This is a very brief discussion of chain sharpening and how to determine if your chain needs sharpening or "touching-up". I can provide more details and in-depth information if you are interested. You can also find much more information from chain saw manufacturers and companies that manufacture chain such as Oregon or Carlton. I have found that Oregon, leading saw chain manufacturer, has excellent information about chain maintenance.
The above drawing shows the various angles you need to control when a chain is sharpened. To ensure proper and safe cutting it is important to control all of these angles when re-sharpening a chain cutter. A precision chain grinder used in professional shops has the ability to control these angles very precisely. If hand sharpening with a file it is best to use a filing guide.
There are several ways to determine if your chain is sharp.
The two logs were cut with the same chain one right after the other. The chain was a Stihl 26RM2™ (the RM means Rapid Super) Rapid Super is Stihl's designation for a chipper low profile cutter. This chain also employs a safety strap between cutters to help reduce kick back. The chain had been sharpen just prior to the cuts. The saw was a Stihl 026 Pro. Sharpening was done on an Oregon grinder using a 3/16" thick Borazon wheel. The top plate filing angle was 30° and the top plate cutting angle was 60°.
It is important to remember that significantly different chips were created with the same chain. The determining factor was the the type and condition of the wood. If you are going to use this method to measure chain sharpness check the chips with a new or sharp chain and when you feel that the chain is starting to dull check the chips once again. If there is more saw dust than chips, the it is time to to install a new chain.
As mentioned at the beginning there is much much more to know about chain, bar, and saw maintenance. I can provide more information if you are interested. Your chainsaw owners manual is also an excellent source of information. A "good" independent dealer is also a worthy place to find more information.
A chainsaw is a very powerful and useful tool. It makes felling trees and cutting up wood a rewarding task. The saw however is one of the more dangerous pieces of equipment you can purchase and use. In the hands of a careless and untrained user it can be deadly.