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

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 smooth finish for many polished surgical implants is specified at 0.025µm to 0.05µm (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 Gorilla 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. Rob Hill from Prysm body piercing has done useful tests for his APP Scratching the Surface presentation.

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

502000N12
251000N11
12.5500N10
6.3250N9
3.2125N8
1.663N7
0.832N6
0.416N5
0.28N4
0.14N3
0.052N2
0.0251N1

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

"Surface Finish Tolerances In Manfacturing" by Emok - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Surface_Finish_Tolerances_In_Manfacturing.png#mediaviewer/File:Surface_Finish_Tolerances_In_Manfacturing.png
“Surface Finish Tolerances In Manfacturing” by Emok – Own work. Licensed under Public Domain via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Surface_Finish_Tolerances_In_Manfacturing.png#mediaviewer/File:Surface_Finish_Tolerances_In_Manfacturing.png

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

Further resources


Figure 3. The relationship between biomaterial surface roughness/chemistry and bacterial attachment is intricate. Bacterial attachment is facilitated by increased surface microscale roughness since larger surface areas (especially when irregular) provide binding sites and protection. Increased smoothness of the surface should prevent bacterial colonization. A similar scenario is found with corrosion associated surface micro-cracks that are prone to infection. On the other hand, exceptionally smooth materials can increase the bacterial attachment via physical forces such as van der Waals interactions (double rows) and by providing a number of molecular contact points.

— Antibacterial Surface Treatment for Orthopaedic Implants
https://www.mdpi.com/1422-0067/15/8/13849/htm

Read more about foreign body response and fibrotic capsule formation for intradermal implants

21 thoughts on “How smooth is smooth enough? Surface finish standards for body jewelry”

  1. je suis navrée de voir que tous les articles qui me semble intéressants sont toujours et uniquement en anglais. Serait-il possible d’avoir une explication du contenu de cet article en français car ” reverso ” ne comprend pas non plus ! ! ! merci par avance , Dominique.

  2. Hey B,

    Any estimates on the cost involved in having independent laboratories provide testing?

    What implants are you looking at in the 1µm – 2µm range? Are you musing over studies that involve osteoblast adhesion or keratinocyte growth or something else?

    • I'd talked with <a href="http://www.namsa.com” target=”_blank”>www.namsa.com about testing, and the estimates vary. There are plenty of labs with profilometers that have scanned jewelry for us as a favor, for no charge as well. Some, perhaps all of the major jewelry manufactures have the equipment as well. The issue with jewelry seems to be in random testing and frequency, and in getting the finish agreed upon. The ISO and ASTM standards are what I've been working with, rather than specific implants, though I've used Smith & Nephew as a resource since I still have a connection there. I've been considering adhesion as one variable, but mostly concentrating on the benefits of cleaning, passivation and a stable and uniform oxide layer.

  3. how about for biological structure like plant leaves…im not sure what ASTM standard that i should follow if i want to measure surface roughness by using non contact 3D profilometer (Alicona)….

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