Brian's Donkey Engine

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Jasonb said:
The large size Benson that I have just got running uses 1/8" & 3/16" ports at the same spacing as your 0.282".

But the cavity under the puck is 0.423" long, total length of puck 0.75" and the eccentric throw 0.117" and it runs fine on 10psi and thats a much bigger piston & stroke.
Click to expand...

I got very close to these sizes when i ran it through my maths (.438 and .748) for 1/8 and 3/16 ports.

If you've done the cad simulation and and it looks OK it probably is OK Brian, we can keep our fingers crossed and hope the black cat comes up trick.

There is more than one way to skin a black cat, and these engines have a fair bit of tolerance for air running its not as though we're running a pumping station with them where ever bit of effieciency matteres.

Its a great project Brian and very interesting.

Stew
 
I just finished going thru all 7 pages of this project. It is fascinating to say the least. Now caught up I won't miss any more.

"Billy G"
 
Brian Rupnow said:
I wanted this cylinder to be buildable without having to buy metric endmills
Click to expand...

On small engines its possible to just used drilled holes for the port face and not worry about machining slots which allows you a lot more freedom with sizes (benson is drilled as you can see )

If you are going to just run this on air I'm sure things will be OK but if the boiler is to work then a bit more working out to get a good balance between power and steam usage would be worth while doing, don't want to run out of steam half way through a lift and have to wait for the boiler to recover!!

You are making good progress.

J
 
Jasonb said:
On small engines its possible to just used drilled holes for the port face and not worry about machining slots which allows you a lot more freedom with sizes (benson is drilled as you can see )

J
Click to expand...

That,s worth knowing

Stew
 
Brian

Those are nice looking cylinders with a nice crisp action. I do have a comment on the valve dimensions. The inside length of the steam chest is 1.031" and the total space required for valve travel ( valve width = .657 plus total movement = .344) is 1.001. That leaves a clearance of only .015" at either end of valve travel. You risk having the corners of the valve hitting the radiused corners of the steam chest. Just a little contact may not prevent the valve from working but would likely make a "knock", even if perfectly adjusted. You can eliminate the problem by filing the inner corners of the steam chest square or by rounding the corners of the valve. This will not have any affect on the valve operation.

Jerry
 
Jerry---Thanks for looking. I actually did have a 1/16" x 45 degree chamfer on each corner in the drawing, but after my video action, it felt like it didn't need it, so I took it off the drawing. If I hear anything knocking when the engine is all set up, I'll know it isn't opportunity and chamfer them then.----Brian
 
My God, those crosshead guides have a beautifull shape!!! Kinda like Marilyn Monroe on steroids.---But what a pig to machine. I started with solid 1.125 cold rolled steel. The drilling, reaming, and boring are pretty straightforeward, but the removal of material from the outer diameter to yield that beautifull profile is a nasty bunch of work on my small mill. That big .625 inch diameter ball end mill I use clunks and thumps like the voice of doom. I may try to get the same result on the second crosshead using a 2 flute .625 inch endmill and plunge cutting at the ends. In the picture you can see a piece of tooling that I saved from when I built the Popcorn engine when I had to machine the crosshead for it.
onecrossheaddone001.jpg
 
Brian,
As you have found out ball mills are not very efficient cutters as far as metal removal goes. The cutting edge is very blunt and that makes them pound against the work instead of slicing off material. Their use requires a very sturdy machine. There are several ways around this, first is to take a rotary tool, Dremel or the like, and using a small cutoff wheel or mounted stone carefully scallop the face of the ball shape back to obtain a sharper cutting edge. The second way is to use a smaller ball mill and by the process of sine/cosine step off the desired radius. Depending on the amount of stepover you use it will require a bit of hand work but I find this easier than beating away with a large ball mill. I'm really surprised that you could even use a .625 ball mill in a small mill. My larger mill would have a rough road trying to use that big of a ball mill. Another thing that you might consider when doing something like these crosshead guides is to use 12L14 steel. It's much easier to cut than 1018 or the like.
gbritnell
 
My last idea was NOT a good one. Making one of these crossheads with a ball end mill is miserable. Making one by plunge cutting is sheer torture, and you end up having to work from the blind slide half the time. Trust me---Don't do it that way. The results end up the same but its twice as much work. On the positive side, I now have a nice matching pair of crosshead guides. I agree with gbritnell---Large ball nosed cutters shouldn't be used on a small mill. I won't do it again.
BOTHCROSSHEADS001.jpg
 
Messing about a bit with the model----I added in steam pipes at 1/4" diameter. I don't think they look to big or "clunky", but beauty is in the eye of the beholder. At 3/16" they just looked too small to me. I moved the boiler a bit closer to the back of the base, and added some beefier connecting rods because of the length. When I get to the boiler, I am going to add a fire door and a pair of ash clean out doors in the base. Speaking of the base---that may be the next thing I build, so I have something to bolt all my little parts to. I find that if I don't have something substantial to bolt all my little bits to, I start to lose them.
FULLASSEMBLYDONKEY.jpg

 
Coming along very nicely. Despite the difficulty you had making them, they sure look good!!!

