Start by resizing the Tinkercad workplane to 250mm wide x 200mm high (Edit Grid at the bottom right of the screen) and set the grid snapping to 1mm. Set the view (icon on the left hand side) to flat / orthographic. Use the view cube to set the view to Top.

Now for a little maths. We’re going to make a 12-sided cuff with one of the sides open to allow it to slip on and off. For a cuff of 190mm circumference, each side will be 190/12 = 15.8mm. Let’s go with 16mm to make life easier.

But the Tinkercad polygon generator (find it in the Basic Shapes) doesn’t allow you to input a side length, only the overall X and Y dimensions. So we need to work out what those should be.

Looking at the diagram of a dodecagon, the length r (which is the radius of the circumcircle) is given by

r sin α = d/2 where d is the length of a side.

α is 1/24 of a full circle, ie 360°/24 = 15°, and in this case d = 16mm.

This gives r = 30.91mm and the width and length of the dodecagon (ie the distance between opposite corners) is twice that, 61.82mm. So, having dragged a polygon shape generator onto the workplane and set the number of sides to 12, 61.82mm is the size you need to stretch it to in both the X and Y directions. Then change its height (in the Z direction) to 60mm.

Now copy this solid body (using copy and paste) and turn the copy into a hole. Resize the hole to make it 2mm smaller in the X and Y directions (ie 59.82mm). This will give a wall thickness of 1mm.

Align the centres of the two bodies — select them both by clicking with Shift held down, or by drawing a selection box round them — then use the Align tool in the top menu. While they’re both still selected, Group them (top menu again) to make them merge and create a hollow cuff. Then rotate it by 15° so that the top and bottom sides are parallel with the X-axis.

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