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How to machine motorcycle swing arm?

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mcarroll_nz

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#1
Hi, my first post here. I'm a machining newbie but having a lot of fun learning along the way.

I have an aluminum box section motorcycle swing arm that I want to open the axle channel by 5mm to accept a larger diameter axle. I'm not sure how to go about suitably holding the piece for machining due to the awkward shape. The only way it will fit in my mill is by clamping one end to the table or securing in the vice but my big concern is chatter ruining the finish.

Any guidance, suggestions, alternatives greatly appreciated. Thanks.
 

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RobertHaas

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#2
If your mill head can not swing left or right where you could let the arm clamp to the table and have the other side under the table..... you could get a bit of steel and build a triangle clamp that too the upper and secure it to your table with standard hold downs.
 

mcarroll_nz

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#3
Thanks for the response.

Yes, the head does swing thru 90 degrees (it's a PM-932M mill). I had thought about this and coming at it from the side for machining. I could well be wrong but I had read that these machines can be a pain to get aligned vertically again?

I would love to be able to clamp the arm that's being machined with the other side under the table. Unless I'm missing a trick, I can't see how to position it this way as the stand is solid and the piece needs to be centered for milling.
 

P. Waller

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#4
You will have to build a fixture to hold them, this may cost more then the machine however.

If it is a hobby job and time and materials are not an issue then have at it.
 

Cadillac

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#5
I would remove the vise then put the swing arm like you have in picture except put some wood between the swing arm as support. Cut wood exact fit if not a touch heavy. Then use some strap clamps with some looong threaded rod to hold it all to the table. Align slot and cut. Two setups obviously.
 

rwm

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#6
Just a thought. Could you move the mill onto a temporary stand that would allow the other side of the trailing arm to go below the mill? A heavy table would work. This will save you a lot of hassle.
Robert
 

38super

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#7
Swing the head, you're going to need an angle plate.
 

markba633csi

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#8
Doing it on a lathe would be easier, or on a horizontal mill. But lacking those, Cadillac's approach in post #5 is probably the best
mark
Tilting the head is a possibility, but you still need to mount the swingarm a considerable distance above the table.
Re-tramming the head is a minor concern
 

mikey

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#9
I think I would build a fixture and try to mount it on the cross slide of the lathe. It would be finicky to set up but would likely be more rigid.
 

BaronJ

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#10
Hi Guys,

5 mm ! Whats wrong with filing it to size ? Its only aluminum !
 

rwm

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#11
The lathe is also a great idea. Clamp it to the cross slide with hold downs and spacers to get the height right.
Robert
 

mcarroll_nz

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#12
All great info and appreciate the responses - thank you.

I don't want to file it as it's a visible part of the swing arm and want it to look nice.

I had thought about clamping it to the cross slide on my PM-1236 lathe but everything I had read about this type of machining was never particularly positive - but maybe that's just from a purist's point of view. Anyone had good success with performing similar machining on a lathe?
 

MarkM

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#13
Well here is my take on this. First I want to make sure we are on the same page and you need the clearance in the swingarm in order for the axle to be adjusted for the chain adjustment is what I am thinking. Use machinist jacks or something and clamp down to them being indicated for the machining. Don t plunge your cuts and use a small endmill and mill on your sides. It s only roughly .100" each side you can creep up to it and there is alot of material in the cutting force direction but the trick is to use a small cutter and keep the forces minimal. You may still have to creep up to it but a good finish I feel can still be achieved on a final pass. Hss is better here!
Since it s clearance take your measurements of location flip over and repeat. Your concern is finish and function. Don t sweat it too much but still be on the ball.
 

hman

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#14
I had thought about clamping it to the cross slide on my PM-1236 lathe but everything I had read about this type of machining was never particularly positive - but maybe that's just from a purist's point of view. Anyone had good success with performing similar machining on a lathe?
Use the "Search" function to look up user "savarin." He's documented several projects, mainly telescopes of various kinds, and he's done some absolutely magnificent milling work on a 9x20 lathe.
 

NortonDommi

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#15
That slot you see is a piece of oval shaped tube welded in place and by the time you mill it out 5mm there will not be a lot of wall left. What equipment do you have? The oval shape extrusion is a standard shape available in different sizes, have you checked what the next size up is and if it is close,(Metric or Imperial),to what you need? You may have to make a custom piece and I'd be sorting that out first. Alumiweld is a low-temp brazing rod that would excellent for this job.
Re-tramming the head is not a big deal and as you are going to have to do it sometime just accept it as something you have to do on some jobs.
If you have a drill press it would be easier to drill out and blend with a die grinder.
I strongly suggest you get an insert that fits your new axle before doing anything else though.
 

derf

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#16
It's tubing, correct? Clamp it up horizontally and use a key cutter to widen the slots, then blend with a file.
 

mcarroll_nz

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#17
I appreciate everyone's input but in the end I chickened out. I dropped it off today to a machinist guy I know who has a mill that swings horizontally and extends so he can clamp the end he's machining directly to the bed.

I still have plenty more machining to do for this swing arm, creating the pivot shaft inserts and aligners and components for the axle locator inserts. But if I mess any of this up then it's easy enough to start again machining a new part.

Thanks again.
 
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