Hey Man. I've written up a guide to the process we use in our shop. Feel free to read through it and let us know if you have any questions. We are happy to help. This is from the blog on our site: http://bedlamwerks.com/blog/80-seatpanguide Follow the link for pictures.
1) Remove your seat and measure the dimensions of the frame where you would like your seat pan to rest. Here is a guide to taking measurements: http://bedlamwerks.com/blog/71-how-to-measure
2)Make a template using bristle board or cardboard of the base of your seat pan, which should include both the seat and backrest section. We always cut the seat to exact dimensions, but leave at least an inch of extra material in length and width to the back rest section to allow trimming and proper fitment to the cowl, which you will be adding later.
3) Trace the template onto your sheet of aluminum. We use .090 3003 aluminum for strength. .063 is also commonly used.
4)Using jigsaw, bandsaw, or nippers, cut your template shape out of the aluminum.
5)You should have a long rectangular or trapezoid shape with a half circle dome shape on the top. Bend edges down to a 90 degree angle using metal break or by clamping the aluminum to a table and bending by hand.
6)Bend or roll your dome shape upwards, the opposite direction of your edge bends. We prefer using a sheet metal roller, then english wheeling the backrest section to ensure a nice clean arc.
7)At this point you should have your seat and back section roughly set. Next you will be making the cowl.
8)Decide what shape you want for your cowl. Here the possibilities are endless. This example is for a standard dome shaped cowl. Dome shapes require more hand shaping than some of the more simple rolled cowls. It depends on your preference. Next you make your pattern.
9)For a cowl pattern, much like the back rest section, we generally find it safer to oversize. For any cowl, especially a dome, oversizing a few inches can save you from wasting time and material.
10)Because dome cowls require a deep draw, it is extremely helpful to anneal the metal. Aluminum work hardens, meaning the more stress you put on it the harder it gets. So in order to get the correct depth and shape, multiple annheals are typically required.
*How to annheal aluminum:
-You will need either an oxy/acet. or an oxy/propane setup with a hothead torch tip. There are other ways to annheal aluminum, but using torches are typically the easiest and fastest.
-Light your torch with only gas going through the torch head. This will produce a very sooty flame.
-Run torch using this flame over your entire piece of metal until it turns completely black from carbon.
-Add oxygen for a typical heating flame as you might when you’re heating to bend steel.
-Run flame back and forth across the aluminum in even, steady strokes. What you are trying to do is bring the temperature of the metal up to around 700 degrees. You know this temperature has been reached when all the carbon has been burned off and you are left with a nice shiny finish. A little warping and misshaping is common, but can be reduced by evenly heating the metal.
-Be very careful not to overheat the aluminum. Overheating will not only cause heavy misshaping, but also can cause the aluminum to melt. This can happen very easily and quickly depending on the thickness of your aluminum, so keep the torch moving at all times.
-Aluminum is not hardened by rapid cooling like steel, so when you're finished annealing your piece, you can either wait for it to cool in the air or quench it with water. Unlike steel, this will not affect the strength or flexibility of your newly annealed piece.
11)After the metal has cooled, begin hammering with a rubber or wooden mallet. We typically use wooden as it works faster and is a bit more aggressive. You can easily make your own using a wooden baseball bat. (insert picture)
12)Begin hammering the metal in a uniform fashion, working in rows starting from the inside and working your way out. The metal will want to begin closing in, so you'll have to go back and forth between hammering and reforming.
13)After the desired shape is formed, it's time to use the English wheel. Choose a die that matches the contour of your cowl. Tighten the wheel and begin rolling back and forth to smooth the metal. Keep in mind that with hammering or wheeling your metal is always changing shape. If you move it in one place, it's going to move in another. So you're constantly going to need to step back and reform things to your desired shape. When you're done, you should have a fairly smooth and symmetrical form. This takes practice. Forms such as bucks can be made in order to ensure symmetry but for something as small as a cowl, it is typically more work than necessary to make a buck.
