Drawing symetrical features

[herroni]

Hi, I'm wondering what other peoples views are on this. Simple example; Say your drawing two holes 50mm apart, which are to be symetrical about a datum plane. It would seem logicalto me to draw a centre line on the datum and use the symmetry constraint on the two holes. One dimension of 50 mm is then placed to distance the holes apart.


In my job I often have to work with many files created over the years by varioususers and invaribly in the example above I will see that there will betwo dimensions, one to space the holes (50mm) and another todistanceone of the holes to thedatum(25mm), instead of using the symetry constraint. The part will be correct but if you modify the 50mm dim then the 25mm dim also needs to modified.


Can anyone suggest whythis method is used? It can be an absolute minefield for anyone (i.e. me) trying to modify the partslater on. Is this just a case of lazy technique or am I missing something?
Edited by: herroni

 

[prohammy]

Herroni,


In general I will use the method you have described, but there are many cases where I want to leave the sketch with two dims (personally I would leave it such that the dims are 25 & 25, not 25 & 50) so that I can control the position of each hole should I change my mind and no longer need the holes to be symmetric


Kev

 

[sip]

Herroni,


You can use the symmetry constraint in sketcher, and still create the 25 mm dimension in sketscher, just make sure it is a reference dim. That way it will always update. You can remove the ref letters if you desire.


My 2 cts Cdn,


Sip

 

[jraquet]

It all depends upon the necessity of the 25 mm dimension.


If the model datum is some type of dimensional/tolerance datum then that dimension may be necessary for manufacturing. If this is true and symmetry is still required you can use a relation to tie the two together or make the datum a GD&T datum and apply a geometric symmetry tolerance constraint.


For manufacturing an actual dimension for locating the feature is necessary, the dimension is implied by symmetry but doesn't appear outside the modeling environment.

 

[Willmsy]

My thought might just be in the design intent of the feature.

 

[dgs]

My thought might just be in the design intent of the feature.

Yep. If you create with only the 50mm dim, that tells me (the next guy to work on it) that you intended it to be symmetric, whatever the appropriate distance is.


If you dim 25mm from center and then 50mm hole to hole, that tells me that it's important for hole 1 to be located to the datum and for hole 2 to be located to hole 1. If the numbers work out right, then it's symmetric. If they don't it's not and it doesn't matter either way.


If symmetry is desired, always use the symmetry constraint. If symmetry is accidental but not important, don't use the symmetry constraint. Remember, every reference selected and constraint applied is telling Pro|E what's important and how to update the model when changes are made.

 

[shankar_me]

Hi all,


well! if symetry is necessary, then its ok to use the symetry constraint. But it should not effect the part accuracy when physically made.

For manufacturing an actual dimension for locating the feature is necessary, the dimension is implied by symmetry but doesn't appear outside the modeling environment.

Thats right.The positions of the holes shall definetly be effected during drawing when you are dimensioning during drafting.


Shankar

 

[AHA-D]

The 25 mm / 50 mm scheme is what ProE offers as a standard and most people don't bother changing it. Connecting the dimensions with relations is IMHO even further off. Intelligent design should use intelligent relations. So definitely make things symmetric if they are symmetric.


Note : very often you can model half or quart a part and mirror as a final (or pre-final) step. However I have had cases where for instance revolved protrusions do "weird things" when mirrored.
Edited by: AHA-D

 

[michaelpaul]

Is this just a case of lazy technique or am I missing something?

I've found that people who are less experienced don't use the symmetry constraint because they don't know any better. Once they gain experience, they learn about it and use it. At least that's been the case with most older models that I work on. Many of the people at my work have been here for quite a while and even they are quietly amused at how they used to construct models.............

Michael

 

[sau]

But sometime it depends on the design intent and how it going to modify in future.