An extract from the web. 

I was not able to correctly explain such a technical point in a foreign language...

This answer your question.

G40 revocates G41 or G42 modal functions.

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Tormach

G40, G41 AND G42 - CUTTER RADIUS COMPENSATION

Details:

To turn cutter radius compensation off, program: G40. It is OK to turn compensation off when it is already off.

Cutter radius compensation may be performed only if the XY-plane is active.

To turn cutter radius compensation on left (i.e., the cutter stays to the left of the programmed path when the tool radius is positive), program: G41 D~. To turn cutter radius compensation on right (i.e., the cutter stays to the right of the programmed path when the tool radius is positive), program: G42 D~. The D word is optional; if there is no D word, the radius of the tool currently in the spindle will be used. If used, the D number should normally be the slot number of the tool in the spindle, although this is not required. It is OK for the D number to be zero; a radius value of zero will be used.

G41 and G42 can be qualified by a P-word. This will override the value of the diameter of the tool (if any) given in the current tool table entry. It is an error if:

The D number is not an integer, is negative or is larger than the number of carousel slots;The XY-plane is not active;Cutter radius compensation is commanded to turn on when it is already on.

The behavior of the machining system when cutter radius compensation is ON is described in the chapter on Cutter Compensation. Notice the importance of programming valid entry and exit moves.

Note: The tool offsets must have been applied with a G43 H~ for compensation to work.

Maybe better.

http://cncprograming.blogspot.fr/2011/07/tool-nose-radius-compensation-g40-g41.html?m=1

Thank you Jean Francois.

I think Mr. Francois explanation about tool radius compensation in normal longitudinal, face and boring operation is complete. there are some experimental hints in CNC turning about tool radius compensation that you may know them are as follows:

1- Longitudinal turning (external) from tail-stock toward spindle needs G42,

2-Longitudinal turning (internal) from tail-stock toward spindle needs G41,

3- Face turning from up to down needs G41,

tool radius errors are usually happen when when the programmable cutting point is different from actual cutting point. It happens when you have free form cutting. in turning the example can be cutting of spherical workpiece.  since during spherical cutting, the programmable cutting point is constant but the actual cutting point changes with the spherical surface. 

I don"t know if the new generation of the turning machines can overcome this difficulty or not.

I know some experimental traditional solutions like considering stock allowance after roughing operation....