How to concave a flat surface?

tj_thorniley

I'm trying to create a model of a microscope objective lens assembly. This is the first element in the series. It's a bit of an unusual shape, but essentially it's a cylinder with a 45° bevelled edge (the chamfer tool worked really nice, by the way). 

It's basically done, but I need to make a shallow indent/depression (concave) in the highlighted surfaces. Kind of like the inverse of a dome, hemisphere, dimple, or parabolic dish (imagine scooping it out with a spoon). If I could invert the chamfer tool (and round off the hard inside edge/corner so that it's a smooth arc/curve - like in the diagrams below) that would be perfect. 

NeilCooke

Please take a look at the revolve feature. You could do that part all in one sketch/revolve. 

tj_thorniley

Okay, sweet. I've been trying with the revolve tool tool but can't seem to get it to co-operate. Would you mind going into more detail? What's the correct procedure? I just need to push it in about a millimetre or two at the deepest point. Thanks in advance, much appreciated.

NeilCooke

You need to draw only half a section something like this (badly drawn, I know):

tj_thorniley

So, it's kind of like using the symmetry tool, but you end up with a 3D cylindrical / spherical object? Surely there's a way to modify the solid I already have? I suppose I could try to split the current model in half and use the 3-point arc tool (and trim) to carve out the concave.

But for more complicated models that's going to be a major hassle. Rather than drawing the whole thing in a single sketch, I usually start with a basic sketch of a single shape, extrude, then sketch from a surface of the new solid, then extrude, and so on.

john_mcclary

Revolve is the best way for this shape.

But if you really like to trim away at a solid you can use boolean operations and making your cuts that way

owen_sparks

Yes as the chaps mentioned
Option 1 Revolve remove the bits you want to cut away from your existing part.
Option 2 New Revolve to create whole part in one go. Note:- Easier than option 1

You can even use the equations you've posted above to drive the sketch for the revolve if you wish.

Owen S.

tj_thorniley

Here, I got it to work using revolve, from scratch.


Then bevelled the edge with the chamfer tool.



Changed the material to glass, edited the appearance to a translucent blue, and hid the intersecting planes.


But I still haven't managed to scoop the concave out of an existing object. This shape is relatively simple, but I anticipate having to do this kind of operation on more complex solids in the future. Having to redraw a completed part from scratch every time I want to make a minor adjustment is very quickly going to become a major headache. Especially if the dimensions are already specified. 

I've tried to be detailed and include diagrams depicting every step of the procedure (not unlike a tutorial). If someone could do the same for me, I would really appreciate it. Thanks in advance.

owen_sparks

Howdy.

Removing from an existing part is exactly the same but instead of selecting the "new" option on the revolve you select "remove" and then which part you want to remove it from. Termed the merge scope, shown below.



The highlighted area is the new sketch used to make the cut away.  (The actual grey part was created earlier from an extrude of a circle.)

Note shown with cross section turned on as it shows the concave nature better, the part is of course whole with that view turned off.



File here:-
https://cad.onshape.com/documents/b6a887c3cdcfbf681b9c9373/w/f16831af92b9443420026b87/e/2b158c9b00f1666108d2bff7

Owen S

john_mcclary

https://cad.onshape.com/documents/28050bca2b1ddeb0acbab720/w/72369ffa7faabbf84de28ab1/e/b15ee4cfd9f19f3a388fd9fc

@owen_sparks beat me too it,

but yea, what he said. above is another example drawing

owen_sparks

john_mcclary said:

https://cad.onshape.com/documents/28050bca2b1ddeb0acbab720/w/72369ffa7faabbf84de28ab1/e/b15ee4cfd9f19f3a388fd9fc

@owen_sparks beat me too it,

but yea, what he said. above is another example drawing

Too slow, chicken marango

Owen S.

john_mcclary

  

owen_sparks

Have an up-vote anyway

tj_thorniley

Thanks for the quick replies fellas. I did try something like that, but to no avail. I'll try it again in a bit, with your examples to guide me. Hopefully this helps someone else too. By the way, how did you do the cross section view? That could be useful.

john_mcclary

There is a lot of neat things you can do with section view, but this is the basic start:

For more info you should check out the OnShape youtube channel. All kinds of stuff to look at there, including a complete run down of this

ollie_morrison

How do you do this with a shape that is not spherical? I have a softly rounded triangular shape that i need to add a concave into. 

Jake_Rosenfeld

@ollie_morrison

It will be easier to help out if you post a link to your document (see instructions here https://forum.onshape.com/discussion/9107/forum-post-checklist-read-this-first#latest)

and some additional context on what you have now and what you would like to achieve.  

john_mcclary

@ollie_morrison
All you need to get a section started is at least 1 flat surface or plane.
Once you have a flat surface selected, you can move/rotate the manipulator wherever you need.

If you grab the circle at the base of the arrow, then you change the start point of the section and you can snap it to any other vertex, including the origin.

jeremy_irwin

This is great! Thanks for the tip. Can you help me make a concave indent on the top of a square object?

glen_dewsbury

Just because I was too slow and everybody agrees revolve is best. Here's a loft version. LOL
https://cad.onshape.com/documents/d2a86f2a038bac6976d055a0/w/aafeaeea2f5793f490b24e03/e/a11571ff46247090ae374903

john_mcclary

I'm not sure how to make a concave square without having sharp edges.

I tried lofting and ended up with more creases that just extruding circles then boolean subracting.

I'm imagining a soap bubble shape is more what you're looking for right?
Which is a bit out of my wheelhouse

https://cad.onshape.com/documents/7049d342b4e7ee052d899f27/w/3de4a12eb3cc3680b0e6edf9/e/09a515e631373149c6cfca54

S1mon

Fill surface is good for adding some crown to a surface. As @john_mcclary says, it's hard to avoid getting some sort of crease or convexity in the corner unless you cheat things a little.

This document has a relatively quick and dirty way to do it:

wayne_sauder

You can also create a sphere and boolean subtract away.