Cylinder printing wrong size

pete_allen835

New to CAD & 3d printing, I created a holder for glue sticks which are 20mm in diameter. It  simple L shape with a cylinder sitting on top of it. The cylinder was 24mm in diameter. I used the shell function to hollow out the center of the cylinder, used a 2mm wall which should have left me with an opening in the cylinder of 22mm. After 3d printing the hole came out 19.88mm. Don't quite understand what happened here. The rest of the dimensions came out correct, including the outside diameter of the cylinder.

My extrusion width is 4mm is that were I lost my 2mm?

owen_sparks

If you post a link to the doc I'm sure someone will look at it for you.

Jake_Rosenfeld

Hi @pete_allen835

If you click on an edge in the system, its measurement will appear in the bottom right corner:



When you click on the edge which you expect to have a 22mm radius, what does it say?

.12mm deviation is probably within tolerance for your 3d printer.  When you design for 3d printing, you often have to underbuild; things often tend to grow a little too large when printed.

owen_sparks

Methinks that is a 2.12mm deviation. 

I presume the extrusion whidth is 0.4mm but that won't influence the diameter of a hole.

owen_sparks

Hang on, we've shelled a 24mm diameter cylinder with a 2mm wall thickness? That'd be 2mm on each side so 20mm hole would be correct.

brian_jordan

A 24mm O/D cylinder with a 2mm wall would leave a 20mm opening wouldn't it? Or am I missing something? If I'm right 19.88mm, while not great, isn't too bad for a first attempt.

pete_allen835

I think I see now how I should have done it. Made my circle the diameter I wanted the inside of the cylinder to be, them added the 2mm wall shell to the outside. Much less math this way, I was under the impression that 2mm wall would shrink it 2mm not 4mm, I get it on a rectangle but not a circle.

Ok I learned something, double the wall thinkness on a shelled circle or put the walls on the outside. Thanks everyone

brian_jordan

@owen_sparks Is there a technical expression other than "Snap" to cover posting the same answer at exactly the same time?

@pete_allen835 My recent use of Onshape has been designing and 3D printing stuff. I find it is essential to measure the final object whenever I alter anything and, if required, to adjust my Onshape model if it is out of my expected tolerances.

Jake_Rosenfeld

@pete_allen835

Maybe it will help your mental model to think in terms of
"A 2mm shell on a cylinder will shrink the radius by 2mm"
rather than the more complicated
"A 2mm shell on a cylinder will shrink the diameter by 4mm"