Problem with Succesive Extrudes

It is useful to overextrude a Shape so that you have better access to the internal edges to do a vertical chamfer. Ss1 the block is 9 in high but say I really want it to be 1 in high. I want to Extrude the top face -8. Ss2 The Extrude tool proposes 5 but I change to -8.

The shape stays the same height but you can see a new line 1 in from the bottom that results from my -8 extrude. (1 in verified by added vertical line)

It seems that you can successively Extrude up but not down.

Is this normal? If so, how to eliminate top 8 in of shape?

I was able to work around this by pushing the top down 8 in but I would expect the Extrude tool to work similarly?

Please comment and thanks for looking at this.

Best,

Tommy

Not sure I get it. Can you post a video?

Here you go… At the end of the operations, the cube is 10 in tall with a line around its periphery at 1 in height. I would expect a cube 1 in high. Thus the example shows that Extrude works successively positively but not negatively. Unless I am unclear on concept… Thanks…

Emailed you the file as forum only has a 15KB limit.

Ah ok, I see what you mean. When you are using the extrude command it will create a new body. So the extrude with the negative value will not automatically subtract the new body from the other one, it will just create another extrusion, but in the other direction.

I guess your rationale is the literal definition of Extrude. Like you can only squeeze toothpaste out of a tube. But my reasoning came from a question I posed here whether Push an Pull were the exact equivalent of Extrude. The answer was yes but this is not quite correct as I can Push/Pull with the expected effects while a negative Extrude will not offset a positive one.

If I follow your logic, the Extrude tool should not accept negative values.

Doing the negative Extrude also seems to result in a curious condition with the 2 shapes: the 1 in high one with the 9 in high one on top of it. I cannot select them independently. If I select the top one and click Delete, it deletes the bottom one! Just the opposite of what I would expect and want. I was also unable to subtract the top one from the overall cube. So the negative extrude approach seems a complete bust to me. Luckily I can Push/Pull.

Thanks

Tommy

No, it is not the same. Push pull operations are offsetting faces, although in this case it would indeed have the same result. So when you are pushing the top face of an extrusion, it will actually move the face down, but the result geometry will be the same of an “extrude down+subtract” operation.

Understood. Perhaps I am a slow learner? Thanks

You are not, Shapr is easy, but 3D modeling is hard