Gallery, Projects and General > How do I??

PMR Coke Bottle Engine

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rleete:
Okay, so I've goaded Bernd into trying to get these engines made.  We both have the kits, but his appears to be mostly brass/bronze while mine has the major structural elements (like the frame) made of cast iron.  I refer you to Bernd's website for a pic: http://www.kingstonemodeleng.com/EngineShop/Model5b/5b.htm

So the question is, where to start?

The frame is, naturally, cast in the coke bottle shape.  [Hint: it's the tall thing in the center of the picture].  There are no straight lines on it.  The top and bottom are uneven, having a casting line through the center, and draft on both sides.  If it is clamped into a vise to square off the ends, and it is not perfectly done, the thing will sit off kilter.  Maybe only a little, but being so small (6" or so), the slightest variation will show up as the whole thing leaning like that famous tower in Italy.

Do I clamp it up, trying to use the fairly thick casting/parting line to get it square?  If, after machining, it appears to be off, to try and shim it and cut again?  This trail and error method (mostly error in my case) seems a bit crude.  I'd mount a shaft in the vise and use that to line it up, but the bore is also cast, and may or may not be perpendicular to the base.  It's also not very round.

So, help a newbie out and let me know where to start.

bogstandard:
This is where making from bar stock is more preferable for a beginner than making from castings. You can easily get a datum face with stock, rather difficult with a casting.

But that doesn't solve your problem. Finding a datum face to work to.

Without a datum, you will be working totally blind and stabbing at it, hoping everything will line up afterwards. I will tell you now, that way just doesn't work.

Not having the castings to handle, it is difficult to imagine the best way to machine them, but by looking at the picture I can only suggest a way to go.

Looking at the main bottle casting, I personally would go for one of two ways.

The first one is the rim around the top.

Holding the square base in the four jaw, get the casting running as true as possible, you can see how it wobbles about when run up. By eye, get the main bottle shape running as true as possible by moving the jaws and tapping with a soft hammer, and hope that it is nearly symetrical. Then using super fine cuts, turn the end rim on it's outside edge until it has just cleaned up all round (do not take it down to size, that can come later), followed by a very fine skim across the end face. These first two operations have to be carried out very tenderly, as any heavy handling will knock it off centre. Once that is done, you will have a good datum.

If you have soft jaws you would bore them out until you can fit the datum into the recess, but if you don't, put your outside jaws into the 3 jaw self centring chuck, and mount the newly cut datum part of the casting into those. By gently tapping and seating the casting into the back of the jaws, the bottle shape should run true, or very close. Now another very fine cutting exercise, face off across the bottom of the base.

That was the first way.

The second way goes like this.

Using outside jaws, mount the casting by holding the rim in the nipped up jaws, and tap the casting until it is running true by looking at the bottle shape of the casting. Fully tighten up the jaws and recheck it is still running true. Very gently tap to straighten it up.  You might need to shim the rim to get the neck part running true as well.
The better you can get it trued up, all following machining operations become easier.

Machine very gently across the casting foot. This will ensure that at least the casting is looking upright when sitting on the foot.
Mount the casting onto your faceplate using the now datum foot, and tap everything straight, bottle shape running true. Now you can machine the rim and it's face, plus maybe the hole in the end as well.

If you can get those two datum faces made, you will most probably find you can mount it onto a faceplate using the base datum, and a mandrel to go into the end hole for mounting the other way.

There is another way, by making a mandrel and fitting it into the core hole in the end, but it all depends on how accurately that core has been placed prior to casting.

Castings have to be machined initially by eye, to get your datum faces. Only then can other machining be carried out.

I hope this has helped a little and not confused you too much.

John

rleete:
Yes, this has helped.  I kinda worked it out myself the same, but was hoping there was a simpler way, or something I'd missed.

Since most of the dimensions are taken off the base (or the foot, as you call it), and that sets the stage for everything being lined up vertically, I think method two is the one to use.  True up the bottom, and take everything off that as a datum.

Bernd:
Bogs,

The place I worked in I used to have to work on rough castings. Of course they had fixtures that they fit in so you could do your maching. So I'm not new to maching rough castings. It's just I would hate to ruin this casting. You've mentioned using the lathe. How about the milling machine to a get some of the maching done? Just thinking outside the box here.

I have the casting sitting right in front of me right now. Looking at it sitting up, just like I have the pic on my web site. You could file the bottom of the frame to get the major burrs off. Just sitting here on the table it looks to sit pretty darn straight. Then you could mill the top off to clean up the casting. Once clean turn 180 and do the bottom same way, just a cleaning cut.

That was one reason I didn't go further on machining the engine. I had a hard time trying to figure out how to begin on the frame. Also remember seeing some were how somebody machined the frame. I did notice a note I had written on the print for the stean chest cover "done 9-30-03. So I haven't touch this kit for 5 years.

BTW, the casting are quite clean, very little flash on it. I'll take a couple of pics and post them.

Bernd

Bernd:
rleete,

How much have you got done on your engine? I have both wheels semi-finished (at least that what I marked on the print) and the steam chest cover is milled flat both on the bosses and the back with the holes drilled in.

I'm going to hold off on the frame until the end. I want to do a bit of searching and see if I can find how to hold the frame for machining. Could be an off the wall idea. I also remember some place I saw how it was done. If I find it it would take the fun out of finding a solution of our own.

Bernd

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