14)At this point you should have your oversized cowl with fairly rough edges. So it's time to match your cowl up to your seat pan. First you need to focus on creating a flat bottom for the cowl while making sure your sides stay symmetrical. The easiest way we've found is to use a sharpie and place the cowl flat on a table. Trace along the inside of the cowl with a sharpie using the table as a base for running the sharpie along. Go to the bandsaw and cut along the line. This may require multiple passes til you create a nice flat bottom and symmetrical dome. You can also use a ruler on the inside of the cowl and trace a line from point to point along the bottom edge and cut this out. This is faster but can sometimes require an extra set of hands.
15)Next you need to match the oversized back rest section of your seat pan to the dome. You should be able to place the dome on the edge of the table and back the seat pan up to line it up straight. Reach on the inside with a sharpie and trace the backrest along the inner edge of the cowl. You can cut this line using a bandsaw, which can be very tricky because it's at an angle. You can also use a jig saw or tin snips, but this can take longer. I typically use the bandsaw at an angle and very slowly cut to maybe a 1/4 inch from the line. Using the bandsaw, you're not going to have a straight cut with this method, which is why I leave extra. But you can go back in with tin snips and trim it closer, finishing up with a horizontal belt sander for final sizing. You're seat pan and cowl should now match up.
16)You're cowl will probably require more trimming along the front edge which can usually be done with tin snips fairly quickly, but once everything is lined up perfectly, you'll want to do a final sand and clean because it's time to weld.
17)Welding: We tig weld everything we make. You can also oxy-acetylene but if you have access to a tig welder it's typically the preferred method. As with any tig welding, make sure you're metal is extremely clean, free of rough edges, grease, oil, dirt, etc., because all these will contaminate your weld. There are a lot of good welding videos online that will teach you how to weld aluminum, and more than anything, practice practice practice.
18)For our seat pans, after it's tack welded into place and everything is double checked, we start by welding the entire outer seam. We also do an inside weld along the inner seam. With good penetration in your welding, this may not be necessary. Welding along the inside can be very tricky. We do it both for looks and strength. It does add a little extra time but we care about the quality of our products and a good strong weld on your seat. After all, it's keeping your ass in check.
19)After you have welded your seat pan, you'll need to clean up the welds. If you have really pretty welds, this is not necessary but it's more common and gives you a much cleaner piece. We start by very carefully grinding the edges down with a soft aluminum grinding disc on a 4 1/2 inch grinder. Be careful not to grind past the weld and into the seat pan face. You wanna leave a little of the weld raised up and we'll do finishing work using files and then sand paper.
20)Filing can be an art in itself but the important thing is to use smooth steady strokes and work your way from an aggressive file to a fine tooth file. Be careful but don't worry too much about scratching because they will be sanded out next.
21)For sanding, you can use either a random orbiting or vibrating sander. We typically start with 60 grit sand paper on a random orbiting sander and sand the entire face of the aluminum. We work in stages starting with 60, then 80, and 120. After 120 grit, we usually switch to a vibrating sander. Orbital sanders can leave 'whiskers' on the surface of the metal that will stay visible after polishing. Vibrating sanders tend to not leave these blemishes. We use the vibrating sander with 220 and then 320 grit. 320 is a good smooth finish for painting. For polished metal, you'll need to go farther and sand from 400 to 600, 800, and finally 1000 grit. To get a nice polished finish, you'll need to buff the surface using a bench mounted buffing wheel or hand buffing wheel. You can also buff by hand. Make sure to use proper pads and polishing solutions. We use Mothers Mag and Aluminum Polish. Sanding and polishing correctly is a very time consuming process. Take your time and wear a respirator!
22)Congratulations! You've made a seat pan. Now you have to mount it onto your frame. This can be done many different ways. Most cafe racer seat pans require modifying the seat section of your frame. They are typically shorter than stock seats so it is very common to cut the back section off of your frame and weld on around the metal end piece. This is not necessary on every bike. Our Honda cb750 seat pans require only cutting off the helmet hanger and wiring clip. It's entirely up to you and what you're willing to do to your bike to create the look that you want. For mounting we try and use the easiest method possible. We want the seat pan to be easy to install and remove, and even more so, we don't want the aluminum to scratch your frame.
Good Luck!
