Since I had the chainsaw up and running, I decided why not rice it up a little and make a custom exhaust system for it. I've just recently gotten into the world of CAD using Fusion 360 and this would be a good way to test my learning and finally MAKE SOMETHING HAPPEN!

I had this idea to take a picture of the exhaust port coming from the chainsaw with the stock muffler off so I can measure the bolt holes and port holes. It probably would have been best to take that picture zoomed in from a further distance to minimize the optical distortion, but I learned about this after I took the picture. 

It might be hard to tell but some things I did wrong here was using every corner as a reference point and having my units in mm on an imperial part. I found that this was on an imperial system by measuring the exhaust bolt Major Diameter and Pitch which was 1/4 inch 20 Threads Per Inch as well as measuring the exhaust port which was 1 inch. 

It probably would have been best to use either 1 corner as a reference point or the center of the flange. This made it quite tricky to machine the part as we had to use an edge-finder on the milling machine to reference the corners to zero it and then drill a hole based on the x and y coordinates from the reference point. Now I see why machine shop guys and mechanics have that hate on engineers for the bad prints and designs they come up with. However, this opportunity allowed me to see the faults in my prints and make better ones for my next part designs.

Before I had this flange machined, I printed out this design as a 1:1 on paper, drilled the holes, and lined up the mockup flange onto the chainsaw port and it worked perfectly. Below is the mockup flange on the chainsaw

Right after I tested the exhaust flange, my muffler from Aliexpress arrived. There was a minor problem, The Major Diameter of the thread of the muffler was 1 inch and it was advertised as 1/2 inch. THOSE LYING BASTARDS! I mean, I did expect there to be some issues as this muffler was only 6 dollars. Using a Pitch Gauge, the Threads Per Inch (TPI) on the muffler turns out to be 14. So this is a 1inch MD, 14TPI type thread on that muffler.

Knowing the the flange is good to go, I further improved my design by adding a short pipe that connects to the square design of the exhaust port and later working on a thread adapter that will allow for the muffler to be inserted into the pipe connecting to the exhaust flange. Below is a print of my design of the completed product from the side view.

After designing everything, I was finally ready to make things happen!

At the machine shop our first priority was to make the exhaust flange. We cut out a steel flange that had a flat dimension of 35mm x 60mm. Once that was cut, Using each of the referenced corners we moved in the X and Y direction on the Bridgeport Milling Machine to find the center of the two holes we drilled. The next job was to cut out the center port for the exhaust gasses to flow through on the flange. To find true center of the exhaust port we had to take the distance from the edge to the port (18.42mm) and add it to half the length of the exhaust port (25.4/2), this would be our movement in the X direction. For the Y direction, I did something similar and took the height. The Y dimension would be (8.16 + (15.77/2)). This would give us the center point and to start the process we drilled a single large hole in the center. Since we are in the center, we move half the total length of the port in both directions since it is zeroed to the center of where the exhaust port would be. This would be done for both the X and Y direction. 

My next challenge I encountered was to now make the square port align with the circular exhaust pipe. This turns out to be a scaling problem, we need to find the angles from the center point of the pipe that would shape out a rectangle with length 25.4mm and height 8.16mm. After finding that angle, I would then make a line cut of about 1 inch deep into the pipe on 4 corners and bend those cuts into a rectangle. To find this angle involves using the arclength equation for a circle, we know that the angle from opposing sides muss be less than 90 or else that would make a square and we are after a rectangular shape. The formula we use is arclength=(2πrθ)/360. I know that I want the arclength to equal either 15.77 or 25.4 (I chose 15.77), and the ID radius of the pipe is 12.7mm (1/2 inch). Solving for θ in the equation we get θ = Arc(360)/(2πr) ≈71°. This means that 71° is the angle between the cuts on opposing sides. My next task is to make the cuts in which I use a protractor to mark 71° and cut 1/2 inch deep into the pipe, bend those circular edges flat, and press them into a rectangle.

Now that the Flange and the pipe are completed, my next step was to weld the two together. We first took the flange and got the surface machined to remove the top layer of paint and sand down the edges of the pipe so that there is a good contact between the metals. We clamped the flange down with the welding table and lined up the exhaust pipe with the exhaust port on the flange. 

After the weld I took the part home to test fit and it turns out I made an incredibly large mistake. The pipe was welded onto the wrong side of the flange so now the bolt holes wouldn't match up. The problem also was within the print, I hadn't labeled which side was the proper facing direction so in the end, this mistake was made.

The next day, I went back into the shop, cut the pipe back out, resurfaced the flange and had it welded onto the correct side. My next task was to design a thread adapter that the muffler can screw into, the thread needs to be 1inch 14 TPI and be a loose fitting as this part will see much vibration and the threads on the muffler aren't the best of quality. There should also be a slot where the OD of the pipe can squeeze onto to help with the welding process.

Now it was time to make the thread adapter, I picked out a nice cylindrical pole of steel and had it cut to size on the lathe. The outer diameter was done first and then the inner diameter was bored out to spec. The thread adapter was moved onto another lathe where it had a select speed for cutting a certain thread per inch. The dial was selected to 14 and the lathe boring bar was pushed in while the thread adapter was spinning at a certain speed. There was a total of 5 passes made to ensure that the thread was the right size. After the thread adapter was made, it was welded with the pipe on the flange.