Stuart 5a stationay build

[Bob Sorenson]

Today is a short update to finish the connecting rod. Round over the end of the con rod on the rotary table. Center the rotary table directly under the mill spindle. Clamp down a slab of aluminum and drill for a 3/8” pin. Secure the connecting rod on the pin and round over the end.

The rotary table is the way to go. The finish is almost glass.

To mill the slot for the cross head, make a slot in an aluminum plate to snugly fit the con rod.

Clamp the connecting rod in the slot and mill for the crosshead.

The plan calls for a fully rounded bottom end of the slot. Future me will discover that a fully rounded bottom end will foul the crosshead. It would be better to finish the crosshead slot with a ¼” diameter end mill.

That’s it for the connecting rod. Next time is the crosshead and piston rod.

At the time of this writing the engine is done and only 3 or 4 little parts remain on the Stevenson reverse mechanism. Hopefully get this monster running on air in a couple weeks. I’m getting nervous.



Take care Bob

 

[Bob Sorenson]

The piston rod and crosshead are one single casting. As with many of the castings, this one is way oversized. The casting has a very nice profile, but it will all go away during machining. I’m not sure what Stuart uses for the connecting rod and the piston rod castings. It turns like some kind of steel, not quite as crumbly as regular cast iron. Whatever it is, it machines very well.

The method to use is straight turning between centers. Start by center drilling both ends.

Get it up between centers. I would say fabricate a driving dog specifically for this part.

Turn the shaft to final diameter and polish. Leave the very top end blank for the time being.

Turn the piston rod end for end and machine the crosshead.

Ultimately, the piston attached to the rod by means of a taper and nut. Don’t try to do the taper for the piston now. It’s a good idea to do the cylinder bottom cover first, then do a “sanity check” on the piston rod. So, hold off on that. Next time we’ll finish the piston rod on the crosshead end.



Take care, Bob.

 

[Bob Sorenson]

Next for the piston rod is get it between centers with the dividing head to shape the crosshead portion. Mill the crosshead to the final width on both sides.

All of the nice-looking cast features on the crosshead are gone. Start profiling those shapes back in. Work the lower end and side cheeks in the mill.

On the rotary table, carve out the wrist pin bosses.

Lubrication for the wrist pin comes from the standard. Drill passageways on both sides of the crosshead.

The piston connects to the rod with a taper and nut. If you really trust the accuracy of the plan, go ahead and machine the taper and thread. I decided to do the bottom cylinder cover first, then do a “checkpoint” before going further. So, next time is the bottom cylinder cover.

Take care, Bob

 

[Bob Sorenson]

Today are the top and bottom cylinder covers. These are a fairly straight forward turning process. The covers are cast iron, with no cast bosses to help in the turning process. However, there is enough material to grip during the machine steps. As with all the other castings, they are way over size and none of the original cast surface will remain.



I decided to do one major departure from the plans. The bottom cylinder cover plan calls for a 1-1/8 x 16 TPI male thread to accept a corresponding blind threaded cap to retain a solid bronze packing bushing. I reviewed some old books I have on steam engine design and did not see this method on other engines. After long deliberation and consultation with my shop mentor, I decided to go with a more traditional two bolt bushing retaining method. But, instead of packing the gland with graphite yard, as is traditional, I also decided to use an O ring for the seal. Kozo Hiraoka has an article with specifications on O ring packings in one of his books, so I went with that.



The piston rod is 13/32” diameter. And, of course, our local hardware store has every O ring in the visible universe, except 13/32” ID. Fortunately, 10mm ID is extremely close, so that’s what’s in there.



Start out by turning the cover to its final dimensions on the lathe.

Drill for #10 x 32 clearance and #4 x 40 for the gland.

Fabricate a gland nut and the bottom cover is done.

The top cover has a very nice domed center. Pretty much have the freehand that feature in.

At this point verify the total length of the piston rod. History shows that errors creep in and sometimes plans are not correct. This time, however, everything was right on.

Set the lathe compound to 6 degrees to turn the taper to retain the piston. Turn a straight portion to 5/16” diameter for the piston nut. Thread with either 5/16 x 32 or 5/16 x 40. The piston rod casting is long enough to do the lathe work in one setup. Do not disturb the compound angle setting until the piston is done.

Next time we’ll get started on the cylinder.



Take care, Bob.

