have been programmed cor- rectly, based on the 22.5 position.
The actual tool motion from block N14 to N15 is pro grammed as incremental distance of 05.0, or 2.5 mm per side. This amount will guarantee that the chamfer will be completed. Keep in mind that the tool nose ra dius offset is in effect.
For the internal cut, there is a major problem in the program – if you run this program as is, you will never bore the part. The control system will issue an error mes sage, informing you about a CRC (cutter radius compen sation) error. This error means that the tool radius cannot fit into the space provided.
This error is quite common, and even some experi enced programmers may make it here and there. It often happens when a large boring bar is used for a hole that is only marginally larger. The reason behind the error is that the programmer is afraid of contact between the bor ing bar and the part on the negative side of the centerline.
Back to the above example. The existing hole in the part is drilled to Ø28 mm, the last bore is Ø30 mm, and the boring bar has Ø25 mm shank. Also to be considered is the minimum boring diameter for the bar. Let’s use this bar to illustrate:
The program would work, IF there were no tool nose radius compensation (cutter radius offset). This offset is necessary, and with it in effect, the program will not work and an alarm at the control will be the result.
Consider the same pictorial representation with the tool nose radius added:
PROGRAM FEATURES TO WATCH
When programming or optimizing the G71 and G72 lathe cycles, some general comments can be useful: 1 P..is the block number where the contour starts
…in second block for a two-block format 2 Q..Is the block number where the contour ends
…in second block for a two-block format 3 Watch the P and Q numbers as they relate
to existing block numbers 4 Change of block numbers, such as re-sequencing,
could influence the P and Q numbers 5 Also make sure that no block number
is duplicated in the program 6 Feedrates programmed between P and Q blocks
are for finishing cycle G70 only 7 Start point for the cycle is defined as the last XZ
coordinate before the G71/G72 is called 8 For safety reasons, use the same start point
for roughing with G71/672 and finishing with G70 9 Cycle starts and ends at the initial start point,
and the return to this point is automatic
Do not move to the start point inside of P-Q block range 10 Start point in the X-axis determines the actual
depth of the first rough pass in G71 or width of cut in G72 11 For external cutting, increasing the start point diameter
will decrease the actual depth of the first cut, decreasing the start point diameter will increase
the actual depth of the first cut 12 for internal cutting, increasing the start point diameter
will increase the actual depth of the first cut, decreasing the start point diameter will decrease
the actual depth of the first cut 13 G41 and G42 tool nose radius offset must be started
before the cycle is called, and must be cancelled by G40 after the cycle is completed
Do not use G41 or G42 Inside of P-Q range 14 Clearances, such as run-off or change in direction, should
always be greater than double the tool nose radius 15 Subprogram cannot be called within the P-Q range 16 Understand Type I and Type Il cycle for G71 (page 187)
Start Point Location
The location of the start point for turning and boring cycles is more than just a convenient and safe position. If you find that the first depth of cut is too light or too heavy – or the tool does not remove any material at all – look at the start point coordinates. In the example, the start point for the X-axis is programmed as X140.0. As the stock is Ø135 mm, the 2.5 mm (0.1 inches) clear ance seems reasonable. However, the result will be that the first cut will be in the air – no material will be re moved. Here is why:
When the tool reaches the programmed start point lo cation of X140.0 22.5, the next block is the cycle call. In the cycle are two parameters that will force the tool to move out of its current position – the first one is the stock amount. In the program, the stock on diameter is pro grammed as 2 mm, The tool immediately moves 2 mm on diameter and now is at X142.0. The Z-location will be adjusted as well, but that is not important for the depth. In addition, the tool wil also move by the amount of wear offset, which we assume is set to zero.
From this shifted position of x142.0, the second cy cle parameter comes into effect – the depth of cut. In the program, the actual depth is 3 mm (per side), so the tool moves back down by 6 mm, to the new start position of X136.0. Now the first cut will start. The problem is that the stock is only $135 mm, so no cutting will take place for the first cut. When the start point in the program is changed – say to X135.0 – the actual diameter of the first cut will be:
135.0+ 2.0+0.0 – (2 x 3) = 131.0
The result is (135 – 131) / 2 = 2 mm physical depth of cut. Still less than the programmed depth, but at least ac ceptable. You can optimize the X-start location to con trol the first depth of cut, to get ‘under the skin’ of some cast or forged materials.
