Three Cylinder Radial Steam Engine

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Jan 17, 2009
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Hi Chaps

This is going to be a bit of a long post but thought I'd bring you up todate with my latest short term project, so I'll apologise in advance.

Any way at Model engineering exhibitions I've alway marveled at the models of radial aircraft engines, now this isn't going to be a petrol engine, but it going to be based on two design of model radial steam engines, I'm going to try and take what I think are the best parts in terms of good design and ease of manufacture from these two designs, plus a bit of bling so that they represent an aircraft engine (cooling fins on the cylinders)

The two design I'm going to work with are:-

The Cygnet Royal by Edgar T Westbury, I got my drawing free from Model Engineer but you can buy them from I understand that you can buy a set of castings for this design but I'm going to use bar stock.

Elmers Radial Engine drawings are available on John Tom

I've been planning this build for some time slowly putting my ideas together and doing a bit of research and I've started on a set of metric drawings as all my kit is metric, and to get round me the material and tools required.


2" dia ally * 250mm length came from ebay, the rest of the stuff came from my stash or as is the case of the chunk of sash weight cast iron from John, and the bearings were donated by Dave Bluchip

To make the boring bar for the crank case I copied out the sectioned drawing at a 1 - 1 scale and used this as a guide to grind the bar, and yes I know its the wrong way but I'm going to run the lathe in revers as its easy to see what's going on that way.


This is it ground up


Then with a chunk of ally in the lathe a start was made on the crank case, skim up the OD, centre drill then a series of drill, and then with a boring bar it was roughed out.


Finished off the bore and then brought everything to finished size, Home made boring bar worked fine, although I had to trim a bit more off it to keep it clear of the job.


Keeping the job in the chuck the whole lot was transfered to the indexer.


First job with a wobbler find the edges of the job:- zero the X on centre line and zero the Y on the front edge.



With the fly cutter I then made the round bar into an hexagon, this is where table stops help, with the stops set I had no worries about running into the chuck.

Forgot to take a pic of the fly cutting but you've seen that before.

Move the Y to where the cylinders will be mounted and zero the dro again from know on all features will be from the centre line of this hole.

Centre drill then roughing drill to get the meat out. Note use of support Jack


And then with the boring head rough out all the to within 0.5 mm


With all the cylinder holes rouged out, brought the first one to size, then as I wanted all to be the same, it'll make things easy down line, the boring bar was clamp up and the other two finished at the same setting.

One of the reasons I've reproduced the drawing in cad was that I wanted all the holes dimensioned from the centre line of the cylinders this way I can make best use of the DRO, so to drill the holes move the table to the x and Y location drill the hole, rotate the spin indexer drill the same hole in the other face, spin index drill the same hole in the third face reposition the table x and Y etc etc

Like this


And here we are with all the holes drilled in the horizontal position including the breather hole tapped 3/8*40 ME I'll tap the M3 holes on my tapping table.


With the job parted off I set it up in the chuck to face it off to length, when disaster struck, the @£$%%^&* thing flipped out of the chuck putting a few dings in it, luckily they are in places that won't be seen when its all assembled together, so set it up in the mill vice to fly cut it off to length.

Now for the port face:-

A little bit more setting up required for this.

First as it will be easy to put features in the wrong orientation, I marked the job up where things were to go checking and double checking I'd got it correct, it wasn't accurately marked just enough to keep me on the straight and narrow.


Next get the job on the RT centre line, to do this you rotate the RT, as I was using a three jaw self centre chuck I had to keep channing the position of the job in the chuck and tightening up on different jaws to find the best place I got it to 0.1mm run out well fit for function.


Next I had to get the hexagon in the correct orientation by clocking up a face level with the RT on zero.


Then get the job on the centre line of the quill (spindle) to do this you hold the DTI in the spindle and rotate the spindle.


Then use the DRO drill the holes in this case one set of 6 and one set of 3.


Ok last bit of work on the crank case:- cut the steam galleries, the drawing calls for the angle from edge to edge to be 60 deg, but for machining the critical feature is centre to centre of the cutter I'm using a 2mm slot drill so drew it up in cad and checked the cutter centre to centre angle 44 deg.

The mill was still set on the centre of the job, move the radius distance to where the ports will be, and rotated the RT 30 deg then off set 1mm to allow 1/2 cutter dia. lower the cutter 1 mm into work and rotate RT 44 deg. To make things simple I worked out the movement of the RT in advance and wrote out a crib sheet.

Like this


Ports cut


Last job tap all the holes:- I used my home made tapping stand, so that they were all tapped nice and square.



And there we are Crank case done.


The cylinders I'm going to make from ally with cast iron liners.

2" diameter length of ally being turned down.


I was hopping to get four cylinders out of this ally (one spare) but I forgot about the width of the parting tool so only got three, I'll have to be real carful

When making multiples its important to get certain key feature as near as posable all the same size, in this case its the width and the bore dia.

