boring for crankshaft

[brickyParticipant @bricky]

The shaft has three cranks and is supported in bearings.The crankcase has two 7mm cross walls to support the mid bearings.Before milling out the waste ,I would like advice on wether to drill and bore from the solid or after milling the pockets between the walls. If there is enough height on my SX3 I will do it on the mill if not I will line bore on the lathe.

[Peter Cook 6Participant @petercook6]

I have never done this – so am no expert. However I would have thought that milling out the waste would stand a significant chance of distorting the cross walls sufficiently to obviate the accuracy of line boring. I would probably rough drill in solid to minimise any distortion of the walls, mill out the pockets then finish line bore the bearings.

But someone will be along shortly to  give you a proper answer!

[JasonBModerator @jasonb]

I would also get the pockets out the way first then rough drill followed by lineboring which will not wander unlike doing it on the mill where the tool is only supported at one end and more likely to wander

[brickyParticipant @bricky]

Thank you both for your quick response to my question I will progress by line boreing as suggested.

Frank

[ChrisLHParticipant @chrislh]

Just to clarify “line boring”, in my own case at least. I made bearing disks that were attached to the crankcase and aligned by close fitting spigots on same. The boring bar ran in these bearings and was driven by the lathe spindle via a flexible connection so that the lathe’s carriage feed  could be used to feed the boring bar the crankcase being clamped to the lathe’s crosslide. My boring bar was rather long which made it impracticable to machine all 5 intermediate bearings with the same cutter. The bar therefore had to be withdrawn to fit the cutter in new positions hence the bearing spigots.

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