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Model Engineering Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Crankshaft

I often use a permanent marker rather than engineers blue as it is a lot less messy and it works just fine for marking out.

The crank is single throw and made from a peice of 0.5" x 1.25" mild steel.


The material held between centres on the Warco WM240B

The faceplate serves 2 purposes:

  1. Adds rotational inertia - flywheel to smooth out vibration
  2. Allows a drive plate to be fixed to the headstock and connect to the part to be machined - see next photo


The drive dog is a right-angled piece of aluminium.

This drive dog reacts against the workpiece, the trouble is there is some vibration and impact as the workpiece is very much out of true at the start of this process.



The permanent pen markings come into their own when you are removing large areas of material with the hacksaw.


With the material removed it is easy to see now that when I hold the main shaft between centres the gap in the shaft where the step is for the crank will struggle to react the axial force that will be applied in the lathe.


I locked 2 nuts onto a threaded bar and machined the faces down so that they were smooth and to give an overall length that would fit the gap between the crank webs.

The 2 nuts with the threaded bar were then placed bewteen the webs and carefully loaded against the webs, not too much force as this will spread the gap too far, needs to be enough to react the axial force between centres.


I used a cable tie just to stop the nuts coming out and whacking me between the eyes. You never know when machining if something comes loose at speed it will launch itself across the workshop.


With one end machined down I then mounted this in the collet chuck and aligned the other with the centre and machined the other end.


Both ends of the crank now machined down to 12mm diameter.


With the cable tie removed you can clearly see the 2 nuts and just see the threaded bar.

Thinking about it now it would be quite neat to use a bolt and a nut and machine down both faces. This would reuce any chance of the short threaded bar snagging and marking the inner face of the web.


The machined face of the nut.


Here you can see the nuts and small length of threaded bar that was used to react the loads when holding it between centres.


The crankshaft in a good state of machining.


One way to machine the big end and to get it true is to mount the crankshaft in a four-jaw chuck, the issue is that the jaws will be holding the main shaft at a large offset and so probably just 2 jaws will be engaged on the shaft and hence little forces to stop it going sideways.

One option is to make a block to clamp and allow all 4 jaws to engage at a large offset.

The block was machined to ensure the ends were perfectly square and then a hole was drilled offset.

The slots were cut with a slitting saw using the milling machine, these allow the block to be tightened in the four-jaw chuck to hold the crank tight in an offset location.


A dial gauge is used to get the big end running true.

You will need a lot of patience and practise to get the shaft absolutely true.

One other thing that is good about the block is that you will not leave marks on the crankshaft from the jaws of the chuck.


The block rotates and th big end runs dead centre.


See also: Conrod, Engine, Gudgeon Pin, Pistons..

Model Engineering Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z