In a utility patent, are there particular good practice rules regarding depicting an invention being used on a workpiece? Should or can the lines of the workpiece be heavier, lighter, solid, broken, or whatever is most clear? Should showing the invention in action on a workpiece be avoid for any reason, or perhaps held to just a few figures? I believe it adds significantly to the clarity, yet most tool patents I have seen do not show a workpiece; is there a reason?
I guess you can find a wide range of opinions about this matter. My opinion is that you should provide as many drawings as necessary to at least properly describe the main embodiments you may claim at some point. The text description is very important for support and sufficiency/enablement, but sometimes it is quite complex to capture the gist of the invention or particular details using text alone; or you forget to describe some features because they seem obvious to you but are shown in the drawings.
At some point you may have to rely on the drawings to amend the claims in some way, so you better have a good set of drawings to be able to extract features from them. Some patent offices are very picky when it comes to amending the claims using the figures as support, so one should spend some time thinking which details should be shown in the drawings in case you need those later on. In this sense, showing the invention both in isolated manner and during operation may be a good idea for this reason, maybe the features you have to extract from the drawings become apparent only while the device is in use.
About the type of lines, that very much depends upon the invention. You do not have to stick to the requirements of design patents, and you should just worry about having drawings that are clear and understandable for the skilled person. So in some cases you will provide 2D drawings, 3D drawings, diagrammatical, non-diagrammatical, block diagrams, flowcharts, etc. it really depends on the type of invention and how it works.