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A skilled CNC programmer

The example will show the offset change in its sim- plest form – wear offset 04 is set to zero. 

G74 Cycle 

Of the two cycles, this one is used rarely for grooving. It is useful for roughing wide grooves oriented along the Z-axis. The format for a two-block cycle is the same for drilling and grooving – the explanation is adapted to grooving: 

G74 R.. G74 X.. 2.. B.. Q.. R.. F.. 

Example 1 – Groove width needs additional 0.015 mm: The offsets are set as expected: 

From all the dimensional possibilities and various ex amples, this is the rarest but also the toughest one in two areas. First, the programmer has to do a really good job to apply the idea, then the CNC operator has to apply the idea in practice. One without the other will present only problems, not solutions. 

For a skilled CNC programmer it is a relatively simple task to program the necessary features. The burden of success is on the CNC operator, providing the program itself is correct. 










Program Requirements – Summary 

When the programmer encounters a drawing that has a groove dimensioned with tolerances, he or she will evaluate the supplied dimensions and evaluate the spec- ified tolerances. The result of this initial evaluation is how the available offsets are utilized. Understanding the program intentions is always a must on the part of the CNC operator. 

A skilled CNC programmer will realize very soon in the program development that to control the groove po sition only can be done with a single offset. The same skilled programmer will also realize that to control the groove width only can also be realized with a single, but different offset. As the wear offset for groove position is always required, the challenge for both programmers and operators is to combine them together. Combining the offset that controls the groove position and the offset that controls the groove width cannot be done with a sin gle offset. 

The solution is to incorporate TWO unique offsets for the single grooving tool in the program. 



In the next section, both groove position and groove width will be combined. 

where … R = Return amount (retract clearance between cuts) 

(first block only) X = Start diameter of the groove z = Final groove depth P = Movement amount in X direction (without sign) 

(equivalent to the width of cut) Q = Depth of cut in Z direction (without sign) 

(depth of each cut) R = Relief amount of the tool at the cutting bottom 

(X-axis clearance after each peck) F = Cutting feedrate 

In essence, this drilling cycle can be used to rough out a groove located on the face of the part, Subsequent fin ishing will be required. Both applications are the respon sibility of the CNC programmer. 


All the discussion here was about the wear offset and how to change it for a particular objective. What about the GEOMETRY offset? 

In most work, once the tool is set and its geometry off set established and entered into the control system, there is no need to make any changes to it. CNC lathes have settings separate for the GEOMETRY and WEAR offsets, and all adjustments should always be done on the WEAR offset screen. 



Under the multiple repetitive cycles in Fanuc manual you will find two that relate to grooving, although they are described as drilling cycles: 

– NOTE – This section specifically addresses offset 

adjustments to control both the groove POSITION and its WIDTH This particular section does not really bring anything new except a couple of examples. If you understand the wear offset adjustments from the last two sections, you will understand this section as well. Take the last exam ple requirement: 

Change of Tool Width 

This subject has already been raised – see the program example on page 202. The only remaining question is how the change of tool width affects the geometry offset. 



End face peck drilling cycle Outer / Inner diameter peck drilling cycle 

Geometry offset measured from the left reference point 

Example 1 – Groove width needs additional 0.015 mm: Although the change requirement is the same, this time the wear offset 04 contains settings related to the groove position – these settings have to be respected: 

Geometry and grooving tool width 

G75 Cycle 

Fanuc lathe controls offer a similar cycle listed under the multiple repetitive cycles section in the manuals – the G75 cycle. This cycle is designed for grooves that also require an interrupted cut, for example, those machined in tough materials, deep grooves, or in any other type of machining where the chip breaking will improve the ma chining process. The G75 cycle can also be used for cut ting multiple grooves that share the same characteristics. While this cycle is not very suitable for grooves of high precision and surface finish, it can be used very success fully for wide groove roughing in X-direction. There is no stock allowance in this cycle, so the roughing param eters must account for that. This cycle is the direct oppo site of the G74 cycle, described in the previous section for a two-block application only. 

