How far can I get away with a piston off vertical on an oscillator?

[student123]

On my 2nd build attempt,having scrapped the first , an (allegedly) simple oscillator based on a design from the little machine shop
http://littlemachineshop.com/Projects/OscillatingEngine.php

On building the cylinder , I've faced the edges & drilled/reamed the bore, on my Taig lathe , holding the piece in the milling vice connected to the cross slide, shims to raise the cylinder.Hit some problems, inc the shim stock moving.

I then measured the distance from the edge of the bore to the edge of the cylinder, on the face that will touch the upright (is there a technical name for that face?)
After the drill/ream , I had 10mm at one end of the bore , 10.4mm at the other

I fly cut to try level the face-that-will-touch-the-upright, getting the difference to 9.95mm & 10.2mm. Another milling attempt to level the face , now has the diff at 9.2mm to 9.75mm.

My thoughts are now to do a new cylinder from scratch (return to go, do not collect £200).
Drilli/ream it with my quite new £35/$50 drill press.
Or to divert off to make a quality Taig vice, eg
http://www.deansphotographica.com/machining/projects/mill/vise/vise.html
But I'm wary of diverting to a subsidiary project with a risk of that spawning more problems…

If I have to remake, first thoughts are to try the cheap & cheerful drill press - I just need to think thru getting the piece vertical for the drill/ream or else I'll repeat the problem.

Perhaps my first question should be , how close do these measurements need to be , how far can I get away with a piston off vertical on an oscillator?


Any other suggestions welcome.

Mike

 

[Twmaster]

Mike,

Sounds like you are experiencing many of the same frustrations new folks run into. Good news, it does get easier.....

I'm not sure I understand what you mean by your question about being off vertical. Many wobblers are designed to be horizontal and even sloped at 45 and other degree angles....

 

[JimM]

Think what you are getting at is that the cylinder bore isn't exactly parallel to the outside edge of the cylinder block ?? If that is the case and it does cause a problem then one possible solution would be to make the link between the piston rod and the crank a little 'sloppy' and/or make the crank slightly thinner, this would allow for a little lateral movement.

Cheers

Jim

 

[student123]

Think what you are getting at is that the cylinder bore isn't exactly parallel to the outside edge of the cylinder block ??

Yes.

Jim. thanks.

Mike

 

[Tin Falcon]

I second jims suggestion. At some point you may want to make things better. but I think there is somthing to be said about moving forward to a running engine.
Do not feel bad I started two engines set them aside do to problems I ran into. then the third one i started was my first rummer then built a another then went back and finished the first two.
Tin

 

[Blogwitch]

Mike,

Like most people who are new to this game, you are most probably trying too hard.

A basic wobbler can be a fair way out on dimensions and still run very satisfactorily. For your first couple of engines, just make them to run, forget what they look like, that comes later.

One of the main problems is getting the portings to line up correctly, and if you make it to drawings as the LMS one, because of your maybe being slightly out in your cylinder drilling and crankpin positioning, they might not match up too well.

Rather than going for the upright ports dimension, you would be better off making a small drilling jig based on your actual dimensions.

I have done a C-o-C to show what I mean. First drill the cylinder pivot point in a little bit of plate, then with a bit of rod thru the plate and into the pivot point, spot thru your crank hole, and drill with a size that matches you crank pin.

Now scribe a line thru the centres of those two holes, and scribe a point from the centre of the pivot hole to where the port should be, and drill the correct sized hole for the port.

If you now insert your piston and rod into the bore, and put a piece of rod thru the crankpin holes in both the rod and jigplate, then do the same thing with the pivot point, you only then have to drill the port hole thru the side of the cylinder and any discrepancy in your being not square will be taken care of. Don't worry about the porting hole not being in the centre of the bore, it will work just fine.

Mark up the plate which side you drilled from, that side must go towards the port face when you come to do the next stage.

Then it is just a matter of using the jigplate as shown on pic 3 and your port holes will line up perfectly when the cylinder swings from side to side.

This only works if you are out of square in one direction, for both directions, the final part is to elongate your crankpin hole in the piston rod in the up/down plane, not a lot, just enough to stop the port faces being lifted apart by being out of square in the bore.

It might seem very complicated, but when you go thru things slowly and by following each step, all becomes clearer.

Later, when you get more experience, and start to get things square and drilled in the right places, you will be able to do things like this very easily, until then, using a 'fiddle factor' like this can almost guarantee the engine will run, as long as you have the other things made correctly.

BTW, if I want to make a wobbler from my own design, I use the jig plate method for drilling the port holes.


Bogs

 

[student123]

Guys,

Thank you.
Bogs, appreciated your taking the time with the explanation & diagram.
Sounds like I should go with the cylinder as is.

Mike

 

[Philjoe5]

Mike,
Although you have received the best advice to which I cannot add much, let me give you my prespective and experience. I've made this same measurement on a variety of steam engines I've built. If I understand correctly you have a difference of 0.4 mm or about 0.015" in wall thickness on opposite faces of the cylinder. From my experience with engines of bores from 1" - 1.5", I don't think this should be a problem though that is about the maximum I've ever gotten. I would go ahead with it because in the worst case you might be able to correct this with a shim if necessary.

I'm not sure what scale you are working on, or your machines, but next time (yes, there's always a next time) try holding the cylinder in a 4 jaw or on the faceplate, or even try boring on a mill. Depending upon your tools you will likely find a combination that gives best results.

Good luck on your build and show us your runner when it's done

Cheers,
Phil

 

[AlanHaisley]

Mike,

After reading this

I then measured the distance from the edge of the bore to the edge of the cylinder, on the face that will touch the upright (is there a technical name for that face?)
After the drill/ream , I had 10mm at one end of the bore , 10.4mm at the other

Perhaps my first question should be , how close do these measurements need to be , how far can I get away with a piston off vertical on an oscillator?

I got to wondering just how you measured the wall thickness at the mouth and base of the bore.

If I were doing it with no more equipment than I have, I'd probably turn a slip fit bar about 3 times the depth of the bore, slide it in, and measure between it and a flat plate with the cylinder side held down. By measuring this bar at a couple of places I could calculate just how far offline the bore was.

A general comment on this build is that getting the pivot screw hole perpendicular to the valve face is much more critical than having the bore canted a little as you describe. If the bore is off that way, the piston rod will want to slide just a little back and forth on the pivot pin. I'd think that so long as it has clearance to slide toward the crank plate, and it's willing to slide, there won't be a problem.

Alan