Elmer’s ‘Tiny’ #23 part two
The crankshaft bearing was completed before moving on to the cylinders.
Having completed the three engine bodies Jim’s build moves on to making the crankshaft bearing before tackling the cylinders
Machining the body was a bit traumatic; three broken drills and three restarts from scratch so for a bit of light relief I made the crankshaft bearing next.
Almost nothing can go wrong with this job, and nothing did. The bearing hole doesn’t need to be reamed, I just drilled it , 2.5mm and then 3mm. I then turned the outside to size, checked that it fitted in the hole in the body and parted it off. After it had been reversed in the chuck and cleaned up, the job was done.
And now the cylinders
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- All this messing around with the angle plate was a pain so when I did the replacements, I cut the cylinders off first and used a couple of parallels to set them up in the vice. With a depth stop set so the bore went just down to the port, the whole job was completed in a fraction of the time and without constantly swapping from Jacobs chuck to slitting saw. This is called experience.
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- Cutting points were spotted between each of the cylinders. In a later attempt, I just dropped the centre drill onto the work without it rotating. This gave a point mark that improved the accuracy of the subsequent cutting. The y axis stops were just pushed up against a piece of silver steel in the chuck to align the work parallel to the y axis.
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- I needed to make 3 cylinders, each 8mm x 8.5mm from 3/8 inch square brass bar so the first steps were to machine the bar to size, spot and drill holes for the port and the cylinder pivot. Once again, the dimensions were taken from one end .
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- I used this Heath-Robinson arrangement to drill each cylinder in turn and then cut it off the stick. I think I was influenced by some articles on workholding in Model Engineers Workshop. If you look carefully, you can see that the hole in the cylinder has been drilled off-centre. And if I had looked carefully, I would not have had to make another set of cylinders.
The pistons and crank disc came next
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- This was my only piece of stainless steel , so everything had to be made from it. I turned each one down to match a cylinder. The small diameter was carefully chewed out with the parting tool and then filleted in with a round file. Blessed be the buffing wheel.
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- Looking at the photos, I have no idea why I chose this method of holding the work to mill the flats and drill the hole instead of putting it in one of the slots. And using an end stop would have been a good idea. Hindsight!
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- These are the first lot of cylinders (before I noticed that the holes were off-centre). You can see the remains of the spots with which I marked the cuts between cylinders. That is why I just dropped the centre drill onto the work for the next lot.
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- Had I studied the plans more carefully I would have seen that the holes were off-centre.
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- The crank disk was turned with a bit of relief to minimise friction, and then attached to the crankshaft with Loctite before being rechucked for truing and cleaning up.
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- Set up in the milling machine for drilling. It is easier to measure in from the circumference than to find a centre from which to offset the hole. Once setup, each of the cranks were drilled in turn.
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- This time, I checked with a dti that the work did not move under pressure from the drill but still, the points were not square
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- It has been my practice to Loctite the crank pins in the holes. But no matter how carefully I drill, the pins never end up square.
Incidentally I noticed that in one of his other designs Elmer uses a press fit for the pin. Despite the difficulties of getting the tolerances right at this size, I might try that next time. This time, I cheated and increased the size of the hole in the piston.