How smooth is smooth enough? Surface finish standards for body jewelry

A superficial proposition:

I believe that professionals and clients have a right to know the roughness of the surface finish of body jewelry from each manufacturer they choose to do business with. Roughness can be measured as Ra to the microinch or Metric: nanometer. (The final buffed finish for many polished surgical implants is 1µin to 2µin). This would alleviate a lot of confusion and allow a more frank discussion of the merits of different processes to arrive at a desired finish.

Smoother is better in our case, but how smooth is smooth enough?

  1. The body jewelry industry should establish an acceptable minimum standard for surface finish and passivation based on current science.
    1. I propose we aim for a Ra minimum of 1µin to 2µin or smoother for initial jewelry for healing rather than the subjective term “mirror finish”.
      Random testing of jewelry samples from Anatometal, Industrial Strength and NeoMetal have shown this to be possible, and samples from Glasswear Studios and Gorrilla Glass have shown that it can be exceeded with surfaces approaching <0.5µin.
    2. Jewelry manufacturers should help us understand what is achievable with their current practices.
  2. Jewelry manufacturers should specify the goal surface finish measurements of their body jewelry in RRq,or RRMS

Why does it matter?

A smoother surface adheres less to tissue during healing, and allows less bacterial colonization and inhibits corrosion. A lower Roughness measurement means a smoother and more regular surface (1µin to 2µin or better should be the goal)

How can I measure the surface finish of my jewelry?

A good magnifying loupe or microscope will help demonstrate the difference in some surface finishes to a lay person or customers, or at least photos taken with decent lighting and a quality macro photo lens, but a non-contact profilometer is required to take an accurate scientific measurement of body jewelry.

  • I’m really interested in what manufacturers QA can propose, as the goal of the polishing process depends on the method and the skill of the worker when it comes to hand polish.
  • I believe that a manufacturer should check the Ra frequently between batches, then establish what they consider an acceptable Ra and state publicly that they aim for a certain smoothness.
  • I would love to participate in a blind analysis of jewelry quality! We can get a third party lab to do the tests, and get contributions to cover it. Raquel and Jeff Martin and I proposed that in 1996, and started an APP committee, collected samples and eventually tested a few. It is past time to follow up with the jewelry review committee.

Applicable references for standards of surface finishing:

10. Techniques for Surface Texture Measurement
10.1 Surface profiles can be measured using both contact and non-contact methods. The former approach involves tracing the surface profile with a stylus that is in contact with the surface; a method unsuitable for materials that are soft or easily damaged. Non-contact methods are better suited to these materials. Non-contact methods include vertical scanning white light interferometers and confocal microscopes (2, 3, 4). — Quote from ASTM F2791

 

Table from ISO 1302: Surface texture measurements

Roughness values Ra

Roughness values Ra

Roughness

micrometers

microinches

Grade Numbers

50 2000 N12
25 1000 N11
12.5 500 N10
6.3 250 N9
3.2 125 N8
1.6 63 N7
0.8 32 N6
0.4 16 N5
0.2 8 N4
0.1 4 N3
0.05 2 N2
0.025 1 N1

In finish milling operations typical surface roughness levels vary from N6 to N9.
Surface roughness monitoring application based on artificial neural networks for ball end milling operations:

Surface roughness is the measure if the finer surface irregularities in the surface texture. These are the result of the manufacturing process employed to create the surface. Surface roughness Ra is rated as the arithmetic average deviation of the surface valleys and peaks expressed in micro inches or micro meters. ISO standards use the term CLA (Center Line Average). Both are interpreted identical.

The ability of a manufacturing operation to produce a specific surface roughness depends on many factors. For example, in end mill cutting, the final surface depends on the rotational speed of the end mill cutter, the velocity of the traverse, the rate of feed, the amount and type of lubrication at the point of cutting, and the mechanical properties of the piece being machined. A small change in any of the above factors can have a significant effect on the surface produced.
—From the Engineer’s Edge:
Materials roughness

SURFACE ROUGHNESS AVERAGE OBTAINABLE BY COMMON PRODUCTION METHODS

Surface Roughness Height Rating Microinches

Table 1: Comparison of surface finish obtainable by different finishing processes

S.No.

Finishing Process

Workpiece

Ra value (nm)

1

Grinding

-

25 – 6250

2

Honing

-

25 – 1500

3

Lapping

-

13 – 750

4

Abrasive Flow

Machining (AFM)

with SiC abrasives

Hardened steel

50

5

Magnetic Abrasive

finishing (MAF)

Stainless steel

7.6

6

Magnetic Float

Polishing (MFP)

with CeO2

Si3N4

4.0

7

Magnetorheological

Finishing (MRF)

with CeO2

Flat BK7 Glass

0.8

8

Elastic Emission

Machining (EEM)

with ZrO2 abrasives

Silicon

<0.5

9

Ion Beam

Machining (IBM)

Cemented

carbide

0.1

Share what you think