Photo 1: Engine frame and jig attached to the lathe in preparation for boring.
Editor's note: This is part two of a four-part series. Read Part 1
Last month I told you about how I became interested in model engines. We also went over a few milling steps that let us get started. This month I want to mill the engine frame as well as bore the cylinder. I think there will even be time to work with the flywheels, so let's get started.
Before the engine frame can be milled any further, it has to be attached to a jig. Some of the milling steps require that the engine frame be held sideways or at an angle. Because of the irregular shape of the engine frame, it would be very difficult to do without some type of jig.
Fabricating a jig
The jig I made is simply a flat piece of 3/8-inch steel that has been milled square on three sides. I made it about 12 inches long and 7 inches wide so there would be plenty of room for the jaws of the vise to grab. By having the three sides milled square, I just have to let the jig touch the bottom of the vise and it will square itself with the table.
With the engine frame attached to the jig, I decided to bore the cylinder. It is important to bore the cylinder before some of the other machining on the engine frame can proceed. This is because the cen-terline of the bore is used as a reference for some of the other steps.
I started by attaching the jig and engine frame to the lathe table. As you can see, I had to use several V-blocks and plates to center the bore with the center of the lathe.
Because I have a small lathe with a minimum speed of 500 rpm, I had two problems. Five hundred rpm is too fast for the 1.250-inch hole that had to be bored, and the largest boring bar I could use was 3/4-inch. However, with a lot of spray lube and a carbide tip, it worked just fine.
Machining the cylinder and bearing cap surfaces
My next step was to face the front of the cylinder, and I began to appreciate the time I took to make the jig. Because the bottom edge of the jig was milled square, I just had to drop the whole thing in the vise and tighten.
I decided to use this cool-looking 3-inch facing tool that came with my milling machine. I set the machine to 120 rpm and got some very good results. Because it did such a good job, I used the same 3-inch tool to face the bearing cap mating surface on the engine frame.
To position the engine frame to the required 25 degree angle, the jig was held in an angle vise. I also added an extra support near the front of the jig, just to be on the safe side.
After milling the bearing caps and mating surfaces, I used a trick I recently learned. I took a little super glue and glued the bearing caps to the engine frame. This held them in position long enough for me to drill the holes for the mounting screws.
After drilling the holes, I used a dead-blow hammer to break the bearing caps free from the engine frame. This let me enlarge the holes in the bearing caps as well as tap the holes in the engine frame.
After removing the old super glue, I screwed the bearing caps to the engine frame and clamped the frame and jig to the mill table. Again, because I milled the jig on three sides, it was a simple task to use a square and get a perfect alignment on the table.
Using my trusty 3/4-inch end mill, I quickly milled the sides of the bearing caps and engine frame. This is one of those procedures that require you to find the center of the cylinder before you proceed. That way, the gap between the bearing caps is perfectly centered.
Drilling the crank and timing gear shaft
Now that I nave a fiat surface on the sides of the bearing caps and engine frame, it is easy to prepare for drilling the holes for the crankshaft.
The jig has to be clamped in the vise sideways. You must to be sure you are the correct distance from the front of the engine frame to the center of your drill point for the crankshaft.
I started with a 1/8-inch bit and worked my way up to 0.625'. The crankshaft is only 0.500', but the extra 0.125' leaves room for the bronze bearing.
While the engine frame is in this position, it is a good time to mill and drill for the timing gear shaft. You can also face the pushrod guide and spark plug locations.
Here is a look at the hole that was just drilled for the crankshaft with one of the bearing caps removed (Photo 3). It is beginning to look like something other than a chunk of cast iron!
Machining the flywheels
The flywheels posed a little bit of a problem. As I mentioned earlier, my lathe has a minimum speed of 500 rpm. It is way too fast for cutting the outside portion of the 8-inch flywheel. I had to borrow a friend's lathe that would slow down to 90 rpm. This turned out to be the perfect speed.
Because these are castings, they are always going to look a little out of round when they start to spin. However, you have to start somewhere. I faced the center of the flywheel with a carbide tool. I then moved to the outside edge and faced this surface.
Before I removed the flywheel from the chuck, I drilled a 0.500' hole in the center of the hub and used a 0.001'-oversize reamer to end up with 0.501'. This will let the flywheel slide easily onto the crankshaft.
