How do I draw a forearm?


I am trying to draw a forearm for my robot.

The elbow end will be a round hole but the wrist hole will be similar to an ellipse (oval). It will not be an ellipse though, it will be two arcs on the ends of a rectangle shaped.

The plastic will be 3mm thick everywhere and the length will be 240mm.

Just like your own forearm, the rectangle /arcs half of the shape will be in the bottom half. I am having trouble smoothly merging a cylinder into the different wrist shape half way along smoothly while maintaining the plastic thickness.

Can anybody help? Please remember that the wrist is not oval shaped, it is a smooth rectangle using two arcs. Thanks.


Tell me if this model is on the right lines, and I’ll describe the steps!

arm.skp (124.2 KB)


Its exactly right. I would extend the wrist a bit further along before rounding-out, but its what I was trying to achieve. Thanks so much! How did you do it?


Select the top edge of a cylinder (not the face, just the edge), and right-click/Explode Curve. You can now carefully select half of the edges of the circle. Just select one half, and only of that end of the cylinder. Use the Move tool to move those edges out by some amount. Then select the other half of the circle edges, and Move those the other way, by the same amount. You should have your rectangle with semi circle ends.

Now select the round end rectangle, and use the Scale tool to shrink it to wrist size. You may need to hold down a modifier key to make it scale around the center of the face.


I can see how that would work but I’m having real trouble selecting exactly 50% of the circle. I find that I have to manually count the segments and then painstakingly count half of them, holding the alt key and adding each segment one at a time. Also, The offest tool doesnt work on the new shape to make the shape hollow with a 3mm thickness.


Before you explode the circle, draw a line across its diameter. Select half the circle on each side of the line, then explode the remaining arc.
An alternate method is create a series of profiles that match a cross sectional shape of the forearm at different points along the length from elbow to wrist. Select the shapes, apply the curviloft plugin to skin the space between each section (selecting them all will do it from end to end). The resulting skin will be a lot more organic looking than what you will get with extruding.

Let me know if you need a concept demo.

Edit: For creating the thickness, copy the shape, paste in place, scale in the x/y directions (assuming the length is in the z axis) explode all, then close the ends off. Regroup and run solid inspector.


For the offset problem, do the offset on the cylinder before you do the other steps.


One thing though, if you do end up scaling the wrist smaller, the 3mm thickness will change too. You may need to get the wrist size to be exactly right just using the Move tool.


I downloaded a plugin called ‘Get Center Point’ to find the centre of the circle. Then I drew a line to the edge. There’s a problem at this point - the distance from the centre to the edge varies even though its supposed to be a circle. ie. I drew a circle with a radius of 51mm but the actual radius fluctuates between 51 and 50.56mm. The centre line therefore potentially fails to cut the circle down the exact centre.

After doing this, the two halves of the circle are recognised as two shapes, so I selected just one. This should be exactly half. Now when I drag the edge out, the line in the middle of the circle, as well as the centre point dot hugely impact the new shape and cause a mess. It doesn’t seem possible to keep the semi circle highlighted while deleting these graphical markers. If I use the Ruler instead of the line tool, the semi circle is not recognised anymore, and its back to figuring out the exact half way point manually again, which is so painstaking that I inevitably make mistakes and need to restart all the time.

Is it essential that EXACTLY half the circle is dragged?

In relation to offsetting, justcheckinm8 seems correct in saying that the resulting image should be duplicated, resized along two axes and centred, although failing to adjust the third axis will result in warping. If three axes were resized, then there would be blade edges later on, but this would probably be easier to fix. I assume the gap would be filled by placing two lines between the gaps along the edges and sketchup would colour in the gap, then I would delete the two lines.

The other thing is that when I want to scale the curved rectangle to specific dimensions during resizing, I’m not sure how to input the correct scale in mm. The resize tool appears to be measure in factors rather than mm, and even if I try to calculate the correct size in factors rather than mm with an equation that takes the existing length x height and see what I need to divide by to calculate the end result length x height (this also requires pre-planning the amount I need to drag the semi circles out by multiplied what I will later divide by), I cannot centre the resizing because the alt key that is usually used to keep resizing centred is disabled if a numerical value is given for resizing. This also means that no changes to the length values of the wrist can ever be made unless I start again each time. Is there no way to simply draw a wrist hole with a 3mm offset, then the elbow hole with a 3mm offset and connect the two smoothly? This method seems uneditable and extremely difficult to calculate.


