Drawing a Mug Mold Using OnShape CAD

Follow me, step-by-step, as I draw a coffee mug case mold, with handle. You'll be able to 3D print this using PLA filament in a standard consumer printer, pour in plaster to make a working mold. If you are coming from Fusion 360 the process will be familiar.

There are lots of online videos showing the use of 3D printing in creating molds for ceramics. But almost no one shows you how they did it. And they almost certainly do not show a step-by-step of the most complex part: Drawing it in a CAD program. Here, I will do this using OnShape, an industrial strength 3D CAD system that is free for hobbyists. The arrival of such software is an inflection point for ceramic artists and craftsmen, one that is going unnoticed by most.

1
After logging in to OnShape.com, I'll by click on my name on the top right and choose “My Account”, then open the Preferences tab.
In the “Units section” I’ll choose “Millimeter” as the default length unit.
Next, I’ll go back home, by clicking the OnShape logo on the upper left, then click the “Create button”, then choose “Document”, enter a name, and click the blue “Create button”.

2
OnShape is parametric, like Fusion three-sixty.
I’ll open the “Variable Table” using this button on the far right.
I’ll need six variables:

For the mug, they are the foot radius of 42mm, base radius of 50, lip radius of 51 and height of 114.
For the 3D print, we need the wall thickness of 0.8mm.
And for the working mold, a plaster thickness of 20mm.
Note that OnShape does not permit spaces in the names.
OnShape exhibits one of its idiosyncrasies here.
You can’t edit the contents of a cell using the mouse.
On each cell, just begin typing to replace what is there. This characteristic is also found in other parts of the user interface, it is a consequence of implementing this in a web browser instead of as an application.

Please pause the video here, if needed, to finish entered the variables.

If you have not already watched our video on understanding the indiosyncrasies of OnShape, compared to Fusion 360, consider doing that now.

3
To start, I’ll hide the front and top planes and then choose the Sketch Tool.

The sketch dialog appears and it is waiting for me to select a sketch plane. I'll click the right plane, and then double-click that face on the View Cube to square it on the screen.

OK, let’s begin drawing the outer profile of the mug.

Choose the line-tool, click on the origin, move rightward to draw the flat foot radius.
Let it infer a horizontal line of about 40.
Draw the diagonal edge of the foot by clicking on a point up, and out, about 10.
Finally, draw wall about 50 high.
Don't let it infer a vertical line, click to get an inward slope.
Then press Escape.

Now, press the “d” key to select the dimension tool.
Click on the first line, pull out a dimension arrow, and click again.
Type “foot radius”, as soon as foot radius is the only item in the list, press the enter key twice.
Now, making sure the dimension tool is still active, click the diagonal point.
Then click the origin.
OnShape guesses whether we want a vertical, horizontal or diagonal measurement.
Just move around until you get what you want. We want this 10mm vertically up from the origin.

Then, I’ll dimension the point to be right of the origin by the “base radius” variable.
Then, I'll move those measurements out of the way.

Notice the line has turned from blue to black. That’s good, it means it is fully defined.

Now, let's repeat by dimensioning that top point

Vertically up from the origin “height times 0.78” mm.

And horizontally right of it “lip radius times 0.88”.

4
OnShape exited that sketch, so I'll right click it and edit it again.
First, I’ll draw a horizontal line for the lip of the mug.
I'll put it about 20 above the endpoint of that vertical side.

I’ll dimension its length as the “lip radius” variable minus four. To do this I have to enter the parameter in the normal manner, then add the "minus four" after. But I also have to finish with "times 1 mm", this is a requirement when added or subtracting numbers from variables.

Next, I'll dimension the left enpoint distance up from the origin as the “height” parameter.

Finally, I’ll choose the “vertical constraint” tool, and then position the left point of the line vertical to the origin.

Next I’ll choose the “point tool” and create a point down from, and to the right of, the right end of the line.
I dimension it 4mm right, and 4.5mm down.

Then, I’ll use the “spline curve” tool to join the lip and the sidewall, clicking on each point and double clicking to terminate at the end.

Next, I’ll use the “Tangent tool” to smooth the transition from curve to straight at both ends of the spline.
For each I'll click the curve (not the curve handle), then the straight line.

To finish the mug profile, I’ll fillet the two corners at the base, to a 5mm radius.

5
Now, let's turn off the "right plane", and offset this profile inward by the “wall thickness” variable.
I’ll do that using the “offset tool”.
I'll carefully click each segment of the mug outline, then use the arrow to get it approximately right.

Finally, I’ll press enter, then type in the "wall thickness" parameter name, and press Enter again.

Now, I’ll create a vertical line, from the origin, up the center, extending above the top line by 10mm.
The reason for this is not obvious, but you will see why shortly.

