Stuart 5a stationay build

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
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.

Con Rod 7.jpg

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.

Con Rod 8.jpg

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

Con Rod 9.jpg

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.

Con Rod 10.jpg

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
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.

Piston Rod 1.jpg

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

Piston Rod 2.jpg

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

Piston Rod 3.jpg

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

Piston Rod 4.jpg

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

Piston Rod 5.jpg

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.
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.

Piston Rod 6.jpg

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.

Piston Rod 7.jpg

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

Piston Rod 8.jpg

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

Piston Rod 9.jpg

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
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.

Top and Bottom Covers 1.jpg

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

Top and Bottom Covers 2.jpg

Fabricate a gland nut and the bottom cover is done.

Top and Bottom Covers 3.jpg

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

Top and Bottom Covers 4.jpg

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.

Top and Bottom Covers 5.jpg

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.

Top and Bottom Covers 6.jpg

Next time we’ll get started on the cylinder.

Take care, Bob.
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.

Cylinder 1.jpg

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.

Cylinder 2.jpg

Now get it rough bored all the way thru.

Cylinder 3.jpg

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.

Cylinder 4.jpg

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

Cylinder 5.jpg

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.

Cylinder 6.jpg

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

Take care, Bob.