 

[Bob Sorenson]

Starting on the cylinder today. The Stuart 5a cylinder casting is wonderful. It is spot on in dimensions and is wonderful to machine. The steam ports and exhaust are right on dimension and very sharp. The cores are clean and uniform all the way thru. No work at all was needed on any of that. The first step is mount up in the three-jaw chuck, rough face the end and rough bore all the way thru, about 2” ID or so.

Opps. The chuck jaws foul the boring bar going all the way thru. So, switch to the four-jaw and use parallels for stand offs.

Now get it rough bored all the way thru.

I was concerned crushing the cylinder in the four-jaw and distorting the bore. So, I switched over the faceplate and gripped the cylinder casting by the flange. The finishing passes were taken with a sharp, broad nosed HHS tool in a ¾” diameter boring bar. Use a telescoping bore gage to creep up on the final bore. Check in several places to make sure the bore is parallel. I got really lucky on this task. The boring bar left perfect finish and everything straight and true. Do a light skim facing cut to finish.

Lightly hone the cylinder bore. It won’t take much.

The cylinder bore is 2.250”. The registration spigots on the covers are 2.312”. If the engine ever needs a rebore, the covers will be undisturbed. Turn a corresponding recess for the cylinder covers about 0.040” deep. Turn the cylinder around and rest on the faceplate to get the other end. Lightly face the cylinder end to its final height.

That’s it for now. Next time we’ll get on the portface.



Take care, Bob.

 

[Bob Sorenson]

Back on the cylinder with some work on the portface. Use parallels to set the portface horizontal to the mill table as closely as possible.

Mill the portface clean and tap the bolt pattern for the valve chest.

And then there was trouble. There is a difference between 1.250 and 1.125 on the DRO. It didn’t look right after center drilling and fortunately I caught it before tapping. I think it will be OK. While in this orientation mill the sides of the portface to match the width of the valve chest.

Use a dial indicator to parallel the portface to the X axis. Drill and tap 10 x 32 for the cylinder covers. There is plenty of material on the flanges for a full thread without poking thru.

While working on the cylinder, I was flipping mental nickels on how to finish the cylinder. Whether to use a cladding jacket or painted cast surface. The cast surface of the cylinder is very nice. Either way, nicely finished flanges would help. Set up the cylinder on the rotary table and skim the flanges. It doesn’t take much to clean them up. Even with my rickety mill and rotary table, the finish was like glass.

The last step on the cylinder is drill and tap for drains. Angle the cylinder enough to ensure reaching all the way to the top and bottom.

Cylinder was a big job. Thank goodness it came out well (except for the one boo-boo). Next time is the valve chest and valve.



Take care, Bob

 

[Bob Sorenson]

The valve chest, valve and valve spindle are next. I did not take a whole lot of pictures on this because it is all straightforward machine work. Nothing remarkable. The packing gland for the valve spindle is the same general design as the cylinder lower cover. It involves a ½” British Standard Pipe (approximately 7/8” x 20 UNEF) male thread to match a blind threaded cap retaining a solid gland. I went with the simple two bolt O ring gland as on the cylinder lower cover. The valve spindle is 5/16” diameter, so that’s an easy ring to find.



The first step is square up, drill, and tap the valve chest to dimension. The casting needs a lot of clean up on the outside. The inside is very close to final size but run a ½” endmill around the inside to knock off high spots.

The large hole on the side is for steam inlet. The two holes near the corner are for the Stevenson reverse bracket.



The valve spindle top guide is a decorative brass turning. Drill the stock for the valve spindle and thread to fit the valve chest. The valve spindle is ¼” diameter at the top end, so a ½” x 20 or ½” x 28 thread will do. Turn a decorative domed top by hand.

Mill a decorative hex as desired and turn into the valve chest.

The valve chest bottom spindle guide is a blank piece threaded tightly into the valve chest and then machined to accept the valve spindle, O ring and gland.

The valve is a well-done bronze casting. The top side is very close to the final dimension. Just clean up the periphery of the driving boss.

Flip over and clean up the portface. Run around the edge and clean up the cavity. Drill for the valve spindle. This was a quick job.

The valve spindle is an easy job straight from the plan. The plan calls for a 5/16” British Standard Fine thread, used 5/16” x 40 instead. Future me, however, will discover that the valve spindle and fork are incorrectly dimensioned on the plan. More on that later. It might be a good idea to hold off on these for now.



Basic engine is all done and ready for valve gearing. Next time is the start of the Stevenson reverse mechanism.



