Potty Popcorn Engine

[sbwhart]

This will be the last post on this job, for over a week going to have a house full of kids and grand kids.

I decided to make the crank shaft with a disc throw, quite a few of the popcorn engines I've seen [ics of have this set up so thought I'd give it a go.

With a chunk of steel in the chuck turn the od and centre drill, then centre it on the mill a off set it by the throw and drill with a number drill just below the size of the crank pin so its a tight fit. One thing they do with this type of crank is maching out a curve oposite the crank pin to balance the crank so using the rotary table mill a 60 deg curve.





Then still in the chuck back to the lathe and drill out for the crank shaft again using a number drill so that its a tight fit.

Then skim it off to correct thickness, the material I'm using is tool steel used to make draw dies from, high carbon with bits and bobs of other things in it, I'm familiar with this steel and know it turn well with high speed tools, but it produces wicked razor sharp snakes of swarf.



Glue the shaft and pin in



Turn up the main bearings and glue them in.



Time for a few hows it looking shots.





Not bad.

Stew


.

[saw]

Nice, I realy love youre flywhile 

[Stilldrillin]

That ensemble is really nice Stew! 


"a curve oposite the crank pin to balance the crank". Is the curve filled with lead, or summat Stew?

Surely, the crankpin area needs lightening. That area needs to be heavier...... Dunnit?

Or am I missing something....... 

David D

[sbwhart]

 

Yes your right Dave the bit oposite needs to be heavier, or the bit behind lighter.

Never mind its only for show

 

Stew

[Bluechip]

That ensemble is really nice Stew!  


"a curve oposite the crank pin to balance the crank". Is the curve filled with lead, or summat Stew?

Surely, the crankpin area needs lightening. That area needs to be heavier...... Dunnit?

Or am I missing something.......  

David D

I'm with you chief ... I reckon there should be added mass there to balance the big-end and the bit of the con-rod which is considered to be rotary..

Often the 'shoulders' of the crank disc either side of the big end are cut away ..

Hmmmm...

BC

Stew nipped in quick .. poke it in anyway ...

[Stilldrillin]

Thanks Chaps! 

It's right nice to be right, occasionally........ 

I thought Stew had changed the laws of physics! (Jim). 

David D

[sbwhart]

I thought Stew had changed the laws of physics! (Jim). 


Well they have got doubts about 

I recon they measured the bit of string wrong.


 


Stew

[AndyB]

Hi Stew,

Superb work! Just love your ideas

I have to ask, after seeing the way you made the steam chest (and the Potty bit), is it your twin overcrank in Model Engineer magazine?

Nice work on that too

Andy

[sbwhart]

Hi Andy

Yes its my the over crank in Model Engineer.

The steam chest is small enough to be made from one chunk of material, if you've got a chunk of the right size, I hadn't to referted to type and fabricated it.

Thanks for you interest

Stew

[sbwhart]

Well back off holiday, and got another part ticked off:- the eccentric.

Using a bit of mild steel bar first turn a 5mm wide groove to 20mm dia, and put a small centre drill into the end



Then over onto the mill and centre under the spindle.

Off set the mill to the throw of the eccentric and put in another small centre.

With a independent four jaw in the lathe and using a woble bar get the off set centre running true.



Put a 7.5mm drill through followed by a 8mm reamer.

Back in the selve centring chuck turn a little boss to take the fixing grub screw, and part off



Set up in the mill a drill and tap M3 for the grub screw.



Thats that one done.



Stew

[Stilldrillin]

"Thats that one done".

And very well done it is, too!  

David D

[saw]

Thank you for nice picture and good description, I always learn something from you. 

[AndyB]

Hi Stew (and everyone else)

Another stupid question I'm afraid ...how do you calculate the eccentric offset please? Is it half the total slide movement? I'm sure there is more to it than that as the ports have to be covered/uncovered correctly.

I really like what you are doing and am thinking about a recycle version, made out of scrap, such as an old cistern weight for the flywheel, the base from an aluminium casting from a load of scrapped office furniture at work, this sort of thing. I just love some of the steampunk stuff, and I have an idea that I could make your engine in the same vein.

Many thanks

Andy (getting brave now )

[sbwhart]

Hi Andy

Theirs no such thing as a stupid question, it's the smart honest thing to ask a question.

The off set is half the total slide movement as you said.

To work the valve spacing out I use this method, I took it from one of tubal Cains books, I've used it for three engines now and found it works well on air running model engine, steam running engines are a bit different as it all depends what you're doing with them as you want to use the steam as efficient as posable, and people will have different ideas on the best design, personally for a model I think they overcomplicate the issue.



For the width of the valve ports a rule of thumb:-for the inlet about 1/10 the dia of the cylinder and for the exhaust 1/4 the dia of the cylinder, I try to go to the nearest standard cutter size, for the web between the ports equals the width of the inlet port.

