Robinson Patent Hot Air Engine

[deverett]

Prologue
Several years ago I managed to get the last two Robinson Hot Air Engine casting sets from Alyn foundry for acquaintances in Washington state. Later, I wished I had kept one, but...

Fast forward to about 5 months ago. I came across Steve’s Workshop http://www.steves-workshop.co.uk/steammodels/robinson/robinsonindex.htm in which he described the machining of a Polly Models 1/3 scale Robinson Hot Air Engine. The next day, I contacted Polly to order a set of castings only to be told they had none and would not be likely to have them for quite a while because they were going to a different foundry for their castings and ‘things’ were still being worked out between them.

Well, the big day arrived a few weeks ago while I was visiting family in England and the parcel with the castings and bits from Polly Models was delivered by the postie; well at least some of the bits. There were a few items missing in the original shipment, so another phone call to Jayne at Polly Models and she promised to have them in the post ‘within the next few days’. The missing parts were the displacer piston, the cover plate and all the screws. I was back in Ireland when the parcel with the missing bits arrived as promised. Oh dear, no displacer piston, but a hot cap. Another phone call to Polly, but red face on my part as I had told them that the displacer Cylinder was missing. Return the wrong bit and 10 days later arrives the correct piece.

Comparing the castings and looking at the drawings and construction notes, it seemed that Steve had done everything in a logical order so I could see no reason to deviate from his approach. Steve appeared to have done most of the machining on a lathe, whereas I would prefer to do some of the operations on the milling machine. The notes supplied with the drawings also were written in the age when milling machines were not very common in home workshops, however the drawings were in metric.

Right, to horse. No more time to waste.

Dave
The Emerald Isle

 

[deverett]

I don't intend to go into a blow by blow account because a lot of it would just duplicate what Steve put in his account. I will, however, amplify things a bit or show a different method where I think it may help another builder.

The Polly Models version is a good representation of the Robinson Hot Air Engine, but I would like to try and make it a bit closer to the ‘Real Thing’ – a 1/3 scale model of a No4 engine. From what I can ascertain, these engines were built by Gardners of Manchester between about 1880 and 1910 and during that time there were bound to have been subtle variations in the build, so my version will not be a copy of a particular engine. Most of the details I am copying are of the one in the Anson Engine Museum, although this is a No.3 engine.

I don’t know what Polly's old castings were like, but so far mine seem to be machining well enough, although I have noticed that some of the allowances are rather tight.

Starting with the stand, I held this in the 4 jaw chuck rather than the 3 jaw for added grip.

Once the top and bottom were machined, I marked out the cutout of the casting which would leave the legs/feet. The feet were not evenly spaced around the base, so a compromise had to be reached, but it is highly unlikely this will be noticed when the model is complete. Holes were drilled in the waste area and the sides were hacksawed. Using a rod blade (AbraFile) in the hacksaw was a quick way to cut the horizontal lines.

Once the roughed out 3 legs were ready, the stand went onto the rotary table on the milling machine. An end mill took the jagged edges off and got the legs close enough to change the cutter to a round noser. This then was used to give the rounded corners to the top of the legs. Note the pencil marking on the edge of the rotary table to remind me where to stop when twiddling the handwheel. This particular mark referred to the end mill used in squaring up initially.

Till next time

Dave
The Emerald Isle

 

[deverett]

Not much to report because I have been slowly working away to Steve's plan.

The platform is now done.

Next part is the displacer cylinder. Machining went well but the casting has several blowholes on the outside as well as in the bore.

Not really bad enough to send the casting back, so what I plan to do is to fill the holes with a mixture of JB Weld and cast iron dust.

The displacer cylinder fits well into the platform. The two parts need to be screwed together - the screws going through the top of the platform into the wall of the cylinder. It would seem that full size engines did not have the top of the platform painted so I have been wracking my pea brain to think of a simple way of attaching the displacer cylinder to the platform without the screws being visible from the top of the platform and yet still being able to take the engine apart if ever it should be required.





Dave
The Emerald Isle

 

[lazylathe]

Looks great so far Dave!!!! ;D

Looking forward to following you on your journey!
I do like the Robinson hot air engines!!! :big:

Andrew

 

[deverett]

Tricky job next. Drilling the transfer passage in the platform. The hole is 7/32" diameter and just under 3" long. There is very little spare metal, so any deviation would be a disaster.


Careful set up with the platform mounted on a 2-4-6 block. That's a piece of newspaper behind the casting to help prevent any slipping. Only two clamps at the moment while everything is lined up. 4 clamps used during drilling.
Dead centre!


Now to join the stand to the displacer.
The hot cap is a close fit inside the stand so it is easy to line up the displacer for the drilling. The drawing calls for 6 x 4BA bolts, but all the pictures of full size engines show only 4 bolts or sometimes studs. I will be using 4 x M4, they being slightly larger than 4 BA.


To join the platform to the displacer cylinder, 6 holes had been drilled and tapped in the displacer and matching holes drilled in the platform previously.
Now it's just a case of bolting everything together.


Slow going, but at least it is progress.