Take care, Bob

 

[packrat]

Very nice job, I am working my reversing gear so will be interesting to see how you are doing yours.

 

[Bob Sorenson]

Hi packrat, here we go!! Today is the start of the Stevenson reverse for the Stuart 5a engine. The basic 5a engine plan includes two types of valve mechanisms. One is a single fixed eccentric, and the other is a slip eccentric. The plan for the Stevenson is sold separately. Stuart sells a casting kit for the Stevenson, but I decided to fabricate all of it instead. There is going to be a lot of bar stock and silver solder fabrication in the Stevenson mechanism. Before starting on any parts, go to Charlie Dockstader’s steam engine valve simulation series and set up a 5a simulation to make sure everything is right. Start with a Zeuner valve diagram. The Stuart plan does not specify the advance angle of the eccentric. However, from the slip eccentric plan, a 30-degree advance angle is inferred. With that and all the other dimensions, go into the Zeuner diagram, enter the settings, and check the numbers.

Everything looks good. The cutoff is 78.8%, that should make a powerful engine.



Stuart’s Stevenson plan calls for a 40-degree advance angle on the eccentrics. That does not seem right. Why would that be 40 degrees when the Zeuner diagram shows an ideal solution at 30 degrees. Use Mr. Dockstader’s Stevenson Outside Admission simulation to verify. Set all the parameters and run the simulation. Here it is.

The simulation calculates 83.6% cutoff at the top end and 78.7% on the bottom end. Close to the Zeuner diagram. The admissions, compressions, expansions, and exhausts all look reasonable. I ran the simulation with 40-degree advance angles as well. It ran, but not very well. Nowhere near as good as 30 degrees. So, I am making the command decision to a 30 degree advance angle.



While there is some time left, let’s do the first part. Not exactly sure what it’s called. It’s a bracket that hangs freely from one of the valve cover studs. The shifting lever clamps to it. Prepare stock to the correct thickness and width. Profile the outside shape on the rotary table.

Profile the shape as desired. This can be done by hand as well, but the rotary table is much more precise.

Make a small button to complete the shape.

Silver solder together and mill a slot down the middle to finish. The length of this slot is important. It provides a stop in full admission in either direction.

All of these reverse parts are fabrications. It’s actually easier to fabricate than dealing with castings. There will be a lot of use for the rotary table as well.



Next time is the eccentric.



Take care, Bob.

 

[packrat]

I do have the castings for the reversing mechanism, I have got to see if I have the drawings.

 

[Bob Sorenson]

I do have the castings for the reversing mechanism, I have got to see if I have the drawings.

Let me know if you can't find your drawings. I'll share mine.

Bob S

 

[Bob Sorenson]

With the math done and verified, time to start on the eccentrics. The basic engine plan calls for the single eccentric or the slip eccentric collar be fixed to the crankshaft with a grub screw. Makes for an easy adjustment. The 5a Stevenson plan calls for the eccentric to be keyed to the crank. The key slot must be perfectly centered between the both eccentric lobes, as well as on the crankshaft. There is no way to adjust these eccentrics with them keyed. First make an eccentric blank from a piece of 2-1/2” diameter cast iron stock turned to final thickness. Center, drill and ream a 5/8" diameter hole to closely fit the crankshaft. Poke a 1/8” wide key slot.

Turn the eccentric on a custom-made stub mandrel. The mandrel starts with a length 5/8” diameter stock. Face the end cleanly and slightly center drill. Mill a 1/8” wide slot straight down the middle. Next, mill a 5/8” wide slot about ½” deep in a piece of aluminum. Mill a 1/8” slot along the center. Cut a piece of 1/8” square key stock.

Clamp the stub and fixture block in the mill vice. Center the mill spindle over the center hole on the stub.

Do some shop math to convert the 30-degree advance angle and 0.187” eccentric radius to X and Y coordinates for the DRO. The X axis is 0.162” and the Y is 0.094”.

Traverse in X and Y and drill small center holes. The plan shows the key in the top position. The corresponding slot in the crankshaft is with the piston at bottom dead center.

Make a mandrel body for the stub. Drill and ream a 5/8” hole in the body and poke a 5/8” key slot. Loctite the stub into the mandrel body. Drill and tap 10 x 32 in the center of the stub.

Blanks and fixtures are done. Next time is machining the eccentrics.



Take care, Bob.

