The authoritative public forum
  for Metal Finishing since 1989
-----

Anodized AL Boss chipping when a taptite screw is inserted

I have a an anodized AL enclosure. It has a boss for a Taptite (thread forming) screw. I am getting chips when the screw is inserted. I know that the hole size is correct and the driver speed is correct. I should not be getting any chipping with a Taptite. I think the anodized surface is chipping. Has anybody experienced this problem?

Michael Ward
engineering manger - Madison, Alabama
2007

---

I'm not totally sure I understand. Are you trying to insert a self-tapping screw into a hole thats been anodized? If thats the case, I would expect the anodize to chip quite easily inside the hole. If you are experiencing chipping on the perimeter of the boss, this could be due to the area around the hole expanding slightly when the screw is being inserted. The AL2O3 coating is much harder, and more brittle than the aluminum, and will probably not "give" like the aluminum substrate will.

Marc Green
anodizer - Boise, Idaho
2007

You will have some chipping of anodize anytime that you do a mechanical operation such as drilling or tapping. Your screw is also performing a tapping operation. One way to minimize the appearance is to very slightly countersink the surface at the hole.

James Watts
- Navarre, Florida
2007

---

The entire cutting edge of threads is converted all the way thru to brittle aluminum oxide. Naturally it breaks off readily with the minimum of pressure. Threads should not be anodized. Mask the threads and you will see no more chipping.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007