Elmer's "scotty" engine........a build log

[55fairlane]

I am a member of The Machinist Group of Northeast Indiana , and this year's group project is to build Elmer's "scotty " engine. All the members are going to build this engine.
I invite you to follow along with me. But first some background on Elmer & alterations to the build.
Elmer Verburg was an electrical engineer with a passion for building running model engines. Thru out the 70's & 80's these engines were published in several magazines. Around 1988 Elmer published a book with all of his engine designs & construction notes in it. A hard bound original copy sells for $200 plus dollars. Luckily Elmer said apon his passing, the book was to distributed free among the masses.
The name "scotty" comes from the scotch yoke arrangement at the crank. Simple put a scotch yoke has a pin the slides in a slot, turning rotating motion into linear motion.
Some of the alterations I am doing include scaling the engine up twice sized, that will make the bore ¾ inch x 1 inch stroke.
Here is a YouTube video of Elmer's "scotty " engine ,

 

[indyblue]

It kind of looks like a sterling engine design crossed with a hydraulic swash plate design.

 

[55fairlane]

Step 1 build the engine frame, simple part , done in 7075 aluminum. The bore for the (plain ) bearing was located (the ¾ hole in the part) and all other dimensions were brought off the bore to keep everything true to the center line of the rotating assembly .

 

[55fairlane]

Next I made outboard connecting rod support, not near as elegant as Elmer's but very functional. I added a leanth of yellow brass (held in place with Loc-tite) as additional bearing support for the "con-rod"

 

[55fairlane]

Next it came time to machine up the cylinder, I chose to make mine from steal, A) This would allow me to lap the bore to a very fine finish, B) allow me to use an aluminum piston (this keeps the rotary mass down) & C) I own the steal, no need to buy a hunk of metal.
This is when things went a drift, it became apparent to me that the steal I have is tool steal , probably a chrome-moly ....ugghhh carbide endmills and go slow. Took a week to get the cylinder down to size.
Next set to drill & ream the ¾ inch bore, disaster struck, as my reamer picked up a shaving on a flute or something, the bore look like I finished it with a concrete drill.
In our next installment, I will make a lap & see if we can get a finish on the bore, before removal from the vise.

 

[55fairlane]

In this picture you will see the aluminum lap & a commercial produced unit I own. What is "lapping " you ask, think ultra fine grinding for a super smooth finish.
Diamond dust, in a grease slurry was introduced to both the lap & the bore, at 100 rpm the lap was run thru the bore. After no more resistance was felt, the bore was cleaned, the lap was expanded & fresh lapping compound was introduced.
A few strokes of my shop made lap, pull the part, wash it down, toss the commercial lap in the lathe & super finish the bore. my next step is to return the cylinder to the mill, drill & tap the 4 attachment points & the intake passage.
You may note my cylinder is open on the end, I will "turn up" a cap when I am done and fix the cylinder closed with the cap & 4 screws.

 

[55fairlane]

Almost for got to show the 99% finished bore

 

[55fairlane]

Cylinder got finished, 4 holes drilled & tapped (4-40) , steam inlet drilled , counterbored for an "O" ring , as a gasket, "O" ring is .070 thick (or profile in o ring terms) counterbore is .068 deap, allowing for .002 press.

 

[55fairlane]

I used a "found" piece of material for the base, did some drilling, tapping, countersink for tapered head screws & the base is mated to the frame. Not real happy with the fit, but that's cosmetic & can be fixed. The cylinder got attached to the frame. Next step is to make the crank bearing to fill that gaping hole in the frame