For the internal cut, the calculation is the same, in the opposite direction. The programmed X-start point is X24.0, the hole is Ø28 mm, depth of cut is 2.5 and stock is 2 mm on diameter. The first cut diameter will be: 24 – 2 +0.0 + (2 x 2.5) = 27.0
Again, no first cut. Changing the start point to X28.0 will make the actual depth of first cut 1.5 mm.
Unfortunately, these are common errors in program ming. A knowledgeable CNC operator will either catch such errors when studying the program, or will be able to change the program after discovering the error during part setup and program proving.
10: 1 Scale The above image shows the position of the tool, actu ally its radius, at the end of the bore, before the retract clearance motion. The cutter radius offset is in effect, and programmed to the left of the contour, as G41. The control always ‘looks ahead’ and calculates the next tool position. In this case, the radius needs to be to the left of the return motion – the motion back to the start point. For that, the new tool location would be as shown next;
The ‘F’ dimension is a standard tool catalogue dimen sion indicating the distance between boring bar center line and the tip of a gage insert radius. The gage radius is also defined in the tooling catalogue. Also in the cata logue will be a minimum bore diameter, for example, we will use Ø27 mm. Also note the tool nose radius – it is a common size of R0.8 mm (R0.0313 inch).
In the program, the last block for boring is block N31: N31 0-2.0
The X-motion will be positive, in order to position the tool properly. That results in overcutting, which the con trol system does not allow – hence the alarm.
Based on these observations, how do they apply to the part program as received in the shop? The items 1 to 9 have been satisfied, and there is nothing to consider in terms of item 13. Items 10 to 12 are all related and should be looked at in more detail. The same applies to item 14. First, let’s evaluate the start point location.
CNC Control Setup for Milling and Turning TURNING AND BORING
The solution? The clearance retract in block N31 must be increased to accommodate the double tool radius plus some clearance. The current tool nose radius is R0.8 mm, the full circle is $1.6 mm per side, or 3.2 mm on diameter. That does not provide any clearance. Chang ing the U-2.0 to U-4.0, for example, would work well for this example, but there is another consideration.
The nose radius R0.8 (0.0313 inch) is one of three most common radiuses used for turning and boring: IRO.4 mm = 1/64 = 0.0156 inches
RO.8 mm = 2/64 = 0.0313 inches R1.2 mm = 3/64 = 0.0469 inches
Here is the tool change clearance for tool T01: N17 G40 GOO X300.0 250.0 T0100 (INDEX POS)
And here is the tool change clearance for tool T02: N33 G40 GOO X250.0 250.0 T0200 (INDEX POS)
Both are programmed as 50 mm (2 inches), which seems reasonable. When you consider the tool tip clear ance in relationship to the actual overhang of each tool, the clearance for T01 may need to be evaluated. The tool setup will be close to these dimensions:
The program will be more flexible, if the clearance is large enough to accommodate the R1.2 (0.0469) nose ra dius. In that case, if the insert is changed to a larger one, no change in the program will be necessary:
The corrected block N31 will be:
N31 0-5.0 (X25.0)
With the physical retract motion of only 0.9 mm per side, there is no danger that the boring will collide with the hole surface.
The key concern here is that when the tool T01 in dexes at only 50 mm off the front face, the boring bar tip will be behind the part by 34 mm (90 – 6 – 50 = 34). In order to move the tool to a position where the boring bar tip will be on the positive side of the front face (20), the clearance programmed in block N17 should be in creased. There is no need for detailed calculation, as long as the clearance is greater than the boring bar over hang. In the example, z125.0 will be sufficient: N17 G40 GOO x300.2125.0 T0100 (INDEX POS)
Tool Change Clearance
Another kind of clearance relates to the tool change. When one tool is completed, the turret moves away, and is indexed to another tool station. In the example, the turning tool (T01) is replaced by a boring bar (T02).
The tool motion away from the part is relatively small, to avoid increasing the cycle time. Usually, the program mer may use 25 to 50 mm or 1 to 2 inches as a common clearance. While such clearance is sufficient, you have to watch the clearance when a short tool is followed by a long tool. This is particularly important when these tools are mounted next to each other in the turret. This is the case in the program example. When you evaluate the program as presented, you may discover a flaw.
There is no problem for indexing from TO2 back to T01 (for a new part), and the X-axis clearances are all reasonable – generally above the largest outside diameter of the part.
Note: In most CNC lathe applications, there is enough clearance between tools and the above discussion may not apply. However, due to the fact that safety is always a major concern, the slight increase may be justified.