Expected Surface Rougness of a Zn Phosphate treatment on a machined steel bar.

[b_unruhly]

Hello all,

Not sure if this belongs here or in the chemical section--apologies if I put this in the wrong spot. Hoping to reach a coating expert here.

We have a machined shaft we are trying to produce to drawing specs--which call for a Ra (Microinch) value of 20 or under.

This is inclusive of coating.

Even through our pre-coated parts are measureing out between 5 & 8, the ZN phosphate brings us up to 80, and then there is a layer of Mos2 that is applied afterwards.

Does anyone have any tips for maintaining surface rougness on Zinc-Phophate pretreatment on a CNC turned shaft? What is typical? Do we have the wrong supplier?

Or secondary processes that can smooth this out, without damaging the protective layer of coating?

Thank you in advance for your valuable input

 

[WKTaylor]

RE ASME B46.1 Surface Texture (Surface Roughness, Waviness, and Lay)... [Units in microinches/Roughness Average]...

Generally speaking... as-is plating's... will roughly double the surface roughness of the base metal it is applied 'over'. IE...16-microinch RA [16-RA] machined/blasted base-finish will 'worsen' to ~32-RA as-plated-finish.

However, ZN Phosphate... which is generally considered a micro-thin ferrous alloy conversion coating... little have little/no surface roughness change, RE the base-metal machined/blasted finish. In service the finish may slightly improve due to natural dry-film build-up/bridging/lubricity... until it degrades [wear/erosion/abrasion, etc]... and the base metal is exposed to corrosive elements.

 

[Compositepro]

Burnishing could work to improve surface finish. The Mos2 would assist with burnishing.

 

[b_unruhly]

@Compositepro, would burnihing makes sense but how do you make sure that it does not remove the Zn phosphate?

 

[MintJulep]

Do we have the wrong supplier?

The supplier is giving you unsatisfactory product, and it's become your job to find an expert to solve their problem?

Yes, you have the wrong supplier.

The expert you seek works for the plating company that you want to use.

 

[Compositepro]

Burnishing does not remove material. But not all materials can be burnished. It is used to flatten high spots on a surface, at a microscopic scale.

 

[b_unruhly]

The supplier is giving you unsatisfactory product, and it's become your job to find an expert to solve their problem?

Yes, you have the wrong supplier.

The expert you seek works for the plating company that you want to use.

Unfortunatly it's not that simple when working in emerging manufacturing markets.

 

[MintJulep]

Unfortunatly it's not that simple

Well, that's the question that you asked.

Are the bath chemicals a commercial product, or a home brew?

The chemical manufacturer might have tech support.