G915 - Excess facing thickness

This tip will detail the programming for excess facing thicknesses, so that you can rework the front face of the part (guide bush operation using thread chaser 1 or 2) and also discuss the consequences. In fact, programming an excess facing thickness involves an end positioning fault of the unit at Z3. This article will provide a small tip to overcome this imperfection.
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G915 - Excess facing thickness

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Excess facing thickness and macro G915

This tip will detail the programming for excess facing thicknesses, so that you can rework the front face of the part (guide bush operation using thread chaser 1 or 2) and also discuss the consequences. In fact, programming an excess facing thickness involves an end positioning fault of the unit at Z3. This article will provide a small tip to overcome this imperfection.

Reminder :
Each DECO single-spindle machine has a model incorporating the programming of an excess facing thickness. To select it, simply choose option B of the model referred to as "Finishing the front face" during stage 2, when creating a new part using the assistant (see picture).

The excess facing thickness value is added to the length of the part (#3003). An additional original offset G54 on axis Z1 must be programmed prior to the first facing operation.

Example :
DECO 13a machine
Part length 50 mm, facing 0.5 mm:

Variable #3003: #3003= 50.5

G54 to be programmed: G54 Z1=-0.5

Problem associated with G915 :
As the facing operation usually precedes the end operations, we are faced with the following problem:

Positioning of the unit end at Z3 by G915 is distorted. As there is no excess facing thickness, the zero part (front face of the part) is now in a different position. Because the system does not know the value of the excess thickness removed, the machining length executed by the end unit (Z3) will be wrong by a value corresponding to this excess facing thickness. The length or depth of machining will now be too short.

Tip :
In order to overcome this problem, it will be necessary to program an additional original offset G54 on axis Z3 for the operation containing the macro G915. The value of this additional offset is the equivalent of that of the excess facing thickness. Hence, axis Z3 will be corrected and the machining operations will be accurate. The additional offset G54 will be programmed in the negative direction (G54 Z3=-...)

Programming :

Particularitey :
Additional offset must, of course, be added each time the macro G915 is called up.

Operation 1:6	Original offset G54 Z1=-0.5 for part facing
Operation 1:7	Part facing Z1=0
Operation 5:1	G915 + G54 Z3=-0.5

ISO operation code:
Operation 5:1:	Macro G915 + G54 Z3=-0.5
ISO code:	G915
	G54 Z3=-0.5

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