Best way to call out surface finish after machining vs after alodine coating 2

[efulm4444]

We have some 7075 AL parts that require a surface roughness of 16 uin Ra or better. It seems that the alodine coating can increase to Ra value of the surface roughness by about 6 uin. We plan to machine them to a 8 uin surface roughness to account for the coating process.

What's the best way to callout a post machining surface roughness and the final surface roughness?

 

[ctopher]

Are you outsourcing them, or in-house? If outsource, ask them about the best process.
If in-house, machine the best finish possible to get the best coating finish.

Chris, CSWP
SolidWorks
ctophers home

 

[3DDave]

"ROUGHNESS APPLIES AFTER COATING"

There isn't a requirement to mention the in-process surface roughness as the acceptance drawing is not a process control document. You might add a "REFERENCE: COATING CAN ADD 6 uin TO MACHINED SURFACE ROUGHNESS - PLAN ACCORDINGLY"

 

[drawoh]

efulm4444,

Alodine is nothing more than a chemical film, that incidentally, is not compliant with RoHS. Your surface is rough due to the etching used to clean the surface. This process is controllable. I agree strongly with 3DDave, above.

--
JHG

 

[CWB1]

Simple, callout the acceptable roughness post-machining on the as-machined print and separately callout the acceptable roughness on the as-coated print.

 

[3DDave]

Making them separate is an excellent way to start a finger-pointing battle between the machine shop and the plating shop with the procuring company a referee in a battle where they control neither side. Not saying you can't, just that it's an additional chance to miss the schedule as non-compliant parts arrive from the plater, who isn't able to machine replacements. Indeed, the most likely result is the plater will buff the surface of all the parts to ensure they meet the limit, rendering the allowable finish from the machine house useless and potentially putting the part out of dimensional tolerance.

Let the shop negotiate directly with who ever they subcontract to so both of them are motivated to work together by not separating the requirement. If they come back too rough to the machine shop, the shop can make new parts with a better finish.

 

[CheckerHater]

Just 2 cents: Specifying surface roughness before and after coating / plating is normal drafting practice:

https://files.engineering.com/getfi...-180b-4c1a-9c6e-5ad312105378&file=Capture.PNG

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

Just 2 cents: Specifying surface roughness before and after coating / plating is normal drafting practice:

That looks like a process drawing rather than a fabrication drawing. When I subcontract fabrication, I inspect whatever shows up on my loading dock. I don't know what went to the coating shop, and I don't care. Specify what your inspector should accept. Let the vendor figure out what to do. I have seen near mirror finishes on chemical filmed parts.

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JHG

 

[CWB1]

Forcing suppliers to subcontract is terrible business practice, many find it annoying, inefficient, and charge accordingly. Not giving your purchasing dept the ability to easily re-source each process is similarly terrible, expensive practice.

Not fully defining prints is terrible drafting and eventually results in bad parts being paid-for and possibly used by your employer. The machine shop needs a fully defined print including roughness of machined surfaces for inspection. The plating shop needs a fully defined print including roughness of coated surfaces, which surfaces specifically are to be coated, and coating thickness. Given all that there's no room for argument, each shop delivers the required parts. If suppliers are defining your parts they'll back you into their process, tolerance, and most importantly their price.

 

[drawoh]

CWB1,

On one or two occasions, I have specified masking of anodized finishes on parts. As a rule, for anodizing or chemical conversion coatings, I want it done all over the part. I don't need a separate drawing. On precision parts, I have seen machine shops fiddle their tolerances to account for the anodizing. I would rather have the fabricator talk to the finishing shop and find a way to ship me what I want. Short of virgin sacrifice, I don't care how they do it.

--
JHG