Bog's Paddleduck Engine

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bogstandard:
Kvom,

If you read the writeup on making the eccentrics, you don't need to use the four jaw, but your mill.

With regards to being critical with the dimensions on the crankwebs. That is because if you don't get the hole centres the same as each other, either the crank won't turn or will wobble all over the place.

If you follow the writeups in the book, it shows exactly how to obtain the correct results, no guessing, no magical solutions, just the correct way to get what is required. Plus with you having DRO's, you should be able to do it with your eyes closed.

Bogs

kvom:
My question was the "engineering" that went into the design. Since you made the block first, did the dimensions for the webs come from the inlet/outlet spacing? My assumption is that the offset of the eccentric is likewise tied to hole spacing in the steam chest. The entire subject of valving in steam/air engines is fascinating. When building the Elmer engine, I just went along blindly following the plans, drilling holes wherever indicated, happy as a clam. It wasn't until I actually looked at the cross-section of the valve and cylinder bore in Elmer's original plan that I had any inkling how it was supposed to work.

Darren:
I don't have an inkling how this all works either, I'm hoping my build will show me the way it all fits :ddb:

bogstandard:
Now I understand where you are coming from Kvom.

In fact, when I designed this engine, I had never worked on a piston valve one before, and I asked around for any formulae, and Sandy (in the book) made some suggestions about timing and porting. From that information I worked everything out myself.

The relationship is the total eccentric lift (6mm), the spool lengths (3mm) (the length of the thick bits on the spindle that goes up and down in the valve block) and the diameter of the holes (3mm) that the thick bits open and close.

When I get a bit more time in the next couple of days, I will knock up a sketch how it all works in unison.

Bogs

kvom:
OK, I have one part made to justify this thread. I decided to tackle the eccentrics, as the technique in the book is interesting. I also had some suitable sized steel cutoffs.

In interpreting the plans I calculated that 1.5mm = .059", so any dimension needed is quickly calculated needing to remember only one number. So the diameter of the eccentric is 18mm -> 1.06".

The initial turning to length and cutting the end flange and groove were not too difficult. I recently acquired a very veteran B&S height gauge and a cheap ENCO granite surface plate, so that I was able to measure the lengths and face to the desired dimension fairly easily, if slowly.

I don't have a rotary table, but I used the following setup clamp the disc in the mill vise and to find the center quite rapidly:

The indicated 5mm hole defines the diameter of the crankshaft, and so would fall between 1/4 and 5/16 inches. I happened to have some 1/4" drill rod, so that's the size it will be. That said, I can always enlarge the hole should a thicker crankshaft be needed.

Then my trouble of the day started, as my run-of-the-mill automotive class die was incapable of cutting a 1/4-20 thread in the end of the hardened rod. So I had to resort to trying to thread the rod using the lathe. Once the threading tool cuts through the hard layer the interior of the rod is a rather gummy steel that resists cutting cleanly. So after any number of tries I finally got some partial threads cut and was able to finish them with the die. Then it was a matter of bolting the eccentric piece against the face of the 5C collet/block as shown here:

The reason for this setup is that the 6-jaw vise won't tighten on a 1/4" rod, so this is my "collet chuck" setup for small rods. The chuck is very accurate, so that the runout with this setup is quite low. It's necessary to really tighten the collet when taking the interrupted cut or else the rod will just turn in the collet jaw. My turning technique was to touch the lathe tool to the innermost point of the boss and set the DRO y-axis to zero. Then when turning I could watch the DRO count down and would know when to stop the cross feed, which was running at the lathe's slowest speed of .0006/revolution.

So after 4 hours work I have 1 part to show:

I used a lock washer under the bolt, which scratched the surface a bit. For the next one a flat washer might be better.

And of course as soon as I was finished I remembered that I had a piece of 1/4-20 threaded rod that I could have used instead of threading the drill rod.

At least I'm "underwauy" :proj:

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