Bouding box behavior driving me nuts

Sice SW didn't think to include rack and pinion mates along with gear
mates, I've had to roll my own. It is useful to simulate any
transformation from rotary motion to linear motion (or vice versa), and
all that is required is to give up a little mechanistic fidelity. I
create a 'Screw Surface' component which is nothing but a cylindrical
extrusion with a diameter much larger than the size of the components to
be mated. With a couple of mates you can constrain the 'rack' of the
rack and pinion such that a point on the rack moves along the
circumference of the 'Screw Surface' and the screw surface is mated
tangent to the pinion surface. In reality, the rack moves in a pendulum
fashion, but by making the radius of the 'Screw Surface' very large, the
motion of the rack is close enough to linear. With a little math you
can calculate the proper ratio for a gear mate which gives you realistic
rack and pinion or screw motions.

The problem is that the large bounding box of the 'Screw Surface'
component wreaks havoc in the assembly. The proper behavior is that the
bounding box for an assembly is governed by the visible comonents in the
assembly. If the large component is visible, the bounding box is
large. If the component is hidden, the box is small. This behaves as
expected, until you put the assembly inside another assembly. Seemingly
at random, the subassembly bounding box will be calculated as though the
large component is visible. When you autoscale the main assembly,
everything becomes tiny. Rebuilding doesn't help, nor does CTRL-Q. If
you go back to the subassembly model and autoscale it, it becomes very
small as well. CTRL-Q fixes the problem in the subassembly
(temporarily) and on return to the main assembly it behaves again, but
only for a while.

I have a main assembly with 4 subassemblies, each of which has one of
these screw mates and I spend almost as much time fixing this problem as
I do accomplishing actual work on the main assembly.

Jim S.

 

I first needed this for a dial indicator model that I wanted to use in
an assembly to verify some complex motions. Then it occurred to me that
I could use it for screws.

No. Helical surfaces have their own problems, so I avoid them.

The large part is used because a gear rack is nothing but a gear with an
infinite pitch diameter. With a large enough diameter, the surface is
nearly flat, like a gear rack.

Jim S.

 

Big files. Slow rebuilds.

Hmmm. I'm using a screw surface with a 50 inch radius to simulate a
#4-40 screw. So, the thing isn't enormous.

Jim S.

 

I found another way. I made a dummy part consisting of a simple sketch
with a circle and a point on the circumference. Bigger the circle,
better the simulation. Bring the dummy part in your assembly and mate
the sketch plane to be coincident with a side of the rack. Mate the
point on the circumference to be coincident with an extreme face of
the rack. Mate the center of the circle at a distance from the plane
passing through the center axis of the pinion, equal to half of the
length of the rack. Create a gear mate with adequate ratio between the
pinion and the circle. Hide the sketch and play moving the rack. If
you want to be a perfectionist, add a limit mate for the movement of
the rack. Note that you don't need to mate the circle tangent to the
pinion.

P. Comand
www.atomat.com

 

What problems should I expect to see with helical surfaces?
your computer will behave sort of like it's got emphysema