mates + best practice

Should cylindrical features be fully constraint even if they don't have to
be? An example would be a shaft and a hole. The shaft has a coincident mate
and a concentric mate. Should the shaft also have a mate to prevent it from
rotating? Which is the better practice? What are the implications of each
case?

Thanks

 

The best practice is to not add a mate to fully constrain the
component. SolidWorks will take time to calculate the mate so adding
the mate will slow SolidWorks down.

If your like me I hate seeing the (-) indicating the component is not
fully constrained. I submitted an enhancement several years ago to
have the indicator change if the component has one degree of freedom
left. (r) would indicate the part rotates along a single axis, (l)
indicates the part travels along a single axis, and (rl) indicates the
part travels and rotates along a single axis. The addition of these
indicators would let the user know how the component is mated by
checking the FeatureManager.

Greg

 

That would be pretty cool - I wonder how much programming it would take to
accomplish it.

WT

 

speaking of mates...

I would like to know if there is a way to use two mates to locate a part
that slides on two pins. Think the top of a dieset sliding up and down on
its pins (sorry I don't know the proper terminology). When I try to putting
a concentric mate on the first pin to its opposite bushing thats fine, but
trying to mate the other pin to its opposite bushing yields an "part is
overconstrained error".

I know I could refernce off parallel faces of the dieset, but adding mates
to the parts that do the guiding in real life is more "natural" to me.

Bill

 

Are the pins the same distance apart as the holes?

Are the pins and holes all parallel to the same direction?

If they're off even a little bit, it will easily overdefine.

 

Are the pins the same distance apart as the holes?

Yes, and Yes.

 

Bill,

NEVER use multiple concentrics to locate pins, holes, etc.. They will go
over defined even if they're perfect. Keep the mates as simple as possible.
On a die set, or mold base, I pick a single pin or hole for concentric. I
then add a parallel, either a plane or a face.

When Matt siad "a little bit" he wasn't kidding. Probably somwhere around
..000000001". SW used to be more tolerant of this.


Regards

Mark

 

Thanks

 

Thanks, thats what I've found. I was just wondering if I was missing
something, like an adjustable tolerance for concentricity, or something.

Bill

 

I agree on what you are saying, but disagree in the practice of it. My
philosophy is, for example, that if you can't mate two holes in a plate
concentrically, then you don't have them dimensioned the same, and I would
like to know about the error right away. Now, I do understand that every
once in a while it's unavoidable, but for the most part, if you want holes
to line up, dimension them properly.

A common mistake is when mixing inches & mm in that someone will dimension
one side to 25mm and the other person will dimension the other side to .984"
because they don't like to work in metric. Maybe close enough to work, but
then again, maybe not. So I tend to practice the more exact method of
precision so my habits are there when it really counts.

WT

 

My experience is that, even when holes are dimensioned correctly and the
center to center distance matches exactly to the eighth decimal point when
measuring, it's still a crap shoot whether or not you can mate both
concentrically.

Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"

 

Wayne,

Well, your welcome to fight with the results if that's what ya want.

There are lots of other less problamatic ways of insuring accuracy.

Regards

Mark

 

Here's my take on concentric-concentric -

Put a 1-2-3 block down on a surface plate with the 3" height facing up
(smalledst face supporting).

It's a lot like that - it will stay, but it's not the most sound state.
Eventually the block will end up on its largest face.

Actually the block is really 6" long in this case.