The next stage was to make the cranks and for these I chose stainless steel. I have never had much success with mild steel but find I can achieve a reasonable finish with free cutting stainless. The piece I had available had to be turned down a considerable amount to arrive at the size required. For this I used an indexable cutting tool which utilises the broad side of the cutting insert for rapid removal allowing fairly heavy cuts.
I used a liberal amount of cutting oil applied by small paint brush to ease the cutting process. I have found that (fine cut) emery backed sponges are ideal for cleaning up workpieces in the lathe. Never be tempted to use a rag as this can so easily wrap itself around the chuck and drag your fingers in before you realise what is happening. Safer to use paper towels but I find the emery sponge ideal.
After centre drilling I ran a 2mm pilot hole in the end of the bar before carefully selecting, in this case, a 3.8mm drill to match the actual diameter of my 4mm stainless shaft that I intend to use for the crankshaft. Do check drill and shaft sizes carefully with your vernier to ensure that you finish up with a nice snug fit. I normally drill slightly undersize as the rotation of the drill does produce a slightly larger hole than you might expect.
Although this next stage could have been completed with hand tools (junior hacksaw and files) it is easier to use the mill. Each crank was set up in the vice using a small length of shaft thro the larger hole and an appropriate drill bit for the smaller hole. Use of the ‘X’ and ‘Z’ readouts gave me control of the cutting area. The backstop allowed me to turn the crank ‘upside down’ for identical machining on the other side. It also allowed me to machine the second crank to the same dimensions without the nead to reset machining limits.
Screws with shoulders to provide a bearing surface for the connecting rod were turned up in the lathe and a die held in the tailstock to provide a 3mm thread. A quick polish on the buffing machine finished this stage of the build.
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- An indexable broad facing tool is used to bring the stainless steel bar rapidly down to size.
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- Fine cut emery sponge proves ideal for finishing and cleaning up turned bar.
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- After centre drilling the hole for the crankshaft is drilled whilst on the lathe to ensure alignment.
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- When parting off rigidity of cutter is important. Use the shortest length of blade possible and lubricate well.
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- A light skim to the final thickness cleans the crank disc up nicely removing burrs and any uneveness from the parting off operation.
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- The profile for the crank webs are formed using an end mill with repeat positioning taken care of with the backstop.
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- Two completed crank webs fitted with shouldered crank pins that were turned on the lathe.
