No, there wasn't any additional distortion that I could measure. For media I'm using glass 'beads' (really ground up glass) purchased from Harbor Freight. A grit wasn't spec'd for the material, but I would guess it started at about 60 and has been getting finer during the past 20 years. - TerryBeautiful work and documentation. I was wondering if the crank distorted any after bead blasting?
I have had some major revelations after the seemingly trivial stress removal from bead blasting.
Again thank you for all your posts.
I saw crank machine from bar stock 144" long and about 8" stoke about 12 cylinder.The journals were turned using side-to-side cutting motions and .005" depths of cut. The depths of these shallow cuts were set while simultaneously moving the carriage and the cross slide in order to achieve the best journal circularities and to reduce the chances of a dig-in due to workpiece deflection. In order to reduce this deflection, the spaces between the rod journal webs were packed with custom ground buttons held in place with lacing cord. All turning operations were done at 80 rpm and feeding was done manually. The finished rod journals wound up with .001" TIRs, and the four workpiece taper measurements remained at their previously measured values indicating a minimum workpiece distortion so far.
The workpiece was returned to the mill where the main journals and inch long ends were roughed in. Back on the lathe and between centers, workpiece deflection was now a major problem even with the rod journal packings. Light thumb pressure on the center of the workpiece easily created a .006" deflection - too much for accurate main journal turning. Some of this deflection was coming from the roughed-in ends which hindsight could have been done later.
I had some 1.250" i.d. seamless tubing on hand that I cut into a number of split pieces in order to stiffen the workpiece during turning. These snapped into place perfectly around the workpiece and were retained with hose clamps. These stiffeners reduced the center deflection to just over .001".
The main journals and their adjacent web walls were then finish turned. The measured TIR's were on the order of .001" with the stiffeners in place. However when they were removed, the workpiece relaxed, and the change in its shape caused two of the TIR's to increase to .004", and one changed to .003". Since the affected journals were still circular to within a thousandth, the increased runout was due to their centers shifting off the crankshaft's main axis. With the semi-finished crankshaft resting on v-blocks, it was apparent that the workpiece distortion had likely occurred sometime after finishing the rod journals and before the main journals were turned.
Next, the weird counterweight shapes were machined into the webs using the 4-axis step indexer still setup on my Tormach. After dealing with the frustrating machining errors on an uninteresting workpiece for so long, it was satisfying to finally see a crankshaft emerge. These operations removed a lot of additional material from the workpiece which added a bit more distortion and another thousandth or so to the main journal runouts. The TIR's of the inner journals were now at .005", .005", .0015", .003" and .0035". These runouts were high enough that if left uncorrected would create difficult fitting problems inside the crankcase, and the final result would most likely be an unsatisfying sloppy fit.
With the off-axis machining operations completed, the end spigots could be safely parted off with the help of a steady rest. I was finally able to measure the runouts with the ends of the crankshaft running in the ball bearings that will eventually be installed in the block. I was hoping the TIR's would improve, but they didn't change measurably.
During my Offy build I came up with a 'scraping' technique that enabled me to reduce the main journal TIR's of that crankshaft by slightly relocating their displaced axes. The journal diameters are reduced in the process, but since work on the crankcase hasn't yet started, this won't be a problem. With the crank resting between centers in the lathe and without any packings or stiffeners, the high areas of the journals were manually rotated back and forth against a razor sharp tool in the tool post. (I continued using the diamond lapped Kennametal carbide insert.) Working carefully while removing a few tenths at a time, the final TIR's were eventually reduced to .002". Progress was monitored with bluing and frequent trips between the lathe and surface plate where the TIR's were measured with the crankshaft running in its ball bearings. The relocated journals received blending polishings with 200g followed by 400g paper to bring them to a common .365"/.366" diameter. All journals then received a final polishing with 600g followed by 1000g paper. - Terry
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Mike,Terry, More gorgeous high precision work for us to admire. Well done.
Quote: "Measurements show the hole had wondered off vertical with a .001" error on one axis and a .005" error on the other. I could probably have lived with the .005" error but, since the workpiece was oversize, it was re-squared around the hole."
So, how did you measure the hole to determine it had wondered off axis by 0.001" and 0.005"? Then how did you re-jig the workpiece to re-square it around the hole?
Thanks for posting your work