How to metalshape a custom fender

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How to metalshape a custom fender

Step 1: Materials

You could use either steel or aluminum for this project. Steel thickness ideally 1.0mm. Any thicker and it will be hard to hammer. Aluminum thickness can be 1.6mm and still workable.

Step 2: Cut your blank

Cutting materialsYou need to measure up “roughly” how much metal you need. In the case of this fender, we just wrapped a tape over the tyre to estimate the size of the blank at 5” x 18”. Leave around ½” or 12mm all the way around that you can trim off to the exact profile as the last step.

Step 3: Annealing (aluminium only)

If you’re using aluminum, you might need to “anneal” or soften the metal to make it easier to work. This is done by dusting the surface with soot from an acetylene torch, then evenly heating the sheet until all the soot is burned off. The temperature the soot burns off is an indication of the tempering temperature. Let it cool slowly in the air. Then you’re ready for the next step.

Step 4: Tools you’ll need

A heavy mallet (the red one is 3 lbs)
Sandbag, approximately 350mm (14”) diameter
Wooden shrinking bowl
Tucking fork (make from a pair of pin punches)
Bowl or panel dolly
Slapping spoon (make one from an old file)
Metalworking file
Sand paper (the white paper I use is Aluminium Oxide)

Step 5: Start with the hammer

Strike the metal hard along the centre of the fender. This will stretch the metal as it is forced into the bag, cause the metal to curl. Straighten the fender out by hand as you work. Continue until you have a rough profile established, and the edges are wavy.

Continue dishing the length of the fender. Now we are working either side of the centreline of the fender.

Straighten the fender by pushing down on the ends. This will help pull the sides in.

Switch between hammering and straightening until the profile is essentially the final shape, even if it is rather lumpy!

Step 6: Shrink the edges

Since the metal has moved from a straight blank into a compound curved shape, the long edges of the fender are now too long and will wave and fold. Use the mallet on a hard surface (curved/dished wooden bowl works well). Hammer out the tucks. Try working from the point of the tuck back out to the edge and “trap” the tuck.

Continue shrinking the tucks and smoothing the fender in the wooden bowl

Step 7: Tuck the edges (if needed)

If you want some more crown in the panel, shrink the edges again. Use tucking forks to make short tucks along the edge.

Hammer those tucks out in the same fashion as before.

Step 8: Planish the surface

With a slapping spoon (in this case, an old file bent into shape), and a dolly (we used a post dolly in the vice), tap out the lumpy surface. This will take a while! But is the best way to proceed if you don’t have access to an English Wheel. Run your hand over the surface: you’ll feel any uneven areas. Keep planishing….more….you haven’t finished yet….keep going…!

Step 9: Wheel the surface

If you do have access to an English wheel, then its possible to speed up the smoothing operations. Select a lower wheel that matches the profile of the fender. Use light clamping pressure (perhaps have the wheels just touching) and wheel the part. You’ll feel the lumps rolling out. When it feels smooth, you can increase the clamping pressure between the wheels and smooth it out even further.

Step 10: File the surface

Using a normal metalworking file, smooth out the crown of the fender. The file will hit the high spots. Keep filing until the high spots are brought down to match the low spots. Take care to not put any flat spots in the fender with the file. Sweep it over the crown in different directions to prevent flat spots.

Step 11: Sand the surface

To go to the next level, smooth out the surface perfectly with a sander. A dual action is best but this old 1/3 sheet sander did the job just fine.
The surface should be perfect at this point. No filler would be needed if you were going to paint the fender.

Right! Now it’s up to you!

You have to try it!

You won’t learn from sitting here reading about it!

MEGA RESOURCE CENTER FOR DIY CUSTOM MOTORCYCLE BUILDERS:

Matt McLeod

About the Author

I teach people how to build custom motorcycles by helping them build skills and confidence with my coaching, articles and training videos. I provide better technical information for custom motorcycle builders. And I shorten the learning curve getting you there.


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