How to extrude around a part

[Dreke]

I'm designing an RC car, for the diffcase I made a revolve, so I have circular shape. I want the bottom end to be a rectangle. So on the top it has to stay round, but on the bottom I want it to be a rectangle, so I can mount it on a chassis.

The problem is that when I extrude a rectangle from the middle of the part tot the bottom it also fills in the space in the center of my diffcase, but it has to stay empty, or clear. Is there a way I can only extrude around the part, without the inner of my part being affected?

Please explain it easy. Thanks allot

[JDMather]

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@Dreke wrote:
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The problem is that when I extrude a rectangle from the middle of the part tot the bottom it also fills in the space in the center of my diffcase,...

Please explain it easy. .

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It would be easier to explain correction if you attached file with this Extruded feature that results in incorrect solution.

[Dreke]

You can see it is in the part now

[JDMather]

One solution-

another one to follow -

and a second possible solution.

[Dreke]

Please tell me how you did this.

[JDMather · Accepted solution]

Example 2.PNGIt is all in the history tree - no secret technique.

Example1

Example2