Cowl Burnishing & Louvers
Before we can make the louvers, the cowl must be burnished. If you did it the other way around, it would be difficult to put the swirl pattern on the raised or curved parts.
Paul Dougherty at the Golden Age Air Museum lent me his louver making pattern. He used this board with some thick gum rubber to press the louvers into the metal. As you can see in the background, the louvers turned out beautifully.
However, the guys in Minnesota who built the Jenny hanging in the history museum in St. Cloud gave me a louver jig, too. It works a little differently.
The jig is variable - the ends slide left and right to make the different sized louvers.
Then a louver die with a hard wood louver shape presses the metal. Instead of the hydraulic press, a mallet is used.
I wanted to practice making a louver this way.
Two holes need to be drilled at the end of the louver. Then a slot was cut into the sheet metal.
I used a die grinder for the cut. A die grinder not the best way to do this because it is hard to cut a straight line and it leaves a ragged edge. But, this was a test piece and I didn't care too much about the cut.
Paul told me he used a sharp razor blade with a steel guide to cut the slot. He said it took a lot of passes with the razor but it cut the steel just fine. I could tell from the pressed louvers that this method made a prettier line. When we make the final louvers, I will do it this way.
Anyway, the hardwood die was hammered into the jig.
You can see it better here.
A finished louver in about three minutes of hammering.
Brian and I also decided to experiment with burnishing. Burnishing was used back then to hide the machine marks in the sheet metal when it was shrunk, stretched or pounded over a form.
Using some photographs we played with several different patterns and spacing trying to find the look that we liked. Eventually, we came to a consensus. The burnishes would be 1 1/4 in apart.
I liked the way the drill press burnished the metal rather than how I made them earlier with the hand drill. But, the cowling is pretty deep and most drill presses can't reach that deep so I was resigned to make them by hand.
But after some thinking, I remembered the Grizzly long reach drill press I bought years ago to drill some angled holes in a stabilizer. The throat was deep enough to reach the center of the cowling.
So, I made an extension arm to go underneath the metal.
We marked the spacing on the sheet metal and started burnishing.
We also tried varying the amount of time the Rylok pad spun on the metal. Three seconds produced a feint swirl. Ten seconds made the swirls deep and dark. Holding down the lever for five seconds worked best.
We also tried various drill RPM's. About 900 to 1000 seemed to be the right spindle speed.
This really is a two man operation. The main job of the second person was to help hold the big piece of metal. But later, we found that if one person could sites down the top line and the other person set up the spacing distance between burnishes, things moved along nicely.
This is what the sheet metal will look like !
Let me warn you - this process takes a long time. What you see here was about an hour's worth of burnishing. There are 21 burnishes in each row. It takes 5 seconds to make the swirl plus about the same time to line up the next one. Doing the math, each row took nearly five minutes. We made twelve rows. That's one hour's worth of work. Whew!
Lastly, the oil tank came back from the welder. Some day I want to learn to weld aluminum with a TIG welder. I've always welded with gas because I couldn't afford a TIG welder but eventually I'd like to learn how to do it.
The tank was made using an old photograph as a guide. I found a picture of a Hisso Jenny without the cowling and tried to make it the same way.
I know having the weld flanges face outwards is not as strong (nor as pretty) as having them face inside, but this is how they made them back in 1917.
That's it for now. More soon. Enjoy