Let’s take a look at the single most common ultrasonic cleaning application there is:

Cleaning oil and particulate off of metal parts. We help customers with this in industries as varied as carburetor cleaning to military weapons cleaning.

Here's a look at a Crest ultrasonic unit cleaning motor parts. Watch how the soil just leaps off the part:

Key points:

• If your parts are heavy, make sure you invest in a perforated tray or basket. These hold their shape better than mesh. This will make a difference in the long run, and make sure you don’t damage the bottom of your tank. A perforated tray is formed from stamped metal – usually with drainage holes between 1/4″ AND 1/8″.

• You are going to need heat. Not just because oil and other petroleum-based soils break down better with heat, but because ultrasonics are just more efficient above 140 F.
• If you find that you need to go to the upper end of ultrasonic compatible temperatures – say 180 f – remember that someone may need to handle the part post cleaning, so worker safety will need to be taken into consideration. 
• Temperature can also be a huge help if you require a dry part. If you have a metal part with an appreciable mass, once you pull it out of the tank you’ll see that it dries by itself.
• The driving issues when you set up your metal cleaning process are these:

How clean is clean? It’s important to have an objective specification for what clean means. Here’s an example of an objective specification: “Viewed under 30x magnification, no more than three particles of soil per cm2  in excess of  0..50 are visible.” Here’s one that is not acceptable: “Gus down in receiving says that it is fine.”

What happens next? If I am going to coat the part with oil and store it, that’s one thing. If it is destined for anodizing, that is quite another. Let your needs drive your process, and let your process drive your equipment selection.

Here’s a good whitepaper Branson Ultrasonics did on this some years ago…
Aqueous Degreasing of Metal Parts (.pdf 1458kb)