Linked dimension? 2

[hititfaster]

When sketching I like to set things equal about the origin so that when the part is formed the planes are in central positions. So, if I want a rectangle with a total height of 100mm, I'll constrain it by dimensioning the total height at 100mm then dimensioning 50mm from the origin to the top/bottom, thus placing the origin in the middle of the sketch.

What I'd like to do is simply link the dimensions so that if I come back and change one value, the other changes. So in this instance, my first dimension is X, and my second dimension would always be X/2. Possible without more effort than typing in?

I know the rectangle is a bad example because I could draw a centre rectangle, but hopefully you know what I'm getting at...

 

[TheTick]

Have you tried extruding/revolving from mid-plane?

 

[hititfaster]

I have. But don't you still have to centralise the sketch if you want the origin to be in the middle, centre in all planes?

I guess this is a bit of a habit more than anything, but I seem to find myself doing it a reasonable amount. My ideal would be when I've dimensioned and the value box comes up for dim no.2, being able to click dimension no.1 and then add /2 and hit enter. I tried it thinking it might work because you can populate tables in this manner but unfortunately it doesn't

 

[mncad]

Instead of linking dimensions, which I have had problems with in the past, I just use a construction line. Draw a construction line and constrain it to the origin with a midpoint. For your rectangle example you can draw a construction line from corner to opposite corner and constrain it with a midpoint to the origin. This will give you the middle for both top/bottom and side/side. Then when you change your width or height dimension the rectangle stays centered on the origin.

mncad

 

[CorBlimeyLimey]

Use the Equations function.

 

[Heckler]

Why do you like to put the origin at the center of the model?

Heckler
Sr. Mechanical Engineer
o
_`\(,_
(_)/ (_)

This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.

 

[Nella95]

For general SolidWorks practice, placing the origin at the center of the model isn't absolutely necessary - unless - unless anyone else ever has to touch the part for editing, or measurements.

(Also, if everyone exposed to SolidWorks creates parts in the same manner - troubleshooting or editing the part is made significantly easier.)

 

[JMirisola]

You do know about the 'center rectangle' sketch tool, right?

Also, I disagree with Nella. I firmly believe keeping the part centered about the origin, or as closely as possible, is a good rule of thumb. I can remember opening models, then zooming to fit, only to find that the model was some vast distance from the origin. I think centering just makes things neater.

Jeff Mirisola, CSWP
Design Manager/Senior Designer
M9 Defense
My Blog

 

[Mekada]

Equations aren`t necessary to do what you want. Simply right click the edge of the rectangle and select mid-point. The ctrl-click on origin to make it horizontal or vertical. Repeat on other side and voila.

Another option would be to use the "center rectangle" option from the drop down menu in the sketch and mate the center to the origin.

 

[Nella95]

We are in agreement Jeff. Though I don't think it's absolutely necessary if the intitial creator models the part 70 inches/mm away from the origin and the next person to edit the part knows that the part origin is so far away from the center of gravity, center of part.

 

[TheTick]

What about symmetry constraints in your sketches? From your OP, it doesn't sound like you are using them. Draw construction lines constrained collinear with the datums and use them to drive symmetry.

 

[ArtL]

Mr Faster I design parts/models in relation to the first part. IE: The first part is centered @ 0,0,0 and the second part is offset from the origin @ X=3.5 Y=-2.00 Z=0.0 Ect... this way the parts are designed to or from the origin of the fixed part in the assy in real time.In the long run this is good as far as placing the parts correctly in the assy, and correcting any variences very easy, by moving the 2nd, 3rd Ect.. parts dimension to or from the origin of the first part. Also I set the origin of the first part concident to the origin of the Assy.I have designed whole engines this way with no trouble. True there are times when this can't be done, but for me it's a really good way to get mates, bores, ect to be aligned correctly.I started doing thes because one of my first 3D modelong jobs, the company used Co- Create where you model in the assy.No seperate screen to individually model parts.

 

[drawoh]

hititfaster,

I vote for the diagonal construction line, centred on the origin. Alternately, you can draw a centreline, and impose symmetry around it.

Usually, I try to model my parts from the datum edges. Much of the time, when I prepare my drawings, I use some other edge as the datum.

JHG

 

[applejack2]

as drawoh and mncad have already said, I use the diagonal construction line. I find it is easy to understand and follow for others who view/modify/reuse my models.

Sometimes it gets frustrating, when working with someone else's model, trying to figure out why you can't move a line, and then you find that it is constrained to some other feature in a way that isn't intuitive to you.

 

[hititfaster]

Wow, I wasn't expecting this level of response to my musings!

Firstly, a star for mncad (I know others have offered the same tip but you were the first) because I think that'll be the winner for me in normal use. Other comments mention increased use of constraints too, so will give it a shot.

Nella95 - because I work in a bureau, our models go out all over the place, and we send everything to our clients so that a) if people want to go elsewhere they can (although we hope they think we do such a good job that they come back to us ;-) ) and b) there is always another back-up of a model if our back-ups failed here. It suits me to make anything and everything we model as absolutely easy to change as possible, because anyone could be doing the changing, either in our office or elsewhere. People tend to remember and curse the original modeller if an assembly is hard to work with!

Also, lots of the things I model are very likely to need a 'tweak' at some point. Clients often only know exactly what they want when you show them something very close to the finished product (this isn't a problem or a moan - it's why they employ me as a design engineer!) so pragmatic modelling makes my life easier down the line.

JMirisola - Perhaps a bad example using a rectangle, but I just wanted a simple illustration of what I meant. Re. an assembly miles from the origin, that would drive me INSANE. Toys out of the pram insane...

ArtL - that's what I aim for. I also drive some features (holes are a regular) with features from earlier parts. Let's say you make your engine block part 1 and mate your heads to it. If for some crazy reason you want to shift the position of one of the bores, it would be handy for the combustion chamber in the head to remain aligned with the bore. Maybe.