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Machining an S50 casting

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Lainchy21/11/2019 10:42:37
244 forum posts
95 photos

Very quick one, I hope... I realise there will be a raft of differing ways to machine a cylinder casting, but my first kit is the S50, and you will have seen my small blowhole issue. Well Stuart did replace that, and the valve chest cover, although that one was chilled too! (I persevered with that, using a carbide endmill to success)

I came to machine the casting, and at first, I tried centering in the 4 jaw... spent some time with the Dial indicator.. faced and bored it through, only to find it off centre at the back end...not by a lot, but I'm doing what I was told NOT to do, and chasing thou's!

Anyway, I ordered another cylinder

I started to machine that last night... making a mandrel first for the rough casting... using that to centre to ensure the bore was as straight as I could make it... faced, cleaned up the edge of the flange (which apparently I could take a small amount off - You can't achieve the plan dimensions if you do IMO) and bored the cylinder.

Took it out and reversed it... and the flange is still off centre. I'm sure in metric, the dimension should be 27.77mm. I'm under that, and It's still not cleaned the edge. Given that it's going to be covered, I really need to not chase it. I believe that the end covers are not going to be bigger than the casting, so I really need to stop worrying! There's plenty of metal to ensure the valve face and the mounting base are flat and perpendicular to the faces.

Have I done this the best way? I can't see anyway of dial indicating the front AND the back that's in the jaws.




Michael Gilligan21/11/2019 10:52:44
15704 forum posts
687 photos

I think the best way might be to first mount the casting between centres, and check everything carefully before you start machining. ... It doesn’t sound like there is much ‘meat’ to play with.


Lainchy21/11/2019 11:01:33
244 forum posts
95 photos

Thanks Michael. That's an idea. I suspect maybe that the bore is slightly off. I'd be amazed if anyone has managed to clean up the flange on this casting without going undersize. I was hoping to clean it up so I had the option of fitting the cleading, or perhaps not fitting it. There's lots of spare meat for the other faces, just not much on the flange.

JasonB21/11/2019 13:11:41
18103 forum posts
1994 photos
1 articles

You should never really go by a cored hole as they are often off and seldom round.

The usual way is to get a couple of bits of hardwood though MDF will do and rough shape then to a round taper and then drive into the cored hole. File off flush with the casting.

You now have two flat surfaces that you can mark out the cylinder center on, some odd leg calipers from say 4 places around the flange will give an approximation that can then be refined, most now would tend to use their digi calipers to do the same thing

You can then use these two points to set out the port face and machine that before holding to an angle plate or in the 4-jaw to bore and face the front (crankshaft end) of the cylinder flange as you want these to true to each other.

Flanges can be turned to 1mm less than the cylinder cover diamater so that 0.5mm cleading fits flush with cover

HOWARDT21/11/2019 13:32:07
556 forum posts
15 photos

Before you start machining blue up the casting and mark it out. This will allow you to centralise the casting. If you have a mill square up the flange ends. Then mark out the bore to give a centralised bore to the casting. As said don’t rely on cores to give true position, some castings can be so far out that some redesign may be required to utilise them.

Brian Oldford21/11/2019 14:23:37
652 forum posts
15 photos

I was taught many years ago, when making patterns, to always put ample machining allowance on a casting. It would seem whoever made the pattern(s) for this was absent from that lesson.
It's easier to take it off than put it on.

Lainchy21/11/2019 14:45:02
244 forum posts
95 photos

Thanks for the advice chaps. I'd have never of thought about plugging the bore and using that to centre. I suspect that would have been a good way in my case.

There seemed to be lots of metal on each end, the port face, and the base... but none spare on the flange sides.

Will certainly bear that in mind for the future.

Many thanks

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