Next, I’ll choose the “Mirror Tool”.
Notice that its dialog wants me to select the mirror line first, then the entities to be reflected.

I’ll just choose the vertical wall section of the mug, but not the base or lip.
Notice the line sections don't stay selected, but they do accumulate on the left.

I’ll finish with the “construction line” tool.
I'll search for it here. Notice that as I type, it highlights, in yellow, the toolbar item it thinks I want.
And, it gives me a list with descriptions and shortcuts.
This is a great way to learn the locations of the tools.
I'll click the first section.
This doesn't look good. Sometimes it is better not to worry about why this happens.
Just undo, and do it another way.
I'll make a selection rectangle around it instead and then click the construction line tool.
I want the reflected contour to be a construction line because it is only needed as a drawing guide later in the process.

6
Next, let’s create three points to describe the handle.
I'll put them in their approximate positions.

I'll dimension the first to be right of origin by “lip radius times 0.7”.
As a reminder, I am going to all of this trouble with parameters so the mug will scale intelligently if I change one of the variables.

Now, I'll dimension this point, up from origin, about three quarters of the height, namely, “height times 0.77”.

The second point is to the right of the origin by “base radius times 1.44”, this is the outer extremity of the handle, well outside the mug body. As I do this, the dimension lines can get cluttered, but that is not a problem, I can move them out of the way later.

I'll position this second point, up from the origin, about two thirds the height of the mug, the "height parameter" times 0.66.

Now, I'll dimension the third point to be, right of the origin, by the “lip radius parameter" times 0.86.

To finish, I'll position it up from the origin by “height times 0.25”.

Next, let's join the points using the spline-tool. This will give us a curve that has a handle at each end, to enable manipulating the direction of the line out of the point, as well as the pull of its gravity.

Finally, I'll adjust the upper part of the curve, using the handle on the starting end of the spline.

7
Next, using the "line tool", let’s draw the outer perimeter of the mold.
I’ll start from the top end of the center vertical.
Then I'll go leftward, drawing line segments to follow the approximate shape of the mug.
For a piece having no handle and no flared lip this is simple.
But in this case, I have estimate the distance to end up with an approximate average thickness of 20mm, that is what we defined the 'plaster thickness' variable as.

Next, I'll dimension the mold base as that same distance away from the foot of the mug.

I can then use that as a visual guide to fine tune the positions of the vertexes that join all the lines that form the perimeter.

Next, I'll add three 13.5mm natch holes.
To do that, I'll use the "center point circle" tool, click for the position, drag the approximate diameter and release the mouse button, enter the diameter, and press the Enter key.

Then I'll press Escape to leave that tool, select it, press command-C for copy, then command-V twice to paste. If you are using Windows that will be control-C and control-V.

Next, I'll position each of them.

Finally, I’ll fillet the corners.

To be continued in the next few days...

v3 Shelled AI Mug Using OnShape CAD

A great on-ramp to learning slip casting

Available on the Downloads page

This picture has its own page with more detail, click here to see it.

This differs from the v2 drawing (below) in that the plaster pouring cavity is formed by shelling (hollowing) the back side (top right). v2 was formed on the front side, by revolving and sweeping the mug profile and extruding the side walls. This v3 shelling CAD design method has several advantages:
-No sketch offset or extrusion was needed to make the outer wall.
-Because the first action is to extrude everything as one solid mass, corners of the outer 3D perimeter can be heavily chamfered (reducing the amount of plaster needed to fill the mold).
-The mug's geometry (offset inward by 0.8mm) is revolved, swept and bevelled by cutting into the block. Shelling to the same 0.8mm wall thickness, from the backside of the block, produces the cavity needed (top right). This approach offers flexibility in the printed wall thickness. Modern 3D printers can handle this thickness well (top right), thus reducing print time and filament use.
-The last steps, after shelling, are chamfering the outside inner corner, cutting the holes for the natches and revolving the pouring spout (as a separate body).
-Our v3 natch system continues to work well with this (lower left).
-Printing artifacts are not a problem for prototype molds (visible op right). Production is asking me to enhance these (a freshly cast mug is shown lower right).
-The plaster mold is of stunning quality (PLA 3D print was carefully softened and peeled using a heat gun and needle-nose pliers).
-Slip cast mugs most often have poor-quality and oval lips. This one stays round because of the outward flare and the quality is better because the 3D printed pouring spout also acts as a cutting guide at the pre-removal stage.
-The PLA pouring spout is deep and absorbs no water. Thus, the slip level does not need to be topped up during casting, the slip surface stays flatter (not developing a bowl shape) so pour-out time can be accurately gauged by its slip level.

There are many casting body recipes that would work with this. DIY CAD skills will enable you to follow me into another exciting world: Low-cost 3D printing of the clay itself! Coming soon.

By Tony Hansen
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