So by putting the job hard up against the back of the chuck and zeroing up the cross slide they were all faced off the same length.


Because of how I'm going to machine things down line I'm not particularly worried about getting the bore concentric with the OD. So sticking the blanks back in the chuck all the bores were centre drilled and drilled out with bigger and bigger drills.


They were all then rough out to within 1/2 mm of finish size with a boring bar.

Then finished off to size with the boring bar:- one after the other at the same setting, running the final cut through a couple of times to take the spring out of the bar.

For length they were all within 0.05mm and for dia 0.02 mm thats well fit for purpose.

Here they all are.


Having slept on it I decided to take some of the meat off the cylinders before I made the liners.

So first job turn up a mandrel thats a nice slide fit on the cylinders with a M12 thread.

Like this


This required some quite heavy cuts and I was a bit concerned the liner would come lose, so decided to get rid of quite a bit of the unwanted material before fitting them.


Then turn down the 35 mm dia sash weight down to 19.25 to give a 0.02 mm interferance fit in the cylinder and part off to length.


Then set back up in the lathe, again concentricity with OD not that important, centre drill and with increasing in diameter drill rough all three out, then they were set back up to finish with boring bar at the same cut so they all ended up the same size.

As I'm going to use the mandrel again to finish off the machining its easyer if they all have the same size bore. The actual size of the bore is not that important, I'll just make the piston to fit, its easy to work to an accurate OD than it is a ID, what's important with the bore is that its parallel and the finish is reasonable.


I just gave the liners a polish to give them a square start into the cylinder, then with a bit of high strength loctite they were squeezed together using the vice as a press.


And her we are all three liners assembled to the cylinders.


I give the loctite 24 hrs to cure before I do any more machining to the cylinders

Set the mandrel back up in the lathe and turned it down to a nice fit on the sleaves, this is why I wanted all the bores the same size, so that one mandrell can be used for all three.

Then same trick as before:- chuck taken off the lathe with the mandrell still in place and fixed on the Spin Indexer.


Her we have one in the indexer with my crib sheet and two assy in the crank.


Close up of three in the crank case,


Just taken delivery of this from the US, I saw these in use a few years ago at one of our subcontractors, he was using a big one 6" dia to hone the bores of some hydrolic cylinders, have any of you guys ever used them and have you any tips.


Keeping the mandrell still in the chuck set it up on the mill and centred the mill quill on the mandrell zeroed the DRO.


Then just as with the crank case indexed and drilled the fixing holes for the cylinder.


With most of the key features machined in the cylinder time for a bit of bling.

Again still keeping the mandrell in the chuck it was transfered over to the lathe, then with one of Johns finning tools some fins were added to the cylinder.


I was going to use some 4mm brass tube for the air ways but the gap between the two pipes would only have been 0.75 mm so I bottled out of that and decided to use 1/8" copper tubes, did a lose assembly to see how the bling would look.


That doesn't look too bad, and I was realy pleased how the cylinders holes lined up with the crank case, this was all due to the accuracy I got out of the DRO, without out it I would have resorted to opening out the holes a fiddling arround to get things to assemble correctly.



That's some fine work. I really like radials, been fascinated by them since I was a young lad.
Keep up the good work, looking forward to more Thm:

What they said... this is really looking good!

On your hone no real tricks good speed 600- 800 keep it wet with your fav juice I like water soluble cutting oil and don't let it sit keep it moving in and out till you have the finish you want. I'm sure you will get it right quickly.But here is a cleaning trick, when you go to clean the bores use automatic transmission fluid and a white rag wipe till it comes out with no honing grit you will be able to see it on the white rag.
And buy the way nice job !!!!
A great start to a build, Stew, and you got quite a bite at it in one go! The copper passage tubes look very snappy next to the silver metal. I like your way of doing things.

Please keep us posted!

I can only add how much I too am impressed with your work so far. Very nice. I'm be watching.
Thanks for you kind comments chaps, its nice to get fead back.

Dave and thank you for the tips on using the hone I'll put them to good use.

Got a bit more work done on the cylinder heads:- cut some 1/4" ally plate leaving 1mm big all round, and made some toolmaker buttons, the plates were threaded M6 and the buttons attached.


Then gripping onto the button a register was turned up a nice fit in the cylinders and the heads skimed off to 4mm thick.



Then on parallels the button was clocked up to bring its centre line onto the quils centre line.


Then it was just a mater of using the DRO to find the position to drill the bolt holes, as I've shown before.

I want to fit a little brass boss on the head as a bit of bling, so first off ground up a radius form tool, I just took advantage of the rad that had worn on the edge of my off hand grinding wheel. Then using this form tool a nice rad was produced on a bit of brass this was then threaded M6 and parted off.


