Senft "Poppin" engine

[b.lindsey]

Looks good from what I can see in the photo. Will be time to try it out pretty soon 

Bill

[Brian Rupnow]

I bought a new #2-56 die today to thread the ends of the cross shaft that operates the valve. I have no idea what happened, but after threading down to the shoulder of the shaft and then backing off (by hand), the diameter of the shaft was decreased but no threads were on it. Very puzzling. Then I ran a #2-56 bolt thru the die, and it matched the threads just fine. Then I ran a #2-56 nut down the same bolt and it threaded okay. I had the shaft ends turned to .085". A mystery, and now I have to make a new shaft.

[Brian Rupnow]

This morning I have decided to put on my design engineers hat. Mr. Senft did a marvelous job on this engine, But---there are a couple of spots that I see as potential problems. The 1/8" diameter cross shaft which operates the valve was originally designed as having the ends turned down to .085" and threaded #2-56 and the flat arm with the roller on it was sandwiched between the nut and the shoulder on the shaft. There was no key of any kind to make that arm hold it's position, and as expected, it slips. I have been warned of this by my own experience on similar mechanisms and by Nick, a gentleman who has successfully built and ran one of these engines. Yesterday when I tried threading the ends of the rod I had made with a new #2-56 die I had just purchased, it screwed up the end of the shaft and didn't leave any thread??? So--this morning, a redesign. The cross shaft now has plain ends with no turn down. The plate "arm" how has a hub with two #5-40 set screws holding it in position on the shaft. The far end of the shaft which originally held a spring clamped between two hex nuts will receive a similar treatment with a hub and set-screws holding the spring.

[crueby]

Good ideas on the redesign. Still curious as to why that 2-56 die did not work - maybe they just botched the edges on it. I've gotten a few drill bits that were ground wrong, with the area of the flute behind the tip projecting higher that the tip, so all it did was rub on the metal. Was the shaft you were threading brass or steel? If brass, it should have cut fine at that diameter, if steel it would need to be a bit smaller. I'm amazed that you have done this many models without making threads that small, I use the 2-56 and 3-48 all the time. Though, George would call those sizes the huge ones, for the tiny work he does!

[Stuart]

I would take a look at that die to see if the entry is ok , but it may have been a oversized stock go for a 60%. Thread in steel , or was it drill rod .

My normal small size is 12 BA tapping drill is #60 but sometimes for scale 16 BA tapping drill is#73,is required

Stuart

[Brian Rupnow]

My shaft was turned to 0.085" before attempting to thread it, however my ability to get nice finishes at that diameter is pretty questionable. Let me put it this way--It miked at .085" but that may have been the average across the high spots. Crueby--the smallest fasteners and taps I have used prior to this engine were #4-40 which have a body diameter of 0.110".

[Brian Rupnow]

I like that valve arm a lot better now. I added a hub with two set screws to the arm and made a separate hub for the end where the spring goes. I have also finished the brass part that bolts to the graphite piston. In fact, the only thing i have left to do is make the graphite piston and then reassemble everything.

[Stuart]

Brian
With regards to the graphite piston , the paper you use in your inkjet printer makes a very good fine finishing media for that last bit

Stuart

[Zephyrin]

if the die is not split, or too tight in the die holder, it may refuse to enter the rod, maybe a little bit too large, and the rod instead of being threaded is chewed by the entry of the die to the core dia.
this may occur when threading on the lathe with small diameter and poor quality dies.

Same hold true if a left handed die is forced in the wrong direction !!

[Brian Rupnow]

I haven't had time to stop and sort out the die, but I will get to it eventually. I finished the graphite piston, and got a good "sizing" on it. The last couple of thou. were taken off using the back side of my 280 grit sanding strips.

[crueby]

I've found with the small dies that filing or turning a bit of taper at the tip of the rod helps it grab in and start the thread better.

[NickG]

I bought some ‘soba’ carbon steel taps and dies which I thought would be ok, but the 10ba one just rounded off when I tried to cut the first thread! Had to resort back to a HSS Presto one which is without a doubt older than me.

Ps that rocker arm looks much better now. It’s a bit fiddley to assemble but enjoyable.

Nearly there.


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[Brian Rupnow]

I was mistooken---Got everything back together and then realized I still had to make the valve rod and the valve itself. The engine turns very freely through half of it's rotation, and has a bind in the other half. Tomorrow I hope to get the "bind" sorted out and make the last two pieces.

[b.lindsey]

Brian, the bind could be the cam acting against the spring if that is installed already. The spring needs enough tension to keep the cam roller against the cam through the cycle.

Bill

[Brian Rupnow]

No Bill, it was a bind in the connecting rod big end. I took it out and removed about .005" on each side of it, and that got rid of most of the bind. I will rig a belt drive from lathe tomorrow and let it run for 20 minutes and that should get rid of any remaining bind. Speaking of the contact between the roller and the cam--That cam is such a strange shape that I'm sure the roller doesn't follow it. I think the downward swing of the 1/16" valve rod must be limited by the 1/4" hole in the face of the frame. I THINK that only the arc in the cam contacts the roller, and the rest of the time the cam doesn't even touch the roller. I haven't actually confirmed this point yet, but will probably know a lot more about it tomorrow.---Brian