This is the fourth in a series on how I built a toy replica dump truck of a circa 1920 Mack AC Bulldog for my grandson. It will deal with shaping the body sheet metal for the front portion of the truck. To go to the first Instructable in this series, click here.
- 20 gauge steel sheet metal (0.030 inch thick)
- 1/8 x 1/2 bar stock
- 1/8 x 3/4 bar stock
- 1/2 inch square tubing
- Pine wood
- Angle head grinder with 1/16 inch cutting wheel
- Angle head grinder with grinding wheel
- Bench grinder with grindstone
- Flux core wire feed welder
- Spring clamps
- Dremel tool with cutting wheels and grinding stones
- Measuring and marking tools
- Drum sander
- Bandsaw for wood
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Step 1: Wood Mockup
I made wooden mockups of the motor cowl, the radiator, the cab, and the dump box. Doing this helped to judge the proportions and make adjustments. Later the motor cowl mockup was used to shape the sheet metal pieces for the motor cowl and to hold them in position for tack welding to the truck frame and to each other.
See the second photo. The motor cowl stands up a little higher than the thickness of a common 2 x 4. I glued a bit of extra wood to it to make the height I needed. If you look very closely, you can see the extra piece is a little different in color.
There were no precise measurements, only "eyeballing" what I thought looked about right. I used a straightedge from the square and I used the larger cutoff wheel to mark the radii for the curves.
I formed the motor cowl by holding the block of wood against a spinning sanding drum. I watched the pencil lines closely as my guides.
Step 2: Forming the Engine Cowl
I pressed paper against the wooden mockup and cut along the creases to get an approximate size for the parts of the motor cowl. I cut the pieces of steel a little large. It is easier to grind away a little excess than it is to add back where I cut something short.
After a couple of attempts with other means, I found the best method was to clamp the steel pieces into a vise with the wooden mockup. Then I pounded the metal to fit the wood as best I could. (I am told body parts for some expensive full-size aluminum cars are made just this way.)
Step 3: Tack Weld the First Piece to the Frame
I placed the wood mockup on the frame and used masking tape to hold the steel in place as I made a few tack welds to secure the steel to the frame. The white paint on the steel form the washing machine and the masking tape produce smudge and black stains that wipe off easily.
Then I realized I needed to make the wooden mockup a little smaller in size. The original did not allow for the thickness of the sheet metal. As best I could, I simply made the wooden mockup a little smaller with the drum sander, but without changing the proportions.
Step 4: More Paper Patterns
In this photo I have already tack welded the other side of the motor cowl in place. I pressed paper against the front edges to determine the pattern for the front piece of sheet metal. (The front sheet metal was pounded on top of about 1 1/2 inches of newspaper to round it a bit for shaping before cutting.) The front piece has been cut and ground to fit. Then I used more masking tape to hold it in place so I could tack weld it in place.
The paper impression shown in the photo gives the shape of the top of the motor cowl. Even at that, there was some careful grinding with a Dremel tool to get a good fit before welding. More masking tape held it in place for tack welding.
Step 5: More Welding
I carefully added more welds to make the pieces more secure and to fill in cracks, all while being very careful not to burn through the steel.
Excess weld material eventually needs to be smoothed and removed.
Step 6: The Radiator
The radiator on a Mack AC Bulldog is behind the engine rather than in front of the engine. At the time, the Teamsters resented trucks cutting into their work. So, they backed their horse drawn wagons into the radiators of trucks with the radiator in front of the engine in order to disable them.
Space was needed for the steering column, so the front sheet metal section for the radiator has an inverted "U" cut out of it to fit the inside of the motor cowl fairly closely. This also meant I could keep clean up of weld material to a minimum by welding the front piece of the radiator to the rear of the engine cowl from underneath.
Then I cut a strip of steel about 1/2 inch wide and formed it to fit the side and upper contours of the radiator. I welded this from underneath, too. That also reduced the amount of weld material that would later need to be filed and ground away. Finally, I cut the rear steel for the radiator. I had to cut into this piece to allow for where the steering column comes through it. This also gave extra support to the steering column and holds it in place very nicely.
As I was building this toy truck, I was always trying to reduce the likelihood of sharp edges that could cut young fingers. The sheet metal was thick enough at 0.030 inch that I could round the edges a little by grinding or filing. I also closed openings that could catch and cut a finger as much as possible.
Step 7: Cab Frame
Because of its unique shape, the cab could be bent if the truck were dropped. I decided to make the frame on each side from 1/8 inch steel for extra strength.
The photo shows one of the frame members laid up from two pieces of 1/8 x 3/4 inch steel bar. The narrower portion is 1/8 x 1/2 inch steel bar. I used the wooden mockup as a pattern.
I wanted the two frame members to be identical. I tack welded them together and ground both at the same time. Then I cut through and ground away the couple of tack welds to yield two identical frame members.
Step 8: Welding the Frame for the Cab
I laid the two frame members next to one another with their back edge on a flat surface. That keeps them in alignment before welding. I welded them to a piece of 1/2 inch square tubing.
Then I welded the cab frame assembly to the frame of the truck.
Step 9: Weld the Skin to the Frame
I began at the bottom and clamped the metal skin to the cab frame. I tack welded each side and moved the clamps up a little farther, tack welding as I went. That made it easier to get a close fit between the skin and the frame.
A crease was necessary in the transition between the back and the top of the cab. I did weld a 3/16 inch steel rod between the front ends of the cab's top. Then I curled some of the skin around this rod and trimmed away the excess.
I ground away all rough edges.
Step 10: Cut the Rear Window
I used photos of Mack AC Bulldog trucks on the Internet to locate and mark the window. I cut it with a Dremel tool and a cutting disc. I checked for irregularities and gently filed them so the window is square and even.
Notice that the frame of the cab extends farther below the truck frame than might be expected. It replicates the original trucks and provides support for the running boards, which run into the fenders. See the next step.
Step 11: Fenders and Running Boards
I am old enough to remember running boards on cars and trucks. In the photo you can see the fenders and how they become the running boards. I used 1/8 x 3/4 strap iron. The fender portion was bent around some of the same pipe I used for the wheel rims. They did not bend as tightly as the radius of the pipe, which is good. Then they angle off at about 45 degrees where they bend to extend laterally back to the frame of the cab. I did add a little spacer between the cab frame and the running boards. I rounded the fender ends. At the point where the fender crosses below the truck frame I welded a 3/16 inch rod and welded the fenders to its ends for front support.
The next Instructable in this series can be found here.