Purpose of construction lines

A construction line will be ignored in a sketch for a feature. For example, if you need to cut a shape out of a plate, and you want to reference something with a dim that is tangent to an arc, or something, then you can put in a construction line that fits the bill and it will not try to be used as part of the outline for the cut. You use it only as a construction tool, not a profile element.
Now, that being said, even in cases where I don't have a problem with solid lines, I like to make them construction lines to denote that they are reference entities, not to be used for a profile.
WT

First, you understand that for feature creation (extrusions, cuts, revolves, lofts,..), using sketches (closed or open profiles), have to be solid (not hidden lines) and construction lines (hidden) can not define a profile. (and yes, a centerline or construction line also does define a rotation center) Think of construction lines (dashed) as reference entities within your normal sketch (solid),.. lines, arcs, curves,.. which serve to reference construction intent and drive your sketch entities (solid). In your sketch, you can always toggle from hidden to solid and back again. The most common or simplest example of a construction line is used with polygon sketches, such as a closed rectangle sketch, where you apply a diagonal construction line to quickly define the center of the rectangle, (via the mid point of the construction line) or using a construction circle to help define the center of a hexagon.
Use construction lines to help construct of your open/close profile sketches,... such as,, you may want to only create a layout sketch, 2D or 3D, which could define your profile intent or open/closed profile boundaries... so, you may have construction entities within your layout sketch to help with defining your open/closed boundaries.
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One thing I like about using centerlines in solids is when doing lathe or cylindrical work. When you dimension from a "centerline" it dimensions the diameter. Seems to keep sketches cleaner and makes you think in ral life, not radius numbers.

Hi Cliff,
What sounds like "Ref"
Construction lines are what they are "Constructions Lines". In Autocadand, Pathtrace as well as Mastercam ( I think) dont want to bother open it.
Where do use "Ref" ?
Daveb

added word construction to above paragraph. (post inserted into useless thread)

I often find on complex profiles, that it is difficult to make a fully defined sketch without extra construction lines beyond just "centerlines" on the main axes.
It is very often I use a construction line between two ends of solid profile lines to define a Mid Point between the two line ends such that I can mirror items in a sketch or dimension to the center of a "rib" for instance.
Lots of good uses for construction lines exist.
Bo

On parts & assemblies to add relationships or mates on difficult to mate parts & assemblies.
Adding relationships for sheet-metal forming tools.
Tom

Ben,
In addition to all of the other reasons, construction lines can be used to mirror sketch items about.
They can also be used to keep track of design intent. You can put construction geometry in a sketch, then use convert edges in later sketches to make "real" geometry.
Jerry Steiger Tripod Data Systems "take the garbage out, dear"