Crankshaft Grinding

[Alan Worland 1Participant @alanworland1]

I have just started making a Stuart Turner No 4 Engine and although not yet attempting the crankshaft yet I was curious to know if anyone has ‘ground’ the big end with the crank between centres?

I was thinking along the lines of using one of those diamond impregnated cutting wheels set up on a spindle in the top slide.

Any thoughts?

[Alan Worland 1Participant @alanworland1]

[John OlsenParticipant @johnolsen79199]

I had the crankshaft on my compound engine ground by an automotive machinist, after roughing out between centres in the milling machine. The only thing on that crank that the lathe ever saw was the outside of the bolted on balance weights. The advantage of this sort of approach is that you can mill out the bulk of the material easily, no need for long spindly lathe tools. Even a long series milling cutter is much less likely to give trouble than turning.There is no reason why doing the grinding yourself should not work.

 

regards

John

 

[Alan Worland 1Participant @alanworland1]

Thanks John, I guess your crank is probably larger than mine although the Stuart crank is as simple as they come. Did it grind well with a good finish?

I would obviously make sure everything on the lathe was covered sufficiently – but when I get to that stage it will give iit a go

[John OlsenParticipant @johnolsen79199]

Hi Alan,

 

I guess it probably is, it has two throws at 90 degrees and the stroke is three inches. It is cast in SG iron and the ground finish came out brilliant, almost a polished finish. He did say I had left too much on to be ground off, the SG iron tended to load up his wheels and he had to dress them often. About ten thou should be plenty.

 

I did an article on milling cranks in the ME a few years back, it has some pictures of that crank and others I have done with the milling first technique. The idea was not mine, it came from Professor Chaddock.

 

regards

John

[Alan Worland 1Participant @alanworland1]

3 inch stroke! The No 4 is only 1 1/4

I suppose to get into the big end area between the webs would require a sort of parting tool to remove the excess before the grind?

[John OlsenParticipant @johnolsen79199]

Hi Alan, That is why I would suggest using a milling machine, if available, and if you are able to set the shaft up between centres on the table. Instead of using a long bendy parting type tool with excessive overhang, you can use a slot drill that is nearly as big as the width betwen the webs. Even if you find you need a long series cutter it is still a more rigid cutter than the parting type of thing, so less prone to disasters. I have tried this both ways…the first one I milled was for a Stuart Double 10 on a Unimat, using the milling attachment. The rotation of the shaft was controlled by hand by turning the normal drive pulley, which is OK in that sort of size. (the motor was on the milling attachment and the lathe pulley was just used as a handle.) For larger ones like my launch engine, you want a more positive control, I had a Vertex dividing head by that time.

 

The shaft is set up in the same way as it would be for turning, eg between centres, with throw pieces when doing the big end, and with spacers clamped between any other webs or between the web and the throw pieces when doing the big end. Actually you could do it in a lathe if you had one of those handy milling attachments that goes on the cross slide.

 

regards

John

[Alan Worland 1Participant @alanworland1]

Hi John, thanks for the info, sounds a sound idea using the milling method – I haven’t heard of that before.

I have a Myford milling attachment, this mounts on the bed and uses the lathe spindle to drive the head, so I would not have access to the headstock centre but I am sure I could come up with something if only on a temporary basis.

 

Alan

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