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mayhugh1

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Even though I decided to scrap the first crankshaft, I finished it up so I'd have a test bed for the remaining setups and operations. I also wanted to be sure there weren't any surprises waiting for me inside the crankcase or with the starter assembly before getting getting too far along with the second one.

Without grinding capability, the Offy's eight inch long crankshaft with its eight half-inch journals is a difficult part to accurately machine. In comparison, the Merlin's crankshaft with its three-quarter inch journals was considerably more rigid and much less troublesome. Even with rod journal packing, .006" deflections could be demonstrated with only modest finger pressure on the center of the Offy's crank. A drawing shows the deflections measured in four different directions with finger pressure on one of the center rod journals with the other three packed. A journal that must be unpacked for machining appeared to be responsible about half of the worst-case deflection. I was able to reduce the .006" deflection shown in the drawing by about half by packing the main journals as well. Even with carefully shimmed packings, tailstock pressure adds its own deflection by distorting the long flimsy workpiece.

The deflections make it difficult to turn a truly round journal to a precise diameter using what is essentially has to be parting tool. They also affect the centerlines of the rod journals' axes and, potentially, their alignment with the main axis. The exact locations of the rod journals' axes isn't a major issue, but they need to be parallel to the main axis to prevent the pistons from binding in their bores.

After semi-roughing a pair of rod journals, I was satisfied with their measured .001" TIR. I continued on and performed the same operations in different setups on the other journals but then discovered the TIR of the first pair had increased to some .010". The subsequent machining operations had apparently changed the workpiece and its reaction to the tailstock. In order to turn the rod journals to their finished diameters, the finishing operation had also to move their axes some .005" in the presence of a .003" workpiece deflection.

Maintaining a consistent tailstock pressure between setups (and throughout an operation in a given setup) can be tricky since the way the workpiece reacts to tailstock pressure changes as material is removed from it. For example, after finishing all the journals, I needed to remove excess material from the nose of the crankshaft so I could trial fit the crankshaft inside the crankcase. This involved turning down a one inch long length to just under a half inch in diameter which I knew would weaken the workpiece and allow it to distort. I watched runout developing in the main journals as more and material was removed from the nose. When finished, the nose ran true, but the TIR of the main journals were now at .005".

Along the way I discovered that centrifugal force on the packed but unbalanced workpiece created an additional deflection. To get around this and its accompanying surface finish problems, the maximum spindle speed was limited to 50 rpm. Some of the final .001" passes on the second crankshaft were actually performed while rotating the spindle by hand.

One of the photos shows the new grooving tool purchased for all journal operations on the second crank. Its style is identical to the one used to machine the first crank's rod journals (as well as the Merlin's), but its narrower .155" width allows it to fit in the narrow space between the cheeks of the Offy's main journals. It was also bifurcated and lapped on a diamond plate until its cutting edge was keen enough to take a consistent .001" depth of cut on a steel test rod. The corners of its chip breaker also had to be lapped flat to reduce the cutter's tendency to dig in during side-to-side cutting. In use, the depth of cut was limited to a maximum .005" (diameter), and the cutting edge was touched up and re-indicated for each journal.

The new workpiece was prepared identically to the first one including the end spigots, reference flats and center-drills. The journals on the main axis were again roughed out on the mill while the workpiece was supported in the vise. Ten-sided polygons instead of six were used this time in order to reduce the trauma to the workpiece during the interrupted cuts. This time I also roughed-in the surfaces for the front and rear ball bearings at the same time.

The workpiece was set up on centers in the lathe and its o.d. immediately skimmed. Left overnight, its TIR creeped up from essentially zero to .0015". All five bearing surfaces on the main axis were then semi-finished to within .030" of their final values using the new tool, and the cheeks were faced to their finished values. The measured TIRs were essentially zero.

The workpiece was returned to the mill where the rod journals were roughed in. After completion, the workpiece was temporarily re-installed in the lathe where I noticed the main journal TIR's had increased by a couple more thousandths. The rod journals were then semi-finished, and their cheeks faced to their final thickness.

On the following day the TIRs were rechecked. The main journals now measured between .005" and .007". The #2 and #3 rod journals were at some .012", and the #1 and #4 rod journals measured .009". All journals were still round to within less than a thousandth and so the entire TIR increase was attributed to workpiece distortion in response to its previous machining.

