Cross-Twin Engine

[Cobra]

Blanks made for the connecting rods. As seen on the top bland the cap has been cut and re-attached in preparation for drill and ream for the large end of the rod.



Large and small ends drilled and reamed.



Centre drilling the small end for the tailstock centre. They will also function as oil holes for the small end.



Thinning down the blanks prior to turning the shafts.



Turning down the shafts.



Connecting rods fitted to the crank and to the pins in the cross heads.

 

[Cobra]

Stuffing boxes for the inboard pistons made in a similar fashion to the cross-heads. Made in a single part and then cut apart.
Packing gland nuts for each of the pistons for sealing the air chamber.



Cross link arms for the inboard pistons cut from flat stock pieces stuck together with double sided tape.



Cross links fitted to the cross heads and the stuffing boxes.



Links are soldered on the stuffing box and pinned on the cross head.

 

[Cobra]

Support bracket for the valve train.




Crank rocker arm for the valve train.

 

[Cobra]

Starting on the valve body. Turned from 1-1/2 inch brass.




Milling the valve slots after the arm had been attached to the top side.
Arm was attached with a fastener so that the length could be adjusted if needed.



Threading the two ends of the valve pivot.
Switched to single pointing when I discovered that I need a new 1/4-20 die. Old one was too dull to cut well.



Link arm from the crank eccentric to the valve link.
Soldered and installed.




As I was installing the arm, I got to thinking about Tom's comments on the assembled crankshaft.

Those pinned shafts look scary. I can't imagine they would last very long running. I've only turned cranks from solid. It's really a great exercise, however.

I had never turned a crankshaft from a solid piece so thought I would give it a try.

 

[Cobra]

Turning a crank from solid turned out to be a little more tasking than I had originally imagined.
Started with the idea of using a parting tool to cut the slots for the two journals.
Started making up the blank from some hot rolled flat bar.



Face milled down to 5/8 inch.



Centre drilled each end for the three rotational axes.



Cutting the first journal.



Got part way and decided that this was a very scary way to make these slots. That blade is sticking out a long way.
About two seconds after that thought, the edge of the blade caught the blank and everything went to hell in a handbasket.



Time to re-think and start over.
Decided that a new tool was in order. I purchased some rectangular tools 1/4 X 3/8 and 1/4 X 1/2



Ground two ends of one of the 1/4 X 3/8 tools to be what was essentially heavy parting tool ends.
One end to cut left and the other right. I put about a 1.5 degree angle on the face of the tool with the idea that the actual journal could be finished by cutting from the centre to each side.



Finished side view of the double ended tool.



Also thought that bandsawing the blank ahead of turning would help the cause.
Sawed out the first journal area and then started on the other before thinking that it would flex.
Made up a spacer to eliminate the width of the saw cuts.
Started cutting with the new tools. Cut very well down the sides of the slot.



Getting near the dimention for the journal and the wide tool dug in and the blank lasted not at all!



Back to making another blank.

 

[Cobra]

Decided that the wide end to the tools could be improved on so ground the ends to leave just a small (1/16") flat on the face.
Again bandsawed out the journal space to get started. No additional saw cuts this time!



Everything was going well for the first little bit. Got the slot cut to the proper dimension, leaving the journal at 0.625" and then started reducing it to the target 0.375" finished dimension.
Just a little in-attention and there went another blank after about another two hours of work. Getting somewhat frustrated with this stupid idea of cutting cranks from solid pieces. This time it tore up the dog and the centre in the headstock.



Starting again on the first journal slot.
Cutting the sides is getting to be routine now.



Switched back to a 1/4" flat nosed tool to finish the journal to the final 0.375" dimension.
To avoid it digging into the job, I put the lathe out of gear and turned the spindle by hand. The freshly sharpened tool would shave off less than a thou at a time, leaving a nice finish.



Made up an aluminium spacer for the first journal slot and started on the second.



Finishing off the final dimension of the second.
Now, just need a second spacer and I can get started on the centre shafts.

 

[Cobra]

I cannot tell you how relaxing it was to be turning the outboard shafts!
Cut the webs to dimension and then just brought the shafts to their 3/8 size.




Finishing up the faces of the crank.



Removed the extra material on the webs.



Thanks Tom! It was a great learning exercise that I would not have done without the push from a member of a great site!

 

[T Bredehoft]

Thanks Tom! It was a great learning exercise

Watching your efforts, I think if I were do to that I'd bandsaw much of the work away before starting on the lathe. But you (and I) learned quite a bit from your experience.

 

[Cobra]

Final assembly and test running.

 

[MikeInOr]

That is a very cool project!... But what I really appreciate is your photos of the machining in action! As a complete novice I am learning a lot from your posts and seeing how you are setting things up and how you are doing the machining. And the blank that got ruined... very cool that you shared that! There always seems to be more to learn from something that went awry then when everything works out perfectly the first time.

Thank you for your posts!