Harold
 
The base should be fairly straightforeward in terms of machining operations. Its going to require a lot of hogging out around the perimeter, and I haven't quite worked out whether the boiler will set in a round counterbored hole or just set on top of the base. ---Probably just set on top. I want to add on a pair of "ash cleanout" doors on the rear side between the cylinders, not sure yet of how they will be attached.
DONKEYBASEPLATE.jpg

ASHCLEANOUTDOORSONREALDONKEY.jpg
 
Brian

You are making good progress. The solid base is a good idea. Mounting the cylinders on the extended base will give you a good stable arrangement.

I have been looking at that picture of the fire door and ash doors for years and hoping for inspiration to build them since the are such a great feature to add to the otherwise bland rear view. The fire door offers a huge challenge, oval shaped and contoured to the boiler. Here is another view that shows additional detail of the fire door:

SteamDonkey001.jpg


In this picture, the pipes rising vertically out of the cylinders are exhaust pipes, that rise to the top of the boiler and exhaust into the stack to aid draft. The steam pipes have completely disappeared in rust but in this picture you can see the relative diameter.

D3418.jpg


Jerry
 
Fire door is probably best done by squashing a suitable piece of tube and flycutting the end with a boring head to the boiler OD plus the thickness of the flange. Bend the flange to fit the boiler and then silversolder onto the curved end of the tube. Finnaly cut off excess tube, file away the flange plate inside the tube and an equal amount all round the outside then rivit to boiler.

Ash doors which look like they also serve as dampers so need a latch to hold them part way open. Can just be flat plate with two strips soldered on for the hinge lugs. The fixed part of the hinge is basically just a "U" shape drilled for the pin and filed to shape, aim for a 1/32" hinge pin.

Interesting to see the pipes better on that last photo, with the 1/4" feed it will need 3/8" exhausts to look in proportion. Also looks like a cladding sheet on the cylinders with insulation below
 
Just in case you didn't know----Aluminum plate can be cut quite handily on a table saw with a carbide toothed blade. Its dangerous, its horribly loud, and its scary as Hell, but it beats standing around ageing waiting for my bandsaw to cut it. Thats 1" plate you see. It has a terrible habit of kicking back and trying to behead you, so you want to hang on real tight, always know where your fingers are, and don't laugh at my table saw. Its probably older than you are. I paid $25 for it 25 years ago at a yard sale and then souped it up with a 2 hp. compressor motor off a house sized propane tank storage unit.
cuttingaluminumplate003.jpg

cuttingaluminumplate002.jpg

cuttingaluminumplate001.jpg
 

Its dangerous, its horribly loud, and its scary as Hell
Click to expand...

Cutting freehand, that is an understatement. It is true that aluminum and wood have the same cutting speed when using carbide and I too often use the table saw for roughing out aluminum plate.

Like Bill said get a specific blade or at least one with a finer pitch.

But above all you must use a properly tuned fence and really should have a riving knife on there.

Please tell me you took the fence off for the photo.

Your parts are looking nice.

 
Brian

Not having a bandsaw, I use my tablesaw a lot. I have cut two inch some and inch and a half mostly. I use a 10" 60 tooth carbide blade and before every cut, I check the gullets between the teeth for chunks. Chunks pop out easily with a pair of needle nose pliers. When I start the blade, I give it a spritz of WD-40 and if it is going to be a long cut, I lay down a puddle of WD-40 on the cut line. I have a board clamped to the miter gauge fence and the work piece clamped to that. If cutting against the fence, I use a feather board to keep the piece tight against the fence.

Cutting a big chunk of aluminum is no more dangerous than cutting a big chunk of oak and a lot less dangerous than cutting a big chunk of yellow pine. I have never had aluminum twist or warp as the stresses are relieved or try to clamp shut on the blade.

If you ever want to sell your table saw, I'll give you twice what you paid for it. A good, belt drive table saw beats the hell out of the new geared universal motors that you find on all but the high end saws today. ( I'm assuming that you'll throw in the 2HP motor since you won't have any need for it without a saw.

Jerry
 
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