Friday, 10 April 2020

QR50 - Crankshaft

With the milling of the oil and fuel tank caps complete, I can revert the lathe to ‘turning mode’ and get on. Next stop is the crankshaft, selected so that I can assemble the cases and bearings and so that I can get the various bits of transmission off the bench and assembled to the frame.

The crankshaft has suffered the usual abuse - mushroomed threads from over enthusiastic tapping. Both ends are like this:


It’s no great problem to fix them as long as the damage is not too bad. I set it up in the lathe with the fixed steady and machine the major diameter parallel again. You need to run the lathe quite slowly - the imbalance caused by the connecting rod will cause a significant vibration. It's strapped in place with a bit of inner tube:


The thread is M10 x 1.25, so we need to set the change gears according the LittleMachineShop Change Gear Calculator. Click on the picture for a link to the calculator:


The calculator shows us which gears to fit in these positions:


You probably don’t want to run the lathe when repairing a thread like this - you are not going to cut very much. What I do is set the tool up using the thread gauge so that it is perpendicular to the shaft:


When the tool is set up, align it with a good thread, one that is undamaged. Set it back so that it clears the major diameter and engage the lead screw using the half nut. Run the lathe slowly or wind it by hand to make sure the lead screw is running in the right direction, and then you can wind in the feed on the cross slide a little. Traverse the whole thread, lightly skimming the damaged area - then repeat the process until you are confident that you have cut the full depth. Test it with a nut:


Looks a lot better. Now I need to do the other end.

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