On 10/20/07 4:05 AM, "Dud.One" <dud.one@xxxxxxxxxxxx> wrote: > on another point , i am building a Rob Roy as my first Loco and i have come to > the point of the castings for the cylinders , how do you get a reference for > the steam chest centre line and the fact of making the port face tangental to > the cylinder bore . I don't think you want to make the port face tangential, if I remember Rob Roy cylinders correctly :) - you want the port face and bore to be parallel. Exactly how you achieve this is going to depend somewhat on the tooling you have available, but in terms of machining personally, I would look to machining the port face first to give you a datum to work from. The casting should be pretty square, so you shouldn't be taking wildly different thicknesses of material when you do that work. Once you have that surface to work from you can then mount against that face and bore the cylinder; with a bore to work from you can mount the cylinder on a mandrel and square the ends... though, as I said, how you do this may vary depending on whether you are doing this with a traditional lathe or if you are in posession of some piece of multi-axis CNC machining center. I've never built with one of the latter; the techiques might be very different... Alternatively you could probably try and cut the bore first, then square the ends to the bore and use the ends as your datum and then cut the port face from them, but I think the former order would be easier. > may be a dead easy question for you guys , but i don't want to screw them up Knowing the right way to do it isn't going to guarantee you don't screw them up, Dave :) Just remember to make sure that your castings are securely mounted when you machine them. There is little as distressing as having put many hours into a casting only to have it move and mangle itself... Tim -- Tim Kirby trk@xxxxxxxxxx MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.