船模发动机制作 船模发动机, 船模发动机制作

boat

转一帖船模发动机的制作过程，帖子非常值得一看。



As a result of a most welcome and unexpected gift at Christmas time from a very good friend across the Ocean, this engine is being machined and erected using a castings set made by Stuart Models. I hope in these pages to convey my experiences in the workshop as the engine takes shape.
The Compound Launch engine is a venerable old design which has been offered for many years. The engine is almost identical to the Stuart Twin Launch version, except instead of two equal cylinders the compound has a small high pressure cylinder and a much larger low pressure cylinder. The Compound Launch is designed to be more efficient, in that steam is only supplied to the small cylinder and the exhaust steam from that cylinder then travels to the large cylinder.
In the Twin Launch engine, steam is supplied to both cylinders which are in size about halfway between the Compound engine's two cylinders. In both engines the cranks are spaced 90 degrees apart so there is always a power stroke across dead centres. In the Compound however there can be dead spots at top and bottom dead centres when starting, since only one cylinder is supplied with steam at the outset. This can be circumvented by the addition of a "simpling" valve which temporarily converts it to a simple engine (verses a compound) by admitting steam directly to the large cylinder which is in mid-stroke when the small one is stuck at the top or bottom. This gives a compound engine the best of both worlds, allowing for higher efficiency and insuring self-starting from any position. These two engines put out the same amount of power to the propeller shaft.
In recent times most of my models have been scratch built, so I thought this would be a good time to show my approach to machining a castings set. Of course I am overjoyed at this prospect as this is refreshing change for me. For those of you who have never tried a castings set, but wondered exactly what to expect I hope this will help to answer some of those questions. While this is not my first experience at this aspect of making model steam engines, I have made some from castings as well as a number of models built from scratch over the years. I don't claim to be any kind of an expert in the machining of castings, but will simply proceed using common sense as my guide.


Please note that constructing this engine involves the use of metal cutting tools and machines which can easily cut people equally well. Everyone should be confident that they will be safe before attempting any of these operations. Use common sense where safety is concerned and be prepared to accept the responsibility alone for any mishaps you cause. Simply thinking out carefully what is going to happen - before you hit the switch - can go a long way toward helping to prevent damage to yourself, the machine or the workpiece.
As always on this website, I am describing how I did things - not how someone else should.
Perhaps you may have wondered, like I did years ago, exactly how rough "raw castings" actually are... That much I think I can answer with pictures. An honest attempt was made to photograph all the lumps of metal in the set as they looked before I performed any machining operations.

boat

This page may prove to be quite an eye-opener if you haven't looked at raw castings before. These are not to be confused with investment (or lost wax) castings, nor will they resemble the apparent beauty of finished castings - that transformation is our job. Just take a look as some of my close-up pictures. The foundry doesn't make the part, that is the task of the machinist. The foundry's job is to provide enough metal - only where it is needed - to allow construction of the part - and that's it. Casting is an art in itself and many years of experience pays off in producing good castings. Among a host of other foundrymens' concerns (including human safety), allowances must be made in advance for shrinkage of the metal during cooling. To me, a well made raw casting is evey bit as beautiful in a different way as its machined counterpart.

boat

Let's have a good close look at the cylinder block first.

boat

There are sixty-two holes to drill in this block of iron and they are also required to be threaded. For each of them there is an equivalent hole to be drilled in another piece. In addition the top, bottom and both ends are to be machined, exhaust ports must be milled and the cylinders are to be bored out and lapped. At that point this casting will be finished.

boat

boat

Think that's a crack in the casting?


Not at all!
Here is the same casting partially machined two weeks later

boat

I know there is going to be some trouble here...

boat

boat

boat