Yes…absolutely agree. Since scaling moves in a flat plane, and not perpendicular to the lofted surface, it won’t be a consistent 3mm thickness throughout the length. A ‘best bet’ approach could be creating 2 sets of section shapes, scale the inner set on the x/y axis, then move them about on the z axis to be adjacent on a perpendicular axis to the outer face. It’s never going to be perfect. Even with the ability to use the offset tool, an offset square, by rights, should have an arc on the corners…because 3mm following around an inner square corner makes it a radius :smile:

Any chance of getting the arm.skp in a SU8 format? (sad but true…but I can do everything I need with SU8 and have better things to spend $1000 NZ on)


It isn’t a very exciting file, but here it is in SetchUp 8.

arm8.skp (141.2 KB)


You guys are very generous on this forum. I will follow your kind lead for others when I learn a few things.

My original method was to create the curved rectangle wrist hole, then the round elbow hole, both 3mm thick and 2d. Then I extracted them both out. After that I attempted to cone the cylinder in towards the extracted quasi cubish cylinder. Its pretty messy, but maybe there’s a way to combine your method and my method to get the best result an allow me to resize accurately?

amateur forearm.skp (122.9 KB)


PS…You said in your OP “it will be two arcs on the ends of a rectangle shaped”

So why not draw a rectangle, then wrap a couple of arcs across each end? Remove the lines across the end of the rectangles, select, resize to your needs, then weld.

I think where the circle cutting method would be failing is because in the x and y cardinal points of the circle, you have a pair of segments pointing left and right. You can fix that by rotating your circle by 1/2 the angle of a segment.

For exaggeration purposes: Create a circle with 4 sides. You’ll see that it doesn’t produce a square, rather a 4 sided diamond. Now explode the ‘curve’, then try and drag one side over…and see the horriblness unfold. Now redraw, rotate it through 45 deg (1/2 the angle of each segment), then cut in half with a line, select one side and move it…and it won’t distort the remaining shape.

Here’s my take on the forearm

OrganicForearm.skp (266.5 KB)


Sound like you would find the subpages under Tweaking Tool Tips useful :wink:


That seems to be the perfect method. How did you close the gaps between your circles?


Thanks, catamountain. I’ll look at that :smile:


It’s the curviloft plugin. It’s also very useful for doing transitions from different shapes (squares to circles, etc). It’s great for doing the organic shapes, but you do have options for skinning the shapes with a more mechanical look.


If you are not opposed to using a plugin, you might consider Spirix:

This only works in inches, so your 51mm radius became 2.000" and the 240mm height became 9.450" and the wall thickness became 0.120" … however, it demonstrates the concept. Note that I use two 24 segment half-circles to create the full circles in order to establish a head-to-toe direction (and then use the Spirix Group function). Consistent with this direction (and using an equal number of segments = 48) I start with a 6 segment 1/4-circle, a line divided into 12 segments, a half-circle with 12 segments, another 12 segment line, and finish with a 6 segment 1/4-circle (and again use the Spirix Group function). After that, it’s just a simple matter of playing around with the parameters. If you don’t like the result, Ctrl-Z and try again with a different one. Or move the Groups c1 and c2 around and re-generate another surface. In addition to the primary surface, you can also create an offset surface as well as the side and end “skirts” that stitch things together to make it watertight.

BTW, SketchUp doesn’t seem to think that this is a solid since it’s actually four mesh surfaces in separate groups. However, they are deliberately constructed as a manifold solid and can be exported un-exploded as STL files to MakerBot (for example):

Spirix Plugin: here

Model: arm.skp (643.4 KB)


Here it is with your file.

By adding transitional sections you can create more definition to the shape.


Awesome demo there Box :triumph:…exactly what I did in my version of the model.