Hope this makes sense

That steam punk job sounds interesting, I always try and make an engine from material I have in my stash.

Stew

[kvom]

For a previous build (paddleducks engine) I followed Bogs' method on the piston, in that it was roughed, then loctited to the rod, and then faced and turned to final diameter with the rod chucked in the lathe.  This ensured that the piston rod and piston were concentric.  Did you find that machining them separately is accurate enough?  or perhaps the packing gland provides any needed adjustment.

[sbwhart]

Hi Kvom

I followed Bogs method too.

I'm sorry that I've misled you guys, but I failed to take a pic of skiming the piston to final diameter griping the rod in a collet so that it would all be running true.

Thanks for following the thread.

Stew

[AndyB]

Thank you Stew

So the eccentric is offset from the centre by Y/2?

That is one of the many things that I have to know and forget to ask when I speak to people about steam engines.

Keep up the great work.

Andy

[sbwhart]

Sorry Andy

I didn't answer your origonal ?.

The slide movement is X-B

So the eccentric offset is (X-B)/2 so for the popcorn the calculation is (14.6 - 9)/2 = 2.8

Stew

[AndyB]

Fantastic Stew

Many thanks.

Sorry, I am by no means a mathemetician...I have to take my shoes and socks off to count to 11  
If I have to count anything unusual I have to ask my mate from Wisbech to take his shoes and socks off...

Andy

[Stilldrillin]

While we're slightly mathematical......... 

Discussed, some months back. What was the thickness of packing needed in a three jaw chuck, to throw to the required offset? 

I seem to remember a simple reply. (Like 2 or 3x throw).....

David D

[sbwhart]

Thanks Andy/Dave

Dave I think Marv has a programme for working that out on his web site.


Made the eccentric sleeve over the past couple of days, every time I make an eccentric sleeve I seem to end up making it a different way than the time before, I think that comes from making them from whatever material I have around at the time.

For this one I had a tub end from a bit of 45mm bar.

Started off in the lathe faced it off, centre drilled it and stuck a 1/2" drill down it.



Then keeping it on the chuck transfered over to the RT centered under the mill spindle, and with a 3mm slot drill cut the shape out.



Whilst I got it centered scribed a line across the middle, this will help set up for drilling a splitting.



Then back over to the lathe things were a bit tight for room but managed to part it off:-still got enough of the stub of bar left for a flywheel sometime.



Then using the scribe marks lined it up level in the mill and drilled and tapped the holes.



Then with a slitting saw cut it in half.



Then set it up in the four jaw, using my centre height gauge get the split on centre, its more important that you get the split centered than getting it exactly in the middle 90 deg from the split.

And bored it out for a good fit on the eccentric.



Her it is assembled.




Stew

[andyf]

While we're slightly mathematical......... 

Discussed, some months back. What was the thickness of packing needed in a three jaw chuck, to throw to the required offset? 

I seem to remember a simple reply. (Like 2 or 3x throw).....

David D

See this page http://www.steves-workshop.co.uk/tips/lathetech/eccentrics/eccentricindex.htm

Andy (Andyf, this time)

[madjackghengis]

Thanks Nick/Arnold/Dave/Saw

Stew, very nice ... a lot of work in that part there. I've often wondered how people did that, nice method ... guess it's one of those that although taking more time to remove metal, it's actually easier done from stock than casting.

Nick

Thats a very good point Nick, I had been wondering myself how you would machine it up if it had bin made as a casting, and i just couldn't see how you would hold it. Had a good natter with John the other night about making this part and he suggested that you could make it in two parts and silver solder it together, which I think would be very veasable also.

Stew

Having taken great pleasure in watching this engine come together, and admiring the real ingenuity shown in working the parts to their final size and shape, I just want to say, working with castings is slightly different, but by no means more difficult, it just follows a different path, using fixtures and means of holding adapted to the castings, and it comes with the knowledge one doesn't have to empty out all the excess metal hidden inside, and often, the very design of the casting makes for the least amount of material removed, making the holding of it easier, because less force is necessary for the machining.  I enjoy making parts and whole engines from blank stock, but I've found similar pleasure in the use of castings and building, and find each has its advantages, and both being a gratifying exercise in engineering.  I only hope I get to the point of making patterns and actually casting my own parts, as that seems to really mean something to those who we watch do it on this site.  I know I'm impressed by much of the skill and ingenuity shown by those who have cast every part of their engine.  Don't think for a moment I would take away anything from the skillful work you are showing in this build log, it is a work of art without a doubt.   Cheers, mad jack

[AndyB]

Not complicated??? 

My mate from Wisbech (they are all like it there)



Andy

[Stilldrillin]

Andyf.
Thank you! 

That's the one I was thinking of........ AND, I can multiply by 1.5..... If me calculator batteries are still ok! 


Tother Andy.
If they're all like that in Wisbech..... They look a happy lot. And they've still got all of their fingers! 

David D