Fallback wobbler

[Jasonb]

With your centring jig move the point back into the bar so it only just protrues a tiny amount. That way it will be less likely to be effected by and tilting of the jig.

As for a scriber that is on ctr height. Turn a point on your scriber, don't grind it. Then mount a piece of bar in the toolpost and a ctr or spot drill in your chuck and drill the bar followed by a drill of the scribers diameter through the bar which will put the scriber point on ctr height whenever you fit it to the toolpost.

[mklotz]

I must endorse Chris' advice to use the mill to directly determine the midline.

I long ago decided that marking out had its uses but for the most accuracy using the tool that will ultimately make the cut is far more accurate.  To that end I fitted both the x and y axes of my mill with two inch dial indicators.  Each indicator is fitted with an adjustable "striker" that allows the indicators to be used regardless of the mill table position.

Once I have the mill table in its working position I set each indicator to read one inch.  This allows me to accurately measure one inch in either direction with the accuracy of the indicator, nominally one thousandth.

If you must mark out, one possibility is to mount the rod horizontally in a V-block/machinist vise/etc. so the face to be marked is fully exposed.  Then set a height gauge to the appropriate height and, with everything sitting on a surface plate, scribe the line.

[Jasonb]

Paul's reason for asking how to do it without machines is due to the fact he does not have  amill.

Don't think he has a height gauge either, just a basic scribing block.

[PaulR]

Yet another way is to use an edge finder on each side, and move to split the difference. A wobbler type edge finder is pretty cheap to buy.

Yes, I used to have on of those when I had a micro mill years ago. I guess I could put it in the lathe chuck with the part in the slide...

[PaulR]

As for a scriber that is on ctr height. Turn a point on your scriber, don't grind it. Then mount a piece of bar in the toolpost and a ctr or spot drill in your chuck and drill the bar followed by a drill of the scribers diameter through the bar which will put the scriber point on ctr height whenever you fit it to the toolpost.

Excellent - why didn't I think of that?!

[PaulR]

I must endorse Chris' advice to use the mill to directly determine the midline.

I long ago decided that marking out had its uses but for the most accuracy using the tool that will ultimately make the cut is far more accurate.  To that end I fitted both the x and y axes of my mill with two inch dial indicators.  Each indicator is fitted with an adjustable "striker" that allows the indicators to be used regardless of the mill table position.

Once I have the mill table in its working position I set each indicator to read one inch.  This allows me to accurately measure one inch in either direction with the accuracy of the indicator, nominally one thousandth.

If you must mark out, one possibility is to mount the rod horizontally in a V-block/machinist vise/etc. so the face to be marked is fully exposed.  Then set a height gauge to the appropriate height and, with everything sitting on a surface plate, scribe the line.

Thanks Marv but as Jason says I don't currently have a mill... but your tips might benefit those that do!

[PaulR]

Making it up as I go along with this one as it's so simple. I made the second little block for the air connection, drilled the ports, the pivot hole and a location hole for the shaft. Better to do that than go to the trouble of soldering the blocks at each end and then muck up drilling (which is the usual source of errors for me).

[PaulR]

Made some sacrificial screws to pin the bits together then soldered them. Cleaned them up, plopped the cylinder in place and realized I've made silly mistake... When roughing things out in CAD I was taking into account the maximum width of the hex bar and not the width of the flats. As a result the ports will be completely uncovered in turn at mid-stroke. It would still work OK I think but will obviously be very wasteful of air. The solution is probably to fit a flat plate to the bottom of the cylinder which is the same width as its max size. Not a big deal as I haven't even thought about the crank end as yet so I can deal with the knock on effect.

[PaulR]

Continuing to make it up as I go along, I filed some 1/8" thick scrap brass to size, made a hole for the port and a larger than necessary hole for the pivot pin and a little widget to clamp them together (I made a centre hole in the widget in case it ended up soldered in place so I could re-drill then cut it off if needed). I carefully added Tippex as a resist around the thread and pivot hole and the port, added flux and used a 2mm drill to align the port (the more critical bit) while tightening the screw. Gave it a cooking then cleaned it up. Also drilled the air passages and cut a little bit of copper to make an exhaust pipe. Here are the bits assembled with temporary screw in inlet hole and studding attached to piston.

[Sanjay F]

Nice, that's turned out well, following along 

[PaulR]

Nice, that's turned out well, following along 

Cheers 

Finished off the other bits yesterday and this morning with a bit of ad lib on the flywheel and chimney (must remember that's not an oil cup or it'll I'll need to clean the ceiling  ). Here it is sat on a random bit of wood with a steel billet to stop it running off. It runs great with the gauge well below zero; in this video I loosened the spring on the pivot pin to let some air escape from under the port face otherwise it just runs too fast. When I first ran it I'd only put a tiny turn on the crank pin which came undone firing the piston, rod and big end into the wall like a bullet - I've done similar before so always keep them pointed away until they're well run in 

Might do some shaping of the crank otherwise it just needs a base. While making this I was thinking about making one of those wobbler-powered garden locos (can't remember what the scale is called) which look pretty straightforward but I doubt I'd be able to get a copper boiler hot enough to solder although I've still got one I made some years ago when I had access to a hearth. There's also the slight problem of having nowhere to run it - a garden railway would be like a hare at a greyhound track to my dogs 

<a href="https://www.youtube.com/watch?v=b7jH5x5grdQ" target="_blank">http://www.youtube.com/watch?v=b7jH5x5grdQ</a>

[Sanjay F]

wow - that was quick, I didn't realise the size of it until the photo of you holding it .... very nice 

I take some comfort from knowing I'm not the only one sending projectiles across the workshop!

[PaulR]

Put it on a simple painted base and that's about done for this quickie project which didn't turn out too bad considering there was no real plan.

[astroud]

Hi Paul
Looks good, I like the 'block' appearance, what's coming next ?

Andrew

[PaulR]

Hi Paul
Looks good, I like the 'block' appearance, what's coming next ?

Andrew

Not sure, I've been playing with the Stirling engine again today. It doesn't run for long due to lack of cooling so I'm debating whether it's worth making a water tank. The trouble is there just isn't much space to get a good contact area. I might make another one with a longer displacement cylinder and a sheet of vermiculite to shield a water tank but need to give it some thought. Alternatively I was thinking about an  boxer style twin uniflow, simply because each power stroke would help overcome the compression at the opposite end (or it might be a total flop!).