I allowed the workpiece rest for a couple days in hope that it might self-heal but no joy. The workpiece was moved back to the mill where the final cheek profiles were machined. When the journal TIRs were later remeasured, there didn't appear to be any further changes.

I should add that rod journal TIR's aren't obvious unless they're measured with a dial indicator. Runout created by a journal that's out of round by only a few thousandths will be irritatingly visible to the naked eye, but 10X that amount due to a displaced rod journal axis won't even be noticed. If one could be sure this displacement doesn't also include a skewed axis, its only effect would be just a degree or so change in valve timing and not worth worrying about. - Terry

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gbritnell

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Hi Terry,
The only way I could come close to having an accurate crank (.002 overall) was to rough mill the main journals then turn them on the lathe leaving .010-.015 stock for final finishing. Go back to the mill and rough cut the rod journals taking the webs to almost finished width. Remount the crank in my offset blocks and turn the throws to finish size and like you said at a very low rpm. With the throws turned I would rechuck the crank between centers and finish turn the mains. I have had tolerances between .0007 and .002 depending on the size of the crank and journals. I wasn't so much concerned with the total accuracy of the throws, square and parallelism as I was with the mains. I would clamp portions of the crank with hose clamps to try and stiffen the cranks while cutting. At best it's a tenuous job getting a truly accurate crankshaft.
As a side note I have even had my finished cranks really close and after weeks of sitting around they would then measure off from the original readings. Stress relief, temperature changes, who knows.
gbritnell
 

petertha

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You guys are scaring me. I was thinking my next project might be an inline. Would cast iron offer any lower stress relief deflections over SP once machined? Or are there more overriding 'con' issues relating to CI for crankshafts in applications like this?
 

gbritnell

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I'm not saying that cast iron wouldn't work but I have used stressproof since I found it worked so well compared to common cold rolled steel. It's not that you can't get very good accuracies it's just that you have to be patient. When I built my 302 V-8 engine I finish ground the crank. I had made a toolpost grinder and I knew a fellow that had a grinding shop so he took a couple of thin (.250 as I remember) grinding wheels and cut the diameter down for me. The grinding was the way to go as it doesn't put much load on the metal like using a bifurcated cut-off tool.
 

mayhugh1

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Petertha,
I suspect cast iron would warp even more although it would be easier to take finer cuts. Cutting out offset rod journals is a pretty traumatic operation, and even Stressproof reacts badly. One thing that would help would be to make the journal diameters as large as possible - three quarters of an inch would be a lot better than a half inch.

It's logical to machine the crankshaft before machining the crankcase since it will be easier to fit bearings to the crankshaft journals rather than the other way around. Although I realized this when I started the build, I knew from my Merlin experience how hard it was going to be, and so I procrastinated as long as I could. Like George says, 'you just have to be patient.' - Terry
 
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mayhugh1

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The next step was to turn a ten degree taper and a 3/8-24 thread on the rear of the crankshaft to secure the flywheel. I used a left-hand thread so I'll have the option of drill-starting the engine from its rear. After roughing away most of the spigot and before turning the taper, I was able to nearly zero-out the rear bearing TIR in the setup at the tailstock using a set-true chuck. Finally, the remaining spigot stub was parted off and the rear end center-drilled for contingency.

Attention was then focused on the front of the crankshaft. Its long skinny nose will receive a Loctite'd pinion gear and a hardened sleeve that will engage the starter's sprag clutch. The crankshaft was set up in the lathe with its rear end in a collet chuck and its front-end spigot supported by the tailstock. Taking advantage of the chuck's .0015" runout, I rotated the workpiece in the collet until a location was found that essentially cancelled the TIR of the front ball bearing surface. This allowed me to turn the 1-1/2" long nose to a .281" diameter concentric with the front bearing. After parting away the remaining spigot, the front end was center-drilled and the nose polished for a close sliding fit to the already machined pinion drive gear.

The crankshaft drawing called for a pair of bosses to be integrally machined with the outside cheeks on each end of the the crankshaft to locate it to the crankcase. To simplify the machining a tiny bit, I instead custom ground a separate pair of spacers.

