Wednesday 12 May 2021

The AJS V-Twin Crankshaft: Part 4 – Machining the Flywheels

Much of the AJS V-Twin project has been carried out during Covid lockdowns. When the first lockdown came in the UK in March 2020 panic erupted as the supermarket shelves were stripped of toilet rolls and baked beans. I also suffered a twinge of panic and immediately went to the workshop and made an audit of all the materials I thought I could possibly need but hadn’t yet ordered for the project – steel, aluminium, brass, phosphor-bronze, bearings, valves, tooling etc. As an afterthought I ordered 2 boxes of tinned haggis on ebay ....just in case; I still have 11 of the original 12 tins in my garage.

Part of this early order for materials included 2x 1900mm diameter x 40mm thick slices of EN24T for the flywheels and a 40mm diameter x 500mm long bar of EN40B for the mainshafts.

EN24T is a tough steel (Tensile Strength 850/1000 N/mm2 and Yield 680 N/mm2) and has been used widely for crankshafts on performance engines. EN40B has similar mechanical properties and can be nitrided to give hardness of around 61 – 65 HRC. It was the intention to harden the shafts by nitriding but, for reasons that will be discussed in a later blog, an alternative route was followed.

The order of machining has to be carefully thought out to ensure that any particular machining operation does not make it difficult or even impossible for any subsequent machining. Before starting, I decided on the following processes for critical operations:

1)    I would ream the mainshaft and big-end internal diameters to get the accuracy and surface finish required. I do not have access to sufficiently accurate internal grinding - I don’t believe that my tool post grinder is adequate.

 2)    Accurate positioning of various holes would be set up using the DRO on the milling machine and then transferred to the lathe for main boring/reaming operations and setup with a dial indicator.

 3)    To ensure that the flywheels were identically machined they would be held together with accurate dowels through the mainshaft and big-end and then screwed together so that both could be machined simultaneously.

The first task was to face-off each flywheel in the lathe and to bore and ream the hole for the mainshaft.

The hole for the mainshaft is bored to 29mm minus 0.008’’ for reaming allowance and there is a 1/8’’ deep counterbore for a shoulder on the mainshaft. The first part of these reamers is tapered to facilitate easy entry and it is important that the reamer is inserted sufficiently that the entire hole is reamed with the parallel part of the reamer.

After reaming both flywheels, a mandrel plus nut and thick washer was made to hold the flywheel on centre so that the outside diameter could be machined. The nut and washer for the mandrel were made first so that the mandrel could be left in the chuck on-centre before machining the flywheels.

After machining the OD and facing off the other side on both flywheels they were put in the milling machine and 3x 5/16’’ BSF tapped holes and a pilot hole for the big-end were put into one flywheel and 3x clearance holes and a big-end pilot hole in the other.

A close fitting 29mm diameter dowel was also made for the mainshaft holes with which the 2 flywheels could be fixed together exactly on centre for further tolerance-critical machining operations.

The flywheel with the 3x threads was then carefully set up in the 4-jaw chuck on the lathe so that the face was perfectly orthogonal to the axis of the lathe bed and was centred on the big-end pilot hole.

The other flywheel was then centred and bolted to the first flywheel so that both big-end holes could be in-line bored and reamed.

There is a lot of off-centre mass rotating in the chuck for this machining operation and it was necessary to run the lathe at very low speed.

After boring the hole for the big-end both flywheels were reamed to exactly 1” ID

and after reaming both big-end holes, the clearance holes for the big-end nut/socket were bored

and the big-end boss was then machined on each flywheel

Each flywheel was then mounted on the rotary indexer on the milling machine and the balance holes were set up and bored. These were left undersize at this stage because the next step was to set up the flywheels on the faceplate so that each balance hole could be set up on-centre, in turn, and both flywheels bored simultaneously. The flywheels are doweled together through both the big-end and the mainshaft holes to ensure they are bored identically.

Each flywheel was then mounted on the dividing head on the milling machine to machine the annular segment.

Both flywheels were then again doweled together through both the big-end and mainshaft holes and the single hole opposite the crankpin was bored to 12mm. This will be used later for alignment of both flywheels for assembly in the press.

The last remaining major machining operation was to face-off the outside of each flywheel to form the mainshaft boss that will be in contact with the main bearings. Any minor adjustments for crankshaft end-float will be made immediately before assembly when all the parts have been made.

Finally, the oil drilling connecting the feed from the mainshaft to the big-end is required. This is a 2.5mm diameter hole and, rather than drilling this in the conventional way with a long drill, I chose to have it spark eroded rather than risk the drill wandering off and emerging in the wrong place.

The flywheel machining is now completed.

It is a somewhat academic point but the introduction of the oil hole creates an extremely small imbalance between the flywheels - one flywheel has the drilling and the other doesn't. This imbalance is referred to as a "Rocking Couple" and is an out-of-balance force along the axis of the crankshaft ....not to be confused with our behaviour back in the days of our youth...

The machining of the flywheels took me a few weeks and generated a few buckets of swarf and, because EN24 is tough steel, the machining ate through plenty of carbide and HSS cutters. The old adage “measure twice cut once” has never been more appropriate; I would not have wanted to have screwed-up the last machining operation!

I was pleased with the final result but the acid test will be when the crankshaft is finally assembled.

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