 

[Bob Sorenson]

Time to machine the Stevenson eccentrics using the custom stub mandrel from last time. Chuck the stub mandrel in the four-jaw and center one of the eccentric lobe centers. Fix the eccentric blank to the stub a screw and washer. Turn the first eccentric to final dimension.

The key in the stub along with the fixing screw holds the eccentric material exceptionally tight. At this point, there is an option to turn the second eccentric. One option is flip the eccentric blank end for end and put it back on the mandrel without changing the mandrel setting in the chuck. The other option is recenter the sub mandrel on the other lobe center and put the eccentric blank back on the same way. Either way will work. Flipping the eccentric blank end for end creates a mirror image with the 30-degree angle on the other side of center. Plus, it puts the blank stock on the outside to make turning easier. I turned the blank end for end. Either way, tune the second lobe.

Put the crankshaft in the mill vice. The geometry of the eccentric key and lobe centers is based on the piston in the bottom dead center position. Mill a 1/8” wide slot.

Make a 1 to 100 tapered key, same as the flywheel key. All done.

Next time are the eccentric straps.



Take care, Bob.

 

[Bob Sorenson]

The eccentric straps are cast iron bar stock fabrications. Cut and mill two slabs to final thickness. Clamp both pieces on the mill table and machine together. Square the slabs to final width, but extra in height. Eventually the slabs are sawn in half, so leave some extra stock.

Saw the eccentric straps in half. Machine the top half to final height. Drill and tap for the eccentric rods. The hole in the center matches a centering spigot on the eccentric rod. The hole on the left is an oil cup. Drill the oil cup about #60 well into the center of the strap. Then drill a reservoir for oil.

Clamp the eccentric strap halves in the vice and center drill. It’s probably a good idea to number the pieces and machine together from now on.

Set up the rotary table to carve out the rounded parts of the straps. Mill out a pocket in a piece of aluminum so the strap halves fit closely.

Clamp the strap half in the jig and work the rounded profile.

After profiling, the straps will need some spot face for the screws. Then screw the strap halves together and mount on the face plate. Bore to fit the eccentric. Use a small grooving tool to cut the recesses to match the eccentric.

That’s it for now. Next time are the eccentric rods.



Take care, Bob.

 

[Bob Sorenson]

The eccentric rods are a 2-piece brass fabrication. It’s possible to whittle them from solid, but that’s kind of wasteful. Mill out the rod arm and saw the slot for the radius link. The cylindrical end of the rod arm will eventually become the spigot to match the eccentric strap. Make that spigot slightly over size in diameter and center drill the end. Make the rod base oversized in length, width, and thickness.

Silver solder together squaring the rod base to the rod arm as closely as possible.

Set up the eccentric rod blank vertically on an angle plate. Use the pointy end of edge finder to center the rod.

Run around with an endmill to finish the rod base to final dimension. Drill bolt holes to match the eccentric strap. The final step, not shown, is turn the rod base and spigot in the lathe to final dimension.

The eccentric rod arm has a little taper from the base up to the clevis. Use a jig to hold the eccentric rod arm in the mill. The jig is similar to the jig used for taped keys.

Drill the clevis end for the radius link pins and round over. These are ready for the buffing wheel.

Next time is the radius link.

Take care, Bob.

 

[Bob Sorenson]

The radius link is a very interesting part to make. My son and I have learned a CAD software for use on his 3D printer. FreeCad is the software we use. It’s a free download, although the author asks for a donation. There are 100’s of FreeCad tutorial videos on YouTube. A handy feature of FreeCad is that it dimensions the center of a fillets and radius. That eliminates a lot of shop math on the mill.

Start by milling up a block of stock well over sized and clamp to the rotary table.

Draw up the radius link in FreeCad or your favorite CAD package. Dimension the centers of the fillets and the radius slot. Drill the fillets and radius ends per the drawing.

Mill the radius slot, the inner and outer profiles.

Profile all the rounded features of the link. In this photo, one of the clevis ends is rounded over. The other clevis end is next. All the clamps are off to better see what’s going on.

Here’s the radius link fresh off the rotary table. All that’s needed to finish is some light blending with a file. The slot turned out very well.

I personally don’t like round ended slots. So, I finished the link by silver soldering some half rounds in the corners. Sand off the mill scratches and buff up on the wheel.

All of the hard parts of the Stevenson are done. Next is the start of a bunch of linkage parts.



Take care, Bob.