Fixturing a swing arm repair advice needed
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  1. #1
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    Default Fixturing a swing arm repair advice needed

    I recently picked up this job and I could use some fixturing advice. First of all, this part is no longer available. I can think of a few ways of doing this but none stick out as the best procedure. I thought I might mount the swing arm on the mill table and bore out all but a little of the original journal and leave that half moon as a reference point. I could bore it all off and then line bore the new journals and hope for the best. I have at my disposal mill, lathe and TIG. I worry about when I flip this thing over to bore the second journal, what would I indicate off to make sure everything is in alignment. Thanks for any suggestions.


    swing1.jpgswing2.jpgswing3.jpg

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    Align it parallel with the axle at the other end of the swing arm. Exact position is less important.

    Use a dummy axle if necessary, line bore or piloted /guided reamer for the second hole.

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    Is your lathe big enough to line bore it using a homemade boring bar mounted between centers?

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    Yes my lathe is big enough to line bore it. I also like the idea of using a temporary shaft in the front holes and indexing from that. I have a pair of Mitutoyo 40" calipers that I could use to index from the temporary shaft. I guess I would need to block up the end of the swing arm so that when I flip it over, the bores would be in alignment. Or as suggested I could line bore it after welding in new journals. Indexing off this temporary shaft would give me front to back data but I still need up and down if you know what I mean.

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    I'd just chop off the broken ends and fab new ones. Then align them on a shaft and weld'em back on.

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    Last week I had to do a rather similar job on a 27' carbon fiber sailboat boom that had its mounting holes damaged.

    Using custom made bushings to guide my homemade "boring bar" I was able to bore one set of ragged holes oversize and re-bush them. The "boring bar" I made from a 25mm annular cutter, a modified arbor, and an extension I made from 20mm drill rod.

    Using that same "boring bar" I was able to line bore the (black) aluminum end fitting to fit an oversize (25mm) pin in the other set of holes.




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    Quote Originally Posted by jancollc View Post
    I'd just chop off the broken ends and fab new ones. Then align them on a shaft and weld'em back on.
    I agree with this, but I'd also be concerned about strength without a fresh heat treatment (if it was made from a HT-able alloy). Any idea what the alloy is?

    And keep in mind you're facing the old enemy of Al, which is fatigue. Explain to your customer that there's a risk of future failure due to the flexing cycles the arm has taken, Al doesn't have an infinite fatigue life.

    If it was me, I'd remake the whole dang thing from plate...

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    This is certainly not my field of expertise but my concern is knowing what alloy the swing arm is made from and what the welding requirements are for that alloy. The motorcycle manufacturers have been known to weld up assemblies from alloys that are considered not weldable by many welders. But because they know the alloy and can closely control the process they have some successes. And I say some because Honda Goldwing motorcycles in around 2004-2006 had aluminum frames breaking at the welds. Broken ones had to be replaced. Cracked frames could be repaired in some instances but the weld procedures were very specific and the frame needed to be aged for about a week after repair before it could take the stress of being put on the side stand.

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    No Milland I don't know what the alloy is but I could investigate. I have informed the customer already that this is a crapshoot when it comes to heat treating etc.

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    The entire thing is a weldment, so you know it's weldable.

    You'll want something on the inside to beef up the repair- a short piece of smaller rect. tubing or a solid piece, to help align the parts and take the strain off the welds.

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    In the OP's shoes I'd cut / burr / file what's left of the old eyes away, rough bore (leaving PLENTY of meat for final machining) a piece of bar long enough to go right across the swingarm, …..knock up a quick jig to hold all in place for welding, ………….and after welding cut out the centre section on tube, then set up and ''line bore'' what have you to size

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    Quote Originally Posted by Bill Simmons View Post
    This is certainly not my field of expertise but my concern is knowing what alloy the swing arm is made from and what the welding requirements are for that alloy. The motorcycle manufacturers have been known to weld up assemblies from alloys that are considered not weldable by many welders. But because they know the alloy and can closely control the process they have some successes. And I say some because Honda Gold iwing motorcycles in around 2004-2006 had aluminum frames breaking at the welds. Broken ones had to be replaced. Cracked frames could be repaired in some instances but the weld procedures were very specific and the frame needed to be aged for about a week after repair before it could take the stress of being put on the side stand.
    I recall the repair program that Honda instituted The program was indeed started in 2004 but it related to the re-designed 1800 cc Honda Goldwing that came out in 2000 or 2001 with the alloy frame. The cracking problem was widespread. I don't know what alloy they may have used, but it was not the correct one. They set up a group of certified welders and gave exact instructions on the procedures. Some of the frames had to be replaced if they were too far gone.

    I had a 1994 Goldwing at the time so I did a bit of reading on this issue. Hopefully, the OP hare will have an alloy that can be welded

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    I read once about the ally el cheapo Chinesium bicycle frames.....five process heat treats,including one to make the alloy weldable,and one to stress relive the weld within minutes........chances of weld repair outside the factory.......zero.

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    I've made quite a few extended swingarm end fittings for friend's bikes.

    use 6061.

    Make the entire end piece new, one piece.

    Cut the welds holding the old ones in

    Press new ones in tubes

    use some sort of alignment bar to line everything up if non-adjustable.

    Preheat a little bit and tig em in there.

    Never had a problem and these are on CR500's and tweaked out CRF450's running big paddles.

    This is an easy cnc mill job.

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    Quote Originally Posted by crossthread View Post
    Indexing off this temporary shaft would give me front to back data but I still need up and down if you know what I mean.
    Get the rear axle parallel up and down too. If eyeballing it parallel isn't good enough you could let the dummy axle rest on two straight edges placed across the lathe bed. (Assuming you line bore on the lathe. Similar approach could be taken if you put the journals on a shaft before welding.)

    If you mean up and down within the swing arm assembly just put in the center of the tube. The whole swingarm moves up and down, that's its purpose.

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    Crossthread,

    Unless this is for a MotoGP bike, you might be over thinking the alignment. The rear wheel has adjustment for chain tension and alignment, so front to back isn't that critical. The suspension height changes with every pound taken on or off the bike, so as long as your pivot point is within the frame it will work. A pair of carpenter's levels will get the pivot bolt and the axle sufficiently parallel to each other for most purposes, a precision level would be even better.

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    Thanks everyone for your valuable input.


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