aluminum surface pitting

[rmetzger]

We have found some surface pitting on a CNC turned aluminum part (7075-T6) due to the machining process. It was previously run on a high speed Hurco machine (without the problem) but recently changed to a older and slower CNC. Anyone experienced this before and have an idea or two to where to begin?

 

[israelkk]

Are you sure it is the slower CNC? Is the pitting found during machining or some time after machining? If the pitting was found some time after machining it is due to corrosion. 7075 is highly sensitive to corrosion, it shouldn't be left unprotected overnight for few days after machining. It sould be cleaned from all machining fluids, then dryed and oiled with a presevative oil immediatlely following the machining operation until it anodized, plated or painted.

If the pitting was found during machining it is probably a bad batch of material.

 

[rmetzger]

The difference between the two parts (as far as after machining prep and cleaning) comes down to essentially the manufacturing of the part. Now due to the imperfections of manufacturing something may have been missed, a process delayed, or one of the other multitude of things that keep manufacturing engineers busy that affected the finished parts - tis is an unknown as these samples came from the production floor rather than a lab. But out of the batch that was run all had this tiny (300um) scars that didn't exist on the previous run on the newer machine.

I will check on the preservation technique we are using and see if that could be a problem. Another thing of note - it that after cleaning these parts they are not subjected to any finishing process that would leave a protective coating (none specified on the print anyway). Older vesions were anodized but prior to my arrival that finishing process was halted for cost reasons. This may be my ammunition to bring it back if it ends up being a corrosion problem.

 

[israelkk]

We had a similar problem in the past and instructed the machinist to clean the part after machinig and oil them. Basicallt to treat the parts as they treat carbon steel after machining and before surface finishing. Leaving the 7075 in the as is condition after machinig will result in corrosion spots and in time even flakes.

 

[rmetzger]

Any recommendations on the proper timing and surface prep to use for 7075-t6 after machining. We don't do plating of surface finishing in house so I'm looking for a good preservative plan to protect the parts before they get finished (several days or weeks later).

thanks

 

[griffengm]

rmetzger,
These may seem elementary but are the two machines running identical tooling at identical parameters? Do new parts from each machine show any differences?

Griffy

 

[rmetzger]

Yea - different code due to language but the programmer "reused" the same tool paths. Same cutter size and tooling but I'm unsure about the surface feeds

 

[solidworkswell]

is there a chance that you old CNC has a worm tool holder. If so there will be radial play in the toolin bit may lead to some problems. But this type of problem, i always seen chatter never pitting.

 

[griffengm]

rmetzger,
My experience with aluminum (admittedly limited to a few cast and wrought alloys) was that some combinations of operating parameters had a tendency to "wipe" material across the surface. When a little oxidation occurred these areas often lifted and left a spot or area with a different appearance.
Could this be what you are experiencing?

Griffy

 

[ctopher]

also check the holding fixtures for mtl and wear.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site

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