The bronze crankcase bearings were originally bored to .5145" to provide what I felt would be an optimum clearance to a length of .513" drill rod used to verify the bore alignment. With a little oil, the rod was snug but turned uniformly without binding. One of the crankshaft's three main journals ended up out-of-round by a thousandth, and since the crankshaft's TIR measured one to two thousandths depending upon the time of day, the main journals were polished down to .511" to eliminate any chance of binding. With the crankshaft installed, the TIR measured at any of the three main journals was between one and two thousandths and outside the front and rear ball bearings it was less than a thousandth.

The starter sleeve was machined from drill rod and hardened before being Loctite'd to the crankshaft. Since it must carry the engine's full starting torque as well as survive kick-backs, a best possible cure is mandatory. It's important to polish away any oxides remaining in the bore after heat treatment since they will interfere with Loctite's cure. A .002" sliding fit to the crankshaft was close to optimum for Loctite 680.

Before permanently attaching the pinion gear and starter sleeve, the crankshaft was assembled several times with the front cover inside the crankcase halves in order to make sure it's fit was consistent. The final step will be to drill interconnecting oil passages between the rod journals and the main journals. These risky operations will require a proper fixture in order to perform them safely. - Terry

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mayhugh1

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The crankshaft's final machining step was to drill the interconnecting oil passages between the main and rod journals. There isn't a lot of safety stock around the drill path, whose angle is a steep 30 degrees. In order to be able to use a reasonably stout bifurcated parting tool to turn the journals, I increased their widths from .375" to .410" by decreasing the thicknesses of the cheeks. This further reduced my working margins for this drilling step and made the accuracy of the angle even more important.

Care was taken to avoid breaking through the inside corner of a cheek or even worse sticking a broken .063" drill. Ron mentioned that this had happened to him when a drill grabbed the edge of its exit hole. Not wanting to spoil a second crankshaft, I spent a few days on a custom drilling fixture.

The initial drilling procedure involved creating a flat area for the drill's entry on the rod journal using a 3/32" end mill. This was followed by a 1/16" v-cutter to spot the hole. The hole was then peck drilled every .050" using a new 134 degree drill that was totally withdrawn after each peck. Compressed air kept the drill free of chips. The entry point for each hole was selected so its exit from the main journal would be in its center directly over the oil groove in the bearing.

The first drilled hole wound up at 27 degrees causing its exit to miss the center of its main journal and the oil groove by .060". After verifying the CAD work and drilling fixture, I began playing with the drilling procedure on the scrapped crankshaft. The same procedure on the same journal of the scrapped crank produced exactly the same result.

After some experimenting, I eventually had a process that drilled the remaining three holes where they belonged. The primary change was using the v-cutter to drill the first 1/8" of the hole rather than just spotting it. I also changed to a 118 degree drill for the first half of the hole's depth and then finished with a 134 degree drill. In separate testing, the 134 degree drill seemed to drill more easily through Stressproof and produce smaller chips that easily cleared a deep hole. Its broader nose, however, didn't seem to like an unsupported angled starting surface even when spotted.

The real test of the drilling process were the oil passages in the #2 and #3 rod journals that shared a common exit hole in the #2 main bearing. The earlier mis-drilled exit hole in the #3 main journal was repaired with a short milled slot which will connect it with the bearing's oil groove. The crankshaft was then moved to the lathe where the journals received their final polishing with 1200g paper. - Terry

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mayhugh1

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The Offy's flywheel was machined from a chunk of 2-3/4" diameter 316 stainless. Compared with the crankshaft machining, the flywheel wasn't much of a challenge. With both ends containing features that need to be concentric, a 4-jaw chuck was used for all the turning operations.

A ten degree internal taper locates the flywheel to a matching o.d. taper on the rear of the crankshaft, and a 3/8-24 left hand nut binds the two together. In order to insure matching angles on both tapers, they were CNC turned on my little Wabeco lathe. I was pretty disappointed when the flywheel wobbled so badly on the crankshaft that was running true. The amount of wobble was different each time the flywheel was mounted and sometimes was as bad as ten thousandths.

Bluing and wringing the parts together was inconclusive, but a tiny bit of 1200g grinding grease brought up a contact pattern showing the two angles didn't quite match. After some head scratching, I remembered the two tapers had been turned using different tools and their contact points may not have been handled properly. Lapping the flywheel to the crankshaft with 600g grease eventually reduced the wobble to a consistent .0015".

