Polishing castings

Hi All,

Just confirming my thoughts....

To polish some stuart castings to a mirror finish, I assume I would use a wet and dry style paper to about 800-grit?

- What stages would you use for polishing a machined part? 150-grit and then 800-grit ok...maybe a step inbetween or no need?

- Would you use any kind of oil on the 800-grit paper to stop clogging? Any oil.....e.g a cutting/tapping oil ok?

Thanks

Matt

The castings would never have been polished on a full size engine. Only surfaces that have other parts like bearings and bushes are machined, never polished. If you polish a casting and paint it it looks like it's made from plastic..

For a realistic looking model, in my opinion, the surface should be left "as cast".

Hi,

Well I was thinking of polishing the flywheel and the valve chest. I wasn't going to take away the casting roughness as I quite like that.

I was however considering a course sanding of the machined parts to remove any milling marks - but I would not polish the majority of the parts to a shine, just think the flywheel and valve chest as mentioned above.

Thanks

Matt

Like engine builder I like a cast surface but if your flywheel rim ran a belt or you have a pully then these would be brighter. On the engines I make for others I had done a brighter finish for them, all I go down to is worn 150g emery cloth which gives a good sheen but not mirror finish.

You'll probably think too messy but any fairly runny oil will stop emery cloth from clogging. Cycle oil / 3 in 1 is ok. It will also cut more efficiently. Water with a drop of liquid soap prevents clogging too, useful for painted surfaces and metal too but for rust.

A related aspect. I honed something down to remove a very large fraction of a thou with a fine ARC diamond stick and was getting no where. Dipped them in honing oil, which they no longer sell, and was amazed how quickly they removed metal. I went 2um too far expecting to have to remove more.

John

-

Lots of good info here:

**LINK**

Honing oil avaible from Axminster PT. This stuff realy does make difference. Started using it a few years ago on woodworking tools as well what a difference it made to the finish. After polishing the ferrous parts coat with Metal Guard to stop any rust forming.

Hello Engine Builder,

With respect, may I disagree with you regarding polishing castings of engine components.

James Simpson & Co's pumping engines had the Corliss valve wrist plates machined and left bright as were the LP steam cylinders back covers and the edges of the flanges of the Corliss valve bonnets just to name a few examples.

In my opinion they helped to make their engines elegant.

Regards, John

One thing I'd throw into the pot - if you are cleaning up castings especially the flywheel, don't round the edges that are meant to be 'square' it can make the engine look like a cartoon toy. (Edges do need to be 'broken' to hold paint (tip from Jason...), but that's different to adding an unintentional radius).

Another is to take care with Stuart castings, some can be surprisingly soft and you can remove more material than intended quickly. I had a bit of this on the spokes of the Victoria flywheel.

Personally, I would not leave as cast some improvement usually improves things - but taking a sole plate say, to a motorcar finish can end up looking like a plastic moulding.

Steve