More cut along Z axis in HMC - intermittent issue

Hello Sir

We have HMC, which is used for making 24 holes ( it is a combination of 4X6 holes) of same size on face of a casting component by using standard combination boring bar - in which initially a small hole, concentric with one of the big 4 holes, is made and then feed changes to make big hole. .

Cycle time is close to 30 minutes

Component is rested against 6 rest pads on Fixture, there is air seat check to ensure positive butting all along between job and fixture

Say 1 st component is alright and may be next component, depth of hole is found to be increased by upto 1mm scrapping the job

We have checked relevant CNC parameters for In-position, acceptable tolerance between commanded and actual position and compared with other similar machines for similar components - found same

Tool clamping force checked and found alright

Z axis repeatability in Auto cycle checked and found OK as per readings on Dial

Compared program with another similar machine - found no change

Tooling and cutting parameters are same between this and other similar machines

Now we have decided to check "Distance to Go" along Z axis and only then allow feed to start, to see if "Distance to go" itself is appearing more than required, which is unlikely in a semi closed loop CNC system

Request your advice