Since ten degrees is too steep for a self-locking taper in steel, I wasn't expecting the flywheel to need a puller. After lapping, the flywheel tended to stick tight to the shaft, and when the engine is fully assembled it will be awkward to remove. A pair of 8-32 threaded holes was added on either side of the taper for pulling screws that can also be used to grip the flywheel while wrenching the nut. The nut was machined from drill rod and hardened, and a backup washer (engraved with a warning about the nut's handedness) was machined from a bit of Stressproof.

Ron added tick marks around the entire perimeter as a timing aid to match those on the full-size flywheel. I used my mill's fourth axis to engrave 72 five degree marks around the flywheel's circumference. For this operation a mandrel was turned using the same tool and taper program originally used on the crankshaft. The wobble of the flywheel mounted on the mandrel was under two thousandths indicating that the error before lapping had been the flywheel's taper.

A timing pointer bolted to the rear cover wrapped up the work on the flywheel. When the flywheel is mounted at final assembly, the tick closest to TDC will be colored in with paint. For now, the machining of the rods, pistons, and rings are all that remain. - Terry

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propclock

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Left Hand thread? Usually a flywheel going CCW has a right hand thread?
Perhaps I am wrong.
As always, thanks for you informative posts.
 

mayhugh1

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Left Hand thread? Usually a flywheel going CCW has a right hand thread?
Perhaps I am wrong.
As always, thanks for you informative posts.
Don't forget this flywheel is at the rear, and so if the engine is started from the rear the nut will have to be cranked counterclockwise. - Terry
 

ozzie46

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Starting from rear counterclockwise means engine turns clockwise looking from front,no?

Ron
 

propclock

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I was confused because I marveled at the beautiful starter splines on the front of the crank.
I use 1 way sprag/Torrington bearings for starters on most of my models. Thanks.
 

mayhugh1

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I was confused because I marveled at the beautiful starter splines on the front of the crank.
I use 1 way sprag/Torrington bearings for starters on most of my models. Thanks.
That spline-looking thing on the front is actually a spur pinion that drives the oil and water pumps and the camshafts. The smooth portion at the very front of the crankshaft is a hardened sleeve that's one-half of a sprag clutch that is built into the front of the engine for starting. If the front-start system is used, the radiator (if located at the front of the engine) will need a hole through its core to pass the drill starter. I'm not yet sure I want to do this, and so I put the LH nut on the rear flywheel so I could rear start it with a drill. If I end up doing this, I'll make up drill starter with its own built-in clutch. Thanks for the comment. - Terry
 

propclock

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Thanks again. I really appreciate all you do for this site/hobby/ obsession.
The tale of 2 cranks was very enlightening.
Or depressing , if it is your own bent crank.
I have the v8 version of the bent one.
 

mayhugh1

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See this thread:

Ron Colonna designed this engine and sold the plans as a book and more recently on a USB thumb drive. He still might be willing to sell you a copy. - Terry
 

mayhugh1

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The connecting rods were machined from 7075 aluminum. The big ends will be fitted with bronze sleeve bearings, but hardened steel wrist pins will turn directly in the aluminum on the small ends. An oil passage through each rod will connect the two. One of the earliest operations in the workflow, and perhaps most important, was machining the big and small end bores parallel with each other.

Machining began by laying out three workpieces containing two rod/cap pairs each. Each workpiece was made up of a pair of aluminum blocks held together with seven SHCS's. Four of these screws were threaded deep inside the workpiece and held together what would eventually become the rod/cap pairs. These screws will be later replaced by the actual cap screws. The other three screws kept the workpiece halves together while the finished parts were machined free from it.

The rods were laid out along the grain of the metal with the rod/cap seams aligned with the seam between the workpiece halves. The rod machining was done through both sides of the workpiece. A unique pair of letters engraved on the faces of each rod/cap pair will insure no mixups later on. After completing the first side, a Devcon quick-set epoxy was used to retain the parts in their workpiece while they were machined free through the other side. For good measure some reinforcing strips were also added. Heat was used to finally release the finished parts.

Additional material added inside the workpiece to the rears of the caps protected the temporary cap screws from the cutting tools. This extra material will be removed in a later operation with the parts free of their workpieces.

For convenience during machining, the half inch thick workpieces were supported on parallels in a vise. Since the span between the parallels might allow cutting induced vibrations in the thin workpieces to affect the surface finishes, a temporary steel dampener was bolted to the underside of each workpiece during machining. - Terry
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