Dave
The Emerald Isle

 

[deverett]

The displacer piston is supplied as a brass spinning with wall thickness of about 1/16”. A piece of brass plate is supplied to silver solder to the top as a cover for the piston but the drawings show a machined plug.

Steve went for the brass plate cover, but he said that there was some distortion to the piston as a result of the soldering process. I chose to go with the drawing version and machined up a top for the piston from some brass bar I had after reducing the inside of the wall of the spinning to about 20 thou, leaving just a small area below the top ridge at the full thickness. This to reduce the weight and also to try and reduce the heat transfer through the brass.


The two parts will be joined together by JB Weld after I have made up the displacer rod.

Dave
The Emerald Isle

 

[deverett]

i have not been idle, although progress has admittedly been slow of late due to other matters and the festive holidays when SWMBO banned me from the workshop!

Steve did all his power cylinder work in the 4-jaw chuck. I milled the base and wanted to bore it on the faceplate but could not mount it safely, so I too resorted to the 4-jaw chuck. The outside of the casting was oval by more than 3/16", so initially I attacked it with the linisher then turned the outside as far as the link anchor at the same setting as for boring.

The model design uses a flat plate to close the rear of the cylinder but I chose to make a real-looking governor, although in this size it probably will make no difference to the speed adjustment.

I should have said remake because the first attempt ended in tragedy on the final cut for the internal length. The design needed some tweaking anyway!

I'll mount the cylinder on a mandrel before honing it and clean up the outside using the dividing head on the milling machine with final cleaning up by file/emery cloth.

Dave

 

[deverett]

I have not been idle, though I wish I had been.

The power cylinder was bored and honed. I made the conrod anchor from 3/8" square and after the piston was machined, I screwed the long bar, prior to cutting it to length, into the piston in order to hold it while I lapped piston to cylinder. Got a beautiful fit, but then disaster struck due to my impatience. Dinner time was nigh and I wanted to get the job finished before shutting down.

For the life of me, I couldn't unscrew the bar from the piston. Tried several different ideas, including warming the piston with a hot air gun, but to no avail. In desperation, I wrapped the piston in several layers of rags and clamped the crown in the vice so that I could get a spanner on the square bar. Crack! That's the end of the piston. Looking back - isn't hindsight wonderful? - I should have held the piston in the lathe chuck. Oh, well just as well I have a bit more cast iron for a second piston.

Try again tomorrow, or shall I have a go at something else while I simmer down?

Dave
The Emerald Isle

 

[Blogwitch]

Dave,

It might even be for the same set of castings as yours.

A friend had screwed up his spun displacer piston and couldn't get a replacement, so I made one up for him from solid and it worked a treat.

If anyone has the same problem, maybe this will help with the rescue.

http://madmodder.net/index.php?topic=4831.0


John

 

[deverett]

I have been away from the workshop for more than a month. So, back home now and it was time to shake hands with the man cave once again.

Wishing to have my efforts look like I was making some progress, I painted the base with heat resistant paint and cured this in 'her' oven - with her knowledge. I'm still married after the event!

Next bit was to screw in the studs that hold the displacer hot cap, insulation and displacer cylinder to the base (in that order). Two nuts were locked together to turn the studs and make sure they were good and tight. Screwing the last stud in and whoops, it just came away in me hand guv.


Workshop esperanto was in full flow as a result.

Fortunately it is a clean break and should be reasonably easy to drill out the broken bit and retap the 4BA hole.

Dave
The Emerald Isle

 

[deverett]

It has been a long time since I updated my progress on this engine. Various 'things' have been getting in the way. But now some advance.

The broken stud in the base has been replaced.

Started work on the flywheel, but a flaw manifested itself on the face of the rim, reaching up to the outer diameter. Even by machining well down below design diameter, it would not go away. I'm now waiting for a replacement flywheel casting.

How do you get neat round highlight circles on pictures? I could only find a freehand line in Paint.

Crankshaft bearing housing. The design calls for brass bushes to support the crankshaft, but I had a pair of miniature caged roller bearings that would fit just fine. Anything to reduce friction in these small hot air engines.

The outside of the casting was oval, so car body filler was applied to the casting, a couple of filing buttons were made up to fit the bore and the use of a file and emery cloth soon had the outside round. I use spray etching primer on all castings, followed by red oxide primer and this is how it stands at the moment. The bore was sealed with the ends of sponge ear plugs before painting.

The model crankdisc was just a slice of bar, but the full size one was a casting incorporating a balance weight which I have tried to reproduce. The crankpin boss was silver soldered in place after machining. A rusty bolt was put through the boss and disc to hold it perpendicular to the surface. Rusty so that the silver solder would not adhere to it.

The original crankshaft was 3/16" dia, very slender to my thinking, so it has been beefed up to 1/4", which was a clever choice because the bore of the bearings is 1/4"! It was stepped down to 7/32" where it was pressed into the crank disc, which was painted before pressing. I also want to key the flywheel to the shaft instead of using a simple grubscrew.

Dave
The Emerald Isle

 

[Jasonb]

Looking good Dave. Oh and if you are using paint then there is the option to draw a circle/elipse.

J