A very common concern with any boring bar is that the bar sometimes will not repeat on bored size. This can be caused by one a few different things, or a combination of more than one.

You set your mike and bore a hole and find the hole is oversized, you reset the mike and the next hole is then undersized, obviously there is something that is not right.

First thing we would look for are scores or scratches in the micrometer face. If the mike is damaged, the bar will not repeat.The mike should be returned to KW and the face reground and repaired.

Micrometer stem bend, will cause the same problems depending on the amount of bend and the amount of stock to be removed. A quick way to determine if the stem is bent is to insert a feeler gauge (.002) between the face of the mike and the tool bit, then tighten the bit. The feeler gauge should stay in place, and if you slowly rotate the mike in the boring head, the tension in the feeler gauge should not increase or decrease. If it does, the stem is bent and requires replacement.

Another thing to check is if the micrometer not locating into the boring head correctly. The stem of the mike has a V-groove in it which works with a spring loaded ball bearing to insure correct position of the mike into the head every time. If the groove is worn or the ball is out of adjustment, the bar will not repeat on size. The micrometer should "snap" into the head and when slight rearward pressure is applied, the mike should snap back into the boring head. If it does not the ball position will need to be adjusted. Contact Tech Services for assistance in this case.

If you tighten the tool bit and the tool bit moves away from the feeler gauge, and the feeler becomes loose, there is a different problem. Either the tool gib, tool holder or boring head is worn. You can examine the tool holder quite easily by simply checking the angle where the gib plate contacts the tool holder for visible wear and also feel it for any step. If worn, it will require replacement. You will need to remove the gib from the boring head to examine it for wear also, if the tool holder is worn, it is likely the gib will be worn also.


The boring head can also be worn, and cause the same problem as the tool gib or holder. If the boring head is worn, then the head will need to be replaced. Rebuilding a boring head is only something that the factory can do, and all heads may not be repairable. A combination of two or more of these concerns may be found on the same machine. These findings are normally on very old or high service units.

These are but a few possible causes for potential boring errors in bore size.

As always please contact KW Tech services for further help if required.