How to counterbore a hole on a sloping surface?

[abennxxx]

I've created a solid with a flat bottom and curved top.  I've created two holes, starting from the bottom flat surface and extending up through the top curved surface.  I now want to counterbore at the top surface so that screw heads will have a flat seating when inserted from above.  I've attempted to do this by creating a larger-diameter hole from the top surface down a few mm, but I can't figure out how to center and align the new hole on the existing hole.  Whenever I click on the exising hole to reference it, the new hole alignment either goes into the side of the existing hole, or perpendicular to the curved surface.  Any ideas, please?

[jhackney1972]

The Forum needs your model.  If you do not know how to attach your Fusion 360 model follow these easy steps. Open the model in Fusion 360, select the File menu, then Export and save as a F3D or F3Z file to your hard drive. Then use the Attachments section, of a forum post, to attach it.

[jhackney1972]

Without the model, this is a guess.  Create a construction plane over the curved surface and the hole coming through.  Create a sketch on it and project the hole center, from the far flat side, to the plane.  Sketch your cbore circle on the center and Extrude.  Example model attached.

[TheCADWhisperer]

@abennxxx 

Revolve works

Extrude Offset works

[davebYYPCU]

Counterbore hole from the offset plane works.

Might help….

[abennxxx]

Thanks jhackney1972 and others.  Creating a construction plane with the initial holes extended up to pass through it as a reference sounds like what I need.  I'll report back with a copy of my model if I'm unable to figure out how to do that.

[TheCADWhisperer]

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@abennxxx wrote:

 Creating a construction plane with the initial

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@abennxxx 

Not needed.  Simply Offset from the original sketch.

[abennxxx]

I understand the concept that's been suggested but I can't figure out how to create a construction plane, or offset the original sketch -- possibly because of the way I've built up the model in two separate stages.  If I go back through the timeline there's two separate 'create sketch' events which I can 'edit' or 'redefine sketch plane'.  Using 'edit' I can modify each part of the sketch (the rectangular base and the curved top) separately, but 'redefine sketch plane' simply makes one of them disappear.

As suggested the .f3d file is attached.

[TheCADWhisperer · Accepted solution]

@abennxxx 

Your Sketch2 is not needed.

Example to follow in a few minutes - check back...

@abennxxx 

Edit dimensions as needed in my example (you might want to change my offset distance to 1.5 - I was not sure of your true Design Intent of the Counterbore depth).

In my example I demonstrate a Revolve method and an Extrude - Offset method.

Go through the Timeline step-by-step and examine each sketch and feature...

[abennxxx]

Ahhh. I saw the 'offset' option in the hole menu, but I couldn't figure how to apply it.  That seems to me to be the most straightforward to me.  I can see how your 'revolve' solution works too, except I can't see how the stepped sketch that you've revolved gets there in the first place.  I'll save your example and replace your revolved counterbore with an offset one for that hole.

Thank you so much for your help.  For me, learning by practical example like this is better than any written or video tutorial.

[abennxxx]

Thank you for the tutorial. Rather than modifying your example, I've now modified my original model to give me the required counterbores using the 'extrude' method you demonstrated. As with most things it's easy when you know how!