With the flywheel faced on one side, I took it over to the milling machine. I placed the face side of the flywheel on some V-blocks to hold it perfectly square with the table. This let me face the center of the flywheel and ensure it would be perfectly square with the opposite side. Now it's back to the lathe.
With both sides of the center faced, the flywheel can be put on a 1 /2-inch shaft. Now all that's left is to face the edge and top of the flywheel.
The second flywheel is prepared the same way as the first, but with a couple of additions. The center of the flywheel has to be cut down on what will be the engine side (Photo 4). This allows room for the governor bracket. You can see the two holes drilled for that bracket.
Two notches also have to be cut into the flywheel for the governor weights. The notches are about 0.128' wide and go in almost to the edge of the hub. With all of this work done, I get to try something new - broaching.
Broaching is one way to cut a keyway into the hub of the flywheel. I wasn't sure how to do it until I saw a broaching kit in a catalog. It looked simple, so I bought the darn thing. I soon found out the hub of the flywheel was twice as thick as the broaching tool was made to work. However, I at least had a sample of what I needed, so I made a bigger one!
I used a 4-by-1/2-inch Grade 8 bolt. The first thing I did was mill a smooth surface on one side. Then mill a 0.128' slot from one end to the other using a 1/8-inch carbide end mill turning at 2,500 rpm. I also had to make a couple of spacers out of some sheet metal (Photo 5).
With all this homemade stuff, I was able to cut a keyway into both flywheels and the crankshaft gear (Photo 6). You are supposed to force the cutter through the material with some type of press. Don't laugh, but I don't have a press, so I just had to drive the cutter through with a brass hammer. It worked and there was no damage to the cutter.
Determining cutter speeds and feed rates
Through this article and the last one, I referred to a lot of different rpm settings for all of these different cutting tools. Are any of you wondering where all of these numbers come from? Well, it's not magic and it is not difficult. But it is important that you know how to come up with the correct speed.
There are two simple formulas you can use to be sure you have the proper rpm settings as well as the proper feed rate. Let me show you an example.
The rpm speed for your tool is as follows: rpm = cutter speed x 4/ the diameter of the tool in inches. There are only three numbers to deal with to get your rpm speed. There is a chart that will give you the first number and you simply measure the diameter of the tool you are using for the third number. Let's do one so you will see how easy it is.
Let's say you are cutting cast iron. Now let's say you are using a cutter made of high-speed steel. Looking at the chart, you will see that the pre-deter-mined numbers for cutting cast iron with a high speed steel cutter is somewhere between 50 and 80. It is suggested that you start on the lower end of the number spread. For this example I will use 60 as the cutter speed.
Now multiply 60 by 4 to get 240. No, 240 is not the rpm needed, but we are close. The 240 has to be divided by the diameter of the tool you will be using. Let's use a 1/2-inch end mill as an example.
If we take 240 and divide it by 0.50, we come up with 480 as the rpm that should be used. It really is simple as long as you have the cutter speed chart to start with. Cutter speed, times four, divided by tool diameter.
Now how fast should you push this tool through the cast iron? There is another formula for that: feed rate = feed per tooth in thousandths, x the number of teeth on the tool, x the rpm.
Let's continue to deal with that 1/2-inch end mill we just determined should have a speed of 480 rpm. The chart says a feed rate per tooth for a 1/2-inch tool in cast iron should be 0.0025 of an inch. This is another pre-assigned number, but it makes things simple for us.
If we take 0.0025, times four teeth, times the rpm of 480, we get a feed rate of 4.8 inches per minute. If you divide the 4.8 inches per minute by 60 seconds, you get a feed rate of 0.080, or 80 thousandths per second. I can't turn a dial 4.8 inches per minute, but it is easy to deal with 80 thousandths of an inch every second.
I hope this has been helpful in understanding how to determine a proper rpm to use and how fast to push a cutter. Next month we will get into some of the little parts and how to deal with holding them in place while trying to mill them to their proper size.
Contact engine enthusiast Richard Allen Dickey at: 246 Skyview Lane, Yellville, AR 72687, email@example.com
Contact the Red Wing Motor Co. at: (660)-428-2288; www.modelengines.com