Trying to create an icon or illustration that has some portions of an item in front of an object with some portions in back of an object can be really tricky. This set of links is a very cool ingenious way to create that link-look to an icon or illustration or chain links. 

Create a rectangle. 30pt stroke, no fill

Select the corner widgets and pull them in towards the the center to their maximum amount. 

Duplicate the shape and double the stroke weight on the duplicate copy. 

With the Direct Selection tool, select and delete half of the duplicate shape. 

Select both shapes and outline the stroke. Object > Path > Outline Stroke.

Move the heavier shape on top of the lighter shape. 

Select both shapes. Duplicate them and flip them. Overlap them as shown.

I made the heavier shapes red to make the next steps easier to understand. 

Select the black shape on top, the red shape on the bottom. Use the Minus Front command in the Pathfinder panel to remove the heavier shape.

This is the result of the Minus Front command. 

Select the other heavier shape. Bring it to the front. Object > Arrange >Bring to Front. Select the top heavier shape and the bottom shape. Repeat the Minus Front command in the Pathfinder panel.

Copy one of the links. Use Edit> Paste in Front to create a copy of the link on itself. Set the reference point on the Transform panel to be in the center, then click on the flip vertical and horizontal buttons.

This is the copy that is flipped on itself.

Select both shapes and use the Intersect mode in the Pathfinder panel.

This is the result of the Intersect mode. 

Duplicate the link to create a chain.

Add the end links to the ends of the chain. 

To create a dimensional chain, open the Gradient panel. Select the chain and select the fill, create a circular gradient that start with yellow, transitions to orange and then to yellow. 

Apply this gradient to the chain links. 

To make it look like a solid chain with no gaps, select the chain, copy it, then choose Edit>Paste in Back.
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With the copy pasted in back, flip the gradient fill to be a gradient stroke. 

Increase the stroke weight on the copy pasted in back until the gaps between the links are no longer visible. This makes it look like a solid chain that has no gaps and all linked together.

Or keep the chain a simple icon with the gaps. 

Creating a 3D spiral look uses the 3D rendering along with Symbols. It is easier than it looks to create this cool 3D spiral.

Create a rectangle for a stripe.

Duplicate the rectangle to create a set of stripes. 

Use the Direct Selection tool to select the right edge and move the stripes up to create a slight angle to the lines. 

Open the Symbols Panel. Window >Symbols. Select all the stripes and drag them into the Symbols panel. The Symbol Options dialog will appear. Name the Symbol, choose Graphic as the Export Type and click OK. 

The Symbol of the stripes will appear in the Symbols panel. You can delete the original stripes you created as they are now saved as a Symbol.

Create another rectangle for the 3D Revolve.

Open the 3D under the Effects menu. Effects > 3D > Revolve. Click the Preview button to see the results. Click OK.

This will create a 360° rendering of the rectangle. Click in the Map Art... button next to the Preview box. 

The Map Art panel is where the stripes are mapped to the artwork. Use the arrows to cycle through the 3D surfaces until you get to the side of the render.

From the Symbol dropdown menu, choose the Stripe Symbols you created. (The only way to map artwork to a render is by way of Symbols.)

With the Symbol place in the Map Art window, you can position the Symbol to the art. 

To fit the stripes to the entire render, click the Scale to Fit button. 

Click the Invisible Geometry check box to make the 3D shape invisible, leaving just the stripes on the shape. ​Click OK.

Use the cube to rotate the shape to get the spiral effect at the desired angle. 

The end of the stripes may not line up when you are previewing the render.

Click on the Map Art… button and rotate the Symbol by hovering outside the corner and turning the stripes until the line ends end up aligning with their ends. There is no magic to this, it just takes a bit of patience and careful rotating. Click OK when done. 

Once the ends are lined up, this is the final render. Choose Object >Expand Appearance to render the object as editable shapes. The invisible shape the lines wrapped around is still there so that will need to be removed. Use the Direct Selection tool to find the edge of the Invisible shape, choose Select > Same > Fill Color to select all the invisible shapes, then delete them.

Ungroup the objects. You may have to ungroup them twice to be able to select the sets of lines. They are grouped in the front set, back set and the tail set. 

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Add colors to the stripes to make it look like they wrap around in a 3D spiral.

Here is how to create a chrome-look volume knob and an indicator that goes from green to red. This uses gradients along the stroke of the line to create these cool effects. And, of course, it is super easy to do!

Create a circle that is half the size of the volume knob you want. 

Apply a stroke the the line large enough that it looks like the circle is filled. The stroke will make the circle twice the size as the stroke is added to the inside ad outside the circle equally. 

In the Gradient panel, select the stroke icon, select the Apply Gradient Along Stroke selection (this is key to making it work)and choose a black and white gradient.

