Surface Treatments
If you get nothing else out of this post, remember that oxiding is for steel and anodizing is for aluminum. This gets messed up all the time, and is always worth an eye roll from the old timers if you make that mistake! Now, for a little more detail......
Black Oxide
This is the most common surface treatment for carbon steel parts, whether they are hard or soft and regardless of the type of steel (except stainless, of course). It adds no thickness to the part, so you don't need to account for it in your designs. One common misnomer about black oxide is that people think it is a rust inhibitor. This is actually not true, and in fact, as you can tell by the name, it is a form of oxidation itself, just like rust. It's relatively inexpensive (probably less than a few dollars for most small to medium details) and can be done as quickly as same day or overnight depending on your supply chain. It is a generally accepted finish in most industrial settings, although not for medical/pharma, but back to the rust thing for a moment, I can tell you from personal experience that if your machine is being sent to Mexico or other high humidity areas like Asia, you will not be pleased with the way the details look after they've been there even a short time (possibly even after transit). Check with your company as to what way to go, but for those situations, electroless nickel is usually a much better choice, although it's several times more expensive.
Electroless Nickel
Electroless nickel is a very thin plating (.0002-.0003" [.005mm]) that has excellent corrosion resistance and fairly good wear resistance properties, although it's not usually used specifically as a wear resistant coating (see plating section of heat treating post for other options). You can generally plate details that are even up to a few feet square or more depending on your plating supplier. Sometimes, the plating does concentrate in holes and tight corners, so some post plating reaming, or pre-plating masking/plugging is done to keep this from creating a problem. Because it does have thickness, on extremely close fitting details, you may have to allow for it in your design, but .0003" is pretty small, so 98% of the time, there's no need to do anything special. When doing medical/pharma work, you generally have a choice between electroless nickel and stainless steel for your details (check to make sure they haven't made that choice for you in their spec). Electroless nickel is expensive (can be $10-20 per detail on medium sized pieces and well over $100 on larger plates and weldments), but stainless takes twice as long to machine than carbon steel, so depending on the complexity of the details, one may be more cost effective than the other. Typically, it's called out as E/N .0003 max on your drawings.
Flash Chrome
This is another treatment for steel parts, although much less common than electroless nickel and black oxiding. It is a plating operation that adds thickness, but is generally much thinner than the hard chrome (see heat treat post for more info on that) version. It's used when extreme corrosion is likely, or if a mirror finish is needed. Thickness is all the way down to the .00005" [.001mm] range. For our industry, it would have to be a special circumstance to call this out, so avoid it in general.
Anodizing
This is the primary surface treatment for aluminum. There are two types that we typically use: standard and hardcoat. Both processes create about half of the anodizing thickness below the substrate and the other half is additive to the part dimension.
Standard Anodizing
Standard anodizing is for aesthetics and reduction in oxidation over time and is generally very thin, adding less than .0003" [.008mm) to the overall dimension. Normally, it's specified as black anodize, but can also be clear and a host of several different colors, which is a nice option for changeout tooling and handles that you want to draw attention to.
Hardcoat Anodizing
Hardcoat anodize is thicker, adding .0005 [.01mm] to .002" [.05mm] to the overall dimension. As the name implies, this is used more for abrasion resistance than aesthetics, although some colors are available here, too. While I wouldn't go as far as saying it's as tough as chrome plating or heat treating, it is a good option for aluminum tooling that will see wear, especially from non-metallic contact.
Teflon Coating
Yep, just like in the skillet. This is a coating that is used for greatly decreasing the coefficient of friction on metal parts. But, just like in the skillet, it doesn't react well to metallic abrasion. It's an excellent coating if you have plastic or fabric rubbing against it, but should be avoided if the details coming in contact with it are steel. Thicknesses are typically .001-.003" [.03-.08mm]. Not cheap, and not super long lasting, but a nice tool to have in the toolbox sometimes.