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Old 08-18-2018, 10:09 PM
Mucci Mucci is offline
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Default Tricks to symmetry on a motorcycle fender?

I'm trying to make some aluminum motorcycle fenders and am having trouble getting the curvature consistent along the entire fender length. My second issue is getting symmetrical curvature from left to right.

After getting the shape close I'll stick a piece of welding rod in the axle and swing it like the hand of a clock to scribe a trim line in the fender. This is the point where it's most obvious the symmetry is off. The scribed line will be concentric to the wheel from side view but from the top it waves in and out.

I'm using a 3" teardrop mallet and 18" shot bag along with a Woodward Fab english wheel. 16g 3003 aluminum, annealed.

My process has been to draw guide lines on the fender before shaping. I try to make the same amount of hammer blows in the same places on both sides and also wheel it in a symmetrical fashion but it still comes out asymmetrical or even twisted sometimes. I'm trying to be methodical about it but could use some tricks if anyone has them.
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Old 08-18-2018, 11:53 PM
Oldnek Oldnek is offline
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One easy way to achieve symmetry and consistency when starting off, is to draw lines along the length of the guard your making prior to cutting it out, space them out around 20mm. This will give you a pretty good guide on wheeling the panel straight along the length and knowing which way you need to manipulate the panel once wheeled.
Good luck with it and post some pics..
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Old 09-01-2018, 06:12 AM
fciron fciron is offline
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Have you made a buck? (Or even better, a flexible pattern if you're trying to reproduce an existing part?)

I just finished my first aluminum fender and there were a lot of small adjustments to do. Wheeling just the low spots to make them confirm to the overall shape. Twists can be removed by literally twisting the piece in the opposite direction by hand.

You need to establish some fixed points both on the workpiece and for the final form in order to evaluate your progress. I'm sure an experienced shaper could read the part just from your trim lines, but I certainly needed more guidance. Thus, the need for a buck.

My buck was simple to build from scrap plywood with a hand held jigsaw. I cut one piece to the lengthwise radius, and several smaller pieces to the crosswise radius. The small pieces were slotted up the centerline and the lengthwise piece along several radii. Slip them together with some glue and sand down any lumps at the joints. I manipulate the work piece by hand to the best possible fit, then wheeled the spots touching the buck until I was satisfied.
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Old 09-01-2018, 01:18 PM
Mucci Mucci is offline
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Previously I was only drawing a grid on the underside to try and keep track of hammer blows. Drawing a grid on the top side helped a lot, making it easier to keep track of wheel passes.

I made 4 or 5 more fenders and am starting to understand how to rework an area that's not symmetrical. I think the first one took me 6 hours and now I can get one down to around an hour. I didn't have a planishing hammer before so I was trying to get all the lumps out by wheeling and the metal ended up getting pretty thin before I could get it perfectly smooth.

Here's one of the latest attempts. You can see the lumpiness in the highlights still.



I've since ordered a planishing hammer.

What also helped was cutting out the exact shape first instead of trimming afterwards that way I can keep track of the shape easier.
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Old 09-01-2018, 03:20 PM
dwmh dwmh is offline
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Looks pretty good to me.
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Old 09-01-2018, 05:14 PM
crystallographic crystallographic is offline
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Hi Mucci,
You get the symmetry and contour by accurate and methodical shaping.
Then you can lose that by planishing.
Then you fight a bit to get symmetry, contours, and surface back and all good ... then it is metal finishing to make the surface nice.
You are getting there.


Marking the outside surface to make and hold the shape is a make-or-break, for me.
Every time.
P1120444 copy c.jpg
Layout of shrinks at start of large reverse
.040 2024 T0

hammering reverse.jpg
Well into the shaping
P1120391 copy c.jpg
Degree of finish for a fussy customer

Lockheed U2, prototype skin for Skunkworks
(fits within .010in.)
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"All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919.

Last edited by crystallographic; 09-01-2018 at 05:40 PM.
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Old 09-01-2018, 06:52 PM
fciron fciron is offline
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Ah, the fenders I'm working on have a lot more shape in them. A buck would probably be overkill for yours. Looks like you're getting it worked out just fine.
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