The G75 cycle also can be programmed in two for mats, depending on the control unit. As this cycle is more practical than the G74 cycle, both one-block and two-block program formats will be presented. 



IX-axis No. 




By Fanuc definitions only, there are no grooving cy cles available for CNC lathes. On the other hand, if you look carefully what parameters each cycle allows, either cycle can be used for grooving as well, not just for drill ing. For details, consult Fanuc documentation. This sec- tion only covers basic descriptions, as these are not primary cycles for majority of grooving. These descrip tions are important to the CNC operators, as they allow correct interpretation of each cycle in the program, and allow informed changes, if necessary. 

In either cycle description, there is the word ‘peck’. That means the cut can be an interrupted cut. Keep in mind that these cycles are basically drilling cycles, not intended for precision grooving, but may be very useful for roughing grooves, particularly those are fairly wide and require a number of steps the grooving tool has to make. 






This is an area where changing wear offsets needs good understanding of the basic concepts and concentra- tion while making the change. It’s all in the numbers. 

Looking at the illustration, it should not affect it at all, because of the tool design, although minor adjustments may be necessary because of tolerances. This is also one major advantage of setting the command point to the left side of the grooving tool, but check the tool anyway. 

CNC Control Setup for Milling and Turning GROOVING ON LATHES 

For a two-block format, the program will change only slightly. This time a basic drawing will be used – input data are also important: 


For the older 10/11/15 Fanuc controls, use the one- block format: G75 X.. 2.. I.. K.. D.. F.. 

where … 

X = Final groove diameter z = Z-position of the groove 

(last groove for multiple grooves) | – Depth of each cut (positive value) K = Distance between grooves (positive value) 

(for multiple grooves only) D = Relief amount at the end of cut F = Cutting feedrate in selected units 


0.5 mm stock 


Spindle speed can also be programmed in the same block. 

A two-block format is used for recent controls, such as 0i/16/18i/20i/211/30i/311/32i Fanuc systems: 

N31 T0500 


N32 G96 S175 MO3 N33 G00 X52.0 Z-34.5 T0505 MOS N34 G75 R0.25 N35 G75 X37.0 217.5 P4000 21000 RO.1 F0.15 N36 GOO X100.0 Z200.0 T0500 N37 M01 

G75 R… G75 X.. z.. P.. Q.. R.. F.. 

where … R = Clearance for each cut (return amount) 

(first block only) X = Final groove diameter z = Z-position of the groove 

(last groove for multiple grooves) P = Depth of each cut (positive value) Q = Distance between grooves (positive value) 

(for multiple grooves only) R = Relief amount at the end of cut (second block) F = Cutting feedrate in selected units 

Also note that the G75 command has to be repeated in both blocks. This is consistent with other multiple repet itive cycles. Addresses P and do not take a decimal point and must be input in accordance of minimum in crement rule for the selected units (0.001 mm or 0.0001 inch). In the example P4000 is 4.000 mm and Q1000 is 1.000 mm. The example groove will leave 0.5 mm stock on both walls and the bottom. Groove finishing is re quired and should be included in the program 

The main difference between the two formats is that the relief amount for each cut can only be programmed in the two-block format. In the one-block format, this amount is set by a system parameter. 

Both formats do support programming of the relief at the bottom of the groove. 

A program sample for a one-block input G75 cycle – drawing is not necessary, just the format and the mean ing of individual data. This is a sample for a single groove. As the example represents only a single groove, note that there is no Z-location and there is no distance between grooves. 

N11 T0100 N12 G96 S175 MO3 N13 GOO X28.0 Z-11.0 T0202 MO8 N14 G75 X20.0 11.0 F0.15 N15 GOO X100.0 Z200.0 T0200 N16 M01 


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