C3 Mini-Lathe Metalworking Beginners
Make a press fit bushing - cheat on the mini-lathe, use the weakness to your strength
In the previous article I described an approach to turning down a bar so that it made a good press fit into a bore of known diameter. The basic idea is to make a step which slip fits, and then measure that, and turn the rest of the bar is a smidgen wider, and thus a good press fit.
This method is good, but there is an even simpler method – so simple, it is only just to the left of cheating. It relies in the inherent inaccuracies in your average mini-lathe to produce minor eccentricity in a part which is turned in two phases.
This method works very well if we are making a bushing out of aluminium. I “discovered” this method when a friend handed me an alternator from a elderly BMW E28 M535i and asked me to make a new bushing. The mounting holes in the casting of the alternator are about 22mm diameter. Originally these holes had rubber bushings pressed into them which had a hole down the middle for an M8 bolt to attach the alternator to the engine.
Anybody will tell you that rubber alternator bushings are a damn silly idea. Besides, BMW won’t sell you a new bushing, just a new alternator, which costs rather more than the car is worth.
So I had to make a new bushing from aluminium. This would be about 2 inches long, with an M8 bolt hole down the middle, and with an outer diameter which would be a gentle press fit into the hole on the alternator.
First of all I chucked up some 1 inch round bar and turned it down to about 0.5mm oversize based on measurements of the alternator mounting holes made using a telescoping bore gauge. Such gauges are not very reliable in the home metalworking workshop, so I was cautious to keep the part oversized.
Next I turned down the right hand half of the tube slowly in stages. I kept turning until the part was what I call a “stiff wobble fit” – the part does not slide into the fitting easily, but will go in with a bit of jiggling… I managed this by turning down the outer diameter in smaller and smaller steps.
Next, I removed the part from the chuck and reversed it to allow me to turn down the part which the chuck jaws originally held. Using a 3 jaw chuck, my test dial indicator and a copper hammer, I centered the part so that the run out was about 0.005 inches.
Then I turned down the section of the bar which protruded from the mini-lathe chuck so it too was a “wobble fit” into the alternator casting.
Now I removed the part from the mini-lathe. If I tried to insert the bushing into the hole, it would jiggle in about half way… and then stop. This is because the turned outer diameter at one end was not perfectly concentric with the other end. Additionally, there will be a tiny burr down the middle.
This slight eccentricity between the two turned ends is enough to convert the two “wobble fit” ends into 1 piece which was a press fit into the hole. Although the two ends are both a slip fit into the hole, the entire piece isn’t – and could be pressed into the alternator mounting with the aid of a vice.
So in summary – turn the two ends of a bush on the mini-lathe by re-chucking each end – the inherent eccentricity introduced by using the mini-lathe 3 jaw chuck will give you a tightly fitting part. Of course, I cant’ guarantee that the centre bolt hole is parallel to the alternator casting – but in this case it doesn’t matter, so it would have been obsessive precision to make it so.
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