# 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.
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Yeah, bounding boxes can be a pain.
I don't know what to do about that, so I'll go a little off topic now.
I'm not sure why your "screw surface" causes a pendulum motion. I'm also not sure why it needs to be large. When I first got hold of 04, I tried to make a rack gear, and failed. Maybe I didn't stick with it long enough. I admire your perseverance.
What you said gave me an idea though, and maybe this idea isn't what you're doing: When you said "screw surface" you made me think of a worm gear. I thought, "That's it!" Make an envelope part with a helical surface having as many turns as the rack has teeth, and at the same pitch. Make a gear mate between the pinion and the helical part, as if it were a worm gear. Mate a point on a rack tooth coincident to the helical surface. When the pinion turns, the worm turns, which should drive the rack at the correct rate.
Is this what you're doing? I don't see the need here for a large part. It seems like it should work.
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I just tried it.... It works pretty well!
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Dale Dunn wrote:

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.
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What problems should I expect to see with helical surfaces?

I think that's where I got into trouble. I tried to get too close to an infinite radius and got frustrated with the side effects. I didn't get as far as bounding box problems.
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Dale Dunn wrote:

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.
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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
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your computer will behave sort of like it's got emphysema