Aesthetic Surface Finish Callout (Black Delrin)

[meanderngineer]

Hello Everyone,

I was hoping to get some input on the surface finishes applied to parts for aesthetic purposes. I have been running into problems when ordering Black Delrin from shops as they meet specs but don't look very good. From what I gather, a surface call-out can look different depending on the method used (if I specify a 64 Ra surface it can look very different if that is achieved through grinding or milling.) My question is, for parts that you want to have a nice aesthetic look, without worrying about functionality, what call-out do you use? Additionally, is roughness not necessarily what makes a part aesthetically pleasing (should I be more concerned with the sizes of valley's / lay direction)?

Thanks in Advance,
Dan

 

[drawoh]

meanderngineer,

The first thing you need is a surface comparator, so that you know what the finishes you are specifying, look like. In my opinion, a 16[μ]in finish looks good. A 64[μ]in finish does not.

Can you grind Delrin?

Ask your fabricator what he can do.

--
JHG

 

[JNieman]

If you specify a specific surface roughness, you may be using a sledgehammer to drive a nail. I mean to say that if you take the roughness down to 16μin, it may still not look good enough for you, but going beyond that is going to make the part very costly to make.

In past days as a machinist, I've had customers complain about rough or ugly parts because they still could see the machining/tooling/cutter marks on the surfaces. It didn't matter that they were well under 32μin, well beyond their requirements. They didn't want to see tool marks on the bare metal (aluminum).

Perhaps what you should require is that "No cutter/tooling marks allowed on finished surfaces" and they can figure out how to make that happen. Whether it's with abrasives, different cutting strategy, different cutter insert geometry, or a post-machining process such as Scotch-Brite or sanding or something... let the suppliers find their best way.

Even parts with a very smooth surface roughness can still look "displeasing" to some due to manufacturing processes.

 

[MikeHalloran]

Specify 'NO TOOL MARKS' or similar as per JNieman.

To me, the ugliest feature of black (or natural) Delrin parts is the porous core that appears at the center of rounds and the midplane of plates. The porosity is unavoidable; it's related to Delrin being a condensation polymer, or something like that. The only solution is to buy grossly oversized stock, so you throw away the core, e.g. for a planar part that's ~1" thick, start with 2-1/2" plate and sawcut the blanks down the middle.





Mike Halloran
Pembroke Pines, FL, USA

 

[meanderngineer]

Thanks for the replies!

I will specify the "no tool marks", that seems like it should clear things up. Thanks for the additional info! I'll keep Delrin's porous nature in mind if I need to be particular about a part.

Much Appreciated!
Dan

 

[dgallup]

It sounds like you are talking about machined plastic parts so the above is correct. For molded plastic parts you can specify the surface finish of the mold itself. The Society of the Plastics Industry has a scale of mold surface finishes from A-1 (smoothest) to D-3 (roughest). There are also companies that specialize in mold surface textures to give a pebbled or grainy texture such as you see in a lot of automotive interiors.

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[Tenkan]

From what I gather, a surface call-out can look different depending on the method used (if I specify a 64 Ra surface it can look very different if that is achieved through grinding or milling.) My question is, for parts that you want to have a nice aesthetic look, without worrying about functionality, what call-out do you use?

if you know what method works best for you, you can specify the exact manufacturing method on the finish symbol or drawing notes. e.g.: 32µin grind, you can be as specific as you want in the drawing notes.

lightweight, cheap, strong... pick 2

 

[MadMango]

Short of having injection molded parts, you will most always see tooling marks on black Delrin. In the past I've had success with secondary flame polishing or "thermal flashing" to reduce or eliminate tooling marks.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[ctopher]

MadMango,
I have seen the same as you.

A good ref table: Link

Chris, CSWA
SolidWorks '15
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