NOTE:
All the measurements in this article are based upon using a 35 gallon barrel like to one you see in the pictures. If you use a larger or smaller barrel all the concepts used to make your composter are the same as described here buy, your measure measurements will be a bit different.

The composter is made in three steps:

1. Prepare the barrel
2. Make the frame
3. Put it all together

Preparing the Barrel
Take your barrel, turn it upside down and drill a hole the same size as the outside diameter (OD) of your 3" tube, about 3 1/4". This will allow the tube to slide through the hole but with the end cap on (photo at right below), it will not go all the way through. Try and place your hole as close to center as possible.

We'll now begin to make the center tube. The center tube is very important because it allows air to circulate in from the outside, giving the the composting microbes the oxygen they need to survive. Using 3" perforated PVC, your center tube already has a number of air holes. My tube is 26" long and when inserted in the barrel it extends about 2" from the top. I thought the tube needed more air so I drilled additional holes.  I also drilled holes in bottom end cap as in the photo below.

Set the center tube aside for moment and go back to the barrel. We're going to drill 4 holes in the barrel for the crossbars  to go through. The top crossbar is the pivot point for the barrel to spin on, and bottom crossbar is to strengthen the barrel.

In order to keep the barrel a little bottom heavy so it does not spin upside down, I measured halfway up from the bottom plus 1" and drilled holes slightly smaller than the crossbar outside diameter (1 5/8") to insure a tight fit. Do this to both sides so a crossbar will go all the way through the barrel. Turn the barrel 90º and drill two more holes 8" below the first set. This will place the 2nd crossbar perpendicular to the top one, but lower.

With the center tube in, place a crossbar through the top hole and note where the crossbar needs holes to go through the center tube. Do the same for same for the bottom crossbar. Disassemble and drill the holes for the crossbars to go through. To help more air circulate through the center tube I drilled some holes where the crossbars intersect the center tube

So composting matter would not clog any of the holes, I wrapped the tube in nylon window screen material and secured it to the tube with some small screws I had lying around the garage.

Dry fit all the pieces together to see how it  fits. The top crossbar needs to stick out a few inches from the barrel on each side. This allows enough space between the frame side for the barrel to spin unobstructed. The bottom crossbar can be cut to fit close to the barrel outside edge. Once it all fits together well, brush PVC cement at all the holes where the PVC crossbars meet the barrel on the inside and outside. This will help hold them in place.

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Fit the top to the barrel and drill several 1/2" holes around the rim. This will help to ventilate the compost. The second reason for the holes is so you can place 1/2" bolts in two hole opposite each other so when the composter is turned upside down the top won't fall off. Your barrel is basically done and you can call it a composter.

Build the Frame.
The frame is made of 1" PVC and various PVC connectors. As you can see in the picture, there are 2 sides and 3 crossbars. 2 crossbars are for stability near the bottom and 1 at top. The top bar which has an outside diameter of 1 1/4" will go through the top crossbar in the barrel allowing it to spin.

The picture  on the left below shows all the parts of the frame side before it's put together. The numbers are the lengths, in inches, of PVC used to make the side. You will need two 5" pieces, 12 2" connector pieces and  4 of each of all of the other pieces .   It's pretty simple to make and goes together like Legos or Tinker Toys. Make two identical sides.

Put It All Together
Dry fit all the pieces together to see how all goes together. If you are satisfied, take apart and use PVC cement to glue all the pieces of each side together. Do not glue the crossbars yet.

When the frame sides are dry, dry fit the barrel to the sides by placing the top crossbar through the barrel and the bottom crossbars to each frame side. If it looks like the picture above and spins nicely, glue the bottom crossbars in place. I left the top crossbar unglued in case I want to remove or replace the barrel.

Once all the glue is dry, your composter is complete and ready for painting (if desired).

We painted our blue barrel green, then we made some bug stencils from card stock.   Yellow spray paint was used to highlight the designs.

Now, we make a new batch of compost every few weeks for our garden and none of our kitchen scraps or yard clippings (grass, leaves, etc.) go to waste. They are recycled back to the earth where they belong.

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