And screwed into the heads, this is how they look, a bit of tidying is still required but things are looking OK.




That's loverly!

And, you just shown me a good tip for finding the center of round stock.

Thank you!
This engine is really coming together nicely. It's going to be a great looking engine and your workmanship is superb.

You've got some nice teachings going on here, very thought out. Looking forward to seeing the posts for the internals.
Great going Stew :bow: - you sure are not wasting time on this one! Thanks for showing!

Regards, Arnold
Yes, things are certainly looking "OK", Stew. A bit of an understatement.
You know how to dress an engine nicely, without making it look like a Mr T ornament.

Looking great. Thanks for sharing your build.


Hi Chaps

Thanks again guys for your comments and interest

Had a right good do in the shed today.

First bit steam chest, turned out of a bit of 1 5/8" ally:- turn od then a big drill down the midle bore to size, transfer over still in chuck to the mill clock up OD to line up with quill and drill the holes on the PCD, the DRO realy does make this an easy job.



Part off and face to length:- job done


Next bit steam chest cover this is a bit more tricky as its not very thick:- turn up OD, turn step then with a trepan tool carve out the groove in the face.

Here we are trepanning


Drill and tap 1/4" * 32 ME


Over to the mill still in the chuck for drilling PCD

Back onto the lathe for partin off

Turn a threaded mandrel 1/4" * 32 ME.


And screw the cover on, then very gently small cuts and a sharp tool face it off to length and turn register step to fit in steam chest.


Thats another bit done.

Next up bearing housing:- Chunk of 2" ally skim up OD then turn up reduced diameter, the drawing calls for this to be tapered, but I'll need to chuck on the taper to do the other end:- forget it, I'll turn it parallel, if I decide I want it tapered I'll set it up on a lose mandrel when I've got the rest done.

First centre drill then bigger and bigger drills finishing up with a 3/4"


Then with my favourite boring bar bore 7/8" for bearing.


Part off


At that called it a day.

This is the steam chest and cover in place and the part finished bearing housing.


Have fun

Hi Chaps

Domestic duties kept me out of the shed today, that and its bloody cold, well for us soft limies it is.

I want to change the design of the crank shaft layout so spent a bit of time in the nice warm house doing a bit of CAD on the crank shaft design,

These are the defferent design options


I got 1) wrong (elmer) it as a fixed master con rod like 2)
This is the one I want to run with


Until I'd drawn it out I thought it would just be a simple off set of the con rod, but no on the origional cygnet design the master rod is quite thin 1/8 this doesn't matter because of the way the bearing is orientated, but the new design requires a thicker rod to give the bearing a bit of width so you have to give the slave rods a bit more crank. Anyway after bit of fidling and head scratching :scratch: came up with this design.


I'll have to machine the con rods up with an angled web, this goes against my origonal objective of keeping the maching simple, and I've not yet worked out how to machine them.

So what do you think mmmmmmmmmmmmmmmmmmmmmmm


Don't see why it wouldn't work, but I don't think it would last very long. The high speeds those things turn and the offset connecting rod I think will bring about major stress problems along with balancing issues. Beleave the connecting rod, crank and piston should stay in plane.
Looks like your outside con rods would be perilously close to the bottom of the cylinder wall at midstroke.

Thanks for you imput Chuck and CMS, you've spotted some of my own concerns plus a few new ones, I think I,ll go back to the drawing board.

The main reason I want to go away from the cygnet design is that I don't like the fact that the big and little ends are ally running on steel or phos bronze, I know some grades of ally have good bearing properties but I'm using scap yard material of unkown spec, so i want to add steel/phos bearings, I think I may be over complicating the design change.

Thanks again for taking the trouble to reply.

Interesting build Stew, you look to be going down pretty much the same road I took. Like you, I shelved the engine until I can come up with a decent solution to the con rod problem - I don't like ETW's layout at all.

Great minds Tel :bow:

Thanks for youre input chaps some good comments their, getting me back on the road.

As some of you guys said the way I was going was too complicated. So I went back to the begining and decided to see how things would look with a 1/8" thick master con rod sharing the same 1/4" dia crank pin with the slave con rods to the sizes of the cygnet design.

I carfully drew up the con rods then assembled them onto a common piston to see how they would fit this showed that they interfeared with each other by about 1mm, decided to redraw taking the interference out, by reducing the thickness of the big end.

This is what I got.


Some of the sizes are a bit odd this is because I've converted from imperial to metric, when I make i'll round down to the nearest 0.1 mm with a tolereance of about 0.2. The straight webs will be quite easy to make I've worked out a what i think will be a easy method.

For the big end I've got some nice oillite bearing that i'll fit for the little ends I'll fit some phos bronze bearings.

First I'll make the crank shaft, fly wheel and pistons so that i can cut and fit to see how it all goes together.



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