What is Bluing?

http://en.wikipedia.org/wiki/Engineer's_blue

"When Prussian blue (pigment) is mixed with methylated spirits it forms a quick drying stain which is known as marking blue or layout dye."

GA...is also known as "dykem" layout fluid.

aka Engineer's blue

I'm not familiar with Dykem, it that a manufacturers name?

I've always known the non-drying blue as Engineer's Blue & the drying type as layout blue but, as we all know, these terms have a habit of changing depending on your trade & location.

I came across a page which shows a good example of the use of Engineer's Blue to check how gears mesh, about half way down this page.

My apologies, Dykem is a "Brand name" and is a layout blue, and Prussian Blue would be the same as your Engineers blue. Dykem may be exclusive the USA because I don't recall seeing any other brand of layout blue. (does come in other colors) Just like a "Allen wrench" is only a hex wrench for socket head screw. not sure if Allen wrench a common reference in the UK?

Jim C

I like Number two's answer. The bluing is not always of the type that drys. In some cases like where a mold has to close well enough to stop the melted plastic from flashing or extending between where the molds come together this non drying bluing will be pressed to leaving only the faintest blue color at contact and progressively leave a tapering darker blue to slightly wider gaps. Some gaps of .0007 of an inch for example can be purposely made to allow gasses to be released but not the plastic. At .0015 of an inch and bigger flashing can occur, even less for other plastics like nylon.

This same non drying bluing can show how square to each other squared blocks are when an assembly of them are put together in a square cavity.

Our Guest, Reply #2 has given a very good response. I would like to add that years ago we did use a method to check gears that did involve using a special paper that would leave an a blue marking on the gears. It was not common carbon paper however. The method was used to check where the teeth first made contact and how they rolled into the next tooth. It would also show you if the contact was in a straight line across each profile (not too useful on helical teeth) and was used on speed reducing gears.

Misalignment could create a situation where there was not a uniform even tooth loading and spalling and flaking of the teeth will result. It could also be a result of uneven bearing wear. Depending on the tooth profile there is a proper meshing point that should be maintained as I recall. If the mark was too high or too low on the tooth profile new bearings or shimming was required to prevent major tooth damage.

I never saw Prussian blue used except during the gear cutting and hobbing in the original manufacturing process.