Purpose of construction lines

Will someone please explain the principle behind making a line a
"construction" line , versus a regular sketched line? What is the
actual reason for it? I see it being used as an axis for a revolve,
also I see it for the placement of text on a model.
I did read the help files on construction geometry, and I must say
that I am now more confused than before I read it. I hope it is not
just me, but it seems that the author of some of the Solidworks help
files actually takes delight in confusing the reader.
My sincere thanks to anyone who takes the time to help me on this,
Ben

 

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

 

Ben,

Construction lines are a great way to show where you're imparting
design intent into your sketches.

Jon Banquer
San Diego, CA
http://www.carsunreleased.com

 

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.

...

 

See if this helps:

http://aec.cadalyst.com/manufacturing/article/articleDetail.jsp?id=140037

This person who wrote this article is the reason that this newsgroup
even exists!

Probably regrets it now. ;>)

Jon Banquer
San Diego, CA
http://www.carsunreleased.com

 

Try this link as well.

http://manufacturing.cadalyst.com/manufacturing/article/articleDetail.jsp?id=133672

Jon Banquer
San Diego, CA
www.carsunreleased.com

 

"A construction line will be ignored in a sketch for a feature."

Gee, what is "design intent" & what are "sketches"?
New buzzwords from where?
And what are you "showing"?

Sounds more like "REF" per honest drafting standards <G>.

IIRC Last week he posted someplace that he was/is a SW user ....
& tried (again) to pose as an expert ....

Pretty good for 2 1/2 axes CAM in a shop where you load
parts or push a broom & the shop actually uses Pro-E & the
programmer uses MasterCAM, is it?

 

Sheesh ... from the link: "driven dimension".
Per all drafting & design standards I ever knew of such things
are not to be shown on prints or designs other than (perhaps) as "REF"
or "For MFG. Purposes Only" and are highly frowned upon.
All *derives* from the actual controlled dimensions which
must fully constrain what the part/design is (though many
entry-level "designers" & detaiers may miss the concept
or it's purpose & not do so. A few rounds with a good
sarcastic checker might cure some of this<VBG>.)

 

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.

 

So you don't know, eh?
LOL ....

 

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

 

Dimensions that do not control but result from those
that do (and are not often used and/or frowned on).
They duplicate information that derives from the dimensions
that do control the features. They are for REFerence only
(often un-toleranced) and may be directed at the
specific intended manufacturing folks.

Another example might be where guage points out in
space control things but are hard to measure so they
might add something else that's easier to measure
for inspection purposes on the shop floor & where
it's sort of an in-house job & they know how it will be
made there. Or even raw stock size for the saw
shop ...

BTW, There are standards that control these things,
often industry specific ones too (which are usually based
on extensions to the core standards).
See, as an example, the GM Drafting Standards which
are based on the ANSI/ISO ones IIRC. Huge manual <g>.

See "Reference Dimension":
http://www.engineersedge.com/gdt.htm
They & their *proper* use can also result in
manufacturing having to hold tighter tolerances
on the REF dimension than the part would otherwise
require. Slightly improper use can easily lead to scrap.

Now, about "Typ (27) places" ...

 

This case of banquerbabble <G>.

BTW, When you consider the two links jb
posted I suspect some confusion.
http://aec.cadalyst.com/manufacturing/article/articleDetail.jsp?id=140037

http://manufacturing.cadalyst.com/manufacturing/article/articleDetail.jsp?id=133672

Those parts look like flanges to me. They exist because stock
is needed to hold the holes apart.
The holes would be the critical things from which all else
follows but that's not how things are shown.
I'd have set zero on the center of the large hole with
the one hole above it & the other two on an axis parallel
to the X axis & then dimensioned the hole locations from
the large hole then the stock about the holes & then the
lines tangent to that, then needed fillets, etc.
Poor jb's latest buzzword/phrase is "design intent" <VBG>.

 

BTW, These parts might even well be symmetrical
and show as such in that orientation (I did not check).
If so, far better to show them as such IMHO.

Speaking of which & his selected examples <G> ....