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Motorcycle Crash Bobbins

October 10/10/2015

These are a bobbin that bolt onto the side of the frame, in the event of a crash the bike slides on these bobbins instead of doing damage to the rest of the bike.

These don't have to be an exact size so I just went with what bar stock I had, 45mm in this case. All of the crash bobbins on the internet are just plastic spools, nothing fancy. I want to make something aesthetically pleasing as well as functional. So I decided to make it in two sections, an aluminium fancy part and an insert that takes the fall and can be replaced cheaply. I spent some time setting up the lathe, removing all of the backlash in the threads and writing a CNC program to do the majority of the work.

 

Motorcycle Crash Bobbins - CNC ProgramMotorcycle Crash Bobbins - CNC ProgramMotorcycle Crash Bobbins - CNC Turning

 

 

 

 

 

 

 

 

 

The roughing out of the first large radius, I managed to get this part from 25 passes down to about 12. I had to find the point where I could rough the most without losing steps on the control motors, 2mm roughing passes were chosen.

 

Motorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC Turning

 

 

 

 

 

 

 

 

 

A fancy pattern on the OD, just a couple of radii. A final pass was then taken with a large flood of coolant, a very shallow pass and a very slow spindle speed.

 

Motorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC Turning

 

 

 

 

 

 

 

 

 

The program simulation below, me flooding it with coolant and the resultant finish.

 

Motorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC TurningMotorcycle Crash Bobbins - CNC Turning

 

 

 

 

 

 

 

 

 

The bore was drilled out and then a step turned on the back, this is what will retain the slide insert. The finish is so good that I would probably dull it if I were to polish them. I made two pairs, one for my bike and the other to sell.

 

Motorcycle Crash Bobbins - Part offMotorcycle Crash Bobbins - Finished Aluminium PartMotorcycle Crash Bobbins - Finished Aluminium Part

 

 

 

 

 

 

 

 

 

The next part was to make the plastic slider. I first drilled it out to accommodate the bolt and then proceeded with the program I wrote. It faced, turned the OD, stopped so that I could set the spindle in reverse and then set to machining the ID.

 

Motorcycle Crash Bobbins - CNC Turn Acetal SliderMotorcycle Crash Bobbins - CNC Turn Acetal SliderMotorcycle Crash Bobbins - CNC Turn Acetal Slider

 

 

 

 

 

 

 

 

 

The part was then cut off in the lathe, this process was repeated four times. I then placed them back in the lathe and drilled a recess in the opposite side to allow for the bolt head.

 

Motorcycle Crash Bobbins - CNC Turn Acetal SliderMotorcycle Crash Bobbins - Part offMotorcycle Crash Bobbins - Manual Drill for bolt

 

 

 

 

 

 

 

 

 

I turned a jig out of aluminium that would hold the bobbins, it's basically the same dimensions as the parts I made earlier. The centre was drilled and tapped, the bobbins were then bolted onto the jig. I set my second program running to finish the outside profile.

 

Motorcycle Crash Bobbins - Slider Holding JigMotorcycle Crash Bobbins - CNC Turn Acetal SliderMotorcycle Crash Bobbins - CNC Turn Acetal Slider

 

 

 

 

 

 

 

 

 

A quick video of turning the outside by CNC, and the finished article. Note: I don't have bolts long enough at the time of writing this to secure them properly.

 

Motorcycle Crash Bobbins - Aluminium and Acetal

 

 

 

 

 

 

 

 

 

 

 

 

 

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