Making a Crankshaft – 8

by John

Finish turned the big end journals today. I decided to make them slightly bigger than the original shaft, because there was a measured deficiency of approximately 0.1mm – 0.15mm (? wear, ? manufacturing error) between the original shaft and the original split bearings.

I had rough milled one of the big end journals a couple of days ago, and Loctited a block into the crank slot of the other big end. I decided to finish turn that journal first, then to remove the loctited block and glue it into the finished crank slot, before rough milling then finish turning the second big end journal. If you follow me.

That all took another half day. And, I experienced my first significant (but not fatal) stuff up in the job.

Copious coolant, 135rpm. Of course the workpiece is centered on the big end journal. Here the tool is approaching the journal to be machined. The packing piece is glued into the other crank slot, and will remain there until the journal being machined is totally finished.

After finishing the first journal and changing the location of the packing piece the second journal was roughed out on the mill. I was much more confident about this method by now, and was more aggressive with the cuts. Used coolant throughout, and the cutter was in good shape at the end. I roughed the diameter to within 1 mm of the final dimension to reduce the time for finish turning. Note the use of my shop made clamp to reduce backlash and vibration. That worked much better than the previous soft wire method.

When I was happy that the journals were finished, I glued in a second packing block, centered the shaft, and turned the curved outside shape on the crank flanges.

As you can see, there is a lot more swarf to be made yet.

Oh yes. The stuffup. After one side of the first journal was turned the insert was blunted, so I rotated the insert. But unfortunately dropped it and chipped it, so it was replaced with a new one. My error was that I did not notice that the tip radius of the new insert was different… 0.8mm radius rather than the original which was larger and made a nice fillet at the join (see above photo). The new insert made a much sharper join, still with a radius, but much sharper. Not fatal, but not ideal.

In the next session I will recheck all of the big end journal measurements. If all are good I will cut off the side flanges at the ends of the shaft, removing those centers for ever.

I will see if my fixed lathe steady will fit into the middle gap between the cranks. If it fits, I will take a smooth light lathe cut of that section and install the steady. Then finish turn both outside sections of the mainshaft. Then move the fixed lathe steady to one of those outside ends, and finish turn the central section. The central section is where the eccentrics are located.

If the fixed steady does not fit in the middle section I will finish turn that section first, after installing the fixed steady on the longest outside section of the mainshaft.

Those possibilities are to keep the mainshaft as rigid as possible during all of those turning steps. (that list is more for my benefit than yours, dear reader).

p.s. So far, there has been no discernable distortion of the workpiece despite removal of over 20kg of swarf. That has been assessed on a granite surface plate, after filing all of the machined edges of all metal tags and lips.