gray dot on gear, what exactly is it?

[PeterKing]

Hi,

I have asked a gear manufacture to make me some gear out of 7075 T6 with hard coating 13um. Please check on the attachment. I found there are lots of gray dots on both sides on the gear. The teeth contact area are prefect, no dot at all. What are those gray dots? What should I tell the manufacture on what to do in order to remove those dot?

Also, I want a darker brown color on the hard coating. How should I express what I want in more technical term? Does the darker color mean thickness and harder coating?
thanks!!

 

[PeterKing]

Is the 7075 T6 already hardened?

 

[tbuelna]

I'm assuming the 13um (.0005") hard coating you specified on your aluminum gear is hard anodize. To be specific, you should have specified a MIL-A_8625, type III, class 2 (dyed).

Of course, even cl. 2 hard anodize color results are kind of "iffy". The final color depends a lot on the alloy used. The hard anodize finishes I've seen have ranged from olive green to medium brown. The spots, or mottling, you see are pretty typical results from hard anodize. I don't think anyone ever gets consistent color results with hard anodize. Mechanical properties are usually more important than aesthetics with hard anodize.

 

[hydtools]

For uniform color on hard anodized 7075-T6 we would specify a final dye finish. Otherwise the parts are mottled, spotted.

Ted

 

[PeterKing]

tbuelna: Thank you for your reply. I'm a grasshopper. I want to know what is MIL-A_8625, type III, class 2 (dyed)? What really does it do to the gear? Sorry for my lack of knowledge.

Hydtools: Thanks for your reply too. If I ask for a dye finish. Will the color feeds out easily? I use this alu. gear with plastic gear.

I understand the gray spots are typical results for hard anodize. And I'm still wondering why there is a gray spots only on both sides of the gear while the face of teeth still looks prefect?

Thank you!

 

[dimjim]

7075T6 is not hardened unless it has been given a stretch
or cold temperature tempering.

7075T651 is hardened by a stretching operation.

 

[israelkk]

dimjim

I think you are wrong. 7075-T6 and T651 has has the same strength and both are aged and therefore heat treated to their maximum strength. The T means thermalloy heat treated. The T651 was stretched to for straightening but this is not a cold drawn stretch to make it stronger. Here is the explanation from MIL-HDBK-5J.

T6 - solution heat-treated and artificially aged.
Applies to products which are not cold worked
after solution heat-treatment or in which the
effect of cold work in flattening or straightening
may not be recognized in mechanical property

T_51 Applies to plate and rolled or cold-finished
rod and bar when stretched the indicated
amounts after solution heat-treatment or
after cooling from an elevated temperature
shaping process. The products receive no
further straightening after stretching.

The reason the spots are only on the sides is probably due the grain orientation of the alloy.

 

[PeterKing]

Thank you for the information.

If I ask for a dye finish. Will the color feeds out easily? I use this alu. gear with plastic gear.

 

[hydtools]

Green seems to work well. Black, if not done well, can be spotty.

Ted

 

[PeterKing]

Thanks very everyone. I know what should do now thanks