Tuesday, August 10, 2010

Cafe racer seat Part II

Now I'm ready to start shaping the seat. I'll use a mandrel to bend the sheet to a starting radius which is hopefully close to my final goal.
It turns out my leg is just about the right shape.

You can see the metal is curved, but only in one direction. 

It still looks flat in profile view. I want what is called a compound curve, which means it curves in two directions. To achieve this I'll use my planishing hammer, an air hammer which strikes the metal over a rounded die, some of which you see here.

The more rounded the die to the more shape you'll beat into the metal. One quick round with the hammer and the seat had tons of shape.

But stretching the middle creates a wavy, bacon-like effect on the edge, so I need to shrink the edge to keep it smooth. You can see here that the right side has not been shrinked and remains wavy, while the left side has been and has a nice curve to it.

After more hammer shaping, notice how the top middle has started extending further than the sides, compare to pic above, this is because I'm stretching the metal just behind that area (to the right in the picture) and the metal has to expand somewhere. This isn't a serious problem, it can be trimmed later, but it does illustrate one of the dangers of over-stretching:

Now that the set has the shape I want, I'll start making the framework which will keep it sturdy enough to sit on and allow me to attach it to the bike. I'm making frame rails out of 1 inch angle aluminum.

I'll be attaching these with pop rivets so I drill rivet holes about every inch, trying (but not totally succeeding) to keep them level and perfectly spaced.
Then transfer the holes to the seat legs:
Ready to be attached!

Cafe racer seat Part I

My current main project is a Yamaha XS400, I'm turning into a cafe racer, or as close as I can get to one. I'm not putting any particular emphasis on period correctness (considering its an 80s Japanese bike meant to emulate 50s British bikes), my focus will be on looks and performance.
A key part of a cafe racer is the seat, there are many different styles, I'm going with a classic butt-stop seat. It has a raised hump at the back to support you under acceleration. I started by making a pattern off of my gas tank, so that the sizing and shape would be at least vaguely similar. I placed a sheet a construction paper against the tank then traced the body lines and cut around them.
I went through several versions of the pattern, adapting them by eye and placing them on the bike until I got the shape I wanted. )If you do this, you'll have to visualize what the flat paper will look like once its been shaped. Its not always easy.) Then I transfer the pattern to my sheet of aluminum. 

And cut it out with tin snips

Remember to de-bur the edges of cut sheet metal, they can be extremely dangerous so just running a file over the edges can save you countless adhesive medical strips. 
This is also a good time to correct any errors in cutting you may have made:
You can see here that I left a hard angled corner at one end.

But the file makes short work of the corner.

My next step was to add some strength to the legs, they were very floppy and hard to manage and I was concerned they might get damaged if they were to flexible. So I folded the bottom of each side into a lip by hammering it over a piece of angle iron.
The metal will probably warp a little because you're stretching it every time the hammer strikes, but that can be fixed easily with hand adjustments.

Thursday, August 5, 2010

Greetings Starfighters

Oh Ma, look! I'm gonna be a blogger!
I figured I needed a place to post any projects I work on, I have several running at the same time usually. I'm kind of a hack, still learning how to do stuff right, if you know a better way to do something, its probably too late for me, but shout it out anyway. Most of my projects will involve metal working, hopefully someone can learn and/or teach me and we can all make some cool shit. I feel a bit silly knowing that absolutely no one is going to read this for a while yet.
Hi mom!