Cutting firewood, either with an axe or with a chainsaw, can be a difficult and dangerous activity. Sharpening your cutting tool and wearing the proper safety equipment are necessary and advisable steps to take, but beyond that, it is a good idea to build a saw horse for cutting firewood. This will provide the safest method for cutting firewood, eliminating the chances of the wood shifting position or binding the chainsaw blade.
Cut the 4-by-4 posts into two 40-inch pieces each. Cut one end with a 30-degree angle and make a square cut on the other end.
Make an X out of two of the pieces. Place the angled ends of the boards on one end to form the bottom of the saw horse frame, and the square-cut ends on the other end. The boards should intersect 30 inches from the bottom of each post. This will create a set of lower legs that are 30 inches long and a set of upper arms that are 10 inches long.
Drill three holes through the posts while they are in the X position and insert a lag bolt through each one. On the other side place a lock washer and a nut on the threaded end of the bolt and use a wrench to tighten them down.
Create three more saw horse frames like the first one. Note that these X frames don't cross in the middle, rather, the bottom is larger than the top for the sake of stability.
Stand the four X frames up and separate them by 24 inches. Place a 1-by-4 on the outside of the lower legs right below the intersection part of the X frames, and screw it in place. Do the same for the other side.
Install the diagonal bracing on the saw horse by placing one screw in a 1-by-4 at the bottom of the far right X frame, and then angle the 1-by-4 upwards to touch the 1-by-4 installed in Step 3 at the far left X frame. Screw in place with two screws. Go back and add a screw to the far right X frame and place two screws in both of the middle X frames.
Repeat this step for the other side, and the firewood-cutting saw horse is complete.
Use the saw horse by placing a log in between the upper arms and cutting it to the desired length on top of the saw horse. This way there are no waste pieces to fall during the cut and bind the blade, since both sides of the cut are securely supported.