Mythical Specifications: Non-accumulative tolerance

This entry is part 4 of 4 in the series Mythical Specifications

Mythical specifications come in many forms.  One realm they seem to haunt is that of repetitive features, also known as patterns.  Many attempts to shorthand pattern callouts are continuously made.  Bad habits die hard as old mistakes are passed down from one generation of engineers to the next.  One particularly bad habit is the use of linear dimensions with the term “NON-ACCUMULATIVE TOLERANCES”, or something similar.  There is no such thing.  Trying to use this shorthand leads to tolerance issues.

Pattern non-accumulative tolerance callout

In the example above, the dimensional callout attempts to simply dimension a pattern without considering tolerance stack-up.  However, this attempt fails since any two non-adjecent holes cannot avoid accumulation of tolerance due to the dimensioning scheme.  Tolerance stack-ups on linear dimensions have accumulation.  There’s no way to avoid it without dumping linear dimensions.

I had originally planned on a short article about this topic.  However, once I started delving into it, I found out that there is a lot of ground to cover.   So, this topic will be addressed in detail within a future article (Feb 23, 2011) where examples of different pattern dimensioning schemes will be explored.

Also see How to dimension feature patterns on drawings.

Black Chem-film (mythical)

An old joke in the Engineering field is to send the newbie to go find some impossible thing. Unfortunately, black chem-film is asked for seriously at times.

An old joke in the Engineering field is to send the newbie to go find cable stretchers on the manufacturing floor.  Someone searching for black chem-film can feel just like that poor newbie.  Chem-film goes by other names, such as chemical conversion coating, iridite, or alodine (some of which are trademarked terms).  However, putting the word black in front of any of those will provide very disappointing results.

Chem-film is sometimes used instead of anodizing as a protective finish for aluminum.  Chem-film leaves a coating that is conductive to electricity.  Though an anodized surface may also be conductive, chem-film is more so.

A chem-film finish is typically yellowish or gold in appearance on aluminum, but may also be brown, gray or blue (depending on the substrate).  This is not the result of dyes.  Lighter processing can result in a clear finish.  Either way, the surface color comes from variations within the process itself, and not the result of any coloring additives.  Factors such as the reaction of the substrate with chromic acid, temperature, inhibitors, concentration, promoters, time, surface finish, and accelerators all play a role in the final surface color.  This contrasts with anodizing, which may be dyed to intentionally achieve a variety of colors.

Be cautious of a vendor that says it can provide black chem-film.  They may be using the term chem-film very loosely (i.e., incorrectly).   That said, if a vendor can legitimately produce a black chem-film coating on an aluminum substrate, they are invited to comment on this article.  Of course, I will ask that verification to any such claims be included within the comment, along with confirmation that results are compliant with MIL-DTL-5541F other similar standards.

On a side note, there may actually be black conductive coatings available for aluminum.  If I can get confirmation, I’ll mention those at a later date.

Reference: MIL-DTL-5541F (.pdf)

MIL-DTL-5541F does not support idea of Black Chem-film.

Clear hard anodize finishing (mythical)

Once in awhile, I’ll run across a requirement to use a specification that isn’t physically possible.  Something I see from time to time is the request to apply the specification of a clear hard anodize finish to the drawing of an aluminum part.  This makes me chuckle (unless the requester is thoroughly convinced that this mythical beast really exists).

A hard anodize (Type III) finish is intended to provide wear and abrasion resistant surfaces with improved corrosion protection due to greater thickness and weight than common anodizing (Types I and II).  The goal when using hard anodizing is to have a wear index of 1.5mg/1000 cycles, according to MIL-A-8625F.

An anodize finish on an aluminum part is achieved by growing an aluminum oxide layer on its surface using direct current through an electrolytic solution, with the aluminum object serving as the anode.  The current releases hydrogen at the cathode and oxygen at the surface of the aluminum anode, creating a build-up of the aluminum oxide.   The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V.

Common methods apply an aluminum oxide layer that is .00002 – .001″ thick.  A clear appearance remains as the thickness approaches .0006″ thick.  Thicker than that, the layer darkens to a bronze, gray, or black color (depending on the purity of the aluminum substrate).  At .0017″, the color is very pronounced.  Hard anodize specification calls for .0020″ (+/-20%) thickness.  This is far above the point where the anodizing process produces a colored finish.   Another factor is that of temperature during the process.  Hard anodizing requires the process to occur as a much lower temperature for the harder surface (higher process temperature = softer surface).  Additional coloration occurs due to the lower temperatures required by the hard anodize process.

Although a balance may be struck between hardness and clearness, the specification of clear hard anodize is not an achievable specification in a strictly technical sense.  Any compromise to get close to this specification is going to have some color and reduction in hardness or durability.

Reference: MIL-A-8625F (.pdf)

Mythical creations

Engineering sometimes calls for the use of the specification called unattainium.  In other words, at times there is just no easy way to find the balance between design requirements and reality.  Other times, someone isn’t knowledgeable enough to make certain specifications, so they come up with specs that may sound right, but aren’t real.  Anyone ever run into a set of parts that were designed with all the mating features being line-to-line?  How many of us have searched high and low for a “black alodine” finish?  Another mythical metal finish is “clear hard anodize”.  I’m going to cover some of these points in future articles.  For now, I’d like to see other misspecifications that people have experienced in the engineering field.  Please comment about what you’ve seen.