Once the stroke gradient is applied, this is the result.

On the gradient ramp, add color stops (by clicking below the ramp). Alternate the colors from white to 70% gray (double-click on the color stop to change the color), making sure you end up with the same color at the start and at the end for a smooth blend. Space out the color stops evenly along the ramp. 

This is the result of the multi-stop gradient along the stroke. 

Add a circle for the indicator on the dial. Apply a gradient fill to this. 

Choose Object > Path > Offset Path and set the off set to -3 and click OK. This will create a smaller circle on top of the larger circle. 

Rotate the new circle 180° to make is look dimensional. 

Create a circle outside the knob. Add a stroke and no fill. 

In the Stroke panel, set the Cap end to rounded (hot dog the ends), check the Dashed Line box and set the dash to 0 and the gap to 30. The higher the number for the gap, the less dots will appear on the line. 

Rotate the dotted circle 45°. Adjust the stroke weight of the line to get the dots to look the right size.

Use the Direct Selection tool to select and remove the lower section of the circle. 

In the Gradient panel, apply a gradient to the stroke and set the stroke option to Apply Gradient Along Stroke. Create a gradient with a green color stop at the start, a yellow color stop in the middle and a red color stop at the end.

This is the result of the gradient along the dotted stroke. 

Apply an Outer Glow (Effect > Stylize > Outer Glow) to the dotted line. 

To make the glow stand out, draw a rectangle, fill it with 90% black and move it behind the knob. 

To create a slip shadow coming off the knob, create a rectangle that width of the knob. 

Rotate the rectangle 45° and set the opacity at 20%.

Apply and Out Glow to the knob to create a bit of dimension. 

You are done. Nice job!

Create type to look like a blueprint. Simple Scribble effects makes the fill look like a blueprint. Add lines and arrows to make it look hand drawn architectural drawing.

Choose a typeface that is a heavy weight and sanserif.

Outline the type. Type> Create Outlines.

Add a blue stroke to the type.

Add the same blue to the fill of the type. Ungroup the type.

With the Type ungrouped, the Appearance panel will show the fill and stroke. If they are group, these attributes will not show. Select the Fill.

A the the bottom of the Appearance panel, click on the fx dropdown and choose Stylize > Scribble. Apply the settings for the desired scribble look on the fill.

The Scribble Effect is listed under the fill in the Appearance panel. 

Add lines to the top and bottom of the words. 

Apply a gradient to the line with the Gradient panel. Select the stroke selector (not the fill) and apply a gradient from the gradient preview box. Edit both gradient color stops by clicking on each circle and choosing the blue. On one color stop, set the Opacity to 0%. Use the Gradient  flip icon under the fill/stroke to change the direction of the gradient on the line.

Add arrows to the lines in the X. 

In the Stroke panel, apply arrowheads to the lines form the dropdown menu. In many cases, the arrows are too large. Set the scale amount to a lower percentage for a better fit.

For the dotted lines, create a line and set the ends to be rounded (hot dog the ends). Click the Dashed Line box, set the dash to 0 and the gap to 12.

Add lines, fade them with the gradient. Add dotted lines for a hand drawn, architectural look. 

Illustrator has many charts you can create with the chart tool. However, there is no radial chart tool. Here is how you build one using simple math and have Illustrator do the calculations. It all starts with a simple pie chart.

Select the Ellipse tool and click on the Artboard to open the dialog box. Set the size of the circle and click OK. 

Open the Transform panel. For this example the pie is going to be 55%. In the Pie End Angle enter 55*3.6. There are 360° in a circle so each 1% is multiplied by 3.6° to calculate the angle of the pie. 

This is the result of the 55 X 3.6. The pie shows 55% (which is 198° of a 360° circle).

Repeat this process for each pie section you want. 

Select the Polar Grid tool and click on the Artboard to open the dialog box. Set the width and height to 150 pt (the same size of the pie sections). Set the number of Concentric dividers to be 2 more than the number of pie sections you want. Set the radial dividers to 0. Click OK. 

This is the result of the polar grid. 5 Concentric Dividers, no Radial Dividers. 

Select the Polar grid and add a stroke to the paths. Increase the stroke until the path nearly touch, leaving a white gap between the lines.

Choose Object > Path > Outline stroke to convert the paths to shapes. 

Ungroup the shapes. Delete the center two circles.

Select all the pie segments and align them. 

Move the circles over the top and align them with the pie shapes.

Select the outer circle and one of the pie segments. Choose Object > Clipping Mask > Make. 

Select the next ring in and select the next pie shape. Repeat the Object > Clipping Mask > Make. 

Repeat the process on the third ring. You have a radial chart!